Tip of the Day 136: Positive Volume and the Zero Line

Over the years we have been challenged to explain why we show “Volume / Injection” as a value that goes up as the material is injected into the mold. The simplest answer is that “Systematic Molding” means thinking from the plastic’s point of view inside the mold rather than inside the machine. We want to show how much material you have put into the mold (a positive number) rather than how much is left in the barrel.

This means that we start the volume curve at the machine’s shot size setting: the point where the screw was when the screw motor stopped. Thus the decompression after recovery (“suck-back”) shows up as a negative number.

As an attempt to help visualize this I have created a video that shows the screw position in relation to the Cycle Graph volume curve. The cycle graph is rotated so that the tip of the screw lines up with the volume curve.


A second video shows the same thing mirrored so that the screw traverse is in the direction it is when viewed from the operator side of the press. In both videos the plastic is going into the mold above the dashed zero volume line.