The tough and rigid Styrolux® grades 656 C, 684 D and 3G 46 are the preferred choices for injection molding. In order to improve the stiffness and dimensional stability under heat, in injection molding as well, Styrolux® is sometimes blended with up to 40% general-purpose polystyrene (GPPS); the production of highly transparent parts calls for a plasticising unit that is capable of a good mixing performance.

Universal screws with a length of 16 D to 20 D are suitable. The pitch along the entire length should be constant at a value between 0.8 D and 1 D.

The shaping tool surfaces have a great influence on the transparency and gloss of the injection moulded parts. The tiniest flaws on the tool surfaces are reproduced, as a result of which high-gloss and polished surfaces are recommended. Care should be taken to ensure that the draft is at least 1°. For the mould parting surfaces, a compromise should be sought between a tight seal – for the eventuality of flashes – and sufficient venting.
The tool surface temperatures should be set between 20°C and 50°C [68°F and 122°F], depending on the grade of Styrolux®. Too low a temperature can give rise to streaks and flow marks. Too high a temperature leads to adhesion, which also causes cracks or fissures. Here, mention should be made once again of the polish in the demolding direction in order to effectuate the best possible demolding.

Generally speaking, a low ratio of the barrel contents to the shot volume, a low rotational speed of the screw and working with little or no back pressure are advantageous.
In the case of short production downtimes, it is usually sufficient to lower the melt temperature and then to pump off the melt; at the end of longer pauses, in contrast, we recommend flushing with a high-viscosity general-purpose polystyrene grade.
The melt temperatures should not exceed 250°C [482°F], and the residence time in the cylinder should not be too long. An injection time that is as high as possible is a decisive factor for the optimal transparency and brilliance of injection molded parts. Here, a slight loss in toughness will have to be accepted.
Any commonly employed type of gate, also hot runner systems, can be employed. With a hot runner system, the configuration must be such that no localized overheating (T > 250°C [482°F]) can occur. Adequately dimensioned runners and gates prevent thermal overloads.

The feed characteristics of Styrolux® are not problematic. It is best for the temperature gradient from the hopper to the tip of the screw to rise slightly. The cross head must be cooled.

The flow test using the test spiral at thicknesses of 1 mm to 2 mm reveals an almost linear dependence of the flow lengths with the melt temperature (Fig. 1). At a wall thickness of less than 1 mm only very short flow distances are possible since the flow resistance rises disproportionally as the wall thickness decreases. Melt temperatures of more than 250°C [482°F] cause cross-linking of the material, and consequently the flowability drops once again.
The onset of cross-linking is also indicated by turbidity and yellowing.

Fig. 1: Flowability of Styrolux® in the spiral flow test; injection pressure = 1500 bar.

Owing to its morphology, Styrolux® adheres more strongly to steel surfaces than high-impact polystyrene. Therefore, it requires a greater force in order to overcome the static friction during the first phase of demolding. It is helpful to select a low injection pressure and holding pressure. Due to the high compressibility of Styrolux®, the part is slightly compressed. Following pressure relief, the part relaxes and is easier to release from the core. Excessive injection pressure and holding pressure cause greater deformations in the part. A possible consequence after the pressure relief is that the part could become jammed in the mold cavity. This should be optimized by means of experimentation.
In comparison to general-purpose and high-impact polystyrene, the lower dimensional stability and softening temperature of Styrolux® mean that the cooling times, and hence the cycle times, will be longer, even in the case of intense cooling. The demolding behavior can also be improved by adding Styrolux® Batch ASE in an amount of 2% to 4%.

The shrinkage values of Styrolux® lie between 0.3% and about 1%. They are the lowest parallel to the direction of flow, somewhat higher transversally to the direction of flow near the gate and highest far away from the gate (Fig. 2 ). The shrinkage and the shrinkage differences can be influenced by regulating the melt temperature. High melt temperatures yield lower shrinkage values and smaller differences between the positions near and far away from the gate.
The temperature of the tool surface and the injection speed are less decisive for the shrinkage.

Fig. 2: Shrinkage of Styrolux® 656 C

Styrolux® is considerably more compressible than high-impact polystyrene. This is why centrally gated rectangular parts can exhibit larger internal stress differences between the shortest and the longest flow distances. In conjunction with the lower dimensional stability, the molded parts can undergo warpage even hours after the demolding.
Since there is a direct relationship between the warpage and the modulus of elasticity, a Styrolux® grade with a high modulus of elasticity should be selected for parts where the aspect of warpage is critical.

