Polystyrene can be processed in principle by all methods known for thermoplastics. The main methods which come into consideration, however, are injection moulding and extrusion.

Predrying

Polystyrol moulding compositions do not normally have to be predried before injection moulding, but predrying at 60 to 80 ºC for 2 to 3 hours is advisable if they have been stored in an outdoor hopper.
Since even small amounts of moisture can cause problems in extrusion, it is always advisable to carry out predrying or to employ vacuum-vented screws.

Compatibility

All Polystyrol grades are compatible with one another. It is possible to change from one grade to another, e.g. from semi-high-impact to high-impact Polystyrol, without any special measures being needed. On the other hand, Polystyrol is incompatible with acrylonitrile-containing products and with polyethylene, polyamide, polyester, acetylcellulose and other thermoplastics. In these cases, the machinery must be thoroughly purged before a change of material.

Reprocessing

Scrap is obtained in the form of trimmings in extrusion, punching scrap in thermoforming, sprues in injection moulding, flash in blow moulding and reject parts in all four processing methods. These can add up to a total of as much as 50 percent.
Polystyrol regrind is fully reprocessible as long as the recycled material has not been damaged by excessively high shear or temperature. The usability of regrind should nevertheless be carefully checked in each case for parts which have high quality requirements or when using special Polystyrol grades (e.g. containing flame retardant).
Indications of product damage having occurred are:

An increase in the monostyrene content A decrease in the molecular weight of the matrix Rubber crosslinking A change in the rubber morphology (fragmentation) Conspicuous tendency to yellowing

Regrind must be free of contamination. To protect the regions of the processing machine which come into contact with the melt, it should, where possible, be passed through a magnetic sieve before being introduced into the feed hopper. Regrind with particles of widely differing substance and with a low bulk density can cause problems in maintaining a stable extruder throughput. Fines have a particularly detrimental effect, resulting in screw slip. They should be removed in a cyclone.

Polystyrene moulding compositions are injection moulded predominantly on screw-type machines. Only in exceptional cases (e.g. mouldings with a marble effect) injection moulding machines with special screws or plungers are employed.
Owing to their amorphous structure, polystyrene moulding compositions have not only a wide processing range but also low tendency to distort and low shrinkage.

Screw geometry

Conventional all-purpose screws can be used. Good results are obtained using three-zone screws having an L:D ratio of from 16:1 to 20:1 and the features indicated in Table 5. Although longer screws melt the granules more uniformly, they at the same time increase the residence time of the moulding composition in the barrel.

Nonreturn valves

The only reliable means of ensuring a constant cushioning effect and follow-up pressure is to install a nonreturn valve that prevents the melt from flowing backward into the front screw flights during the injection and follow-up phases. Since designs giving excellent flow are available, a nonreturn valve should always be used as a matter of principle when manufacturing precision parts. However, the screw must then not be allowed to rotate during injection, since otherwise damage to the machine may result.

Nozzle

Since polystyrene melts are more viscous than, for example, nylon melts, they allow open nozzles to be used. Open nozzles offer the advantage of a very simple design that favours easy flow.
Nozzles with shutoff devices can be recommended if the back pressure is high, cobwebbing is to be avoided and mouldings with thick walls are to be produced.
Thick-walled mouldings frequently require cycle times of several minutes. If, in such cases, the injection moulding composition is not removed completely from the nozzle bore, it may cool excessively there and thus give rise to streaking in the next shot. Best results have been given by nozzles with mechanically or hydraulically actuated needle valves, although the pressure drop in such a nozzle may be considerable.

Gating and mould design

In principle, all conventional types of gating are possible. The gate cross section must be sufficiently large to avoid unnecessarily high melt temperatures and injection pressures which could lead to streaking, burn marks, voids and sink marks. The VDI 2006 guidelines for gating and mould design also apply to the Polystyrol moulding compounds.

Demoulding

As a rule, polystyrene can be demoulded without difficulty. A taper of 1:100 or 0.5 º on one side is a sufficiently large draft. In fact, if the mould is polished in the machine direction, drafts of down to 0.15 º are adequate.

Use of inserts

Metal inserts do not obstruct the smooth flow of polystyrene, but they should be heated to 80-120 ºC before being placed in the mould in order to avoid molded-in stresses. They must be thoroughly degreased and secured in the mold by means of milled edges, circumferential grooves or the like. The metal edges should be well rounded off.

