Injection Molding Cooling Time Calculation — How Do You Estimate the Minimum Cooling Time from Wall Thickness, Resin Thermal Diffusivity and the Mold-to-Ejection Temperature Drop?
Last updated: 2026-09-04 · Reading time: about 6 min
Cooling time is the seconds the part solidifies in the mold before ejection — normally 50–80 % of total cycle. The rule: cooling time scales with wall², so doubling the wall roughly quadruples the time. The slab equation: t ≈ (s² ÷ π²·α) × ln[(Tmelt − Tmold) ÷ (Teject − Tmold)], where α is thermal diffusivity (~0.07–0.15 mm²/s). Wall and diffusivity are the strong levers; temperature terms sit inside a logarithm and move time weakly.
See also related article.
Key facts
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Process parameter — A controllable variable in the injection molding process: melt temperature, injection speed, holding pressure, cooling time, mold temperature. Each parameter affects part quality — deviations cause defects. Parameters are set during T1 and locked for production. See also related guide.
What drives cooling time in the slab equation?
The slab equation models a flat plate cooling by conduction to temperature-held surfaces. Four inputs determine the result. Two dominate; two are secondary.
| Term | Meaning | Typical Value | Effect on Time |
|---|---|---|---|
| Wall thickness (squared) | Distance heat must travel from center to steel | 0.8–6 mm | Time ∝ wall² — the biggest lever at design |
| Thermal diffusivity α | How fast the resin transmits heat internally | PP/ABS/PC ~0.08–0.11 mm²/s; filled grades higher | Time ∝ 1/α — filled grades cool faster |
| Tmelt − Tmold | Starting thermal gradient driving heat out | Melt 200–350 °C, mold 20–120 °C | Inside ln — weak (logarithmic) effect |
| Teject − Tmold | How cool the core must get before ejection | Set near resin HDT | Inside ln — lower target = more time |
Cooling time (injection molding). The portion of the cycle during which the part solidifies in the mold before ejection. It is estimated from the slab conduction equation and scales with the square of wall thickness divided by the resin’s thermal diffusivity. Cooling time is normally 50–80 % of total cycle and is the phase most sensitive to part design decisions made at DFM.
How do you go from the formula to a validated cycle?
Four steps. The equation gives the shape of the answer — which wall to thin, which resin to consider. The controller value comes from stepping down against actual ejection quality.
- Establish the four inputs. Take the thickest functional wall that governs ejection (not the average — the part ejects when its hottest feature is rigid enough). Look up the grade’s thermal diffusivity or compute it from the datasheet. Set melt and mold temperature at the values you will actually run — the surface temperature the tool holds at steady state, not the coolant setpoint. Set the ejection target from the resin’s HDT.
- Compute the theoretical floor — and treat it as a floor, not the cycle. For a 2.0 mm PP wall (α ≈ 0.10 mm²/s, melt 230 °C, mold 40 °C, eject ~80 °C) the equation gives roughly 4–6 s as the pure conduction floor. The real cycle is longer: mold interface resists heat flow, fill and pack occupy time, and the part must be rigid everywhere, not just at one centerline. Practical cooling is commonly 1.5–3× the floor (10–18 s for this part in production).
- Shorten the floor at design and mold level before touching process. The two highest-value moves: reduce the governing wall (thin to minimum that holds function, core out bosses) and raise effective heat extraction (bring lines closer to hot features, use baffles or bubblers on cores, consider conformal cooling or BeCu inserts at hot spots). Each lowers the computed time at the source — worth more than any process-parameter squeeze.
- Validate against ejection quality and lock the number. Step cooling time down in small increments while watching for sink, warp, ejector-pin dimples and short shots. The last cooling time before defects appear is your real minimum with margin. Confirm with a stability run (part weight and a key dimension over several hundred shots) and document the locked cycle on the process sheet.
“The cooling equation tells you the shape of the problem — wall thickness dominates, filled resins cool faster, and the temperature terms are secondary. The cycle is largely locked the moment the wall thickness map is frozen. That is why cooling review belongs at DFM.”
— Process Engineer, aCatMould
Frequently asked questions
Why is my actual cooling time much longer than the equation predicts?
The slab equation models one idealization — conduction to a perfectly held surface — and real molding adds resistance on top.
The part-steel interface has contact resistance; the mold surface rises above coolant setpoint under load; the part must be rigid across its whole geometry, not just at one centerline; and fill and pack consume seconds not in the formula.
The equation’s job is to rank options — that wall dominates, that filled resins cool faster. The controller value comes from stepping down against ejection quality, typically landing 1.5–3× above the theoretical floor.
Does a hotter mold ever shorten the cycle?
Rarely for total cycle. Raising mold temperature reduces the gradient and increases conduction time. The indirect cases: a hotter mold improves surface replication so you can thin the wall or fill more easily and end up net faster; for semi-crystalline resins, a warmer mold lets crystallization complete at higher temperature so the part reaches ejectable stiffness sooner.
If you are fighting a long cooling time, the productive answer is a thinner wall, better cooling at the surface, or a higher-conductivity insert — not adjusting mold temperature on the theory that colder is always slower.
How much does glass or mineral filler cut cooling time?
Enough to matter but not to overturn wall thickness. Fillers raise thermal diffusivity, so a 30 % glass-filled grade cools roughly 10–25 % faster than the neat resin of the same wall. Filled parts also eject at a higher temperature (stiffer at higher temp), which shortens the hold further.
But this is a multiplier on the same wall² term — a 30 % glass part with a 4 mm wall still pays the 4× penalty versus 2 mm. Filler helps; pairing a faster-cooling grade with a properly thinned wall and good cooling is how cycles come down. If your program needs a cooling analysis at DFM, tell us before the deposit.
Key takeaways
- Cooling time scales with wall² ÷ α. Doubling the wall roughly quadruples the cooling time — wall thickness is the dominant lever.
- The slab equation gives a theoretical floor.
The real cycle is 1.5–3× the floor because of interface resistance and the requirement that the whole part be rigid at ejection. - Shorten the floor at design (thinner wall) and mold level (better cooling) before touching process. Validate by stepping down against ejection quality.
Cooling time is not a process parameter to be squeezed — it is a design consequence. The wall thickness map, the resin diffusivity and the ejection criterion lock the floor before the first shot.
At aCatMould we run the cooling calculation at DFM, thin the governing wall with you, and hand you a process sheet where the cycle is grounded in the physics of the part, not copied from a similar job.
Sources
Plastics Technology — cooling time, cycle optimization and thermal analysis
Society of Plastics Engineers — injection molding cooling theory and practice
Plastics Industry Association — cycle time reduction guides
MatWeb — resin thermal diffusivity and property database
Autodesk Moldflow — cooling analysis and cycle time simulation
Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet