A

SEMINAR REPORT

ON

LASER BEAM WELDING OF PLASTIC

SUBMITTED IN FULFILLMENT FOR

SEMINAR [ME – 403] COURSE

BY

DEEPA RAM

(201IUME1498)

UNDER GUIDANCE OF

Prof. Gopal Agrawal

Dr. G.D. Agrawal

Dr. Gunjan Soni

DEPARTMENT OF MECHANICAL ENGINEERING

MALAVIYA NATIONAL INSTITUTE OF TECHNOLOGY JAIPUR

J.L.N. MARG, JAIPUR-302017

SEPTEMBER 2014

ii

© Malaviya National Institute of Technology Jaipur, 2014

All rights reserved.

iii

ACKNOWLEDGEMENT

First and foremost, I express my deep sense of gratitude and respect to Dr. G. D.

Agrawal, Associate Professor; Dr. Gopal Agrawal, Associate Professor and Dr. Gunjan

Soni, Assistant Professor Department of Mechanical Engineering, MNIT Jaipur for their

invaluable comments, editorial help, and overall support in the completion of this seminar

report.

Last but not least, I am especially indebted to my parents for their love, sacrifice and

support. I wish to express my deep gratitude to all those who extended their helping hands

towards me in various ways during compilation of this report. And all the members of

Department of Mechanical Engineering, MNIT Jaipur.

(Deepa Ram Suthar)

2011UME1498

iv

ABSTRACT .

“The laser was invented in 1960…. So new was the tool that our thinking had not

caught up with the possibility” William M. Steen. New possibilities are still arising as a

consequence and cause of the laser's increasing reliability, decreasing price and the

diversification of laser characteristics.

The demand for consumer goods, namely, food and medical products, to be

conveniently packaged in plastic materials in order to preserve quality and hygiene, is

constantly increasing, as are the number of packaging styles and materials.

The replacement of traditional tools, used to cut or weld in the plastic packaging

industry (hot knives, ultrasonic heads or hot air), by laser tools, can be justified by the increase

in the reproducibility of the process (no tool wear), simplicity of processing moving parts (no

need to `stop and start a production lines) and increase in productivity moving the laser beam

over the material faster than the mechanical counterpart. Not to mention the well-known

general advantages of laser materials processing, as a non-contact, non-contaminant process,

flexible and easy to control and automate.

The first few communications on plastic welding by laser appeared in the literature

in 1972, welds of low-density polyethylene sheets up to 1.5mm thick were achieved with a

100W CO2 laser at speeds of 10mms~1. However, it has been during the last decade that

research in this subject has seen greater development, regarding increasing speed, new laser

sources mathematical modelling and industrial applications. It is very likely that much more

proprietary industrial work has been done, but not published. The component-conserving and

clean process offers numerous advantages and enables welding of sensitive assemblies in

automotive, electronic, medical, human care, food packaging and consumer electronics

markets. Diode lasers are established since years within plastic welding applications

v

Title Page……………………………………………………………………….. (i)

Acknowledgement…………………………………………………………….... (iii)

Abstract…………………………………………………………………………..(iv)

Table of Contents………………………………………………………………. (v)

List of Figures…………………………………………………………………... (vi)

List of Tables.…………………………………………………………………... (vii)

List of Abbreviation……………………………………………………………. (viii)

1. Introduction……..……………………………………………………………….. 1

2. Laser Plastic Welding……………………………………………………………. 6

2.1 Fundamentals of laser welding of plastics

2.2 Lasers used for welding plastics

2.3 Welding of plastics of the same type

2.4 Joining of dissimilar plastics

2.5 Welding of plastics of the same type

2.6 Advantage

2.7 Application

3. Hybrid Technologies of Laser Welding of Plastics………….………….……….22

3.1 Hybrid Laser Welding

3.2 Laser welding of plastics to other materials

4. Fibre Laser Welding Assisted by Solid Heat Sink……….……………………..28

4.1 Principle of the welding method

4.2 Choice of infrared radiation source

4.3 Results

5. Quality and Process Control ………………………………….………………….32

5.1 Quality Control

5.2 Process Monitoring Methods

5.3 Cost Comparison

5.4 Boosting Efficiency

6. CONCLUSION………………………………………...………………………… 49

REFERENCES….…...…………………………………………………………… 51

Table of Contents

vi

LIST OF FIGURES

Sr. No. Title of Figure Page Number

1.1 World’s demand for selected constructional

materials (Hyla 2004)

2

2.1 The Process of Transmission Laser Welding 6

2.2 Method of Welding Plastic Component 8

2.3 Assembly of Automobile Light 17

2.4 Material Compatibility 17

3.1 Equipment for Light Laser Welding using

Nd:YAG or CO2 laser

24

3.2 Hybrid Welding Equipment by LPKF Company 26

4.1 Principle of novel infrared radiation welding

procedure with a transparent heat sink

29

4.2 Transmission Spectra of Plastics 30

5.1 Galvo pyro combination 33

5.2 Melt Collapse 35

5.3 Reflection Diagnosis Concept 36

5.4 Camera view of flaw 37

5.5 Cycle Time Comparison 39

vii

LIST OF TABLES

Sr. No. Title of Table Page Number

2.1 Comparison with Bonding 11

2.2 Main technical characteristics of the individual

lasers used for welding plastics

12

2.3 Weld-ability of different plastics 15

viii

LIST OF ABBREVIATION

Sr. No. Abbreviation Full Form

1 DPSS Diode Pumping Solid State

Laser

2 LFTPC Long Fibre Reinforced

Thermoplastic Composites

3 PS Poly styrene

4 ABS acrylonitrile butadiene

styrene

5 PMMA polymethyl methacrylate

6 Nd : YAG Laser Neodymium-Doped Yttrium

Aluminium Garnet

7 LPKF LPKF Laser & Electronics

AG

8 TTIR Through Transmission

Infrared Laser

9 LASER Light Amplification By

Stimulated Emission Of

Radiation

1

1. Introduction

Plastic materials form such a large and varied group, characterised by so wide a range

of properties that it is now normal to call them ‘mass material’. It is difficult to conceive of

plastics being substituted for by other materials. In certain circumstances, this group of

materials is a substitute for traditional materials, such as steels and non-ferrous metals. The

high level of experience that is associated with current methods of manufacture of semi-

fabricates in plastic materials provides designers with unlimited possibilities to produce new

products, often of complicated shapes. Modern bonding methods, such as laser-welding, make

it possible to widen the range, and this in turn, gives a chance to free selection of shape to be

made, and properties of new product to be developed.

The constantly growing utilisation of plastics in the industry creates new possibilities

for constructional solutions, lowering of cost and mass of fabricates, and also for the rising of

durability levels, resistance to corrosion and action of many chemical agents (Hyla 2004). All

of the above listed factors lead to call for more plastics and for their participation in the world’s

production of constructional materials to rise at a high rate (see Figure). Over1000 various

types of such materials are currently available on the world markets. From the point of view of

bulk, they constitute over half of the production of steels. According to data produced in 2000,

the use of these materials in the world in 1993 amounted to 90 m tons. It rose in 1999 to about

a 100 m, and is expected to reach over 120 m tons by 2020 (Zuchowska 2000).

In line with the popularisation of plastics as constructional materials comes the

development of bonding methods and the provision for selection of welding techniques correct

from the point of view of the level of yield and the economy of bonding.

The need for effective and reliable bonding methods and requirements for improved

quality of products are undoubtedly reasons for the development of new processes of bonding

plastics (Boron 2000), which also include laser-welding. Not only does this method provide

high efficiency, the highest possible levels of quality and strength of bonds, but also the

maintenance of high manufacturing precision and cleanliness of the joint area.

2

The laser welding of plastics is the advanced technology of joining of sheets, films or

shaped components produced from polymers in heating with the focused beam of laser

radiation. Laser welding was demonstrated for the first time in the 1970s and has been regarded

for many years as an expensive process in competition with the conventional technologies of

joining of components. Nevertheless, since the middle of the 1990s, laser technology has been

widely accepted as a result of advances in the area of laser methods.

The laser welding systems are most efficient in the applications in which the welded

components require careful handling (electronic components) or sterile conditions (medical

tools, packing of food products, etc.). The very high speed of laser welding makes this method

especially valuable in applications in the assembly lines of plastic components. Laser welding

Fig 1.1 : World’s demand for selected constructional materials (Hyla 2004).

3

can also be used to join components with complicated geometry which are difficult or

impossible to weld by other methods. Engineers often note special advantages of laser

technologies which will result in the active growth of the number of industrial applications –

the absence of contact of welding equipment with the welded components; very low labour

content; the possibility of joining materials of different composition and colour; high quality

of welded joints; only slight heating of components and minimum deformation; possibility of

welding in areas of difficult access and different spatial position; simple automation and

robotization; efficient use of electric energy and filler materials; comfortable working

conditions and ecological efficiency.

