Laser Welding Handbook

7/27/2019 Laser Welding Handbook

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New advances in polymer laser welding 2

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1 Summary and conclusions

2 Introduction

3 Basic process information3.1 Potential of laserwelding3.2 Parameters and their influence3.3 Materials3.4 Equipment

3.5 Weld geometry3.6 Weld quality

4 Applications

Acknowledgement, authors

Appendix (references)

This document may be distributed freely, under the provision that no changes are made. Parts of thispublication may be reproduced if due reference is made to this document.

The picture on the frontpage shows the 3D Contour welding of a rear light using a robot as well as a fibre coupleddiode laser system.(picture by courtesy of ILT)


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T h 8 p y v

Polymer laser welding is an emerging joining technology. It offers several advantages compared to

existing welding techniques, such as:n

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No or hardly any weld-flash is formed No damage to product exterior as only limited mechanical load is exerted during welding. .

n

X v q r h y v p h i v y v )

Laser welding is ideally suited for v v h v h v

Good-quality bonds can be obtainedeven if the available welding area is very small, while process-times can be < 0.5 s.

On the other hand, laser-welding can also be used to advantage for r y h t r q p withseveral meters of weld-length

. Thermoplasticr y h r

can be welded.

n H v v h y v s y r p r u r q p ) The heat-affected zone is very small. As a result, built-in stresses are not large. Sensitive components or parts are not affected, even if these are close to the weld zone. No

electrical fields or mechanical vibrations are generated, contrarious to most other weldingprocesses.

n

X r y q v t s " 9 u h r q r y q v t p

n Q p r s y r v i v y v ) With the proper welding equipment, it is quite easy to switch from one product to another.

Some boundary conditions as to materials and product design have to be fulfilled. The most important

condition to the materials is that one product part should be transparent for laser radiation, whereas theother part has to be absorbent. As regards product design, the most important aspect is the geometry ofthe weld region. A large number of shapes can be used to obtain an optimum welding result.

This publication describes the potential of polymer laser welding and presents information on productdesign, materials, process parameters and equipment.

Attention is also given to the latest developments in the area of diode-lasers. This new type of laser isparticularly attractive because of its interesting price/performance ratio. Their optical beam quality issomewhat less compared to conventional laser systems, but this is usually no disadvantage at all in caseof polymer welding.

Finally, the usage of the process is illustrated by meansof a number of (potential) applications. These involvevarying industrial areas. Product sizes range from thevery small to the very large:

- Miniature components for optical informationstorage

- Miniature products for biomedical applications- Encapsulation of electronic components

- Housings of personal electronic products- Automotive components

- Double-walled window systems.

Fig. 1.1 Microfluidic device during filling withred-coloured liquid

(STEAGmicroParts)


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Today, typical joining methods for plastic parts are screwing, snap-fitting, adhesive bonding and welding.

Welding is applied when high strength and liquid- and air-tightness are required and when the materialsare compatible.

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Among the classical welding methods, laser welding has recently become an interesting alternative andoffers a number of advantages. These include, improved optical appearance, small heat affected zone,miniaturisation, less thermal, mechanical or electrical load upon the product, less flash or no flash at all,feasibility of 3D weld geometries and the possibility of welding thermoplastic to elastomers.

Laser welding is a versatile process. It is suitable for both small-series and mass production. Moreover, itcan be used for micro-components in high tech devices, but also for very large products such as double-walled window systems.

The weld strength is similar to conventionally welded parts. The weld-line width can be varied over a widerange, from some tenths of a millimetre to several millimetres; even complete surfaces can be welded.Therefore the process can be adapted to the application. Precise and locally limited energy inputguarantees that the mechanical and thermal load, which is exerted upon the parts, is kept to a minimum.This allows welding in the close proximity of, for example, functional layers in bio-sensors, sensitiveelectronic components or sensitive components in micro-engineered parts.

Conventional welding techniques, such as ultrasonic, induction or hot plate welding fall short in several ofthe above-mentioned aspects.

Furthermore, laser welding has been made attractive for industrial application by the recent advent ofrelatively inexpensive diode lasers. The radiation of the diode laser can be fed into an optical fibre,

allowing simple and easy beam handling in industrial applications.

X u h v y r r y q v t 4

Two process variants of laser beam welding of plastics can be discerned: overlap welding butt-welding.

