Terblend® N (ABS+PA) is easy to process. It is a highly flowable material and reproduces very fine surface structures, so there is often no need to use matt lacquer. Good flowability means that even complex parts can be injection molded without difficulty. A particular feature is excellent demoldability of injection moldings made from Terblend N.

Terblend N is usually processed on injection molding machines with screws. The functions of the screw here are conveying, plasticizing, and injecting. Other types of machinery (e.g. plunger injection molding machines) are usually used only for specialty applications (such as production of marble effects).

The particular type of screw used has a substantial effect on the properties of the eventual molding. If the screw selected is unsuitable for the material the result can be non-uniformity of melt. Poor screw geometry can expose the polymer melt to high shear stresses and lead to thermal and mechanical degradation. This impairs both optical properties and mechanical properties in the molding.  Three-zone screws (Fig. 7) have proven successful for processing Terblend N under industrial conditions. As their name implies, screws of this type have three different zones (feed, compression, and metering zones), each of which has a specific task. The tip of the screw also has a non-return valve which prevents any reverse flow of the plasticized melt present in the space in front of the screw during injection and hold pressure phases.  Modern standard screws generally have effective screw length of 20-23D, the length of the feed zone being about half that of the screw. The length of the compression zone and metering zone are approximately equal. Pitch is usually 0.8-1D. Flight depth ratio in the feed and metering zones is usually between 2 and 3. A compression ratio which has proven successful for injection molding Terblend N is 2 to 2.5.

The design of screw tip and non-return valve are important in achieving good melt flow in the plastifier. Non-return valves are essential for maintaining a steady melt cushion and long hold pressure times. The gap between cylinder and non-return valve should be between 0.02 and 0.04 mm at operating temperature (Fig. 8). To prevent the melt from exerting back-pressure, flow cross sections in the different areas (A, hA, H) should be designed with the same dimensions. The screw tip needs to be designed for good flow, and the angle C should be identical on the screw tip and at the inlet to the nozzle, to minimize the amount of melt which can remain at the head of the cylinder or in the nozzle. If the screw tip is incorrectly designed, thermal degradation of the melt can occur due to prolonged residence time in the plastifying cylinder. The result can be a reduction in the mechanical and optical quality of the component.

Open nozzles

Open nozzles (Fig. 9) have rheological advantages, giving simple cleaning and flushing, and also rapid change of material or color. To prevent thermal degradation, the diameter of the nozzle aperture should be at least 3 mm. The angle at the nozzle inlet should be the same as that on the screw tip.

Shut-off nozzles

The use of shut-off nozzles permits retraction of the nozzle during plastification, reducing heat transfer between the temperature-controlled nozzle and the mold, which is at a lower temperature. Shut-off nozzles are also useful when operations require relatively high backpressure and when the problem of stringing has to be avoided. Mechanically or hydraulically operated needle valve nozzles are very suitable for Terblend N (Fig. 10). One disadvantage of shut-off nozzles is that pressure losses are higher than with open nozzle systems.

Gate design

The design of injection molds for Terblend N needs to include sufficiently large dimensioning of feed channels and gating. Resistance to flow during injection and hold pressure time needs to be kept to a minimum. A good general rule is that the diameter of the gate should be at least half the wall thickness of the molding. Under-dimensioned gating can lead to problems such as streaks, charring due to shear, and delamination. Premature solidification of the melt in the gate is often the cause of sink marks and voids in the molding, due to lack of compensation for the volume contraction of the melt during the hold pressure phase. Styrolution has rheological calculation programs needed to dimension and balance gating and feed systems.

Wall thickness

The wall thickness needed is calculated from strength and stiffness requirements and takes into account the economic cycle time for the molding. To reduce or eliminate sink marks, the gate should ideally be in the region of greatest wall thickness, since this is where the hold pressure has its longest action.
If gates are in regions of lower wall thickness where hardening is relatively rapid it becomes impossible for additional material to move onward to compensate for volume contraction in the thicker regions. This generally results in sink-mark problems with varying levels of severity.  Reinforcement ribbing should generally be designed with walls thinner than the main walls and with a radius where it joins the main wall. The radius eliminates flow problems and notching effects.

Wall structure

A factor which needs to be considered in relation to wall structuring or texturing is that eroded mold surfaces are superior to etched mold surfaces in terms of scratch resistance of the resultant moldings. Assessment of scratch resistance is also easier when the mold wall has high surface roughness (Fig. 11). This also makes flowlines more visually acceptable.

Venting

Venting channels usually need to be incorporated into the injection mold at the end of the flow path and where the melt streams coalesce. The consequences of inadequate venting are mold filling problems, deposits, and sometimes even charring.

Demolding

Drafts of 1º are generally sufficient for Terblend N, but these must always be at least 0.5º. To avoid deformation during demolding, ejector pins or stripper plates should be designed with the largest possible area. This can sometimes also permit earlier demolding and thus a shorter cycle time.
Terblend N features excellent demolding properties. The demolding force required to remove moldings from the injection mold is lower than for other plastics (Fig. 12).

Temperature control

Uniform control of molding temperature should be taken into account at an early stage, i.e. when positioning the mold cavity within the mold. Effective mold temperature at the surface of the mold cavity has a decisive effect on cycle time, surface quality (gloss, surface structure, flowline visibility), shrinkage, and warpage.  One of the important factors for uniform and rapid cooling of moldings is the size of the coolant duct, together with its position with respect to the surface of the molding and the flow rate of the fluid used for temperature control. It can sometimes be necessary to use separate coolant circuits for temperature control of certain sections of the mold surface.