Laser welding is used extensively in electronics in assembling keyboards for different

systems, mobile telephones, a large number of contact devices, etc. and also in the car industry

in the production and assembly of automatic door locks, devices for keyless access, heating

models, the bodies of transmissions, sensors of sections of engines, the bodies of the driver

cabins, the oil tanks of the hydraulic systems, filter casings and many other systems. In

medicine, laser welding is used for assembly of containers and filters for liquids, joining of

pipes, bags for patients with intestinal problems, implants and micro jet elements used for

analysis, etc. The technology of melting the edges of thin plastic films for hermetic packing

items is used widely.

Laser welding of plastics is a very ‘young’ technological process. As a result of the

development by technologists and also rapid advances in laser technology, the methods of laser

welding are being constantly improved. The authors of the present article have already

discussed this subject many times. At the same time, it is believed that the laser welding of

plastics is an independent section of laser technology and has a considerable scientific and

industrial potential. It is therefore convenient to consider separately in this article the current

state and dynamics of investigations and developments and also the prospects of this advanced

technological process.

4

Traditional methods of welding plastics

Plastics are used as structural materials in engineering, the car industry, aviation,

instrument making, electrical engineering, shipbuilding, etc. Polymer materials are used as

films, adhesives and fibres. The process of welding of plastics consists of the formation of a

permanent joint as a result of the formation of interatomic (intermolecular) bonds between the

surface atoms of two welded components. Thermoplastics are welded using the heat of

secondary sources (gas heat carriers, heated filler material and heated tools) or by the

generation of heat inside the plastic material in conversion of different types of energy (friction

welding, high-frequency current welding, ultrasonic welding, welding with infrared radiation,

etc.). Thermosetting plastics (thermosets) are welded by the method based on the chemical

interaction between the surfaces directly or with participation of filler material (the so-called

chemical welding).

As in the welding of metals, in welding of plastics it is necessary to ensure that the

mechanical and physical properties of the material of the welded joints and the weld zone differ

only slightly from those of the parent material. The strength of the welded joints in the plastics

is greatly affected by the chemical composition, the orientation of macromolecules, the

temperature of the environment and other factors. The most widely used methods of welding

plastics include welding with a gas heat carrier with or without a filler, with an extruded filler,

contact-thermal welding, welding in a high-frequency electric field, ultrasonic welding,

friction welding, beam and chemical welding.

It is well known that not all the types of plastics can be welded. In particular, the

thermally hardened plastics (which do not melt under heat) cannot be welded. On the other

hand, thermoplastic materials (melting during heating) can be welded by a large number of

methods. It is believed that only plastics of the same type can be welded because every type of

plastic material has its own typical molecular structure and welding temperature. The joining

of plastics by welding takes place if the three constant conditions are fulfilled:

.

Higher temperature which should reach the level of the viscous-fluid state of the

welded materials. The transition of the polymer to the viscous-fluid state should not be

accompanied by thermal degradation of the material. Every plastic melts within a

specific temperature range;

5

.

Tight contact of the welded surfaces. Pressure enables the molecules of the plastics to

mix with the formation of the welded joints. The quality of the welded joint decreases

when the pressure is reduced below or is higher than the optimum pressure for every

pair of materials;

.

Optimum holding time because the plastic material requires a certain period of time for

melting and a certain period of time for cooling. It should be mentioned that the

temperature coefficient of linear expansion of plastic materials is several times higher

than that of the metals and, therefore, welding and cooling are accompanied by the

formation of the residual stresses and strains which reduce the strength of the welded

joints in the plastics. In this case, acceleration of the welding process may cause higher

stresses in the region of the welded joint.

There are a large number of systems for welding plastics on the market but no universal

welding technology is available.

6

2. LASER PLASTIC WELDING

2.1 Fundamentals of laser welding of plastics

Laser welding rightly occupies the leading position in the group of the advanced

technologies of beam welding of plastics. Special features of the laser welding processes,

resulting in very rapid introduction into various branches of industry, are the absence of direct

contact between the emitter (heat) and the welded surfaces, and also the possibility of

controlling a wide range of the heating conditions as a result of changes of radiation power,

and the heat and light absorbing capacity of the welded materials. At the present time,

transmission (penetrating) laser welding is used widely. This type of welding has a number of

significant advantages in comparison with the traditional methods of welding plastic materials,

such as vibration, contact thermal or ultrasonic welding.

Transmission laser welding is based on the physical effect in which many polymers

efficiently absorb radiation in the near-infrared range. A relatively narrow wavelength range

(800—2000 nm) is used for welding.

Fig 2.1: The process of transmission laser welding: (1) laser beam; (2) clamping force; (3) melt;

(4) the component permeable for the laser beam and (5) heat transfer.

7

Usually, the components to be welded are placed in a clamping device which

compresses the components together with maximum force. In the conventional device, the

material of the component which is the first to be affected by the beam is selected to ensure

that it transmits maximum radiation (Figure 1). The material of the second component should

be capable of absorbing laser radiation. The laser radiation beam passes through the first

(‘transparent’) component and is absorbed by the material of the second component of the joint

generating a large amount of heat. Since both the components are tightly pressed to each other,

the heat is transferred from the absorbing layer to the transmitting layer and heats both

components. The thin layers of the plastic, situated on both sides of the joint, melt, mix together

and form strong joints during cooling. The main critical process parameters are temperature,

holding time and pressure.

The energy density required for welding is associated with the temperature of the

component and the duration of the process and is determined by the laser power, the size of the

working spot of radiation on the component, the radiation time (for stationary processes) or

welding speed (in the processes with relative displacement of the components). The energy

density in this case is proportional to the radiation power and inversely proportional to the area

of the focused beam on the processed surface and the speed of travel of the beam in relation to

the surface.

If the level of laser radiation energy in the welding zone is not sufficiently high, heating

may prove to be insufficient and, correspondingly, the welded components are not held for a

sufficiently long period of time in the heated state for the formation of a strong joint. On the

other hand, in excessive heating the polymers may degrade in the joint zone resulting in the

formation of porosity, charring or burning. In practice, there is a wide range of conditions for

each specific joint in which the joints of acceptable quality form. The majority of polymers are

welded using an laser energy density in the range of 0.1– 2.0 J/mm2. Regardless of the fact that

the energy density in the welding zone can be used to characterize the process, many authors

believe that this correspondence is only conditional. The heat transfer from the welding zone

in the welding process should be taken into account and this makes the process non-linear. This

means that the application of the same energy density results in the same quality of the welded

joint. For example, at a constant size of the focused radiation spot, doubling the radiation power

8

usually increases the welding speed by more than 100% whereas the welding characteristics

remain the same:

where Ew is the radiation energy supply to the welding zone; P is the laser radiation power in

the vicinity of the welding zone, and v is the speed of travel of the beam in relation to the

welded components.

In cases in which the welded plastic components are not compressed to each other or

the compression pressure is not sufficiently high, the contact between the components is not

sufficiently tight. This may result either in inefficient heat transfer from one component to the

other or in limited mutual diffusion of the polymer chains on both sides of the joints. In both

cases, the strength of the welded joint is reduced. Therefore, reliable clamping and securing of

the welded components in the weld zone is an important technical condition. The clamps

Fig 2.2 : Methods of welding plastic components: (a) with the moving object in

welding; (b) with the moving beam; (c) with the fans shaped distribution of radiation;

(d) simultaneous (synchronous) welding around the perimeter (contour) and (e) with

the scanning radiation beam.

9

Laser welding is used for joining components of different shapes, size and

configuration. There are several main methods of laser welding differing in the methods of

generating the forces in the range of 0.1–1.0 N/mm2 are used in most cases. Transmission

welding using Nd:YAG or diode lasers has been used to weld successfully the plastic

components more than 1mm thick at a linear welding speed greater than 20 m/min. The welding

speed in welding of films using CO2 lasers can be even higher, up to 750 m/min, although

technologists frequently mention the restrictions of carbon dioxide laser. relative displacement

of the welded components and the laser radiation beam (Figure 2).

At a stationary radiation beam, the components to be welded are moved to produce a continuous

joint (Figure 2(a)). In most cases, this displacement is produced using a table with movement

along one or two coordinates and can be easily programmed. This method is used only in cases

in which welding in three coordinates is not required.

The optical system for the radiation beam, supplied by the optical waveguide, or the

head of the diode laser can be installed in robotized equipment, including the three coordinate

systems of the hand. In these cases, the laser or the final element of the optical system travels

along the trajectory (the contour) corresponding to the future welded joint. In welding along

the contour, the layers are gradually welded by the laser beam which travels and melts the

material along the welded joint. In a different variant of this welding system, the components,

compressed to each other, travel in relation to the stationary laser beam. In the automatic

systems, the displacement of the laser beam is often combined with the displacement of the

components. In synchronous welding with several beams, the laser radiation from, for example,

several laser diodes is directed to the contour line of the welded joint which is to be welded

resulting in simultaneous melting and welding of the entire profile (Figures 2(b) and (d)). One

of the varieties of this welding method is based on the radiation of a single laser split into

several separate beams which are subsequently applied together on the component to improve

the strength of the effect. In some cases, it is recommended to use quasi-synchronous welding

, which is based on the combination of welding around the contour and synchronous welding.

