Asahi Kasei Plastics reinforced thermoplastics are readily molded on conventional, reciprocating screw, injection molding machines. Some desirable machine features are described in the General Molding Machine Considerations section. Most standard injection molding practices employed for non-reinforced thermoplastics are applicable to reinforced compounds. Table I outlines the fundamental injection molding parameters for our major thermoplastics categories.

This document details recommendations for injection molded reinforced thermoplastics.

Material Considerations

Amorphous Plastics: Compounds based on amorphous plastics (e.g.: ABS, polycarbonate, polysulfone) generally do not achieve as smooth a surface appearance as their crystalline counterparts.

Amorphous plastics do not have a sharp melting point. The melt viscosity is relatively high at standard molding temperatures. The stiffness of the melt inhibits wetting of the reinforcement at the molded part surface.

Due to their melt stiffness, amorphous plastics tend to have a strong jetting tendency. That is, to enter the mold cavity from the gate as a rope like strand. Subsequent material then backfills the cavity until the mold fill is completed. This condition results in major irregularities in material orientation which may be evident at the part surface. Fan gating or placement of the gate where the entering material will be forced to impinge on a mold surface or core pin will help minimize jetting by inducing turbulence. Higher mold temperature will minimize premature chilling of melt in the cavity thereby reducing surface irregularities.

Crystalline Plastics: Compounds based on crystalline or semi-crystalline plastics generally have relatively low melt viscosities. These materials readily flow into the mold. However, due to their sharp melting point it is essential that adequate melt temperature be maintained to prevent premature solidification in the cavity. Mold temperature requirements of between 80°C and 125°C are not uncommon for the higher melting point crystalline materials such as nylon 6/6, polyphenylene sulfide and Thermylon^®.

Areas on the molded part surface that are often interpreted as regions of high glass fiber concentration are in most instances actually regions where the melt partially froze during mold filling. Subsequent shifting of the partially solidified melt during final cavity packing upends fibers and partially detaches them from the melt creating the apparently "glass rich" areas.

Higher mold temperatures and more streamlined gating can often eliminate this surface problem.

Drying Recommendations

The maximum allowable moisture content of thermoplastics prior to molding is governed by the requirements of the base polymer. The drying time and temperature are essentially the same as that for the base polymer.

For the maximum allowable premolding moisture content, dryer temperatures and residence times required, please refer to Table I (located below). The recommendations are based on a typical dehumidifying dryer capable of maintaining a dew point of approximately -18°C.

Thermoplastic pellets and regrind tend to liberate finer particles and dust than their unreinforced counterparts. For this reason, dryer filters should be checked frequently and cleaned when required to insure adequate air flow.

Molding Parameters

The thermoplastics we manufacture generally have higher melt viscosities (lower melt flow rates) than their corresponding unreinforced counterparts.

We have the ability to compensate for the stiffening effect of the reinforcements by the addition of lubricants and flow enhancers. Additional compensation is generally accomplished by proper selection of molding parameters.

Barrel Temperatures

As compared to unreinforced thermoplastics, our polymers generally require higher melt temperatures. The additional temperature may vary from 5°C to 10°C for mineral filled crystalline compounds (such as polypropylene and nylon 6), to 40°C for highly reinforced Thermylon^®.

Table I:
Recommendations For Molding Characteristics Of Reinforced Thermoplastics 

TABLE I outlines the recommended melt temperatures for the material exiting the nozzle and processing parameters required for satisfactory injection molding of reinforced thermoplastic parts.

Barrel Temperatures Continued

Semi-Crystalline and amorphous thermoplastics differ in their behavior during the mold fill process.

Crystalline thermoplastics display a sharp melting point above which they are essentially liquid. For the melting points of our various plastics, please refer to Table I.

Amorphous plastics do not have a sharp melting point. They display a less distinct softening behavior as their temperature is elevated. Table I lists softening point values for the amorphous thermoplastic based compounds. These values can be interpreted as minimum permissible processing temperatures.

