In Injection plastic mold , the first thing to occur in the process is filling the mold with a given resin. Improper fill rate can result in various part defects, both in part strength and aesthetics.

Below, we cover the 2 most important considerations for plastic injection molding fill rate: Speed and Gates.

Plastic Injection Molding Speed
With today’s machinery, plastic molders are able to inject resin very fast, very slow, or variations of both throughout the process, but within the same injection phase.

Plastic molders who fill too fast (usually to save time or reduce process cycle time) will end up causing the plastic to trap gas in the vents, resulting in mold burns.

*Certain plastics have a high level of shear and are easily burned because of fill rates that are too fast. Shear is less likely to occur with slow fill rates and large gates into the part.*

If filling is too slow or too fast it will cause voids, and high levels of molded-in stress, and surface blemishes.

Plastic Injection Molding Gates
When diagnosing surface blemishes, gas venting, as well as the size and type of gates into the cavity, are very important.

Gate size will depend on the resin being used and the size of the part being molded.

*The larger the gate, the better and easier it is to make high quality parts.*

Small gates provide a better appearance, but they require longer mold time or higher pressure to fill properly. While large gates allow for better resin flow and shortened mold time.

If gate size is not sufficient, it can cause defects like mold warpage and/or black streaks in the molded part. When plastic is injected directly into the cavity with a heat-controlled nozzle, the molded part is far less likely to have dimensional or surface blemishes, like black streaks.

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