The mirrors direct the laser beam at a high speed (at least 10 m/s) along the component which

is to be welded. The entire contour of the component is then gradually heated and melts.

10

Welding with a template is slightly different. In this welding method, the laser beam is

applied to the component through a specially produced template which does not cover small,

precisely defined areas of the underlying plastic layers which will be melted and bonded.The

method can be used for producing precision joints with a high resolution, up to 10 mm.

In cases in which the laser power is adequate, welding is carried out with the fan-shaped

distribution of radiation in which laser radiation is distributed in a flat diverging beam and

forms a line on the surface of the component (Figure 2(c)). During welding, the beam or the

component travels in a specific direction. Masks which protect sections of the component that

should not be subjected to radiation are used in some cases. This method is often used in two

dimensional welding of small components with the complicated configuration of the welded

joint.

If it is required to produce a large number of identical short welded joints or weld spots,

it is recommended to use the matrix of diode emitters which is shaped according to the shape

of the component and assembled taking into account the number of welded joints. This method,

which is used if simultaneous laser radiation is to be applied along the entire length of the joint,

is usually automated. This is carried out using basic equipment for ultrasonic welding in which

laser technology efficiently replaces the ultrasonic process in the technology of joining

components sensitive to vibrations and where high-quality welded joints are to be produced.

The technology of simultaneous welding permits both two dimensional and three-dimensional

configuration. A particular advantage of this method of laser welding is the larger allowance

for the welding operation.

The laser radiation beam in welding with scanning (Figure 2(e)) is deflected by two

orthogonal mirrors controlled by the direction of propagation of the beam in space. The

working zone in the systems of this type has the transverse dimensions from 50 x 50 to 1000 x

1000mm in two-dimensional welding. Generally speaking, the main problem when increasing

the treatment area is the appropriate increase in the difference of the working path of the laser

beam so that it is necessary to ‘under focus’ the beam. An efficient method of coordinating the

focusing of the beam in different areas of the treated surface is the application of several

scanning optical systems, and the combination of the systems increases the length of the

treatment zone. As in simultaneous welding, these welded joints overlap the entire joint zone

11

and are characterized by high shrinkage of the material and potentially larger welding

allowances.

Table 2.1: Comparison with Bonding

12

2.2 Lasers used for welding plastics

Several main types of lasers are used in industry for welding of plastics at the present

time. These include CO2 lasers, solid-state lasers (with lamp or diode pumping) and fibre

lasers. Recently, high-power diode lasers have also been used widely in certain areas of

production.

Table 2.2: main technical characteristics of the individual lasers used for welding plastics

CO2 lasers are used at a wavelength of 10.6 mm in the infrared range. Usually, these

lasers generate a beam of highly collimated radiation with a diameter from several millimetres

to several centimetres. A significant shortcoming of the CO2 lasers (like of any gas laser) is

the low efficiency (the radiation power, related to the electrical power in pumping) resulting in

high production costs. The second shortcoming of the powerful gas lasers is their large

dimensions. Both factors introduce a number of restrictions on use in the actual technological

process. In addition to this, the radiation of CO2 lasers cannot be sharply focused because of

the multimode structure and large wavelength of radiation (laser radiation of the majority of

lasers) and, therefore, equipment based on CO2 lasers is used mainly for welding of films.

The collimated radiation beam at the exit of the solid state lasers, where the pumping

of radiation from the lamp or a group of light diodes is focused injected into the laser bar or

discs, has the wavelength in the near-infrared region (usually 1.064 mm) and is transferred to

the treatment area through the light waveguide. The solid-state lasers with lamp pumping are

13

characterized by the relatively low efficiency and, in addition to this, the pumping lamps

require regular and relatively frequent replacement. This greatly increases the service costs.

Nevertheless, the commercial technologies of welding using these lasers have been developed

quite extensively. The lasers with diode pumping (DPSS) have a considerably higher

reliability, longer service periods and the low cost of materials and components. However, the

initial price of these lasers is slightly higher and this requires higher initial investment and

actually equalizes the service costs of the solid-state lasers of both types.

The fibre lasers with doping the active substance with rare-earth elements generate

radiation with the single wavelength in the range of 1.0–2.1 mm. Usually, fibre lasers are quite

small and the active element can be cooled by water. Taking into account the large amount of

data obtained in recent years, the optimum possibilities of fibre lasers are used in particular in

welding plastic materials, including precision welding and welding of films and fabrics. As

regards the concept, the fibre lasers resemble solid-state lasers with diode pumping, with the

only difference being that the role of the laser medium is played by the optical waveguide. The

single-mode fibre lasers are capable of generating the radiation beam at the same wavelength

and similar power as the DPSS lasers. A particular advantage of these lasers in comparison

with the solid-state generators is that they generate a sharply directed beam which can be

simply focused into a very small spot.

A number of important developments in the last decade in the area of diode laser

technology expanded the possibilities of laser welding and also greatly changed the economic

parameters of these processes. Consequently, the diode lasers of the new generation are

replacing conventional sources of laser radiation in many industrial technologies, including

welding. The diode lasers are characterized by the compact form, the relatively low initial price

and service costs, high efficiency (up to 60% on the emitter) and a large number of variants of

the radiation wavelength (e.g., 405, 640, 790–1060 and 1450 nm, which is convenient in

welding different types of plastics). On the other hand, as a result of the relatively simple design

of the resonator, the laser diodes do not have a facility for efficient adjusting of the radiation

beam, which results in low coherence and a wide radiation spectrum (sharper focusing of the

beam is not possible), and also the resultant short working distances .

14

The efficiency of the laser is the ratio of the emitted power to the power required by the laser

in the standard mode, the quality of the beam (Table 1) is the possibility of sharper focusing of

radiation with a high power density in the working spot.

2.3 Welding of plastics of the same type

Laser welding of two identical or similar (as regards chemical composition) plastics is

a widely used technological process, especially if it is necessary to produce high-quality joints

with high productivity. In many cases, laser technology is the result of optimization of the

technology of joining and selecting the material. In transition from the traditional welding

technologies to laser treatment, it is often necessary to change the materials and this may result

in considerably lower costs in comparison with the conservative process. For example, in

analysis of the production problems, the experts of Barkston Plastics Engineering (the well-

known British producer of unique elastic components) noted the low repeatability of the

characteristics of a series of propylene bags for the storage of liquid chemical reagents in

lithographic equipment for the production of printed boards. In the technology used by the

company, these bags were assembled by manual gas welding.

As an alternative to traditional welding, transmission laser welding was selected

in the modernized process. In designing new technology, it was important to take into account

two essential requirements: the strength of the joint should not be lower than in standard gas

welding, and the joint should be leak tight. Previously, the bags were produced from natural

polypropylene. To increase the technological parameters of the new process, including laser

welding, the edges of the end sheets were produced as previously from the same polypropylene

which is transparent to laser radiation in the near-infrared region of the spectrum, whereas the

transverse sections of the bags are pressed from polypropylene sheets which efficiently absorb

laser radiation. The welded joints, produced by the new technology, have the same strength as

those produced by the standard technology – no failures were detected in the welded joints in

both cases. The new process greatly simplifies the design of the transverse sections and

produces efficiently leak tight joints. Subsequently, the process was automated and introduced

into the production cycle.

15

At the present time, the bags are welded by a robot of Motoman company with six

degrees of freedom, controlled by a diode laser manufactured by Laserlines. The components

are secured and compressed using a ring shaped sliding clamp controlled by a pneumatic drive.

Laser welding of identical plastics showed the highest efficiency in lap welding of

thermoplastic films.

The application of laser radiation for joining the reinforced thermoplastic composites

offers new possibilities for overcoming the shortcomings of the traditional technologies. In this

case, laser welding is ecologically clean because no chemical additions or adhesives are

required. The accuracy, flexibility of laser technology and also the high-quality welded joints

are already utilized in a large number of industrial applications.Laser transmission welding has

been introduced in the industry in the manufacture of thermoplastic composites with short

fibres, where the laser efficiently replaces welding with a hot air jet. Of special interest in recent

years has been the possibility of welding reinforced composites with the long fibre structure

(long fibre reinforced thermoplastic composites LFTPC – thermoplastics with the fibre length

greater than 6 mm). The authors of developed technical fundamentals of laser technology

utilizing the natural properties of the material. The new technology is based on the layer

welding and the mechanical characteristics of the welded joints were determined. The test

results show that laser welding is a superior and highly promising technology for joining many

combinations of materials used in the automobile and aviation industries and is characterized

by considerably better ecological parameters and safety of the processes in comparison with

the conventional adhesive bonding technologies.