P r y h r y q v t

Overlap welding is illustrated in fig. 2.1. The parts to bewelded are held together by means of a moderate clampingforce (not shown). The laser beam traverses one product part(which is transparent for laser radiation, wavelength in thenear infra-red region between 800 and 1100 nm). Thereupon,the laser radiation is absorbed in the top-layer of the otherpart. This top layer is heated, heat is transferred to the upperpart, the surface layers of both parts melt, and after coolingand solidification a reliable bond is formed.

Parts that are transparent to near infra-red laser radiation are,generally speaking, also transparent to the eye, whereas laser-absorbing parts usually have an opaque appearance (colouredor black). There are, however, a few exceptions to this, whichcan be utilised to increase the freedom of design with regardto colour.

Fig. 2.1 Overlap welding withlaser beam (serialwelding)


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! 7 r y q v t

Butt-welding is not the ideal configuration for polymerwelding although most papers that were published in earliertimes referred to this variant.

Due to the low thermal conductivity of polymers (contraryto metals) only those parts of the work piece will be moltenwhere laser energy is absorbed. In order to achieve a strongweld seam, a melt has to be created throughout the wholejoining volume. This puts important restrictions on the laserabsorption of both parts, and consequently on theirpigmentation.

6 r y h w v v t u h h p u v q r h r h s h y v p h v p h r q i r y q v t u r r h v q r s

u v u h q i x v y y i r r v p r q r p y v r y u r s r

W h v h s y h r r r t v

Regarding the method by which the laser energy is supplied to the work pieces, five major variants can bedistinguished, serial welding, simultaneous welding, quasi-simultaneous welding, line-scan welding andmask welding.

T r v h y r y q v t

With serial welding the laser spot is guided along the weld contour (see Fig. 2.1). Therefore a new

welding geometry can be adapted by simply changing the contour program, creating less consequentialcosts and giving this welding process an unexcelled flexibility and the possibility for joining devices witha 3D weld contour.

! T v y h r r y q v t

For simultaneous welding (see Fig. 2.3), a special arrangement of the laser-diodes and an appropriate,product-specific optical system shapes the laser-beam in such a way as to illuminate the whole weldcontour simultaneously. No relative motion of the partsand the laser beam is necessary and very short processingtimes are obtained.

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The laser beam rapidly scans the complete weld contour.If required, the scan is repeated one or more times. Thus,irradiation is serial but the whole contour is in theweakened / molten state at the same time, if the time-interval between successive scans is sufficiently short(this is possible because of the low thermal conductivityof polymers).

# G v r p h r y q v t

A line-shaped laser spot moves in the direction that is

perpendicular to its length, so as to irradiate a surface.

Fig. 2.2 Butt-welding with laserbeam

Fig. 2.3 Overlap welding withsimultaneous energy input


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$ H h x r y q v t

Here the shaping of the laser beam is done by a contact perforation mask. Very fine and elaborate weldlines are possible. In principle, mask welding can be combined with any of the four above-mentionedmethods. Fig. 2.4 shows a combination with line-scanwelding.

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" Q r v h y s y h r r y q v t

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Compared to other techniques, laser welding has all the properties of conventional welding techniques.Apart from those already mentioned in chapter 1, extra advantages are:

n @ p r y y r v h y h r h h p r )

Hiddenoverlap joints can be made without affecting part surface. No or hardly any weld-flash is formed. No damage to product exterior as only limited mechanical load is exerted during welding. .

n

X v q r h y v p h i v y v )

Laser welding is ideally suited for v v h v h v Good-quality bonds can be obtained

even if the available welding area is very small, while process-times can be < 0.5 s.

On the other hand, laser-welding can also be used to advantage for r y h t r q p

with

several meters of weld-length.

Thermoplastic r y h r can be welded.

n

T r r q )

Very short processing times are possible, depending on product type. Laserwelding can bepractically as fast as ultrasonic welding.

n

H v v h y v s y r p r u r q p )

The heat-affected zone is very small. As a result, built-in stresses are not large.

Sensitive components or parts are not affected, even if these are close to the weld zone. No

electrical fields or mechanical vibrations are generated, contrarious to most other welding

processes.

n X r y q v t s " 9 u h r q r y q v t p

n Q p r s y r v i v y v )

No extra product part (as compared to induction welding) and no additional materials are

necessary. No additional structures are necessary in the product, depending on circumstances.

Fig. 2.4 Line-scan welding with a mask-

shaped laser beam


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A v t " Testing

f hermeticity (SIEMENS)

bulging

no bubbles

at weldseam

some bubbles atconnection piece

5 bar

Materials with large differences in processing temperatures have been welded successfully

(Hytrel to Vectra, PA 6 to PA 46).