The moisture level of the pellets used has to be kept below a specific limit if the moldings are to have satisfactory surface and good properties. The moisture level in Terblend N pellets should not exceed 0.1%. Drying at 80 to 90 ºC (preferably under nitrogen) for 4 hours is needed to achieve this using material from freshly opened packs - 16 hours if the packs have been open for some time.  Depending on storage conditions, ambient conditions and time elapsed, the moisture content within the packs can rise. Particularly in winter, if the packed pellets are moved from a cold storage area to a warmer processing area and then opened immediately, condensation onto the pellets can occur when the air around them cools. Condensation can be avoided if the closed pack is stored for some time in the processing area or brought to room temperature in an intermediate silo.  After removal of pellets, part-filled packs need to be sealed carefully and immediately. The pellet hopper on the machine should be covered by its lid.

The heat needed to melt the pellets is supplied by external heating of the plastifying cylinder and also by the friction from rotation of the screw. The temperature profile options shown in Fig. 13 are available when setting heater band temperatures.  A horizontal temperature profile is used when maximum heat has to be supplied quickly due to low residence time or to make full use of plastifying performance. A rising temperature profile is used when residence times are comparatively long. It can provide fairly mild conditions for melting the material. A temperature profile which first rises and then falls away toward the nozzle is mainly used with open nozzles to prevent escape of the melt and stringing.  The aim of any temperature profile has to be that the melt temperature in the space in front of the screw is the correct processing temperature. The actual temperature of the melt can be measured using the temperature sensor incorporated in the nozzle head (cf. Figs. 9 and 10 above), and the temperature profile of the heater bands can be adjusted, as can the screw rotation rate.

Melt temperature

The melt temperature for processing Terblend N is 260 to 280 ºC. It is advisable to check the melt temperature by using a needle thermometer within the melt downstream of the screw.
Processing temperatures are very similar for glass-reinforced and unreinforced grades. For reasons of lower flowability, and to achieve high surface quality and good mechanical properties, the upper end of the relevant temperature range is always preferable.

Mold surface temperature

Mold surface temperature is one of the most important parameters in the entire injection molding process. For Terblend N it should be 50 to 80 ºC. The ideal mold surface temperature not only improves surface quality (gloss, surface structure, visibility of flowlines) but also gives better mechanical properties, weld line strength, and dimensional tolerances for the molding. Local differences in mold temperatures result in differential cooling of the molding and thus to cooling-related shrinkage variation and warpage.
Higher mold surface temperature generally slows solidification of the melt. This provides a longer period for compensation of shrinkage and permits production of moldings with less internal stress. The risk of tiger lines is also minimized.

Hold pressure compensates the shrinkage of the polymer as it solidifies. It is important here that the hold pressure changeover takes place precisely at the moment of volumetric filling of the cavity.  Depending on the shape of the molding, choice of an appropriate level of hold pressure and hold pressure time can substantially reduce the occurrence of voids and sink marks. In cases of non-ideal design or gating, the use of extremely high hold pressures or extremely long hold pressure times can lead to overloading of the moldings and thus reduce component toughness.

The crystalline polyamide content in Terblend N gives this material higher shrinkage than pure ABS. Other factors affecting shrinkage are the shape of the molding (molding design, wall thickness, and gating) and processing conditions.  Process conditions which have a decisive effect on the mold shrinkage and post process shrinkage of Terblend N (Fig. 14) are hold pressure and mold surface temperature. Mold shrinkage decreases considerably with increasing hold pressure, but post-shrinkage shows less variation. Both types of shrinkage increase as mold surface temperature rises.

In the event that finished parts are stored, storage time has little effect on total shrinkage, which is the sum of mold shrinkage and post-shrinkage (Fig. 15). However, storage temperature does affect total shrinkage.

Moisture absorption leads to lower shrinkage values, since moisture causes swelling and thus a degree of linear expansion (Fig. 16). The resultant difference from the shrinkage values for dry finished parts is almost constant.

Mold shrinkage of Terblend N is comparable with that of ABS and PC/ABS (Fig. 17). However, post-shrinkage of newly injection-molded parts (80 °C/20% rel. h./1h) is higher than that for the amorphous materials ABS and PC/ABS, due to the crystalline content of Terblend N. Post-shrinkage of conditioned parts (70 °C/70% rel. h./15d), on the other hand, is lower than for ABS and PC/ABS. The reason for this is moisture absorption by the PA content of Terblend N, resulting in some degree of swelling. The total conditioned shrinkage is the decisive value for practical purposes, and is similar for Terblend N, ABS and PC/ABS (about 0.7%).  The shrinkage performance of Terblend N is therefore comparable with that of ABS and PC/ABS in relation both to injection molding and to storage of finished parts under industrial conditions.

When processed correctly, Terblend® N melts are thermally stable and do not create any hazard due to molecular degradation or evolution of gases or vapors. Like all thermoplastic polymers, Terblend N will decompose if subjected to excessive thermal stress, e.g. if overheating occurs or burn-off methods are used for cleaning. The decomposition products formed are gaseous. Above about 300 ºC decomposition accelerates, producing mainly carbon monoxide, ammonia, caprolactam, styrene, butadiene and acrylonitrile.

Incorrect processing, e.g. excessive temperatures and/or long residence times of the melt in processing machinery, can lead to formation of vapors which have a pungent odor and are hazardous to health. These problems are accompanied by brownish burnt streaks on the moldings. In such cases the cylinder of the processing machinery needs to be flushed by injection with the mold removed, at the same time reducing cylinder temperatures. Odor can be reduced by cooling the degraded material rapidly, e.g. in a water bath.

Further information is given in our safety data sheets for Terblend N.