Mould temperature control

A well-designed mould temperature control system is of great importance because the effective surface temperature of the mould exerts a critical effect on the finish (gloss, brilliance and absence of flow lines), the strength of weld lines, the resistance to warpage, the shrinkage and the adherence to tolerances. Depending on requirements, temperatures of from 10 to 70 ºC are customary. Very thin-walled parts which have to be produced with short cycle times can also be injection moulded at mould surface temperatures of less than 10 ºC. At even lower mould temperatures, brine has to be used as coolant.
One means of counteracting any tendency of the moulding to warp is to cool the two halves of the mould separately to different temperatures.

Injection temperature

Polystyrene can usually be injection moulded at melt temperatures between 180 and 260 ºC. The melt temperature has a significant influence on the toughness of the finished parts, particularly that of the rubber-modified Polystyrol (GPPS, HIPS) moulding composition.
If the residence time of the melt in the barrel is relatively long, the temperature should not be in the upper end of the range or even above it, since otherwise thermal degradation and/or an increase in the residual styrene content can result (Figure 39). Thermal degradation can usually be recognized by silver streaks or burn marks. A change in color can also occur. The melt temperature is best monitored by means of a penetration thermometer on the pumped-out composition.

Feed characteristics

The feed characteristics of polystyrene are influenced by screw geometry and rotational speed, back pressure, the temperature settings in the plastification and feed sections and also by the shape and nature of the granules (externally lubricated or unlubricated).
As a rule, polystyrene can be plastified uniformly and without thermal degradation even at high screw speeds. Normally, the plastification capacity increases with a rise in temperature.
Frequently, the individual heating zones of the plastification barrel can be set to the same temperature. However, if the processing temperature is in the upper end of the batch and/or if the cycle times are long, the temperature of the first heating strip (close to the hopper) should be set at a somewhat lower value. This prevents premature melting of the granules in the feed section (bridging).

Filling of the mould

A general rule for polystyrene is that the mould must be filled as rapidly as possible to prevent marks at the weld lines and to ensure that the weld lines are as strong as possible. Another advantage of a high injection rate for most polystyrene grades is that it yields glossy and brilliant moldings. The only polystyrene grades for which very fast injection can have a detrimental effect are the high-impact grades (GPPS, HIPS) with high heat distortion resistance, in which case matt concentric zones around the screw may occur under some circumstances. Dark-coloured products are particularly prone to this effect. Fluctuations in the injection rate may also cause matt streaks in these products. In such cases, slower injection at higher melt and mould temperatures leads to more uniform flow and improves the surface.
Steps must be taken to ensure that air can easily escape from the mould at suitable points to prevent burning as a result of compressed air (Diesel effect).
To obtain perfect injection-moulded parts and to prevent void formation, the follow-up pressure and screw forward time must be sufficiently high to compensate for the volume contraction on cooling of the melt. This requires the gates to be large enough to prevent the melt from solidifying in their vicinity before the screw forward time has elapsed and thereby stopping the follow-up pressure from acting on the moulding while it is still plastic in the interior. However, the volume expansion experienced by Polystyrol as a result of heating can be more than compensated for by high pressure. The dependence of the specific volume vs. temperature T and pressure p is shown for general-purpose polystyrene (GPPS) in Figure 40.

Flow

The Polystyrol product line includes grades having different flow characteristics. The measure usually used for the flow is the melt volume index in accordance with ISO 1133. Information of greater practical significance is obtained from flow tests in which coils of various thicknesses are produced in a spiral mould (Figs. 41 and 42). At a given melt temperature, mould surface temperature, screw advance rate and the corresponding injection pressure, the length of the spiral can be regarded as a measure of the flow of the product.

Fig. 42: Flow in a spiral mould for high-impact polystyrene

Cycle time

A factor that decidedly affects the injection cycle is the time taken for the polystyrene to cool from the melt temperature to the solidification point. The lower this temperature difference, the shorter the cooling time and thus usually also the cycle time. The difference between melt temperature and solidification point depends on the particular polystyrene grade.
A measure of the solidification point is the Vicat softening temperature.
The higher the flow of Polystyrol, the lower the temperature at which it can be processed, so grades which flow easily and at the same time solidify rapidly can be processed most quickly.

Shrinkage

Polystyrene undergoes considerably less shrinkage than partially crystalline plastics. Although it is primarily a material property, it is also decisively influenced by the geometry of the mouldings (restricted or free shrinkage) and by processing conditions such as melt temperature, mould surface temperature and follow-up pressure. Local interplay between these parameters may cause great differences in shrinkage within one and the same moulding.
As a rule, the processing shrinkage is between 0.4 and 0.7 %, but in exceptional cases it can be significantly below that range. It may even be zero in zones within the moulding that are subjected to a high follow-up pressure (vicinity of the sprue). After-shrinkage can be neglected in most applications; it accounts for about 10 % of the total shrinkage.