It may be necessary to use an adapted design of the weld zone

Additional degrees of freedom for product design. With the proper welding equipment, it is quite easy to switch from one product to another.

" ! 7 q h p q v v

The joint should always be made of an IR transparent part and an IR absorbing part. Polymeric materials should be compatible, just like other welding techniques. However, some

combinations have been welded by laser, whereas they could not be welded by other techniques.

Homogeneous pressure should be applied during welding. The clamping force is often lower than with

other welding techniques.

Stresses can build up during welding, although these will be lower when compared to the other

welding techniques. Therefore, this will usually present no problems. Still, there are ways to solve

problems, if any, depending on materials and particular applications.

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The joining result is influenced by different parameters. These parameters can be divided into certaincategories, which will be explained in the following.

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Laser properties describe the laser in terms of wavelength, power, intensity distribution, operation mode

etc. The intensity distribution, which can be influenced to some degree by the optical system, may affect

the welding result. The focal length of the optical system influences the free working distance as well as

the minimum spot size. Laser radiation can be applied in various ways: directly onto the workpiece

without additional optics, by means of an optical fibre plus focussing optics, or shaped by an additional

optical device. Defocusing will influence both the intensity and the intensity distribution, but may cause

irregular results if a diode-laser is used without intervening optical fibre. Finally, the laser source can beoperated in continuous wave or in pulsed operation.

Fig. 3.2 Testing of water-tight bonds

(Siemens)

Fig. 3.1 Testbox with welded, water-tight

lid (Siemens).


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" ! ! H h r v h y Q h h r r h q Q r v r

The most important material properties for applying the laser welding process are the optical properties of

the joining partners. The laser radiation has to be absorbed by the polymers and transformed into heat

energy, while generating a temperature rise above melting (plastification) temperature. The transformation

takes place within the optical penetration depth, which is determined by the absorbency. The optical

properties are mainly: reflection, transmission, absorption, and scattering. These are affected by pigments,

additives and other additional ingredients. There are also chemical properties which influence the welding

process as well as the results, such as the compatibility of the joining partners.

" ! " Q p r Q h h r r h q Q r v r

Process parameters hold the key for optimising the process. At the time a process describing parameter

called line energy is commonly used. The reached tensile strength vs. the line energy supplies a char-acteristic curve (Fig. 3.3), which shows an opti-mum. This optimum defines the parameter set of

laser power and feed rate for reaching the strongest

joints. At lower line energies only adhering occurs,

while at higher line energies decomposition occurs.

Furthermore, the process is influenced by the way

of irradiation which means the process strategy.

The two extremes are contour welding and simul-

taneous welding. Since both strategies show dif-

ferent heat up and cool down behaviour also the

time scales of the process as well as the process it

self is influenced.

Furthermore, scanning a beam over the work piece

or applying mask technology are possible.

" " H h r v h y

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Thermoplastic polymers are made of molecules in which monomeric repeating units are attached together

into long chains. When two products made of a thermoplastic material are welded, the polymer chains

diffuse across the interface and a bond is formed by entanglement of the chains. This applies to all

welding techniques of thermoplastic materials. In simple overlap joints, flow of molten polymer is notnecessary; the bond is only formed by diffusion. Diffusion does not seem to be a limiting factor: We have

evidence that materials with a very high melt flow index (Hostaform H4320) can form strong welds.

The low thermal conductivity of thermoplastics keeps the cooling rate after irradiation sufficiently low for

the formation of strong bonds. This is an important and advantageous difference with metals where heat is

easily transported away from the weld area.

Since the formation of a bond requires diffusion of polymer chains, some degree of miscibility is

necessary for welding. Usually, different types of polymers are not miscible and the strongest welds can

be made when both parts of the joint are made of the same polymer. Welds of dissimilar materials can be

made as long as the materials have some degree of compatibility.

In the case of laser welding, the energy which is required to melt the materials near the interface, isdeposited into the material by an infrared laser. In most cases overlap welding is used. For this variant

Fig. 3.3 Characteristic CurveWeld Seam Strength vs. Line

Strength Characteristic Curve

E S := Laser Power P / Feed Rate v

adhering

decomposition

optimum

Line Energy E S


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of laser welding, one part has to be transparent and the other should absorb the laser, as described in

chapter 2. Since most polymers are transparent for the infrared laser radiation, which has a range of 800

to 1100 nm, absorbing additives are needed in the absorbing part of the joint. For colored parts which donot absorb in the IR, special IR absorbing additives (or IRAs) should be used with a low level of visiblecolor.

