Dimensional Stability of Plastic Injected Parts

  Dimensional stability problems occur when a part is inconsistent in part size or in part weight from shot to shot. Plastic injected parts that are underweight can suffer in structural integrity; overweight parts can also suffer from integrity due to molded-in stresses and increased part costs. Here are some causes and solutions below for dimensional stability of plastic injected parts.

When injection pressure transfers on time or hydraulic pressure, plastic injectedparts weight and size can vary since any change in hydraulic pressure can cause too little material to fill one shot and too much material the next. Transfer due to screw position is found to provide improved object weight and dimensional stability. Undersized nozzle or sprue size can restrict the amount of material that enters through the gate and into the cavity. Too short a closed mold time will cause the part to shrink more.

High mold temperatures can result in too small parts. This can be noticed even more in semicrystalline material such as polyethylene and polypropylene. Plastic injected parts can be made that are oversized, indicating that not enough shrinkage is occurring. Excessive packing pressure can push too much material into the cavity. A hotter mold will slowly cure the part, and upon ejection, will be warm and continue to shrink.

The screw position can be set to fill the cavity about 95%, and then the remaining 5% can be filled on hold pressure. The pack pressure is increased to fill the cavity with sufficient material, the injection forward time is increased, and the fill time to force more material into the cavity faster is also increased. Enlarging the nozzle and sprue will permit more material to flow into the gate and cavity. Increasing mold closed time will keep more pressure on the melt and cavity and increase the part size.

Lowering mold temperatures will allow the material to cure faster in the mold and reduce shrinkage. Reducing packing pressure permits the material to shrink away from the cavity. A cold mold surface will cure the part faster, resulting in a cooler ejected part that will not shrink much more.

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