Injection Molding Machine
The shot weight should stay between 20% and 80% of the maximum melt volume the screw can accumulate in front of it. Below 20% the material waits a long time in the barrel and thermal degradation and color yellowing begin; above 80% no cushion is left, the holding pressure cannot be transmitted to the part and shot consistency suffers. The calculation must be made with melt density, not solid density.
The screw diameter and metering stroke determine the theoretical shot volume of the machine. Applying the melt density to this volume gives the maximum shot in grams. A common mistake is to use the solid density: because melt density is 10–25% lower than solid density, the fill ratio looks better than it is.
The lower limit of the rule is about residence time. Molding a small part on a large machine keeps the material in the barrel a little longer every cycle; with heat-sensitive materials (PVC, POM, PET) this means degradation.
If there is a smaller screw diameter option, switch to it; if not, calculate the residence time and compare it with the material's limit. With tolerant materials such as PP there may be no problem; with PVC and POM there will be.