The IR laser radiation penetrates the transparent part and irradiates the interface between the product

parts. In many polymers there are several phenomena, which lead to scattering of the incoming IR

radiation. Sources of light scattering are mineral fillers, glass fibers or crystallinity of the polymers. An

example of a strongly scattering material is PBT. All these phenomena result in a broader intensity

distribution at the weld area.

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D q p v

Polymers can be joined by a variety of methods which are already in use (eg glueing). However, thedurability of the joints is often poor and the opportunities for joining especially small (particularly micro)

components, is limited.

It is possible, however, to obtain very good joints by incorporating organic IR absorbing materials into a

polymer and then laser welding onto a non-IRA containing polymer. More details of the IR absorbing

materials, the key parameters they need to have for, and how they can be used in laser diode welding, are

given below.

D S 6 i i r

The key properties needed for laser diode welding include

good-high stability (especially to the temperatures reached during extrusion)

high absorption at wavelength of laser diode being used

polymer compatible

high strength/low colour

cost effective

non-toxic

The most well-known organic IR absorbing material is carbon. Potential benefits of carbon in polymer

welding include its low cost, excellent stabilty and good polymer dispersibility. A major disadvantage is

its colour - which means that it is not possible to weld coloured (non-black) polymers to each other

without having an effect on the overall polymer colour.

Alternatives to carbon are provided by special IR absorbers. An example is Avecia s range of IR dyes -the PRO-JETs. These dyes have the desired features listed above and they can be used at concentrations


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" " " Q y r H h v

The idea of the Polymer Matrix (Fig. 3.4) is giving a rough overview of thermoplastic polymers welded

within the POLYWELD Project.

However, the Polymer Matrix does not lay claim to completeness.

It is not possible to integrate all influencing parameters and aspects within the Polymer Matrix as well as

making all the information comparable.

Therefore, some aspects influencing the weldability should be mentioned very briefly offside from the

Polymer Matrix it self (see also chapter 3.2).

1. A considerable fraction of the experiments were performed using flat test samples of 2 mm thickness.

The transparent joining partner contained no pigments at all, whereas the absorbing partner contained

one or more of various pigments; in many cases, the pigment was carbon black in a concentration of

0.3 w.%.

2. The weldability of polymers may depend on the supplier and the manufacturing history.

3. Optical properties change with thickness, pigment concentration, pigment supplier, glass fibrecontent and processing history.

4. The assessment of weld seam quality involves strength and optical appearance, while the latter is

more or less subjective.

5. The weld quality was not optimised for all materials.

Ranking scale:

+ strong welds are obtained showing good optical appearance

+/- depending on the supplier both strong welds showing good visual appearance were obtained, as well

as very poor or moderate welds.

- no or no good weld was realised

open space: not tested so far

Please keep two things in mind:

a. Each assessment sign is based upon quite some information covering a wide range of aspects.

b. This matrix has a relative value since the data behind it is much less compared to the data available

for classical welding methods at the moment.


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Fig.3.

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PolymerMatrix,givinga

roughoverviewofthermoplasticpolymersweldedthecontributors.

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atrixhasarelativevalue!

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aredtodataavailableforclassicalweldingmethodsatthemoment.


12/22


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The "many-sided tool" laser has its firm place in modern manufacturing technology. Even if laser materi-als processing of metal is the most prominent application, there is a huge market for marking of polymers.

Joining of polymers with lasers could not reach a reasonable market share so far. This may change by the

appearance of a new, reliable, cost effective, small sized laser source: high power diode laser.

There are different lasers available, while they can be classified under different view points, such as their

active medium or the emitted wavelength. Different active media correspond to gas- , semiconductor, dye

or solid-state-lasers. The emitted laser light ranges from ultra-violet to the infra-red radiation (diode la-

sers). The most prominent lasers for materials processing, CO2 (gas), Nd-YAG (solid-state), diode (semi-

conductor) and excimer (gas) lasers.

Since approx. 1995, high-power near-infrared GaAs-based laser diodes are becoming available for

laboratory use. These lasers (wavelength 800 1000 nm) became attractive due to their lower price. Ithas been demonstrated, that this technology with its small size, good beam quality and life time is suited

for polymer welding applications.

