Why does adding regrind shorten hopper residence time?

Regrind usually has a lower bulk density, so the same hopper holds fewer kilograms, and each leaves sooner (Conair, Novatec). Weigh a known volume of your actual blend and enter that density.

Will a faster cycle starve the drying hopper?

Yes: residence falls in proportion to cycle time. At the defaults, cutting the cycle from 30 to 22 sec drops residence from 4.5 to 3.3 hours, under the 3.5 hour drying time.

Can resin sit in the drying hopper too long?

Yes, for some resins. Novatec reports some PBT grades lose properties after 8 to 10 hours at heat, and Teknor Apex says over-dried nylon flows worse and natural grades yellow. Run a lower hopper level or a smaller hopper.

My hopper is rated in pounds. What volume do I enter?

Enter its volume in liters: one cubic foot is 28.3 L. A pound rating holds only at the bulk density it was rated for, and your resin may be lighter.

Injection Molding calculator

Hopper Residence Time Calculator

Check that each pellet gets its full drying time in the hopper, and no more than its over-drying limit. You need hopper volume, bulk density, shot weight, cycle and datasheet times.

What this calculator does

How long resin sits in the drying hopper at your shot and cycle, against drying time and any over-drying limit.

Formula used

Resin held (kg) = hopper working volume × bulk density
Press consumption (kg/hr) = shot weight × 3,600 ÷ cycle time ÷ 1,000
Residence time (hrs) = resin held ÷ press consumption
Shortest cycle for full drying = drying time × 3,600 × shot weight ÷ (1,000 × resin held)
Volume for full drying = consumption × drying time ÷ bulk density; largest volume uses the over-drying limit

Inputs explained

Drying Hopper Working Volume: Volume below the loader's fill level, from the hopper spec sheet.
Resin Bulk Density: Pellet bulk density from the resin or dryer maker's table.
Actual Shot Weight per Cycle: Parts plus runner and sprue, weighed from one shot.
Cycle Time per Shot: Measured cycle time from the press monitor.
Supplier Drying Time: Hours at drying temperature and dew point, from the resin datasheet.
Over-Drying Limit: Longest time at drying temperature the supplier allows; 0 if none.

How to use the result

Best suited to qualifying a hygroscopic resin on a press, checking a cycle-time cut against the dryer, chasing splay on a moisture-sensitive part.
A hopper that channels sends some pellets through faster than the average; confirm moisture at the throat. Does not check dryer air output or dew point.

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

Why does adding regrind shorten hopper residence time? Regrind usually has a lower bulk density, so the same hopper holds fewer kilograms, and each leaves sooner (Conair, Novatec). Weigh a known volume of your actual blend and enter that density.
Will a faster cycle starve the drying hopper? Yes: residence falls in proportion to cycle time. At the defaults, cutting the cycle from 30 to 22 sec drops residence from 4.5 to 3.3 hours, under the 3.5 hour drying time.
Can resin sit in the drying hopper too long? Yes, for some resins. Novatec reports some PBT grades lose properties after 8 to 10 hours at heat, and Teknor Apex says over-dried nylon flows worse and natural grades yellow. Run a lower hopper level or a smaller hopper.
My hopper is rated in pounds. What volume do I enter? Enter its volume in liters: one cubic foot is 28.3 L. A pound rating holds only at the bulk density it was rated for, and your resin may be lighter.

Last reviewed 2026-10-01.