Products

The most suitable polystyrene products for extrusion are those with a high viscosity, i.e. products with a melt volume rate MVR 200/5 in the lower end of the range between 1 and 7 ml/10 min. However, materials with a higher MVR are also used for multilayer composites.
The desired ratio of rigidity to toughness can be obtained by blending high-impact and general-purpose polystyrene. An important point to observe for ensuring a homogeneous melt is that the flow characteristics of the blend components do not differ too widely (the flow is indicated by the final letter in the product designation). Equipment items required are a metering and mixing device upstream of the extruder inlet and a mixing element in the melt region upstream of the die.

The bursting pressure test on finished, thermoformed cups gives a good guide to the toughness of the material used. The bursting index as a function of the mixing ratio is shown for two examples of blends in Figure 45.

Processing conditions

The processing temperatures for polystyrene lie between 180 and 240 ºC, in individual cases even a little higher. As a rule, the grades with a higher viscosity are extruded at a higher temperature.
Pressure and temperature of the polymer should be measured where possible by means of suitable devices. In practice, the screw pressure ranges from 100 to 200 bar. For safety reasons, a maximum pressure warning device should be fitted.

Vented extruders

Vented extruders allow the extraction of volatile constituents (e.g. low molecular weight fractions, moisture) from the melt and the removal of entrapped air. The classical vented extruder screw can be likened to two three-zone screws arranged in tandem (Figure 46). The length of modern vented extruders is in the range 30-36 D. The compression ratio should generally be between 1:2 and 1:2.5 for Polystyrene. Compression ratios of 1:3 can also be employed when processing up to 50 percent of regrind.

In principle, polystyrene can also be extruded without the use of a vented extruder. In this case, the granules should be predried, depending on the Vicat softening temperature of the Polystyrene used, at temperatures between 60 and 70 ºC for 3 to 4 hours, e.g. in a hot air drier or a vacuum drying oven. It has been found in practice that predried granules cause fewer production and quality problems in the long term, although films which have been produced without the use of a vented extruder have inferior organoleptic characteristics.

Dies

Slit dies are used for the extrusion of both flat film and sheet. In both cases, the length of the parallel zone is about 20 times the slit width. Sheets are usually produced using an adjustable choker bar, while films are produced without it. When a choker bar is employed, it should be set obliquely (Figure 53). The lower lip is interchangeable in order to cover a greater thickness range.

Calibration rolls

On leaving the die, extruded sheet is led through calibration rolls. The roll temperature should be as high as possible to keep molded-in stresses to a minimum during cooling. A rule of thumb is that the temperature at the surface of the center roll should be about 5 K below the limit at which the extruded sheet sticks to the surface of the steel. This gives an idea of the most favorable temperature settings on all the rolls for a given product under given conditions. If the melt is fed downward, the temperature of the upper roll should be set to a value well below that of the center roll, e.g. 10 K, because of the small contact area at the upper roll (theoretically, the contact is merely linear). The temperature setting on the lower roll should be between those on the upper and center rolls.
If curvature of the sheet occurs after passing through the rolls, the temperature settings need to be optimized. As a general rule: the concave side corresponds to a roll which is too hot and, conversely, the convex side corresponds to a roll which is too cold.

Orientation

The orientation or even prestressing of sheet intended for further shaping can be assessed by heat treatment as described in ISO 11501. Increasing the melt temperature is the most effective means of achieving low shrinkage, but this must obviously be kept to a level which does not cause appreciable degradation of the product. Other available parameters, which have varying degrees of effectiveness, are die gap, distance between die and roll, roll temperature and tension on the sheet.

Coextrusion enables the properties of several materials to be combined. It requires matching of the flow properties of the materials used. The extruded product is a multilayer composite in which the individual layers should adhere to one another. If the adhesion of the layers to be combined is nonexistent or inadequate, a layer of bonding agent has to be interposed. A distinction is made between adapter and die coextrusion.

In adapter coextrusion, the composite is built up in front of the die and is extruded as if it were a single-layer melt. In die coextrusion, the layers are formed separately in a special die and are subsequently joined. General-purpose polystyrene (GPPS) is often used as a gloss layer on a high-impact substrate. In this combination with high-impact polystyrene (HIPS, PS-I), no bonding agent is necessary. In most other cases, a bonding agent has to be used and the choice depends on the components of the composite.

Coextrusion of high-gloss composites

The use of general-purpose polystyrene as gloss layer reduces the impact toughness of the composite. The drop in the impact toughness with increasing gloss layer thicknesses is shown by way of example in Figure 54.