9 v q r G h r

Since diode lasers are most commonly used for polymer welding they are explained in more detail. From a

single diode laser element only a few milliwatts can be extracted from the pn-transition; such elements are

e.g. used for communication or in CD-

ROM's. But the power of course is by far

not sufficient for materials processing.

Therefore, many of such basic units are

integrated in one semi-conductor element,which is called a diode laser bar. A laser

bar (see fig. 3.5) has a size of ca. 10000 x

1000 x 115 m. The special way how the

light is generated in the pn-transition leads

to emission characteristics, which are

unusual for a laser: the emission shows a

large divergence from a very narrow slit in

the direction of the pn-transition ("fast-axis") and a smaller divergence but a wide emitting stripe in the

plane of the pn-transition ("slow-axis").

A single diode laser bar can be driven as high as 50 or 60 A. This in turn results in a laser power of 40 to

50 W per diode laser bar. Two or more diode laser bars integrated in one lasers system are called a diode

laser stack, which allows to increase the output power even more.

As mentioned above diode laser radiation shows different divergence angles perpendicular to the emission

direction. Sophisticated optical design allows coupling the radiation into an optical fibre or using it for di-

rect irradiation, while the intensity distribution can be adapted to the process needs [pat. 2, 3].

G v s r v r h q 6 t v t @ s s r p s 9 v q r G h r

For an industrial application, long lifetime and as few service breaks as possible are mandatory. Diode la-

sers are almost service free. Semiconductor elements typically do not show sudden defects, if they are

manufactured properly, but slowly degrade over a certain time. Thus, definition of lifetime for high power

diode lasers has been defined (not yet as an international standard, but rather as a silent agreement) as a

20% loss of optical output power at a constant current. Practically, the current is increased to

electrical contacts

emitter groups

GaAs-bar

(magnified)

laser mirror

Single emitters

Fig. 3.5 Principle Scheme of a Diode Laser Bar


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maintain constant optical output power for the process. Using this definition, a lifetime up to 10.000

hours is guaranteed from laser manufacturers, before the diode laser has to be replaced.

G h r T r s Q y r X r y q v t

Diode laser systems for serial welding typically provide several 10 Watts, which can be either delivered

directly from the laser bar, a system consisting of stacked bars (fig. 3.7) or from an optical fibre (fig. 3.6).

If no fibre is used, the spot has a rectangular or square shaped beam profile.

" # ! Q p r 8 y T r

As known from previous sections, laser welding has to take place within a certain process window

(fig. 3.3). If only a narrow process window exists, i.e. the process becomes sensitive to laser power: Too

low power is not sufficient for melting, whereas too high power leads to decomposition. In some cases the

acceptable temperature range may

cover only a few 10 C. In thesecases, little deviations from an ideal

surface or from an accurate feed

rate may cause severe damage of

the material. In order to obtain a

reproducible and high quality weld,

it is important to get a constant

temperature at the interface

between the joining partners.

Therefore, process monitoring and

control systems are being developed

in order to increase the productivity

and maintain constant quality of the

products.

An active control system is shown

in fig. 3.8. The laser head consists of a laser beam delivery, a camera for easy positioning of the laser

beam as well as a pyrometer. The pyrometer detects the heat radiation and converts it into an electrical

Fig. 3.7 ROFIN DLx15 150 W diode laser system

(min. spot size 0.6 x 1.2. mm)

A v i r

8 y y v h v y r

X x v t v p

I p u s v y r )

T400-700

> 85%

T1300-2500

> 80%R

800-950> 99,5%

H y v y v r s v y r )

R1400-2500

> 80%T

vis(

l

) > 15 - 90%

Tvis. average

> 50%

p h r h r r

Fig. 3.8 Laser head with

beam delivery, camera

and pyrometer control.

Fig. 3.6 ROFIN DFx03 30 W diode laser system

(400 m fiber beam delivery)


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signal which is proportional to the surface temperature of the weld zone. The pyrometer signal can fed

back into the power supply as a control signal (feedback loop) adjusting the laser power keeping the

surface temperature in the weld zone constant (fig. 3.9). Therefore, this process control system regulates

the laser power (and/or the welding speed) to keep the process temperature constant.

Figure 3.9 shows detected pyrometer signals with and without the feedback loop as well as the

corresponding welding results. While in the top pictures the laser power is kept constant the weld shows

defects in the corners of the triangle. If the feedback loop is used, the temperature is maintained at a

(almost) constant level while adjusting the laser power via the pyrometer process control (feedback loop).

The weld does not show any defects anymore.

