Residence is a division, and it moves
A hopper holding 150 lb of resin, feeding a machine that shoots 96.3 grams every 24 seconds, is losing 31.8 lb an hour out of the bottom. That is 4.71 hours of residence. If the datasheet asked for 4 hours, the hopper is holding material for 117.8 percent of that time and there is 22.6 lb of headroom in the charge.
Now speed the job up. Take the cycle to 19 seconds — a perfectly ordinary improvement — and consumption goes to 40.2 lb an hour, residence to 3.73 hours, and the same hopper that was comfortable is now under the time it was set up for. Nobody touched the dryer. The dryer has no idea anything happened.
Working it the other way
Two lines on the page run the calculation backwards. For a 4 hour drying time at 31.8 lb an hour you need 127.4 lb in the hopper, which at 38 lb per cubic foot is 3.35 cubic feet. And with 150 lb in the hopper, 4 hours of residence supports 37.5 lb an hour, which at a 96.3 gram shot is a 20.4 second cycle.
That last figure is the useful one when somebody asks how fast the job can run. If the drying time is fixed and the hopper is fixed, the cycle has a floor that has nothing to do with cooling or with the machine, and it is worth knowing which of the two constraints is actually binding before spending money on either.
What residence does not tell you
Whether the resin is dry. Residence is time in the hopper and nothing else. Drying is a function of the temperature the bed is held at, the dew point of the air reaching it, whether that air is genuinely passing through the material rather than channelling round it, and how much moisture the material had when it went in. Regrind that has sat open on a shop floor is a different starting point from a sealed bag, and the same four hours will not do the same job on both.
None of that is measurable from a weight and a cycle time, which is why this page reports the division and stops. The drying time itself is a field for the same reason: it belongs to the resin datasheet, at the temperature and dew point the datasheet names.
Downtime cuts both ways
The running share field scales the hourly consumption. A machine up 70 percent of the hour pulls 22.3 lb instead of 31.8, and residence stretches from 4.71 hours to 6.73. More time is not automatically better — the material is sitting hot the whole while, and some resins have something to say about that in the same section of the datasheet that gives the drying time. Reading only the minimum and ignoring the rest of that paragraph is a common way to get this wrong in the other direction.
Questions people ask
How do I work out residence time in a drying hopper?
Divide the weight of resin in the hopper by the weight the machine consumes per hour. A 150 lb charge feeding a machine that uses 31.8 lb an hour gives 4.71 hours. The consumption comes from the shot weight and the cycle: 96.3 grams every 24 seconds is 150 shots an hour, 14.4 kg, or 31.8 lb. The runner counts, because it goes through the dryer along with the parts.
What size drying hopper do I need?
Multiply your consumption per hour by the drying time the resin datasheet gives. At 31.8 lb an hour and a 4 hour drying time that is 127.4 lb of resin in the hopper, or 3.35 cubic feet at a bulk density of 38 lb per cubic foot. Working charge rather than catalogue capacity is the figure to use, because a hopper that runs part full holds less than its shell size.
Does more residence time mean better drying?
Not on its own. Residence is only the time the material spends in the hopper; the drying itself depends on the temperature, the dew point of the air reaching the bed, whether the air is really passing through the material, and how wet it was to begin with. Long residence also means the material is hot for longer, which the datasheet may have something to say about. This page reports the time and makes no claim about dryness.
Why did my residence time change when I sped up the cycle?
Because a faster cycle pulls more resin per hour out of the bottom of the same hopper. Going from a 24 second cycle to a 19 second one on a 96.3 gram shot takes consumption from 31.8 to 40.2 lb an hour and residence from 4.71 hours to 3.73. Nothing about the dryer changed. This is the usual way a job that was fine quietly stops being fine, and it is worth recalculating whenever the cycle moves.
Does regrind change the calculation?
It changes the bulk density, which changes how much weight a given hopper volume holds — ground runner is normally lighter and less uniform than virgin pellets, so the same cubic feet is fewer pounds and less residence. It also arrives with its own moisture history, having been open to the shop air, which is a drying question rather than an arithmetic one and belongs with the resin supplier.