Table 2.3: Weldability of different plastics

16

2.4 Joining of dissimilar plastics

Transmission laser welding differs only slightly from other welding processes in

joining of the different combinations of dissimilar plastics. Many authors have noted that the

dissimilar plastics cannot be welded with high quality. Nevertheless, there are also exceptions

from this rule. One of them is the technology of melting sections of rear lights of vehicles

where the polymethyl methacrylate (PMMA) lenses are joined to the body made of acrylonitrile

butadiene styrene (ABS) by the contact method. The results of introduction of laser welding in

assembling of identical sections have also been published (Figure 3). Dissimilar combinations

which can be welded should be chemically compatible and also have similar glass transition

temperatures (amorphous polymers) or similar melting points (polycrystalline polymers).

Table 2 gives the data for welding dissimilar plastics. It may be seen that the materials not

listed in Table 2 can be welded usually only to the plastics of the same type.

Almost all thermoplastics and thermoplastic elastomers can be laser welded. Some

structural plastics, such as polyphenyl sulphide and liquid crystal polymers, are difficult to

weld by laser welding because of low permeability for laser radiation. In order to make the

lower layer capable of absorbing the energy of laser beams, carbon soot is often added to these

layers. Laser welding is used to join both non-saturated polymers and also polymers reinforced

with glass fibres. Lasers can be used for welding coloured plastics but the permeability for laser

beams decreases with an increase the concentration of the dye or pigment.

Many authors have published tables of the weldability of dissimilar plastics. The most

detailed results for laser welding are found in the tables of regularly updated website

www.laserplasticwelding.com maintained by the enthusiasts and professionals in laser welding

technology.

17

Fig 2.3 : Assembly of the automobile light (by the method of laser welding using a glass sphere

which focuses the laser beam and also acts as the clamping device).

Fig 2.4 : Material Compatibility Chart

18

2.5 Welding of transparent plastics

Until recently, in the introduction of welding technology it was necessary to use

standard materials, and in the majority of cases both welded materials were visually non

transparent. The technological restriction of transmission laser welding – the upper layer

(component) should be transparent to laser radiation – and the lower layer – should absorbs

laser radiation, considerably limited the design possibilities. In transition to laser technology,

this usually included the change of the material to a suitable material or addition of a dye to the

plastic to increase its absorptivity. At the same time, for example, in the medical industry, today

are a large number of tasks in which one or frequently both welded components should be

transparent. This circumstance has greatly restricted the areas of application of laser

technology. Therefore, in many processes of this type it has been necessary to develop a

welding technology without using additional absorbing materials. However, such materials are

either very expensive or have different colour shades which are not acceptable in components.

A breakthrough in the welding of transparent plastic was made by the British company

TWI which reported the development of a new technology in a patent in 2003. Clearweld

Technology is based on the application of plastic materials with a high absorptivity as the laser

radiation wavelength (and at the same time the minimum absorptivity in the visible range of

the spectrum). This approach can be used to produce welded joints with the minimum effect

on the external appearance of the component, and offers considerable flexibility in the selection

of materials and colours. The absorbing material is represented by an additional coating or the

lower layer of the welded pair.

Many systems of pair plastics with the selection of appropriate colours providing

further possibilities for designers have been developed. The only condition in the pair is the

coincidence of the visual colour and the large difference in the absorptivity of the radiation

wavelength of the working laser. The very first application of the method was welding of two

visually black materials; at the present time, a large number of systems of pairs, including white

materials, are available. The project Poly Bright awarded to the scientific and research

organizations of the European Community countries has been formulated for detailed

investigations of laser welding of polymers and for the development of technological

conditions of high speed and flexible industrial laser technologies. The key aspect of the project

19

is the extensive application of the fibre lasers with the power of up to 500W which can be used

to optimize not only the thermal parameter of the welding process but also the wavelength to

increase the efficiency and speed in welding and the quality of the joint.

New laser welding systems have been developed, the resolution of technology has been

improved by using dynamic masks in high-frequency scanning systems for transporting and

focusing the beam.

The experimental results show that the most promising results in laser technology can

be obtained by selecting laser radiation with the wavelength at which the welded plastics have

the required properties.

The latest technology and prospects for laser welding without absorption of radiation

on the example of the components of transparent PMMA polymer films using the accurate

selection of the wavelength of laser radiation and radiation techniques have been published in

research.

Regardless of the completely different physical principle of welding, the authors have managed

to obtain the highest technological parameters of the process. The welding speed reached up to

100mm/s at the laser radiation wavelength of 1550, 1700 and 1908 nm. The best spectrometric

results have been obtained at the radiation wavelength of approximately 1700 nm which is used

by many fibre lasers. To supply the radiation to the weld boundary of the transparent materials,

the authors developed a special lens optical system in which the focal point is situated at the

interface with high geometrical accuracy.

2.6 Advantage:

1. Lower Joining Cost

2. Minimal Part Stress

3. Joint Strength

4. 3d & Complex Shapes

5. No Particulate Development

6. Precision

7. Aesthetically Pleasing Weld

8. Weld Different Material

9. Process Monitoring

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2.7 Application

Nearly two decades old, laser plastic welding is gradually gaining a foothold in the

plastics joining universe. It started out as a niche welding method providing specialized welds

for cutting edge applications that could not be joined by traditional methods. Today the

technique is becoming more widely used.

Plummeting costs of laser sources and I higher demand for the systems themselves are

driving the cost to adopt laser welding down. In fact, the cost of a standard laser welding system

is now on par with traditional methods such as ultrasonic and hot plate welding.

Currently you can find the majority of applications in three major industries:

automotive, medical and consumer products. However, with the cost barrier alleviated, laser

plastic welding is beginning to look more attractive to wider range of applications and

industries.

Automotive

The automotive industry created the foundation for laser welding. The original use was

welding housings for electronic components. A simple task, but as electronics become more

prevalent in cars (approaching 35% of the total cost of a vehicle) protecting those electronics

is becoming increasingly important.

A stress free, reliable and highly monitored process allows for tightly sealed housings,

with no additional material costs and a near perfect reject rate. The high volume applications

in the automotive industry clearly benefit from such a process.

The flexibility of laser welding does not stop there. Through the use of robots, laser

welding was able to expand its abilities in the automotive industry to include lamp welding.

Clean, strong joints have been sought after for automotive lamp assembly ever since plastics

AUTO

CONSUMER

MEDICAL

21

replaced glass for exterior lighting. Laser welding is a stress free process and clean,

aesthetically appealing joints are easily achieved. But, possibly even more important is its

ability to work on large, free-form shapes with complex curves, a vice of most traditional

welding methods.

Other applications in the automotive industry include welding of instrument panels,

keyless entry remotes and even fuel tanks.

Medical

The medical device industry is quickly growing, requiring joining of plastic devices

ranging from catheters to microfluidic devices. The surgical nature of laser plastic welding

makes it well suited to handle the delicate devices and precision joining.

Besides hermetic seals and a high precision requirement, medical devices often require

perfectly clean joints. This task is often difficult for other joining methods. Adhesives can cause

contamination, especially at the micro level where many of these devices are operating and

traditional welding methods such as ultrasonic and vibration leave dust-like particulates behind

that can also contaminate the device.

Consumer

The consumer products industry is a large one, but as of yet it still lags behind

automotive and medical. The main reason for this is because typically consumer products do

not require the specialized needs of laser plastic welding (precision, clean joints, stress free

welds, etc.) or if they do, it is not to a degree that other welding methods cannot accommodate.

However, as laser welding systems become less capital intensive it is likely that

adoption will increase in this industry. Consumer products are often at high enough volumes

that they will benefit from the low production costs associated with laser welding. Also, as

consumer electronics become smaller the precision and low-stress capabilities will become

increasingly attractive.

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3.Hybrid technologies of laser welding of plastics

3.1 Hybrid Laser Welding

In addition to the large number of advantages of laser welding, there are also a number

of restrictions associated with the requirements on the optical properties of the welded

components. The laser beam should be absorbed by the second (in the direction of movement

of the beam) welded component and, consequently, the dye (for example, soot) must often be

added to the composition of the component. This addition results in the black (or some other

dark) colour, which is not always permissible.

One of the possibilities of avoiding the use of special absorbents is the introduction of

a layer of a light absorbing substance into the zone between the welded components (the so-

called welding method using intermediate layers). In this case, the laser beam is absolved

exclusively by the intermediate layer with the generation of thermal energy. Heat transfer

results in the plastification of the surface areas of the welded components which are in contact

with the intermediate layer. These layers can be in the form of a film containing light-absorbing

dyes or pigment. As shown by the experimental results , the intermediate film layers, pigmented

with soot, ensure strong bonding of the components and the welded components are optically

transparent. The shortcoming of this variant is the additional cost associated with the use of the

intermediate film. In addition to this, in certain conditions it is necessary to carry out an

additional treatment of the intermediate film in order to ensure that its configuration is in

agreement with that of the welded joints. This is associated with considerable difficulties if the

geometry of the welded joint is complicated. Therefore, it is desirable to produce welded

components, containing the appropriate light-absorbing zones, in a single-stage process.