Fig. 3.9 Welding results applying a feedback loop based on pyrometer technology

top : no process control, bad welding results (decomposition in the corners)

bottom: applied process control, good welding all over the weld seam

(ILT)

All applied systems are based on pyrometer technology, while the detector can be applied as internal or

external (outside or inside the laser head) process control unit.

" # " Q q p 8 y h v t

Q r

Three reasons can exist for using a clamping tool:

1. To keep the top-part in its proper place (only necessary if positioning is not done automatically by the

product design).

2. The formation of a bond requires thermal contact and diffusion across the interface. Therefore, the the

upper product part has to be pressed against the lower part along the complete welding contour. This

is particularly important if gaps between both parts are present due to dimensional tolerances and the

weld has to be closed hermetically.

3. During cooling (after the laser-beam has been switched off), some pressure is needed in order to

ensure that the molten polymer in the weld zone has a strong, compact structure after solidification.Depending on the product and the material, the weight of the product can already give sufficient

pressure but an additional force is usually needed.


15/22


Q v p v y r

In principle, hardly or no material displacement has to take place with laser welding. There are situations,

however, where some materials displacement occurs, for instance if gaps between both product parts haveto be closed. In that case, the upper product part will move a little downwards during welding. Therefore

it is important that the clamping force be maintained in spite of this movement. If an air-cylinder is used,

the slip-stick effect should be minimal, as with all other polymer welding processes.

The clamping force has to be applied close to the weld zone. Preferably directly on top of the weld. This is

only possible if the clamping tool is transparent for the laser radiation. This leaves two strategies for

clamping.

H r u q

1. The clamping tool is provided with an opening directly above the weld zone, so the clamping force is

applied in the immediate vicinity of the weld.

2. The clamping tool is transparent and is positioned on top of the weld zone.

In case of method 1, a special situation arises if the weld has to be made on the outer fringe of the product

and has to be hermetically closed. If the weld would not be on the outmost fringe, the clamping force

could be applied simply outside the weld contour. If the outer fringe is the same as the weld zone,

however, clamping has to be done inside the weld contour so the laser beam is blocked by the fixation of

the clamping tool. Therefore, the fixation has to be transparent to the laser radiation, which can be

realised by any one of several methods.

" $ X r y q B r r

Just as with every other welding process, the design of the weld geometry is an important factor forsuccessful laser welding. There are cases where products designed for ultrasonic, induction welding or

adhesive bonding could be very well laser-welded without any change of the product design. However, in

general the design of the weld zone deserves special consideration, while it has to be distinguished

between external and internal welding geometry.

" $ 7 h v p 8 v q r h v

All considerations concerning the weld geometry start with a definition of the requirements.

These can refer to e.g.:

- strength

- visual appearance- Should the weld be hidden ?

- Is a special weld-flange allowed ?

- Should the weld be completely closed ?

- Should the weld be air-tight, water-tight, dust-tight, resistant to high electrical fields ?

- Do both product parts fit closely or are substantial gaps present in the weld zone ?

The properties of the weld contour constitute another important point:

- Does the weld consist of one or more separate points or is it a continuous line ?

- Is the weld contour 2- or 3-dimensional ?

A 3d contour will usually give rise to gaps in the weld zone due to moulding tolerances.

Also the materials or the material combination to be weld has to considered:


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- High thermal expansion of polymers will cause mould-shrinkage and irregularities of the product

shape, and will therefore lead to gaps between both parts.

On the other hand, the same phenomenon makes it possible to bridge gaps, because of the expansion of

polymer during the welding process.

- The difference between the decomposition temperature of the material and its melting point or

softening range has influence on both the process and the product shape in the weld zone. If the

difference is large, the requirements are not very tight and vice versa.

- If the laser-transparent part has a high absorption or scattering of laser-radiation, its thickness has to

be limited. This may necessitate a special weld-flange or a special wall-shape, see figure 3.10.

" $ ! @ r h y X r y q B r r

The external weld geometry is considered as the (macroscopic) geometry of the two joining partners where

they are in contact. Different possibilities of the design of the external weld geometry of overlap welded

parts are shown in Fig.3.10. These different designs allow applying overlap welding while obtaining

different shapes of the weld area.

Fig. 3.10 Different designs of the external weld geometry

" $ " D r h y r y q t r r s y h v y y

The internal weld geometry is considered as the (microscopic) geometry of the area where the weld seam

is generated. It will be clear that simple recipes cannot be given for the design of the internal weld

geometry. However, some general remarks may be given:

- generally, it is wise to make the gap between both joining partners as small as possible over the

complete welding area.

