Barrel Residence Time Calculator for Injection Molding
A small shot on a large press leaves resin cooking in the barrel for too long. This calculator estimates residence time and shot utilization so you can decide whether a job belongs on a smaller machine.
Your numbers
From the machine spec sheet, usually quoted in grams of general purpose polystyrene.
Solid density of the resin on the datasheet, used to scale the polystyrene capacity.
Melt in the barrel, screw flights and nozzle compared with the maximum shot. Two is a commonly used planning figure.
Maximum safe time at melt temperature, from the resin supplier. Heat sensitive resins are far lower than polyolefins.
Your results
A planning estimate using a common rule of thumb. Actual hold-up depends on screw design, back pressure and barrel length. Confirm limits with the resin supplier.
Residence time (min) vs shot weight (g)
Sweeps Shot weight (parts plus runner) from 20 to 400, holding your other inputs fixed.
Match jobs to the right press
Optional: we can review how job shot weights and press capacities are stored in your ERP and flag mismatches.
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How to use it
Enter the machine maximum shot from its spec sheet, normally in grams of polystyrene, and the density of your resin so the capacity can be scaled to what the machine really holds in your material. Add the shot weight including runner, the cycle time and a resin residence limit from the supplier. The result shows percent of capacity used and an estimated time at melt temperature.
The formula
Capacity in your resin is the polystyrene capacity times the resin density divided by 1.05. Residence time in minutes is capacity divided by shot weight, times cycle time in seconds divided by 60, times a hold-up factor to account for melt in the screw flights and nozzle beyond one maximum shot. The factor of two is a widely used planning number, not a physical constant.
- Shots below roughly 20 percent of capacity raise degradation risk
- Shots above roughly 80 percent leave little room for recovery and consistent plasticizing
- Heat sensitive resins such as PVC, POM and some polycarbonate blends need short residence
Reading the result
With the defaults, a 90 g shot on a 450 g polystyrene-rated press in polypropylene-density resin runs about 17 percent of capacity and an estimated residence near 9.5 minutes, which is over an 8 minute limit. The fix is a smaller press, a lower hold-up through a smaller barrel, or a shorter cycle. The chart shows how steeply residence time rises as shot weight falls.
Common mistakes
Using the shot weight of one part rather than the whole shot with runner, ignoring density, and forgetting that downtime multiplies residence: a press stopped with a full barrel degrades resin quickly, so purge procedures and temperature setback matter. Also check the real melt temperature, not the setpoint.
Frequently Asked Questions
What shot size percentage is recommended?
A common guideline is to keep the shot between about 20 and 80 percent of machine capacity. Below that, residence time grows and consistency suffers. Above it, recovery time and shot control can become limiting.
Why scale capacity by density?
Machine ratings are given for polystyrene. A lighter resin such as polypropylene fills the same screw volume with less mass, so the maximum shot in grams is lower for that resin.
How do I know the residence limit?
Check the resin supplier datasheet or technical service. Polyolefins tolerate many minutes at melt temperature, while heat sensitive resins may allow only a few. Use their figure for your melt temperature.
Is the hold-up factor of two accurate?
It is a rule of thumb. Screw design and barrel length change it. If you have measured purge or color change data for the machine, use that to calibrate the factor.
Netray can match job shot weights from your ERP routings against the presses that exist on your floor and flag oversized machine assignments before they cause rejects.
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