An important improvement in this area is the combination of the methods of two-

component casting under pressure and laser welding. The light-absorbing material, used as the

second component, is deposited in the form of a thin layer on the surface of one of the welded

components by injection into a mould. Although this technology is associated with additional

expenditure on equipment and casting moulds, it does make it possible to produce high-quality

components in a single cycle of casting under pressure (using laser welding) and no additional

materials are required.

23

Specimens were welded by welding around the contour using welding equipment in

which the radiation source was a laser with a semiconductor diode (LDF 400-90 model) of the

company Laserline GmbH with the maximum output power of 80W and the radiation

wavelengths of 940 nm. The laser beam is supplied to the components using a fibre light guide.

Optical equipment was assembled on the basis of a RV-4Asix-axis robot (Mitsubishi Electric)

in which the laser beam can be moved along the required trajectory of the welded joint. The

diameter of the beam in the welding zone is 1.5 mm.

In Research it was established that even a small amount of soot results in distinctive

colouring of the weld zone in the components (they become black). As a result of the

optimization of the weld zone of the components and selective injection of the light-absorbing

material, the size of the black sections can be greatly reduced. For example, when only 0.025%

of soot is added to the absorbing material, the welded joint can be clearly observed, and at a

small thickness of the layer the natural colour of the absorbent is difficult to distinguish. Since

the consumption of the relatively expensive dye is very low, this factor is very important,

especially in the manufacture of large components. The reduction in the amount of the

absorbent to the thin intermediate layer makes it also possible to eliminate the problems

associated with the formation of the large zone of the thermal effect because this zone is

bounded by the light-absorbing layer and by the sections of the light-permeable components in

the immediate vicinity.Thus, the two-component technology can be used to produce strong,

almost completely transparent welded joints.

Problems in the organization of operations in the welding of polymers are associated

with the selectivity of the optical properties of these materials as regards laser radiation, and

also with the high price of laser equipment. Consequently, it is important to find new methods.

One of the possible solutions is the development of welding methods combining two heating

sources. In particular, in Research it was proposed to use a hybrid heat source for welding

consisting of a polychromatic light emitter and a monochromatic laser source. The description

of this method of welding polymath is also the aim of this study. The system was used

previously with considerable success for hybrid light laser welding of thin metals

Taking into account the fact that the fixed wavelengths of the lasers (0.8, 1.06 and

10.6mm) are not always sufficiently absorbed in the volume of the polymer materials resulting

in restrictions of laser welding, and the radiation of the wide-band polychromatic lamplight

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source overlaps the transmission spectrum of many polymath, gas discharge metallic

dismountable lamps with an internal reflector have been proposed for use as the source of

auxiliary power for polychromatic radiation. The efficiency of laser light technologies depends

on the spectral and geometrical parameters of the polychromatic source of radiation. As a result

of the application of the non-traditional output window in the light gas-discharge lamp, the

radiation power has been increased 1.7 times, and the radiation spectrum has been widened to

0.2– 0.6 mm. The size of the working spot in the focal plane is 2–4 mm, and the power density

in the heating spot is higher than 104 W/cm2.

The combined effect of coherent laser radiation and the focused radiation of a powerful

polychromatic light emitter results in the formation of a heat source in the polymer whose

spatial distribution is determined by the superposition of the radiation fluxes from the laser and

the light emitter. The selected circuit (Figure 4) is used for light laser heating, and the mutual

position of the laser and light focal spots on the component is varied. The combination of the

two heat sources has been used to develop the following welding techniques:

Figure 3.1 : Equipment for light laser welding using Nd:YAG or CO2 lasers in combination with powerful

monochromatic radiation sources: (1) laser (CO2 or Nd:YAG); (2) polychromatic light source; (3) welded

components; (4) temperature substrate and (5) welded joint.

25

with preheating of the polymer components to be welded using a polychromatic

radiation source for subsequent welding;

quasi-spot welding using a pulsed periodic Nd:YAG laser and a lamp heat

source;

Joining of polymer materials of different thickness with minimum deformation

in welding through a template.

The power of laser radiation, required for producing the welded joint, is approximately

50% lower than in welding without lamp heating, the welding speed is several times higher

than in welding with laser radiation only and the reliability of the hermetic joints in the

polymers is higher.

Similar results were obtained and presented at the International Exhibition of Plastics

and Rubber ‘K’ in 2007 by LPKF company specializing in welding of polymers. The method

combines laser radiation with infrared radiation with a wide spectrum and is used mostly for

efficient welding of large plastic components of complicated shapes in the car industry. Prior

to introducing this process components of complicated shapes were difficult or impossible to

weld together. When using traditional laser or other welding methods the quality of welded

joints was relatively low. New patented hybrid systems manufactured by LPKF represent

significant achievements in this direction (Figure 5). In this technology, welding equipment

combines the laser radiation beam and the thermal radiation beam generated by conventional

halogen lamps. This superposition of the beams results in a large increase in the speed of both

two and three-dimensional welding. It should be mentioned that the development of light laser

welding technology is a natural result of investigations in the direction of increasing the

efficiency and utilizing the technological possibilities of lasers. The website of the LPKF

company reports on patents for the latest equipment for hot laser riveting – a technology which

combines laser welding with riveting.

As reported by the LPKF company, the method is contactless and retains all advantages

of laser technology, including ecological efficiency. High-quality welded joints are produced

in pairs which were previously regarded as unweldable, for example, welding of components

of toxic and heat-resistant polychlorinated diphenyls to plastic body components.

26

Laser welding of plastics to other materials

3.2 Laser welding of plastics to other materials

Modern science and technology in many branches of industry are oriented to the

development of very light structures having also higher strength. In many cases, this result can

be obtained by the manufacture of hybrid components, including plastic and metallic elements.

Consequently, it is important to join such materials. One of the new approaches to

solving the problems of joining plastics and metals is laser ablation – the removal in the first

phase of the welding process of part of the surface layers of the metallic component in order to

develop a system of shallow grooves with undercut sides. In melting the plastic component

under the effect of laser radiation, the material fills the grooves in the metallic part of the future

combined joint. An important role in this process is played by the application of the specialized

clamping device.

The plastics can be joined with dissimilar materials by the methods of laser welding.

An important factor in this technology is that the part of the joint heated by the beam should

Figure 3.2 : The two-beam equipment for hybrid welding by LPKF company.

27

have a higher melting point than the plastic component used in welding. Temperature-stable

materials including, in particular, metals, ceramics, glass or heat resisting plastics. The high

density of energy in the radiation beam results in the high heating rate. In Research it

demonstrated the possibility and optimization of the parameters (the travel speed of the beam,

radiation power, and compression pressure of the welded components) of the contour laser

welding of T shaped joints in nylon 6, reinforced with 30% glass fibres.

To optimize the welding conditions, experiments were carried out to determine the

relationship between the parameters, in particular, it has been shown that the strength of the

welded joint unexpectedly decreases with an increase in clamping force. The processes of

joining components in micro technologies should satisfy special requirements associated with

mechanical or thermo mechanical loading and also with the accuracy of adjustment of these

components. In similar cases, laser radiation as the energy source satisfies almost completely

the requirements of welding of the micro components because it ensures minimum and spot-

like heat input. Investigations of different absorbents and dyes show that they can be used in

welding plastics with silicon and glass substrate. The application of microelectronic optical-

mechanical systems makes it possible to split the radiation beam and control the distribution

of the laser radiation power density in the cross section of the beam. The precision systems of

this type can be used for controlled welding of three-dimensional components of complicated

profiles of different types, and also for selective ‘healing’ of defects in microelectronic and

optical components.

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4.A fibre laser welding assisted by solid heat Sink

4.1 Principle of the welding method

One of authors has developed a novel welding technique for overlapped

thermoplastics without causing surface thermal damages from the point of view of heat

transfer. The principle of this infrared laser welding technique is schematically shown in

Figure. It is based on the combination of the heating due to penetrating infrared laser and the

cooling due to thermal diffusion of a heat sink. The solid heat sink transparent to the laser

placed in contact with an irradiated surface of thermoplastic plays an important role in the

system during radiation heating.

