What Shot Weight Includes
Shot weight includes all plastic injected per cycle: molded parts, runners, sprue, cold wells, and sometimes an allowance for cushion or process variation. In hot-runner molds, runner volume may be much lower than in cold-runner molds.
Machine selection should consider shot size, plasticizing capacity, clamp tonnage, injection pressure, screw diameter, residence time, and material behavior. Shot weight alone is necessary but not sufficient.
- Process engineers use it to check machine utilization.
- Mold designers use it to compare runner options.
- Estimators use it for material cost per cycle.
How to Calculate Shot Weight
The formula is: shot weight = (part volume x number of cavities + runner and sprue volume) x material density. Use consistent units, such as cubic centimeters and grams per cubic centimeter.
For example, a part is 25 cm3, the mold has 4 cavities, the runner system is 20 cm3, and polypropylene density is 0.90 g/cm3. Total volume is 25 x 4 + 20 = 120 cm3. Shot weight is 120 x 0.90 = 108 grams. On a 200 gram rated machine, that is 108 / 200 = 54 percent of rated shot capacity.
Machine Capacity and Material Differences
Machine shot capacity may be listed in ounces of general-purpose polystyrene or another reference material. Because plastics have different densities and processing behavior, the usable shot for nylon, acetal, polypropylene, or filled materials may not equal the nameplate number directly.
A common processing target is to avoid using an extremely small or extremely large percentage of barrel capacity. Too small a shot can cause long residence time and degradation. Too large a shot can limit cushion, pressure control, and consistency.
- Convert machine capacity to the actual resin density when needed.
- Check cushion requirements with the processor.
- Consider regrind, colorant, and moisture-sensitive materials.
Common Shot Weight Mistakes
A common mistake is counting only the part and forgetting the runner. In a cold-runner multi-cavity mold, runner weight can be a large share of every cycle and can change both material cost and machine fit.
Another mistake is assuming CAD volume is final. Molded part weight can shift with shrinkage, packing, wall thickness changes, inserts, venting, and process settings. Use first-article data to refine estimates.
- Do not mix ounces, grams, and cubic centimeters without conversion.
- Do not ignore filled-resin density.
- Do not select a press from shot size without checking clamp tonnage.
Frequently asked questions
Is shot weight the same as part weight?
No. Shot weight includes every cavity plus runner, sprue, and other injected plastic for the cycle. Part weight is only the finished part.
Why is machine shot capacity often listed in polystyrene?
Polystyrene is a common reference material. Other resins have different densities, so practical capacity should be converted or verified.
What shot capacity percentage is best?
There is no universal number, but very low or very high utilization can create process problems. The processor should verify the window for the material and machine.
Does a hot runner reduce shot weight?
Usually yes, because less runner plastic is ejected each cycle. The manifold still contains material, but it is not normally part of each shot weight.