Where do I find melt density if the datasheet lists only solid density?

Ask the resin supplier for its flow-analysis data, which lists melt density by grade. DuPont gives 1.16 g/cc for Delrin 500P against a solid density of 1.42 g/cc.

Should shot size include the cushion?

No. Shot size is the screw travel from the end of recovery to the final cushion, so set the recovery position at shot size plus your cushion. Decompression is excluded too.

My press is rated in grams of polystyrene. What do I enter?

Enter the theoretical shot volume in cc from the same spec sheet instead. A polystyrene rating in grams holds fewer grams of a lighter melt such as HDPE, and more of a denser one.

Is a shot near 80% of the barrel acceptable for acetal?

Not ideally. DuPont's Delrin guide puts optimum productivity at or below 50% of injection unit capacity, with 1 to 2 screw diameters of travel. Above that, screw speed must drop late in recovery.

Injection Molding calculator

Shot Size Calculator

Convert part and runner weight into melt volume and screw travel, then check the shot against the barrel before you schedule the mold. You need cavity weights, the resin's melt density and the press's screw size.

What this calculator does

Screw travel a mold's shot needs, and whether it sits inside 20 to 80% of the barrel.

Formula used

Shot weight = part weight plus runner share per cavity × cavities
Melt volume (cc) = shot weight ÷ melt density
Shot size (mm) = melt volume × 1,000 ÷ (π × screw diameter² ÷ 4); in screw diameters, ÷ screw diameter
Barrel use = melt volume ÷ barrel shot capacity × 100, judged against 20 to 80%
Shot weight window = 20% and 80% × barrel shot capacity × melt density

Inputs explained

Part Weight Plus Runner Share per Cavity: Weigh one part with its share of cold runner and sprue.
Cavities: Cavities filled every shot.
Resin Melt Density: Melt, not solid, density from the resin supplier's flow data.
Barrel Shot Capacity: Theoretical shot volume from the press spec sheet.
Screw Diameter: From the press spec sheet or barrel nameplate.

How to use the result

Best suited to scheduling a mold onto a press, choosing cavitation for a new tool, setting a starting shot size at mold trial.
The 20 to 80% window is a general rule; DuPont puts optimum Delrin productivity at or below 50% of capacity. Does not check residence time, clamp force or injection pressure.

Current U.S. benchmarks

The producer price index for plastic resins and materials stands at 280.569 (BLS, Aug 2026), up 6.6% from a year earlier. Quotes priced off last quarter's material cost miss this move.
The U.S. has 9,635 plastics product manufacturing establishments employing about 677,302 workers (Census County Business Patterns, 2023).

Common questions

Where do I find melt density if the datasheet lists only solid density? Ask the resin supplier for its flow-analysis data, which lists melt density by grade. DuPont gives 1.16 g/cc for Delrin 500P against a solid density of 1.42 g/cc.
Should shot size include the cushion? No. Shot size is the screw travel from the end of recovery to the final cushion, so set the recovery position at shot size plus your cushion. Decompression is excluded too.
My press is rated in grams of polystyrene. What do I enter? Enter the theoretical shot volume in cc from the same spec sheet instead. A polystyrene rating in grams holds fewer grams of a lighter melt such as HDPE, and more of a denser one.
Is a shot near 80% of the barrel acceptable for acetal? Not ideally. DuPont's Delrin guide puts optimum productivity at or below 50% of injection unit capacity, with 1 to 2 screw diameters of travel. Above that, screw speed must drop late in recovery.

Related guides

Last reviewed 2026-10-01.