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Lesson 1Basic Princip les of Plast ics

Key Questions What types of polymeric materials (plastics andelastomers) are there?How do they behave under the influence of heat?How does melt viscosity vary with temperature?Which plastics are used in injection molding?

Contents 1.1 Classification and Identification of PolymericMaterials (Plastics and Elastomers)1.2 Deformation Characteristics of Plastics1.3 Molding Compounds and Shaping Methods1.4 Plastics MeltsReview Questions

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Lesson 1

polymers

1 I Classif icat ion and Ident i f icat ion ofPolym eric Materials (Plast ics an dElastomers)Plastics and elastomers are the names given to synthetic organic polymersmanufactured from organic compounds of low molecular weight, calledmonomers, (e.g. ethylene, styrene, vinyl chloride). The manufacture of plasticsand elastomers can even be based on natural macromolecular substances (e.g.,natural rubber, cellulose, proteins).

methods of synthesis The methods of polymer synthesis consist of:Polymerization chain or addition polymerizationPolyaddition (polyadduct formation) andPolycondensation (step polymerization).

Plastics possess different material characteristics, which exert their decisiveinfluence during processing, such as injection molding.

groups There are three distinguished overall groups of polymeric materials. Figure 1 - 1

Figure 1 1 Classification of polymeric materialsnames each of these groups and also quotes examples.

thermoplastics Thermoplastics (thermos=warm; plasso =shape, shapable) are fusible.They can be fused repeatedly and are capable of dissolving or at leastswelling in many solvents. At room temperature, they vary from a tough, softcondition to a ductile or rigid hardness. Quantitatively, hermoplastics have thelargest share of the plastics market.

amorphousthermoplasticssemi-crystallinethermoplastics

Amorphous (amorph=formless) thermoplastics are similar to glass withrespect to molecular structure. They are distinguished from semi-crystallinethermoplastics, which have a milky, opaque appearance. When a plastic is astransparent as glass, it is usually safe to assume that it is an amorphousthermoplastic.

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Basic Principles of PlasticsVarious types of CD cases are available in different materials. The mostpopular one is the Jewel Box however, produced in an amorphous plasticscompound. Most tops of CD cases are produced in that material, because of itstransparency, so that the list of titles can be read through it. If a colored case isdesired, it is also quite easy to pigment the transparent material. Even the CDitself is produced in glass-clear plastic and therefore transparent. However, it iscoated (sputtered) with a reflective layer (usually of aluminum, but gold is alsoused) by a vacuum deposition process and then printed. The aluminum or goldcoating acts like a mirror, so the laser beam does not pass through the CD butis reflected instead (Figure 1-2).

CD cases

Figure 1-2 Compact Disk and case

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Lesson 1elastomers

thermosets

IS0 I043

Elastomers (elastic=flexible; meros =part) cannot be fused or dissolved, butcan be made to swell. They cross-link in a wide mesh pattern and thereforeexist in a soft, elastic condition at room temperature. Examples of moldedparts made from elastomers include seals, bellows, and tires. Natural rubber(NR) epresents a special case within the group of elastomers. It is obtainedfrom the sap (latex) of tropical trees.Thermosets are hard. They are cross-linked with a narrow mesh in alldirections. They are impervious to plastic deformation, infusible, and highlytemperature resistant. Because thermosets are densely cross-linked, they areimpossible to dissolve and quite difficult to swell. They are hard and brittle atroom temperature. Electrical socket-outlets are manufactured from thermosets,for example, because resistance to high and low temperatures is very importanthere. Thermoplastics are incapable of meeting these requirements.

IS0 1043 provides for each plastic (polymer) to be identified by a sequence ofsymbols (an abbreviation) which indicates its chemical structure. Additionalletters (codes) identify its use, fillers, and basic properties, such as density orviscosity. An example is given in Figure 1-3.

Figure 1-3 Identification of a plastics materialIn this instance, we merely wanted to introduce these properties. They arediscussed in greater detail later in the lesson.

