How injection shot weight is calculated
Shot weight is the resin that fills the cavities plus the runner and sprue on every cycle. In US plants the math is run in ounces: shot weight (oz) = (part volume x cavities + runner volume) x specific gravity x 0.578, where 0.578 oz is the weight of one cubic inch of water. Common specific gravities are PP 0.91, ABS 1.05, PC 1.20, POM 1.41 and 30% glass-filled PA66 about 1.37.
American molding machines are rated in ounces of general purpose polystyrene (GPPS, SG 1.05), not in ounces of the resin you are actually running. This calculator converts the rated capacity to your resin before reporting barrel usage, so a 12 oz press running a 1.41 SG acetal shows a higher usage figure than the nameplate suggests. Shops normally keep the shot between 20% and 80% of rated capacity: below 20% the melt sits in the barrel long enough to degrade, and above 80% there is not enough cushion to hold consistent dimensions.
Throughput is shot weight x (3,600 / cycle time), converted to lb/hr. If the plasticizing rate on the machine data sheet is lower than this number, the target cycle is not achievable. Add 5-10% to the calculated figure when ordering material to cover scrap, startup purge and color changes.
Frequently Asked Questions
Multiply the rated shot volume in cubic inches by the specific gravity and by 0.578 to get ounces of that resin. A 12 in3 barrel running ABS at 1.05 SG holds roughly 7.3 oz.
No. Reusing regrind lowers how much virgin resin you buy, but the total mass injected each cycle is unchanged. Set the allowed regrind percentage separately based on the part specification.