Calculations

How to Calculate Rotational Molding Metrics: Powder Charge, Wall Thickness, and Cycle Time

The five core rotomolding formulas worked end to end, from powder charge weight to oven cycle time, with real densities, units, and where each input comes from.

Rotational molding math starts with powder charge weight, because it sets wall thickness and part cost together. The formula is charge (kg) = surface area (m squared) x target wall thickness (m) x resin density (kg/m cubed). For a 1.2 m squared tank in LLDPE at 940 kg/m cubed with a 6 mm wall, that is 1.2 x 0.006 x 940 = 6.77 kg. Add 3 to 5 percent for grinding fines and transfer loss, so charge about 7.0 kg. The Powder Charge Weight calculator does this and lets you swap density for crosslink PE (935), PP (905), or nylon (1140).

Wall thickness is the same relationship solved the other way. Wall (mm) = shot weight (kg) / (area (m squared) x density (kg/m cubed)) x 1000. Weigh a molded part at 6.4 kg, divide by 1.2 m squared and 940 density, and you get 0.00567 m, or 5.67 mm actual against a 6 mm target. That 0.33 mm gap flags either short charge or uneven pickup. The Wall Thickness Estimate calculator maps charge to nominal wall so you can dial in the shot before cutting a section and measuring with calipers at 8 to 12 points.

Oven cycle time is driven by heat penetration through the wall, not part size directly. A working rule for PE is oven time (min) = wall thickness (mm) x internal air temperature factor, where most shops see 4 to 5 minutes per mm at an oven setpoint of 300 C. A 6 mm wall lands near 27 to 30 minutes to reach a peak internal air temperature (PIAT) of 200 to 220 C. Use the Oven Cycle Time calculator with your oven setpoint and wall to estimate the target, then confirm PIAT with a Rotolog or datalogger rather than trusting a fixed timer.

Cooling cycle time usually runs longer than most operators expect and is often the real bottleneck. Cooling to a demoldable 60 to 70 C internal temperature typically takes 1.3 to 1.8x the heat time for a given wall. Forced air alone on a 6 mm PE part runs 18 to 25 minutes; adding a water mist stage cuts that to 10 to 14 minutes but raises warpage risk if applied before the polymer sets below its peak crystallization range near 100 to 110 C. The Cooling Cycle Time calculator separates air-only and mist stages so you can size the tradeoff.

Venting is small but load-bearing math. Vent area scales with mold internal volume so pressure equalizes without deforming the part. A common sizing rule is vent tube inside diameter (mm) = k x cube root of volume (liters), with k around 6 to 8 for PE. A 120 liter mold gives roughly 6.5 x 4.93 = 32 mm, usually met with one or two PTFE-lined tubes packed with glass wool. The Venting Allowance calculator returns tube diameter and count; undersize it and you get blow holes or a sucked-in flat wall as the enclosed air contracts on cooling.

Pigment and additive dosing is a ratio problem tied to the powder charge. Dry-blend color is normally let down at 1.5 to 4 percent by weight of resin, so a 7.0 kg charge at 3 percent needs 210 g of pigment masterbatch. UV stabilizer packages add another 0.2 to 0.6 percent. The Pigment/Additive Usage calculator converts your target loading percentage and shot weight into grams per part and per batch, which matters because a 0.5 percent dosing error across a 200-part run wastes 7 kg of pigment or leaves parts under-protected outdoors.

Tie these together before you commit a schedule. Total cycle = oven time + cooling time + servicing (load, unload, spray, close), where servicing on a manual carousel is 4 to 8 minutes per arm. For the 6 mm tank that is roughly 28 + 20 + 6 = 54 minutes per arm turn. Mold Arm Utilization then tells you how much of your arm real estate that part actually consumes against the swing diameter, and Mold Capacity Per Shift converts cycle time and arm count into parts per 8-hour or 12-hour shift for realistic planning.

A quick sanity pass catches most mistakes. Check that charge density matches the resin grade actually loaded, that area is the molded surface and not the flat blank, and that oven time is quoted to PIAT rather than a shelf number. Rerun the numbers whenever you change wall by even 1 mm, since a jump from 6 to 8 mm raises charge by 33 percent and can add 8 to 10 minutes of oven time plus more cooling. Keeping the Powder Charge Weight, Wall Thickness Estimate, and Oven Cycle Time calculators in one loop keeps those three linked inputs honest.

Published 2026-07-01.