1.2 Deform ation Characterist ic s of Plast icsthermoplastics: Polymeric materials display different characteristics when heated. Thesestatesltemperature characteristics depend on various factors, including the chemical structure ofranges the polymer molecules and the type of intermolecular bonding forces.8

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Basic Principles of PlasticsInjection molding is a primary processing method. In other words, the materialmust exist in a plastic state so that it can be processed. This plastic state ischaracterized by the viscous flow properties exhibited by raw plastic materialssuch as powders or granulates.

plastic state

The characteristics displayed by polymeric materials when subjected to heat-known as deformation characteristics- re illustrated using an amorphousthermoplastic as an example (see Figure 1-4).The polymer is a hard material at room temperature. Because the individualmacromolecules barely move, they are held together by intermolecular forces.An increase in temperature causes the macromolecules to move with greaterintensity. The intermolecular bonds loosen and the strength of the materialdecreases, while its extensibility and ductility increase.

Figure 1-4 Deformation characteristics of amorphous thermoplasticsLegend:

T, =glass transition temperatureTf= low temperature rangeTd=decomposition temperature

Once the glass transition temperature (Tg) has been exceeded, theintermolecular forces have become so weak that the influence of externalforces can cause the macromolecules to slip apart from one another. Thestrength declines steeply, while the elongation leaps upward. In thistemperature range, the plastic exists in a rubber-elastic or thermoelastic state.

deformationcharacteristicseffects oftemperature

glass transitiontemperature Tg

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Lesson 1flow temperaturerange q As the temperature continues to increase, the intermolecular forces are almostcompletely eliminated. The polymer proceeds in a continuous manner from

the thermoelastic state to the thermoplastic or molten state. This transition isdescribed as the range of the flow temperature (q).his temperature cannotbe specified precisely. Primary processing methods such as injection moldingare carried out within the thermoplastic range.

decompositiontemperature T d is destroyed.

If heating continues beyond the decomposition temperature (Td) , the polymer

The deformation characteristics of a semi-crystalline thermoplastic can be seenin Figure 1-5.

Figure 1-5 Deformation characteristics of semi-crystalline thermoplasticsLegend:

T, =glass transition temperatureT, =crystalline melting temperatureT d=decomposition temperature

crystallite meltingtemperature

In contrast to amorphous thermoplastics, two side-by-side conditions virtuallyexist in crystalline thermoplastics. On the one hand, there is the crystallinezone, in which molecules are packed in tight, ordered formation. Then there isthe other, amorphous zone, where molecules are spread further apart, in adisordered fashion. The intermolecular forces holding the crystalline zonetogether are considerably stronger than those in the amorphous one. Theamorphous content of the polymer softens above its glass transitiontemperature (Tg). The crystalline zones of the polymer melts, when thecrystalline melting temperature (T,) is exceeded,

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Basic Principles of PlasticsSemi-crystalline thermoplastic possesses great strength and increasing temperatureelasticity between Tg and T, . The softening temperatures of the amorphouszone of conventional partially crystalline thermoplastics lie far below roomtemperature. Within the range of practical application (i.e., at roomtemperature and above) most semicrystalline thermoplastics are both toughand strong. When the crystalline melting temperature (T,) is exceeded,strength decreases sharply and elongation reaches its highest value (in thethermoelastic range). Once the high temperature has completely eliminated thecohesion within the crystalline zones, elongation decreases sharply andsuddenly.

behavior

This difference in the deformation characteristics of amorphous and deformationsemicrystalline thermoplastics can be attributed to their different molecularstructures.As already mentioned, the intermolecular forces operate with muchgreater strength in the crystalline than in the amorphous state.

characteristics

1.3 Molding Compounds and Shaping MethodsThe shaping methods employed in the machining and processing of plastics,can be divided according to the physical status (temperature ranges) of thosematerials. Figure 1-6 shows the assignment of shaping methods tothermoplastics compounds in relation to their status ranges.

shaping methods

Figure 1-6 Shaping methods11

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Lesson 1injection molding Injection molding is one of the primary processing methods. It is carried out in

the thermoplastic state, i.e. above the flow temperature range(q)r crystallinemelting temperature (T,) and below the decomposition temperature (Td) .