A typical temperature profile in the plastic at a certain elapsed time after laser

irradiation is also illustrated in Fig. After the radiation beam passes through a solid heat sink,

then, it penetrates into the thermoplastic (A). As the thermoplastic (A) is an absorber of the

radiation, the intensity of the radiation beam decreases exponentially in the thin layer near the

surface where the absorbed energy is then converted into heat in the thin layer. Then, the

transparent heat sink placed in contact with the surface of the thermoplastic diffusively

absorbs the thermal energy from the thermoplastic through the contact surface, because the

thermal diffusivity of solid heat sink is about 100 times larger than that of plastics. The

temperature of a thin layer near the irradiated surface is therefore suppressed from rising;

consequently the irradiated surface is restrained from thermal damage. Additionally

evaporation of some gas emitted from the damaged surface is often restrained.

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Fig 4.1 Principle of a novel infrared radiation welding procedure with a transparent heat sink

The emergence of the temperature peaks in a certain depth inside the thermoplastic

shown in Fig. was theoretically verified. The reason is that the thermal energy due to the laser

beam arriving at a little deeper region from the irradiated surface cannot diffuse back to the

heat sink due to the very low thermal diffusivity of the thermoplastic compared to that of the

transparent heat sink. We can say that the thermal energy is trapped within the inside of

plastics and contributes to temperature rise in deeper region instead of the surface of plastics.

The appearance of a temperature peak suggests that an optimized welding can be initiated at

that depth. The features of this infrared laser welding procedure with a transparent heat sink

are as follows:

The top part of the thermoplastic irradiated by a laser beam should be an absorber of

radiation light.

The solid heat sink placed in contact with the surface of the top part of the

thermoplastic should be transparent to radiation light and should have much higher

thermal diffusivity than that of the thermoplastic to be welded.

30

It is not essential for the bottom part of the thermoplastic to be a strong absorber of

laser light as in the conventional TTIR welding.

No pigmentation of thermoplastic parts is required, and no evaporated gas is emitted

from the irradiated plastic surface.

Evaporation of gas from the plastic is restrained.

4.2. Choice of infrared radiation source

A few infrared radiation heat sources are possible to be used: a diode laser, YAG

laser, CO laser, CO2 laser and so on. A choice of laser is dependent on the optical properties

of plastics to be welded. We reported that the feasibilities of welding of polyolefin and

fluorocarbon polymer films using CO2 laser assisted by a transparent solid heat sink have

been shown in the past studies. However, it is impossible for even this procedure to weld

thick sheets (more than 0.5mm) of some ordinary engineering plastics that strongly absorb a

CO2 laser in a wide region of wavelength. Therefore, sufficient laser energy cannot be

supplied to the place to be welded. In order to predict the possibility of welding using lasers it

is important to measure the transmittances of plastics in advance.

Fig.4.2 Transmittance spectra of Tested Plastic sheets

31

The infrared spectral transmittances of eight kinds of plastics such as polybutylene

terephthalate (PBT), polyethylene terephthalate (PET), polycarbonate (PC), polyamide 6

(PA6), polyoxymethylene (POM), polypropylene (PP), polymethyl methacrylate (PMMA)

and polyvinyl chloride (PVC), are seen to be strongly dependent on the wavelength as shown

in Figure. The transmittances of all these plastics have very low values, at 10.6 micro-meter

(wavelength of CO2 laser).

On the other hand, these plastics have moderate transmittance and absorption for

Thulium-doped fiber laser (Tm: fiber laser), which has a emitting wavelength of 1.94 �m.

Therefore, the laser energy can penetrate into the plastic. The welding with no surface

damage using the fiber laser by a novel procedure is expected as shown in Fig. If a CO2 laser

is used in this case, bubbles appear near the irradiated surface, and welding is not successful.

4.3 Results

The overlap novel welding method using Tm: fiber laser has achieved non-thermal

damage on the surfaces for four kinds of plastics (PC, PA6, PP and PVC) often used

in manufacturing medical equipment and devices. The similar CO2 laser welding was

unsuccessful in the welding.

The materials (PC, PP, PVC) having high transmittances require lower velocities of

laser head on the surface because of ample heat generation cannot be expected by low

absorption at the interface due to higher laser moving velocity.

The melting temperature does not clearly influence on choice of the welding velocity.

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5. Quality and Process Control

5.1 Quality Control

As with other joining methods, the issue of quality control also arises in laser welding.

How can one realize during the welding process, whether a weld meets the requirements or not

and separate good parts from bad parts accordingly? How can the number of parts be reduced

by using an appropriate process control? One opportunity for a quality assessment, which is

also used in ultrasonic welding, is measuring a set path during the welding process. Here, the

welding contour is melted by the laser homogeneously and uniformly, which can be realized

by using a so-called galvo scanner that deflects the laser beam in the working plane very

quickly by two internal mirrors.

The laser beam is rapidly deflected over a programmable welding contour and the

contour is melted almost simultaneously. By pressing the upper part with a mechanical

clamping device into the molten material, a defined collapse can be measured. Given that the

joining partners are compatible and weldable, it can be assumed that if a certain collapse has

been reached within a predetermined time, the required weld quality has been achieved. If the

predetermined time-distance curve has not been met, the welding seam quality may not be

sufficient and the part can be rejected.

Another way to assess the plastic welding process qualitatively and quantitatively is by

using a remote temperature measurement. A pyrometer is used, which detects the temperature

of the molten material during the welding process. This detected temperature is a measure for

the bonding quality. Such processing heads with an integrated pyrometer in combination with

the diode laser allows a rapid control of the welding temperature and the detection of welding

defects. The advantage of a temperature control during the welding process (so-called closed-

loop process) is becoming clear when the components to be welded show a certain

inhomogeneity with respect to their optical properties. Such inhomogeneity can occur when

the components are reinforced with glass fibres, for example. Density fluctuations or various

orientations of the glass fibres within the plastic lead to different transmission and absorption

properties. This, again, requires adapted laser power during the welding process to achieve the

required melting temperature. The pyrometer keeps the melting temperature constant within

certain limits by adjusting the laser power automatically. By this online process control, the

33

components can be processed in an optimized process window. This affects the weld strength

and the stability of the process positively. A defect welding area cannot be compensated by the

pyrometer control, but can be realized by a sudden increase of the temperature signal. Such a

temperature increase may have several reasons. For instance, a contaminated surface can lead

to higher absorption of laser radiation and burning of the material. Another reason may be poor

or non-existing mechanical contact between the two welding partners. Hence the heat of the

molten lower joining partner is not absorbed by the upper part which leads to overheating. This

is visible in the pyrometer signal. Likewise, it is recognized when the required temperature is

not achieved, e.g. by lack of laser power. If the upper and lower temperature limits, defined

within the software, are exceeded, the affected components can be separated. An individual

process number for each single weld allows mapping and corresponding traceability. A

combination of the above described possibilities for process control is provided by a Galvo

scanner with adapted pyrometer.

Such a combination combines the advantages of fast beam deflection by mirrors with

fast, remote temperature measurement. Since the measurement wavelength of the pyrometer is

in the range between 1800 and 2100nm, it is necessary to adapt the optical system accordingly.

By using special designed and coated optics, it is ensured that laser focus and the pyrometer

spot are congruent.

Fig. 5.1: Galvo pyro combination combines the advantage of fast beam deflection and online

process control (Source: DILAS GmbH)

34

5.2 Process Monitoring Methods

Laser plastic welding is often selected as the joining method of choice for precision

and intricacy. Many of the parts in question have high tolerances requirements, therefore, the

quality assurance and validation must be equally as precise, able to measure the tiniest

deviations from the tolerance abilities boasted by laser welding.

In itself, the process of laser plastic welding is extremely reliable and repeatable.

However, the process can be hindered by deviations in the parts to be welded. The two causes

for concern are geometric and optical deviations.

There exist five different types of process monitoring techniques for laser plastic

welding, ensuring that any part, regardless of its nuances or the variations in the part, are able

to be monitored effectively.

5.2.1 Melt-collapse Monitoring

The most robust and often used process monitoring method is collapse monitoring.

This technique makes use of the natural convergence of the joining parts as they move

together under clamping force.

Typically, parts are designed with a collapse rib, such as you see in Figure 3. This rib,

once melted and introduced to clamping pressure, will collapse. The measurement of this

collapse can be used to determine weld quality.

The laser welding process itself is highly-reliable, but deviations in part dimensions

can result in poor welds. If the two joining parts are warped, this can leave gaps. Gaps of

more than 0.05mm are known to degrade weld quality significantly.

Introducing a collapse rib that is greater in height than the part tolerances can ensure

that the distance of collapse will overcome the tolerances. Once an adequate melt-collapse is

determined in testing, parameters are entered into the system. If a part fails to fall within the

proper collapse parameters during production it will be marked for rejection and all data will

be stored for later evaluation. The device that measures the collapse is known as a linear

voltage distance transducer. It is accurate to less than 0.01mm, which is overkill as even the

most precise injection moulding processes are incapable of producing dimensional tolerances

of less than 0.02mm.