- Some sort of a weld rill is certainly required if all of the following conditions are met:

the weld has to be hermetically closed

thermal expansion of the material is not high

Gaps of the order of 0.1 mm or larger may exist between both parts

the laser-transparent part is not flexible

process speed is important


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- Also other reasons may exist to apply a special weld rill, for instance special requirements on strength.

" % X r y q h y v

The most important criteria for the determination of weld quality are the strength and the optical

appearance of a weld. Both are discussed in this chapter.

The strength of a weld arises from the diffusion of polymer chains across the interface between the welded

parts. The optimum strength is achieved when polymer chains in the weld form entanglements, just as they

do in the bulk of the polymer. Tensile testing experiments suggest that the geometry of the weld is

probably more a limiting factor for the development of strength, than the welded interface itself.

Figures 3.11a and 3.11b show typical cross-sections of welded overlap joints. In figure 3.10a the

absorbing part is in the lower part of the picture and the transparent part in the upper one (the white line

in the picture is an artifact resulting from the preparation of the sample). The laser radiation entered the

weld area from the upper part of the picture. The rather vague, dark line indicates the outside of the heat-

affected zone. A schematic representation of the weld zone is displayed in figure 3.12. In opticalmicroscope images, the boundary of the heat-affected zone can clearly be indicated, because of

recrystallization. The depth of penetrationof the laser beam into the CB containing material is given bydCB, whereas the depth of the heat-affected zone in the natural material is given by dNAT. The width of the

weld is indicated as w.

Fig. 3.11 a Fig 3.11 b

Fig. 3.11 a and b Optical transmission images of welds in PP

Fig. 3.12 Schematic representation of the weld area(see text for explanation of the parameters).


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!

( "

!

v v t p u h i r

y v q

v t v h y

s y v q p y

p u h t r q

s y v q p y

h y r

v y r

When carbon black is used as an absorber for the IR laser, strong bonds can be formed in most cases

when concentrations are used ranging from ca 0.1 to 1 wt%, depending on the welding parameters.

Sometimes a hole is formed in the middle of the weld, as shown figure 3.11b. The origin of this hole maybe shrinkage. In most cases observed however, the presence of a hole did not deteriorate the strength of

the weld.

The optical appearance of the weld does not pose serious problems in most cases, since the weld is hidden

between two product parts. Depending on the type of product, problems may arise when optically

transparent materials are welded and the weld is visible. Also, when dCB is similar to the dimensions of the

product, sink marks can appear on the surface of the product. However, a typical value for dCB is in the

order ofh s r u q r q U u v v h y y u h v q p v x h x q p p q CB increases

with decreasing carbon black content. dNAT does not vary strongly with carbon black content.

# 6 y v p h v

In this chapter some product areas are discussed which illustrate some of the special advantages of

laserwelding

7 v r q v p h y h y v p h v

In this field, laser welding technology has come closer to practical use for products such as disposable

for body liquid testing. Due to the potential of laser welding for making accurate, high quality,

miniature joints, advantages in function, process time and economics are expected, which are

promising for a wide range of these applications.

Fig 4.1 Fluidic test device where the welding seams are required to provide the sealing function between

adjacent fluid channels, and where the fluid channels have a complex geometric shape (STEAG

microParts)

H v v h r q p

In the area of miniature products the introduction of polymer laser welding technology is related to

some general trends: continuing miniaturisation and increasing use of polymers in miniature products.

Especially for optical information storage devices laser is an alternative for the present-day adhesive

bonding process. Driving forces are yield and product quality.


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Fig. 4.4 Air-intake manifold (FIAT CRF)

Fig 4.2 Optical unit (PHILIPS)

@ p h y h v s r y r p v p

p r

Important opportunities are seen with the encapsulation of electronic components and with welding ofhousings in general. An important advantage compared to adhesive bonding is the absence of

contamination. Another advantage is that thermal and mechanical load are applied only locally, and

not to the whole product.

6 v r

The automotive industry is a high potential area for

introduction of laser welding technology; for exterior

applications, welding of lighting systems is under

investigation. One of the requirements, however, is that the

joint design must be specific for laser welding. Many of the

lighting assemblies currently produced are designed forvibration and hot plate welding and have a poor fit on the

joint. Due to the low stresses induced by laser welding after

treatment is not necessary.