molding compound Molding compound (or, more specifically, injection molding compound)denotes the raw material, from which molded articles are produced. Moldingcompounds are supplied by manufacturers of injection molding materials as:

Granulate (extruded molding compound, die-face cut into grain-sizedgranules)Powder orGrit

delivery

moisture

thermosetselastomers

viscositypow-resistance

Molding compounds are supplied in metal drums (contents: 10kg), paper- orplastic bags and in some cases by rail- or road transported bulk-carriers even.Injection molding compounds must be stored in dry conditions, because theirmoisture content must not exceed the permissible limit of 0.1% to 0.3% (forthermoplastics). However, damp molding compounds, such as polycarbonate(PC),can be dried at temperatures of 120 to 130C (250-270F) for about 8 to12 hours.The situation is different with thermosets and elastomers. They are usuallyintroduced to the machine in the form of strips (elastomers) or liquids (siliconerubber). Thermosets are supplied as powders or liquids. Thermosets reinforcedby long glass fibers are sometimes loaded into the machine as sauerkraut.

1.4 Plastics Melts1.4.1 ViscosityIn the injection molding process, the polymer exists in a thermoplastic state. Itflows in a viscous manner. The viscosity of a liquid is a measure of its flowresistance. For a Newtonian liquid, such as water, the viscosity is defined as[ q ] , shown in Figure 1-7).

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Basic Principles of Plastics

Figure 1-7 ViscosityTo illustrate what viscosity means, we can imagine two containers of equalsize. In the bottom of each container is a closed hole, each with the samediameter. If we fill one container with water and the other with a viscouslacquer, then open both holes at the same time, a given quantity of lacquer willrequire much more time to flow out of the container than will the samequantity of water. We can see that the lacquer is more viscous than water. If aliquid possesses an especially high viscosity, it is said to be highly viscous.

viscous liquid

1.4.2 Shear Stress and Shear RateWhen a liquid flows through a tube, the highest rate of flow occurs in themiddle of the tubes cross section.As we move out toward the wall of the tube,we find that the flow rate decreases. At the wall of the tube, it finally reacheszero, because of the liquids adhesion to the wall. The result is a profile of flowrates. We can therefore imagine a flowing liquid as a series of liquid layersflowing at different rates (see Figure 1-8).

$ow rate profile

Figure 1-8 Speed rate profile of a liquid passing through a tube

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Lesson 1shear stressshear rate

viscosity

temperature

processing data

Shear stress results as the flowing layers of macromolecules slip past oneanother. The shear rate is the difference in the rates of flow or the sheargradient- hat is, the change in the rates of the flowing layers across the radiusof the cross section. The viscosity decreases with increasing shear rate.1.4.3 Viscosity and TemperatureIf a liquid is heated, its viscosity decreases. For example, if we heat the lacquerin our previous example, it requires less time to flow out of the container. Theviscosity of a thermoplastics melt thus depends not only on the shear rate, butalso on the temperature (see Figure 1-9).

Figure 1-9 Viscosity function of a plastics materialViscosity decreases as temperature increases, so the plastic melt begins to flowin a less viscous manner. In the case of polyethylene, for example, theviscosity measures approximately 138 Pa.s at a shear rate of 1000 l/s and atemperature of 150C (300F). However, at the same shear rate and atemperature of 270C (520"F), the viscosity measures approximately 52 Pa+.This example applies to polyethylene (PE).Aside from viscosity, other factors also influence the processibility of plasticsin the injection molding process. The summary table in Figure 1-10 showssome of the types of plastics used in injection molding, as well as thecharacteristic material properties which are important in processing (e.g.,shrinkage in processing, flowability, and processing temperature range).

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Basic Principles of Plastics

Figure 1-10 Important material characteristics

It can be seen from the table that the range of processing temperatures is verynarrow for some thermoplastics but very broad for others. For example, PVC-U (unplasticized or rigid PVC) can only be processed within a very narrowtemperature range (19O-21O0C, 374-41OoF), while POM can be processedbetween 180 and 230C (356446F). Cif. no. 10 and no. 11 in Figure 1-10).

processingtemperature range

The subsequent Table (Figure 1-11) shows the material properties of PC, fromwhich CDs are produced, as well as the setting parameters, which were usedfor programming the injection molding machine employed.