In extrusion processing, particularly the tough Styrolux® grades 684D, 693D ----- and 3G 55 in blends with general-purpose polystyrene (GPPS) are extruded to form films and then thermoformed into dimensionally stable finished parts. The mechanical and optical properties of the film and of the deep-drawn parts are determined to a large extent by the Styrolux® grade, by the mixing ratio and by the GPPS type (see “Styrolux®: mechanical and optical properties”).
The biaxial stretching during the thermoforming considerably increases the toughness of the finished parts; therefore, highly stretched parts such as, for instance, beverage cups can be successfully manufactured with high contents of GPPS (> 50%). Blends containing 50% to 80% Styrolux® have proven their worth for the majority of thermoforming applications.
Moreover, efforts should be aimed at optimizing the homogenization of the components, and this is achieved by a good mixing action of the screws used. Here, the viscosity of the GPPS component in the blend plays a role; it should preferably be close to the viscosity of the Styrolux® and it should lie in the melt volume-flow rate (MVR) range of 8 to 16 [cm3/10 min].

As a rule, the Styrolux® granules do not have to be pre-dried. However, in the event of unfavorable storage or transportation conditions involving severe temperature fluctuations, moisture can condense on the surface of the granules and this then has to be removed in a pre-drying step. The granules should be pre-dried in a dry-air dryer for 3 to 4 hours at a temperature of about 50°C [122°F].

Styrolux® and blends with GPPS can be processed on all conventional extrusion installations that are also suitable for high-impact polystyrene or polyolefins. A high L/D ratio is advantageous for the homogenization and for reducing the sensitivity to pressure fluctuations. However, with an eye towards keeping the residence times short, this ratio should not be too large either. As a rule, L/D ratios between 28:1 and 34:1 meet both of these requirements.
Vented extruders are recommended for the production of high-quality packaging films. They permit the extraction of volatile components and moisture from the melt as well as the removal of entrapped air.
Screws with medium compression ratios (2:1 to a maximum of 3:1) have been found to be optimal, that is to say, universal screws (PE, PP) can also be employed. Excessively high shearing elements (barrier zones, shear sections), however, can cause damage to the product (cross-linking, gel specks). Generally, the use of a melt pump is recommended.

Chrome-plated and polished flat film extrusion dies with an adjustable die gap (flex lip) and choke bars are recommended, and the die gap should be set at +5% to +10% above the desired film thickness in order to minimize orientations in the film. The line speed should be set in such a way that the melt cushion in the bead of plastic material remains very small. Otherwise, if a larger melt volume were fed in, there would be a risk of the formation of waves and other visible surface flaws. Optimal roller temperature control is important for purposes of attaining high quality in terms of the transparency, gloss and surface finish of the films. It is recommended to initially set the first deflector roller at a temperature up to the sticky limit and then to lower the temperature by 10°C [18°F]. For this purpose, the temperature of the polishing stacks should be regulated individually (up to about 80°C to 90°C [176°F to 194°F]). Polishing stacks that are too cold leave markings on the surface.

When film reels are produced, care should be taken to ensure that the film temperature is lowered as far as possible, for instance, to about the ambient temperature (between 25°C [77°F] and 35°C [95°F] at the maximum) and that the winding tension is kept as low as possible. If the films are wound at a temperature that is too high, then the shrinkage that occurs during cooling causes high tensions in the roll of film. In the worst case, this can lead to a partial or total blockage of the films. Therefore, if the machines are operated at high throughput rates or if the lines are operated at high speeds, it is generally recommended that post-cooling rollers are used.

In spite of the outstanding stabilization of Styrolux®, product damage can occur in case of long residence times in the extruder (dead spots, accumulations in the mixing section). An indication of this is the greater incidence of gel specks, which often only appear for a few seconds but which nevertheless greatly impair the film quality.
Such problems require that the extruder be flushed, preferably with general-purpose polystyrene. This procedure is also recommended when the product is changed or before the extruder is switched off in order to reduce the thermal stress to the Styrolux® melt caused by the downtimes.