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5.2.2. Pyrometer

Pyrometers measure the temperature within the welding zone. Temperature

inconsistencies are directly correlated to inconsistencies in the weld. Pre-defined upper and

lower temperature limits are developed in testing. If anomalies from burned contaminates or

inconsistent part dimensions cause the radiation to fall outside of the defined “temperature

envelope” the part will be flagged and the data stored.

Pyrometers are typically used when melt-collapse monitoring is impossible, for

example in the case of radial welding of a catheter, where pressure is created via an

interference fit and not clamp tooling.

5.2.3. Reflection Diagnosis

This method measures reflected light, but rather than measuring the intensity or

temperature this method measures the divergence of the reflected light from the surface of the

part. Parts that are correctly molded, fit together perfectly, with no gaps between them;

therefore, the only surface of the part as a whole is the upper layer surface.

Figure 5.2 – Melt-collapse

36

Parts incorrectly developed may have gaps. In such an occurrence the part would

technically have two surfaces, the surface of the upper layer and the surface of the lower

layer.

Figure 5.3 – Reflection diagnosis concept

This system is able to compare the divergence of the reflected light. Parts with gapping and,

therefore, two surfaces, are identified by two peaks of light, whereas properly fitting parts

will only reflect a single peak.

5.2.4. Burn Detection

Burn detection is used to recognize surface scorching. Scorch marks are typically

caused when the laser strikes a contaminant on the surface of the plastic, resulting in a burn.

Such burns emit radiation that is outside of the typical reflected wavelengths and can

therefore be distinguished.

Although scorch marks, usually no more than a few tenths of a millimeter across,

rarely have the capability of compromising the weld quality, they are often unacceptable for

aesthetic reasons.

5.2.5. Camera-Assisted Vision Systems

In some cases weld quality can be determined by a simple visual inspection. However, being

a manual process, this is very impractical and unreliable.

Vision systems are capable of monitoring the weld seam automatically and to a much

higher accuracy. Figure shows an example of the system detecting a visible seam

37

inconsistency. This was most likely the result of a gap or particulate hindering thermal transfer

at the weld interface.

Figure 5.4 – Camera view of a flaw

5.3 Cost Comparison: Is Laser Plastic Welding Economical?

5.3.1 Introduction

Laser plastic welding is becoming well known for its ability to join complex and

detailed parts. In fact, the technology is driving innovation for products that were once not

possible in the medical, automotive and consumer electronics industries.

However, there still exists a stigma that laser technology cannot compete economically

with other joining methods. Despite the technical advantages, the initial capital investment of

laser systems has been a stumbling block for the technology.

While there is truth in this and no laser system manufacturer will deny the higher

investment, this is only one factor; many other substantial cost factors are often overlooked.

5.3.2 Laser Sources Becoming More Economical

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Typically the largest expense for any laser system is the laser source itself. However,

with technological advancements and widespread adoption of lasers in many fields, not just

plastic welding, the cost of laser sources have dropped significantly.

Naturally this lends to a noticeable price decrease for the overall laser system.

Although this change has yet to bring the cost of laser systems below those of other

joining methods, the heavy weight of the initial capital investment is becoming more bearable.

Much more bearable, in fact, once other advantages of the process are realized.

5.3.3 Total Cost of Ownership

When considering the total cost of ownership it is important to understand all of the

cost factors involved in the system itself as well as process costs.

5.3.4 Laser Cost Advantages

The following is a breakdown of potential cost advantages for a laser plastic welding

system.

a) Cycle Time

Laser plastic welding is capable of very short cycle times. This does depend, somewhat,

on the laser process method (contour, scanner or simultaneous), but overall, laser has the

potential to reach cycle times comparable to ultrasonic welding.

Possibly of greater importance, laser welding does not require a time intensive

annealing or curing step, drastically reducing the overall process cycle time and the need for

additional equipment during this step.

Also of note, is LPKF’s hybrid welding process. Using halogen lamps to assist the laser

in heating the plastics, cycle-times can be reduced by up to 50%. The halogen energy also helps

soften the entire joint allowing for better part contact.

39

b) Maintenance

Laser systems are extremely low maintenance. A typical laser source can last 20,000

operating hours with virtually no maintenance.

Systems are designed to run full-time and can be either manually operated or integrated

into an existing automation system.

The only scheduled maintenance on a typical machine is a recommended, annual

change-out of the water filter for the laser cooler, a painless and inexpensive step.

c) Quality Assurance

Multiple process monitoring techniques give laser system users excellent quality

monitoring capabilities. Pyrometer readings, reflection diagnosis, collapse monitoring and burn

detection are tracked for each part.

Parts that fail any of the above controls can be flagged for discard. Also, data for each

part, good or bad, is stored for evaluation, allowing for process adjustments.

Fig. 5.5: Cycle Time Comaparison

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The continuous measurements from this series of evaluation techniques lay out the

platform for Six Sigma process performance analysis.

d) Consumables

Laser welding does not require any consumables. Common consumables for other

joining methods include: adhesives, tapes, screws, rivets or filler plastics.

e) Flexibility

Switching applications for a laser system is often as simple as a quick clamp tool swap

and the loading of a new process/parameter file. This can typically be done in less than a few

hours.

f) Extra Steps Avoided

Because laser welding leaves clean joints there is no need for post-welding part

cleaning.

Also, as mentioned earlier curing and annealing steps are not required.

5.3.5 Laser Cost Disadvantages

a) Additives

Most thermoplastics are naturally transparent to infrared laser energy; therefore,

additives must be doped into the lower plastic layer to create absorptive properties.

Fortunately, the most common, and best, absorbing additive is carbon black, a cheap

additive that is already used to give plastics a black color.

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b) Part geometries

The process of laser plastic welding requires excellent contact between the two joining

parts. Because of this, part tolerances are very strict for laser plastic welding. Warpage and

gaps in the parts can result in poor weld quality where the joint is not in adequate contact.

Although not a significant difference this may result in higher costs, as more stringent

requirements will be required of the injection molder.

5.3.6 Equivalent Cost Factors

a) Labour

Actual operation of laser systems does not require any special technical skills, once the

process software and tooling is set-up by the engineer.

Therefore, laser systems require no more man power or expertise than any other method.

In summary, laser plastic welding is well suited for high volume applications, where

process cost savings can outweigh the initial capital investment. When neither speed nor quality

can be sacrificed laser plastic welding provides an excellent solution.

5.4 Boosting Efficiency: With Laser Plastic Welding

Lasers have gained an excellent reputation for joining plastic components together.

They create very efficient joints and boast qualitative as well as economic benefits. Please see

the article on the current state-of-the-art for laser plastic welding below.

Joining plastic components is something that has exercised the minds of industrial

experts for more than half a century. The search for economical methods to join components

became urgent with the development of injection molding. And even today, the majority of all

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plastic products still cannot do without a joining process which seals the component after

assembly.

In the early days, this was frequently done with screws, or the parts were simply glued

together. These methods are comparatively expensive and time-consuming. The development

of more economical and reliable joining techniques suitable for production lines was given a

major boost when the automotive industry began to build more and more plastic components

into their vehicles. This gave rise to the development of plastic welding.

5.4.1 High Energy Density

During metal welding, two parts are welded together by heat and pressure. The same

applies to laser plastic welding, but with one big difference: the laser energy is not usually

effective on the top surface of the component but only precisely where the welding seam is

required. How is this possible?

Because a laser works with light, the laser beam can be concentrated with lenses, and

guided by mirrors or fiber optic cables. This transmits the energy to the workpiece.

An important aspect is that not all of the laser energy becomes effective. The energy

which is beamed-in is partially reflected, partially transmitted through the component, and

partially absorbed. Only the absorbed portion of the laser energy becomes effective as it is

converted into heat. A high energy density can be achieved even with a relatively small laser

capacity. The laser beam is focused onto a point only a few micrometers in diameter. The

energy density at the focus of the laser beam can easily exceed that on the surface of the sun.

5.4.2 Different Welding Methods

In the transmission laser welding technique, the process depends on having a laser-

transparent material and a laser-absorbing material. The laser light passes through the upper

laser-transparent component with almost no absorption and melts the surface of the underlying

laser-absorbing component. The application of a precisely calculated amount of pressure

guarantees good thermal conduction so that the underside of the upper component also melts.

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This principle is used in a variety of welding methods, and has led to the development of

optimal welding processes for a range of products.

In the simultaneous welding method, the whole welding seam is warmed up

simultaneously by a laser beam. This method requires very homogenous distribution of the

output density – even around corners and over changes in height. It needs the application of

several lasers or special masks and is now only used when extremely high production volumes

are involved.

The quasi-simultaneous welding technique works in a similar way, but in this case, a

laser beam runs along the contour repeatedly until the specified welding result has been

achieved. The rapid movement of the laser beam gives rise to the quasi-simultaneous

plastification of the whole welding seam. One of the advantages is that this enables the melt-

travel to be controlled and can thus compensate for tolerances in the injection molded parts

being welded together.

