How to Achieve a Fill-Only Part

The fill-only part is a critical principle of plastic injection molding process development and troubleshooting. A fill-only part is what we call the first stage, or the filling of the cavity. Most people call that 95-98% full, but this can be subjective—does it refer to weight or visual? Our goal in developing this process was to get something a little more defined.

Before beginning, it is important to remove the pack and hold so that we don’t have any second stage hold pressure on our process. Then follow these 5 steps:

Step 1: Create a shot where the most full part is full, but not packed out. The part will not be short but will contain sinks.

Step 2: Weigh the entire shot, without the runner, and record the weight. (Example: 29.920 grams)

Step 3: Multiply the entire shot weight by .99 to establish a fill-only shot weight. (Example: 29.920 grams x .99 = 29.621 grams)

Step 4: Adjust the shot size or transfer in order to obtain the fill-only part weight. (Example: 29.621 from above)

Step 5: Turn pack and hold back on, and you should have your full part.

Exception: This works on 98% of parts that are out there, but there is one exception: this process won’t work for parts that have moving hinges. Because of the pressure losses across the hinge, we get a big difference in density upstream (high) and downstream (low) the hinge. This would result in a very short, even if the weight is at 99%.