Injection Molding: The 2 Most Important Considerations For Fill Rate

How to Play a Key Role in Plastic Mould's Injection Molding Success

In plastic injection molding, the first thing that happens in the process is filling the mold with a given resin. Improper fill rates can lead to a variety of part defects, including part strength and aesthetics.

Below, we'll cover the two most important considerations for plastic injection molding fill rate: speed and gating.

Plastic injection molding speed

With today's machines, plastic molders are able to inject resin very quickly, very slowly throughout the process, or both, but in the same injection stage.

Plastic molders who fill too quickly (usually to save time or shorten process cycle time) end up with the plastic trapping gas in the vents, causing mold burn.

Some plastics have high levels of shear and burn easily due to too fast filling rates. Shearing is less likely to occur with slow fill rates and large gates into the part.

Filling too slowly or too quickly can result in voids, high levels of in-mold stress, and surface imperfections.

Injection molding gate

When diagnosing surface defects, venting and the size and type of gates entering the cavity are important.

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

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

Small gates provide a better appearance, but they require longer molding times or higher pressures to fill properly. The large gate allows better resin flow and reduced molding time.

Insufficient gate size can result in defects such as mold warpage and/or black streaks in the molded part. When plastic is injected directly into the cavity through a thermally controlled nozzle, the molded part is less likely to show dimensional or surface imperfections, such as black streaks.