In the contour welding technique, the laser moves relative to the component. Only a

small part of the welding seam is melted at any given time. Contour welding is particularly

good when rotation-symmetrical or very large components have to be joined together without

any melt blow-out.

5.4.3 Plastic Welding: An Innovative Joining Process

A whole range of different welding methods has been developed for production

processes. The first plastic welding technique to be developed was the hot-element method.

This was followed by the development of ultrasonic and vibration welding. Laser plastic

welding is the most modern development.

The main difference between these technologies is the way the energy is applied.

Today, the following methods are commonly used in production lines:

- Friction welding (vibration and rotation welding)

- Ultrasonic welding

- Microwave welding

- High frequency welding

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- Heating-element welding

- Laser plastic welding.

Every method has its own special advantages and disadvantages. The selection of the

method partly depends on the properties of the plastics which are used, as well as the

application and the associated specifications.

5.4.4 Restricting the Energy Precisely to the Welding Zone

In heating-element welding, the plastic touches a heated plate. This method is

unsuitable for plastics which could stick to the heating element. Vibration welding is only

possible on components which are almost completely flat because the relative movement

between the components would otherwise cause serious damage. The process involved in

friction and ultrasonic welding automatically gives rise to the formation of particles around the

welding seam.

Laser welding suffers from hardly any of these process-related restrictions. And in

addition to the very wide range of application areas, this method also has other qualitative

advantage which makes its use even more attractive – for instance, the energy can be restricted

very precisely to the welding zone.

Modern laser welding machines are gaining more and more ground because the price

of laser sources is continuously sinking and the development of new process methods opens

up many new options. This gives rise to the continuous development of new applications. None

of the other welding methods available today can boast such a huge application variability.

5.4.5 IN FOCUS: Large Free-form Components

Large components such as car taillights are a very challenging joining problem,

particularly when they involve difficult material combinations and visible seams. The robot-

assisted LPKF TwinWeld-3D welding system combines a halogen lamp and a laser beam to

join together any type of free-form components.

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The compact head sits on a robot with seven degrees of movement. No special clamping

tool is required because the necessary pressure is supplied by a special pressing device attached

to the head. All this means minimal tool costs and the highest welding seam quality without

any damage to the surface and the visible areas. These systems are currently used to weld car

taillights. Other large components such as solar modules and containers are in the pipeline to

utilize this technology for serial production.

5.4.6 Continuous Improvement in Cost Ratios

Modern diode and fiber lasers have lifetimes of more than 20 000 operating hours and

are guaranteed to work without any problems even in industrial conditions. The costs compared

to other methods have also improved simultaneously. This cost efficiency is emphatically

highlighted by the use of this method in the permanently cost-driven automotive subcontracting

sector – where the transmission laser welding of thermoplastics cuts a very good figure in the

total cost calculations. This has moved it from a niche application to the technique of choice.

The laser welding of plastics reduces the amount of consumables and boasts

outstanding welding quality. Laser welding is a non-touch method which reduces the

mechanical stress on a component to an absolute minimum. The only pressure applied to the

product is the orthogonal joining pressure. And there are no vibrations which could damage

the plastic housing or harm components on the inside of the product.

It was precisely this advantage which led to the breakthrough of laser welding, because

this made it ideal for welding electronic housings of all kinds. These components cannot be

joined together by ultrasonic welding because of the damage this could cause to electronic

components. Screwing or gluing is expensive – which means that laser welding is the obvious

choice.

And because laser welding is hygienic and boasts very high levels of process safety, it is also

optimal for applications in the medical sector.

5.4.7 Broad Areas of Application

Laser welding is currently predominantly used in the automotive and medical sectors.

The applications in the automotive market are frequently found amongst the subcontractors.

The encapsulation of sensors and control units is a typical application.

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Laser plastic welding is also gaining ground in the consumer market: precise welding

seams in the visible parts of components are now found in many everyday goods.

Medical technology applications specify very high levels of hygiene in their production

lines. And laser plastic welding is the method of choice for microfluidic applications: using

lasers for the highly precise creation of the welding seams, and the absence of any particles

produced during the laser welding process, is the only way of guaranteeing that sensitive

microfluidic channels remain open and function perfectly.

Balloon dilation catheters for widening constricted coronary arteries have a zone which

can be dilated during an operation. This zone is welded by a laser in a rotation process.

5.4.8 Better Safe than Sorry

Critical processes demand high quality – and this quality has to be permanently

maintained and documented. A whole number of methods are already available during laser

plastic welding to determine the perfect welding parameters. If the necessary interface is

installed, laser plastic welding can be done with continuous tracking & tracing. Real-time

recording during the welding process makes subsequent controls unnecessary, and in many

cases enables the laser itself to automatically correct the identified fault.

Melt travel monitoring can be used during quasi-simultaneous welding. The criterion

for successful welding in this case is defined by a specific amount of settling across the whole

component.

In the pyrometer control method, a temperature curve is measured during the welding

process: if this curve deviates from a defined upper and lower boundary, this may indicate

faulty material or a foreign body which prevents proper welding. When necessary, the

temperature signal can be used to directly control the laser output.

The scorch diagnosis method identifies thermal defects, whilst camera monitoring measures

the width of the welding seam and visually inspects the homogeneity.

5.4.9 Reflection Diagnosis

Reflection diagnosis is a completely new method that can be ordered as an optional

accessory for welding systems from autumn 2010.

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Reflection diagnosis takes advantage of the physical effect of the total reflection of light

at boundaries.

Such a boundary exists between two parts before they are joined. When light –

including laser light – hits this boundary, a certain amount is reflected. The boundary changes

when the zone is welded, reducing the amount of reflected light. Measuring this change in the

amount of reflected light enables faulty seams to be detected.

This method can be used to check the quality of the welding seam in real-time.

Depending on the product, the methods can be combined to enable the continuous tracking &

tracing of specific components.

5.4.10 Laser Staking

Laser staking combines riveting technology with laser plastic welding. It creates a form-

fitted join between twocomponents. Because the riveting tool does not come into contact with

the plastified material, no plastic adheres to the tool, so this method can be used to join together

components that cannot be welded directly – for instance, fixing printed circuit boards into

plastic housings. The LPKF LQ-Spot system is completely inline-compatible.

5.4.11 Positive Development

Laser plastic welding will continue to conquer new areas of application in future – by

virtue of its cleanroom compatibility, the integrated process controls, and the outstanding visual

results. Laser plastic welding also makes economic sense. Completely new products are

possible when combined with other methods – such as using laser structuring to apply circuit

tracks to a sensor housing, and then safely sealing the housing using laser plastic welding.

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6. Conclusions

Laser welding of plastics is a highly specialized technology of joining components

which can be used most efficiently in applications requiring high-speed welding and welding

of brittle components or components requiring sterile conditions. The laser beam welding often

has technological advantages in comparison with the traditional technologies of welding

components. Service experience shows that industrial equipment for laser welding of polymers

has a number of special practical

Advantages

high-quality welded joints and the possibility of producing leak tight joints;

the possibility of packaging components sensitive to vibration because the components

do not move during welding;

reduction the degree of distortion of the components in welding of heat-sensitive

components because the size of the heat-affected zone is small;

reduced contamination of the environment and reduced amount of welding fumes

because the molten material is situated inside the joint and is in contact with the

equipment;

reduced energy requirement because laser radiation is focused only in the zone of the

formation of the welded joint and only the small volume of the polymer is remelted so

that the efficiency of the process is very high;

the high degree of automation of the process resulting in higher quality of the

components and reproducibility of the results;

Flexibility of the process and equipment – the laser system can also be used in other

production lines.

Comparison with CO2 arc Welding:

In comparison to CO2 gas arc welding, LBW can significantly reduce the welding

deformation

The range with high longitudinal tensile stress in the joint welded by LBW is

significantly narrower than that generated by CO2 gas arc welding. Moreover, the

maximum value of longitudinal residual stress generated by the former is smaller

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than that caused by the latter.

Laser welding of plastics is an established process that is used in more and more applications

in different markets and is increasingly displacing the traditional welding methods. In medical

device manufacturing, for instance, cleanliness is absolutely mandatory. Hence, laser welding

is particularly well established in this market. In the automotive supply industry the parts are

equipped with sensitive electronic components or guide and contain fluid - here laser beam

welding is the method of choice. In combination with process control, the diode laser will make

its way to a variety of future applications.

The use of lasers in the industry for the bonding of plastics has increased in last decades.

According to the literature (Herzinger, Schloms 1995), some 25% of the industry employing

lasers is concentrated in Japan. This is due to the development of the industry and economy of

that country. It is expected that in the future some 10% of all of the joints in plastics will be

laser-produced (Grande 2004).

The laser technology offers novel solutions that permit to off-set limitations, often

imposed by conventional methods. Technological progress that has taken place in last years

and the requirements that the development of the industry poses indicate that this technology

of bonding plastics will further develop.

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References

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