Build the complete shot first
Add every cavity’s part mass, cold runner and sprue mass, then consider the process cushion. A hot-runner job may have little runner scrap, but it still needs a documented shot mass and a stable cushion.
Shot mass = (part weight × cavities + runner weight) × (1 + cushion)
Separate volume from weight
Barrels are fundamentally volumetric. Convert the complete shot using an appropriate melt-density basis before comparing it to screw stroke or injection-unit volume. Do not assume that a machine’s quoted polystyrene shot weight transfers unchanged to every resin.
Read the machine data sheet carefully
Manufacturers commonly publish maximum stroke volume and theoretical shot weights for defined materials. For example, ARBURG publishes both calculated stroke volume and PS shot weight, while material suppliers publish density and processing data by grade. Use the actual machine/injection-unit documentation and the exact resin grade for the decision.
Check the usable process window
Too little of the barrel used can increase residence-time risk; too much leaves inadequate cushion and pressure-control margin. The acceptable window depends on the polymer’s thermal stability, screw geometry, actual capacity rating, and process. Establish it with the machine maker, resin supplier, and a validated trial rather than treating a generic percentage as a rule.
FAQ
Is a PS shot-weight rating a capacity for every polymer?
No. Treat it as a machine-specific reference. Convert the actual shot with the appropriate material basis and check machine documentation.
Why does cushion matter?
A stable cushion supports packing control; its suitable value depends on the machine, screw, material, and validated process.