Differences in the thicknesses of the layers in the composite may give rise to dull areas. In order to avoid this, the gloss layer must have a minimum thickness. A gloss layer thickness of from 12 to 15 µm in the finished part is a good compromise. Since the layer thickness is subject to a certain degree of variation and adapter systems allow very little scope for correction, undesired thicker zones can incur a risk of brittle fracture.

The optimum conditions for blow molding bottles and other articles depend greatly on the type of machine and the nature of the molding and should always be determined beforehand. As in the case of injection molding and extrusion, changing the colour of a given molding compound may require alterations in the processing conditions.

Owing to their wide viscoelastic range, polystyrene sheet and film are particularly suitable for thermoforming. The forming temperature should be between 130 and 150 ºC.
Polystyrene extrusion grades can achieve high draw ratios. Thus, draw ratios of, for example, 5:1 in production of beakers and in the thermoforming of interior containers of refrigerators are common. This value can sometimes be even higher, as for the reinforcing ribs at the bottom of components or the depressions in the corrugations.

Another advantage of polystyrene is that it absorbs little heat up to the thermoforming temperature, which has a favorable effect on the energy balance and the cycle time. Furthermore, heat absorption is uniform and can therefore be more easily controlled. This behavior is shown in graph form in Figure 56 where the heat capacity (enthalpy) is plotted against temperature.

Polystyrene does not present any fundamental difficulties in forming lasting and uniform beading on modern machinery. It must be ensured that the screw channels remain free of deposits and allow smooth passage.

The two extremes in the Polystyrene range are, to some extent, exceptions: the general-purpose products because of their brittleness and the very high-impact grades because of their great elasticity (tendency to recover their shape after beading). However, beading machines have been designed to provide a remedy for these products as well.

Semi-finished polystyrene parts can easily be machined by punching, sawing, drilling, milling, turning, etc., using the tools and machines customary for metalworking or woodworking. Owing to the low thermal conductivity and the relatively low softening temperature, the cutting surfaces have to be cooled by a current of air or with water. High-impact polystyrene (HIPS, PS-I) has a lower tendency to splinter and can be machined more easily than general-purpose polystyrene (GPPS).

Welding - Preference is given to ultrasonic welding.

Adhesive bonding - Polystyrene parts can be bonded together with the aid of solvents such as toluene or methylene chloride, but only to parts made of the same material. We recommend that all questions regarding adhesive bonding be directed to a commercial adhesives manufacturer.

Polystyrene can be easily and durably coated and printed by various techniques. We recommend that manufacturers of surface coatings or printing inks be consulted in individual cases.
Polystyrene articles can also be provided with a mirror-like, metallic surface by high-vacuum metallizing.

When polystyrene is injection molded, thermoformed or extruded, small amounts of styrene and other degradation products are given off into the surrounding air; the actual amounts depend on the processing conditions.

Inhalation of relatively high concentrations of styrene can, like other organic solvents, have a reversible effect on the nervous system (tiredness, loss of concentration, etc.). Such effects are not to be expected if the workplace concentrations are below the prescribed threshold limit value of 20 ml/m3 (ppm) (cf. TRGS 900).

In our experience, the styrene concentration will not exceed 1 ml/m3 (ppm) if the workplace is well ventilated and extracted (e.g. 5 to 8 air changes per hour).

A review of emissions of volatile compounds in the processing of thermoplastics may be found in M.J. Forrest et al., Ann. occup. Hyg. Vol. 39, No. 1, 35-53 (1995).

The possibility of a carcinogenic effect of styrene has been addressed in an assessment by leading members of the German Commission on Maximum Workplace Concentrations. They propose classifying styrene in a new category: Substances which have a carcinogenic and genotoxic action but whose action is considered to be so weak that no appreciable contribution to the cancer risk for human beings is to be expected if the maximum workplace concentration (20 ppm for styrene) is adhered to.
(Published in: Deutsche Forschungsgemeinschaft. MAK- und BAT-Werte-Liste 1998; Senatskommission zur Prüfung gesundheitsschädlicher Arbeitsstoffe; Bulletin 34; Wiley-VCH; p. 121).

Workplace emission and venting residues

An exhaust hood with condenser and collector should be installed above the die.
Under normal extrusion conditions, the condensed residues from the vent amount to 100-300 ppm of the product throughput, the actual figure depending on the melt temperature and the shear. The condensed products consist essentially of water, lubricants, stabilizer, monostyrene and oligomeric styrene.
The nonaqueous fraction of the condensate can be disposed of under the same conditions that apply to waste oil. However, it should not be mixed with waste oil, since the latter is usually reprocessed.
There have been no reports of adverse effects on health which have resulted from correct processing of Polystyrene in well-ventilated areas.