Most glass fibers utilized in plastics are less than 0.001 inch in their minor dimension. It is for this reason that our polymers will flow through extremely small gates without total blockage.

However, much of the property enhancement achieved is related to maintenance of the fiber length. Restrictive gating ultimately reduces effective glass fiber length by forcing the fibers to come in contact with the gate wall and with each other. Such contact may either weaken the fiber due to surface scoring or fracture the fiber outright.

Elevating the melt temperature to compensate for inadequate gating may further deteriorate the final part properties by increasing polymer degradation. For these reasons, generous gating is recommended whenever conditions permit.

Sprue Gating

Sprue gating directly into the body of the molded part maximizes part properties by minimizing damage to the glass filler and polymer degradation.

Sprue bushings should be generously tapered and polished. The low mold shrinkage exhibited by most of our compounds tend to keep the sprue in contact with the sprue bushing even after melt solidification. Generous taper will help prevent the sprue from sticking in the sprue bushing.
The bore of the injection nozzle should be slightly smaller than that of the nozzle end of the sprue bushing. This will help prevent sprue hang-up caused by material overlapping the end of the sprue bushing.

The intersection of the sprue and the molded part should be generously radiused to promote smooth material flow from the bushing into the part. A dimple on the surface of the part opposite the sprue will assist the flow of material in thin-wall parts.

Diaphragm Gating

Diaphragm or disc gates are recommended for cylindrical parts requiring good concentricity and elimination of weld lines for optimal strength. De-gating is required after molding in order to remove the diaphragm.

Asahi Kasei Plastics materials have been shown to work well in this type of gating. It is advantageous to have the thickness of the diaphragm decrease to a thin section just before the gate area. This causes a dramatic increase in material velocity and aids in flowing through the gate.

The sprue should have a generous cold well on the opposite side of the diaphragm, and it should include a sprue puller. The "Z" type pullers work better with reinforced materials than the undercut type.

The diaphragm or disc gate is recommended for all cylindrical parts requiring good concentricity and weld line strength. De-gating will require a post-molding operation to remove the diaphragm.

Tunnel Gating (Sub-Gating)

This gate design permits automatic de-gating of a part from the runner system during ejection. Asahi Kasei Plastics materials work well in these types of gates.

As in the case of conventional gating, the size and number of gates must be sufficient to minimize fiber damage and polymer degradation. Minimum exit diameters need to be larger than with unfilled materials; however, gates as small as 0.050 inches have worked in some applications. Our Technical Service Personnel are available to assist you in selecting gate size and location.

As in the case of conventional gating, the size and number of gates must be sufficient to minimize fiber damage and polymer degradation. Minimum exit diameters need to be larger than with unfilled materials; however, gates as small as 0.050 inches have worked in some applications. Our Technical Service Personnel are available to assist you in selecting gate size and location.

Edge Gating

Gating into the side of the molded part is accomplished in the traditional manner. Short land lengths and generous gate thickness are recommended. This will minimize fiber length attrition and polymer degradation.

Reinforcements generally increase the melt strength of the material as it enters the mold cavity from the gate. If the gates are small or insufficient in number, there will be a tendency for the melt stream to extrude in a strand form jetting toward the opposite end of the part. The rapid set-up time characteristic of many reinforced thermoplastics will encourage this initial strand to stiffen prior to complete packing of the part, leaving a visible impression of itself on the part surface. Generous gating, location of the gate where material will impinge on an obstruction such as a core pin, and elevated mold temperature will help minimize this situation.

RUNNERS

Conventional Runners

Conventional runner systems are readily employed in molds for our composites. The melt flow of tends to be higher than that of unreinforced polymers, and the melt tends to set-up faster. Therefore, full-round runners of generous diameter are recommended. This provides an unrestricted flow path for the melt between the sprue and the part gate. A generous cold well should be provided at the base of the sprue and at any abrupt change in runner direction.

Alternative to generous runners is the use of carefully designed undersized runner systems. In principle, small runners will convey material at a rate that will minimize premature chilling while reducing runner scrap. Such systems should be carefully designed for adequate material flow and balance. Mold and melt temperature may have to be increased as compared to traditionally sized runner systems and performance may be more sensitive to processing and material variations.

Hot Runners

Hot runner systems are regularly employed in plastic molds. The prime considerations in selecting and designing hot runner systems are:

•    Melt stability of the base thermoplastic
•    Type and percentage of reinforcement

Thermoplastics based on polymers of low thermal stability (e.g. Acetal and PVC) may not be suitable for hot runner systems due to the added melt resistance time of such systems. Such situations must be examined carefully, taking into consideration projected melt residence time, runner configuration, and temperature control and uniformity throughout the runner system.

Hot runner systems for plastics containing high levels of fibrous reinforcement must be carefully designed to avoid obstruction to melt flow. Acute angles, large variations in runner size and stagnation regions may promote the collection of high glass fiber concentrations in sections of the hot runner system.

Accurate temperature control throughout the hot runner system is essential. Material will have a strong tendency to freeze off in cold regions of the hot runner system.

Conversely, in overheated regions of the hot runner system, the reduced melt viscosity that often accompanies the initial stages of degradation may lead to separation of the reinforcement from the melt. This may lead to build up of high concentrations of the reinforcement in the hot runner and eventual blockage of the runners and/or nozzles.

Mold Layout

Just as it is important to fill a cavity evenly, so too is it important that the runner system be designed so the material reaches all the gates at precisely the same time. If the mold is not balanced in this manner, some parts could be over packed while other parts are barely filled.

Following are some examples of runner system design.

Parameter Effects on Shrinkage

Molding parameters that affect orientation and the degree of crystallization also influence shrinkage and warpage. The optimum combination consists of a melt hot enough to remain in liquid form long enough to fill the mold cavity with minimum viscous shear. Packing pressure should be high enough and gate freeze delayed long enough to fully pack the part. The cooling rate must provide enough time for the stressed layers of resin to relax, but still be fast enough to prevent a higher level of crystallinity than needed, since shrinkage goes up with crystallinity.

These parameters include raising the temperature of the melt and the mold in many cases, as well as increasing cooling efficiency. This does not necessarily increase overall cycle time as our compounds set up faster than unfilled materials.

An experienced molder can make most marginal designs work, but at the price of cycle time and/or scrap rate. So it is important to consider processing at the design stage.

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.