CD-materialcharacteristics (PC)

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Lesson 1

Material propertiesSolid, rigid, impact resistant up to -1OO"C, (-148"F), high heatresistance, crystal clear, non-toxic.Resistant toOil, gasoline, diluted acids, alcohol, waxes, fats, simple soapsNot resistant toStrong acids, alkali solutions, benzene, amines, ammonia, some solventcomponents.Flame retardant, extinguishes away from flame, bums brightly, producessoot, chars, forms blisters, smells of phenol.Heating zone 1 23&260"CHeating zone 3 260-320CHeating zone 4 26&320"CNozzle zone 280-330CVery high injection pressures ( 1 300-1 800bar=19.00(1-26.000psi) arerequired, as the material is extremely viscous.Pressure usually amounts to about 404 0% of the injection pressure.5&150bar (725-2175 psi)Subject to length of Aowpath and wall-thickness. Fast injection for thinwalls. Where good surface quality has been specified, injection should bea little slower.

Screw RPMHigh screw torque required, therefore medium screw speeds should beapplied.Melt cushion2-6 nun (0.08-0.24"), subject to feed volume.Mold temperatureNot lower than 85C (185F). Mold filling and article quality improvewith increasing temperature.High mold temperature increases cycle timeonly marginally, as the glass transition temperature is at 145C (293F).4-12 hours (high-speed drying oven 2-5 hours) at 100-120C (212-248F). Optimum elongation, hardness and notched impact strength areobtained at a moisture content below 0.02%.0.7-0.8%0;0.1-0.5Y0 ith PC-GF (glass filled).1545% of the respective cylinder volume.If production is stopped over night, purge the cylinder of material andkeep heat on at 16&18OoC (32-356F).

Material characteristics

Cylinder temperatureHeating zone 2 250-300C

Injection pressure

Holding pressureBack pressureInjection speed

Pre-drying

ShrinkageInjection volumeShutting the machine down

Figure 1-1 1 Processing values of polycarbonate (PC)16

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Basic Principles of PlasticsThe terms used in the table are explained in greater detail in subsequentlessons.It will be obvious by now, that many factors must be considered in order toproduce high quality moldings. The processing values can also vary withinwide ranges. The molding of CDs requires a PC of very low viscosity and aprocessing temperature range of 320-360C (610480F). Low viscosity isneeded, in order to enable the melt to reproduce the fine geometry of the pitson the information side of the CD with high precision. If this is notaccomplished, data will be lost. Although CD-players have built-in errorcorrection, this is capable of correcting lost information (non-existent or onlypartially formed pits) to a limited extent only.

CD

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Lesson 1Review Questions

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Question

Three overall classes of plastics are distinguished from one another. Theyinclude thermosets, thermoplastics and monomerssynthes iselastomers

Thermoplastics are soluble and dense ly c ross- l inkedfus ib lenonfus ib le

Amorphous thermoplastics arenot combined with fillers or similar additives.

when they are t ranspa ren tm i l ky opaque

Polycarbonate (PC), from which Compact Disks are molded, isa@) thermoplastic. amorphoussemicrys ta l l inecannot be fused or dissolved but can be

swelled.Thermoplas t icsElastomersThe rmose ts

Thermosets are nonfusible and s o f tdense ly c ross- l inked

The intermolecular forces which operate in the crystalline state are con-siderably than those in the amorphous state.

w e a k e rs tronger

The abbreviation for polyamide, as specified by I S 0 1043, is PSPAPC

Processing temperatures are for thermo-plastics than for metals.

higherlower

Viscosity is a measure of the of a melt. hardnessf low p rope r t ies

As temperature decreases, the viscosity of the melt increasesdecreases

Injection molding is described as a method. reshapingp r imary p rocess ing

Injection molding requires that the polymer is in a state describedas

thermoplas t icthe rmoe las t i c

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