Other exterior applications for laser welders will be in the

production of all-plastic cars (body and frame) and for

interior applications, such as carpeted panels and

instruments panel skins that are currently glued together,

process that required long time and labour intensive.

A lot of the potential application for polymers laser welding are

under car hood; many electronic modules

requires hermitic welds as well as

gasoline filter housings or for example

the oil tank.

Other applications for under-the-hood

are in welding of air-intake manifolds; as

these components evolve away from lost-

core technology, vibration welding will

have to became more and more complex

in order to handle the welding of multi-

dimensional manifolds. Laser welding

may be able to overcome some of theproblems faced with vibration welding.

Fig. 4.3 Laser welded gasoline

Filter housing

(FIAT CRF)


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Fig 4.5 Laser welded double-walled windows(THERMOFORM)

Q r h y p h r q p h q q r v p h y v h p r

In these products the styling aspect is of increasing importance. For welding this implies: welding

without causing any damage, 3D-weld-contours and freedom of design. In addition, water tightness isbecoming increasingly important. Laser welding of polymers is tuned in to this trend in the market of

consumer products.

9 i y r h y y r q v q r

Laser welding technology is under developing to

replace the existing gluing process; the effort spent

for the final objective will be justifiable, technically

as well economically. In this area the request from

customers becomes continuously more demanding

to maintain the actual price level of the products. In

order to meet their request, technological innovationas well as materials and process development are

the right parameters to work with so as to guarantee

the quality of the products and the employment

which this production entails. Introduction of laser

welding technology seems to fulfil all these

requirements.


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6 p x y r q t r r

This publication summarises results and insights gained from the Polyweld project. This is a BriteEuramproject aiming at the development of laser welding as a new joining technique for polymeric materials.

The European Commision is acknowledged for financial support under the IMT/SMT programmes(1994-1998). The Polyweld project started in 1998 as BriteEuram project BE 97-4625, contract no.

BRPR-CT98-0634.

6 u

P. van Engen Philips CFT

F. Lupp Siemens

L. Bolognese Fiat CRFM. Hempel STEAG microParts

B. Palfelt Thermoform

F. Bachmann Rofin Sinar

U. Russek FhG-ILT

J. Campbell Avecia

R. Korbee DSM Research

K. Grim KU Leuven


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% 6 r q v r s r r p r

G v r h r

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Oct. (1995).

2 Bonten C., Serienschweien von Kunststoffteilen: Ein berblick zum Stand der Technik,KU Kunststoffe , ' Jg. 89 (1999)

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5 Grimm R.A., Fusion welding techniques for plastics, Welding Journal, March (1990).6 Jones I.A., Taylor N.S., High speed welding of plastics using Laser ( CO2 & ND:YAG),

ANTEC 947 Klein H., Perspektiven fr die Mikrosystemtechnik. Laserdurchstrahlschweien, Diodenlaser,

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10 Maier C., Design guide 35 laser welding, Asian plastics news, May (1999).11 Nielsen S. E., Laser material processing of polymers, Polymer testing, " , No.4, (1983)12 Ou B.S., Benatar A., Albright C.W., Laser welding of PE and PP plates, ANTEC 9213 Pinho G.P., Laser welding of thermoplastics, SAE Technical Paper 1999-01-3146 (1999).14 Potente H., Korte J., Becker F. Editor: Harris R. M., Laser transmission welding of

thermoplastics: analysis of the heating phase, Coloring technology for plastics, N.Y. plastics designlibrary (1999) (Book).

Q h r

1 Dilas Diodenlaser GmbH, Mainz, Germany, DE 195 00 513 C1.

2 Fraunhofer-Institut fr Lasertechnik, Aachen, Germany, DE 44 38 368 C2.3 Fujitsu Ltd., Welding 2 kinds of moulded plastics - using laser beam irradiation,

JP 62216729.

4 Proaqua-Provita Deutschland GmbH, Laser appts. for welding plastics parts of medical andlaboratory filters - has low reject rate and uses same laser for both cutting and welding operations,

DE 4225679.5 Thermoform AS, Method for the manufacture of a plastic window, EP 942139 A1.6 Tjaden J., Laser welding continuously moving thermoplastics sheets - using changing angle of

incidence of beam on sheets and additives in thermoplastics to increase energy utilisation,EP 126787.

7 Toyota Jidosha KK, Joining different kinds of synthetic resin materials together - using laser to meltplastics and gas blast to mix melts, JP 60225736.

Documents

Laser Welding Handbook