Indicated barrel temperatures generally do not correspond to the temperature of the melt within the barrel. The melt temperature at the nozzle is often 5°C to 15°C higher than the setting of the final barrel zone. High back pressure will tend to increase melt temperature due to increased melt shear. Actual melt temperature is best determined by measuring the internal temperature of exudates from the nozzle.

Mold Temperature

As a general rule, reinforced plastics require higher mold temperatures than their unreinforced counterparts.  This insures full, uniform mold fill. Recommended mold temperatures for our products are outlined in Table I.

The temperatures indicated are mold surface temperatures. The temperature of the heat exchanger fluid (e.g. water, oil) may differ from these values by several degrees.

To achieve mold temperatures of 95°C and above, it is often necessary to use oil heating systems. Adequate mold heating capacity is necessary if the property performance potential of the thermoplastic compounds is to be realized, thus justifying the expense of oil heating systems for applicable materials (See Table I).

Injection Parameters

For any given mold configuration, somewhat higher injection pressures are required for our plastics as compared to their unreinforced counterparts. By using moderate to high injection rates, the mold cavity can be filled prior to significant melt cool down and injection pressures can be moderated.

For our advanced compounds, mold design and temperature may strongly influence the required injection rate and pressure. Molding situations that require unreasonably high injection rates to compensate for restrictive cavity design or inadequate mold temperature may result in burn marks on the part in inadequately vented regions of the mold.

Our thermoplastics typically require short injection hold times due to the rapid solidification of the melt in the sprue and runner system.

Back Pressure

Moderate back pressure (50 to 100 psi) is generally required during the screw filling phase of the injection molding cycle. This assists removal of entrapped volatiles and provides better melt uniformity. High back pressures are to be avoided. High back pressures tend to damage fibrous reinforcements and increases barrel and screw wear.

Regrind Utilization

The permissible percentage of regrind that can be added to virgin pellets is dependent on the condition of the regrind and the end product requirements. Regrind levels of 10% to 15% often yield satisfactory property results. Contaminated or thermally degraded parts should not be ground for re-use.

Materials such as polycarbonate, thermoplastic polyesters and the nylons undergo an irreversible hydrolytic degradation when molded in an inadequately dried condition. Resulting parts should not be ground for re-use. Color shift after molding may restrict the use of reground parts in color sensitive applications.

Cycle Time

Typically, the higher elevated temperature stiffness of our plastics result in shorter cooling time requirements as compared to non-reinforced compounds.

The faster set-up time displayed by reinforced compounds offsets generally higher mold and melt temperature requirements. This results in faster part ejection and shortened cooling time requirements.

Table II: Molding Recommendations

Remedial Action Increase  Remedy  Decrease	Situation
Remedy	Brittle	Burn Marks	Discoloration	Flashing	Sink Marks	Short Shots	Splay	Sticking	Surface Roughness	Warpage (Long Term)
Pre-dry Material*
Adjust Feed
Injection Pressure
Injection Rate
Injection Hold Time
Cure Time
Mold Temperature
Stock Temperature
Clamp Pressure
Contamination Check
Regrind Check
Polish Mold
Vent Mold

*Moisture Sensitive Materials

This chart provides a handy reference for correcting the undesirable situations most commonly encountered in injection molded thermoplastics. As many of the remedies are directed toward requirements of the base polymer, the recommendations are generally applicable to non-reinforced plastics as well. For additional assistance call our Technical Service Personnel at 800-993-5382.

Because we cannot anticipate or control the different conditions under which this information and our products may be used, we do not guarantee the applicability or accuracy of this information or the suitability of our products in any given situation. The information and products referenced herein are intended for use by persons having technical skill and understanding, at their own discretion and risk. We cannot anticipate or control conditions of information and product usage. Users of our products should make their own tests to determine the suitability of each product for their particular purpose. WE MAKE NO REPRESENTATIONS OR WARRANTIES OF ANY KIND, INCLUDING ANY EXPRESS OR IMPLIED WARRANTY OF MERCHANTABILITY OR FITNESS FOR ANY PARTICULAR PURPOSE. Also, statements concerning the possible use for our products are not intended to be nor are they recommendations to use our products in the infringement of any patent.