Cooling is the longest phase of an injection moulding cycle - typically 60 to 80% of the total - and it is the phase a buyer can most influence before tooling is cut. The plastic mold cooling time calculation behind that number is simple enough to run on a spreadsheet, and understanding it explains why a 0.5 mm wall change moves part cost more than any process tweak. This guide covers the cooling time formula, the inputs it needs, benchmark values for common resins and the questions worth asking a moulder.
The plastic mold cooling time calculation
The standard estimate treats the part wall as a flat plate cooling by conduction into the mould: tc = (s² / (π² × α)) × ln((4/π) × (Tm − Tw) / (Te − Tw)). Here s is wall thickness in millimetres, α is the thermal diffusivity of the resin in mm²/s - thermal conductivity divided by density and specific heat - Tm is melt temperature, Tw is mould wall temperature and Te is ejection temperature. Worked example: an ABS housing with a 2.5 mm wall, α = 0.09 mm²/s, Tm = 230°C, Tw = 50°C and Te = 90°C gives about 9 seconds. The same part at 3.0 mm wall takes roughly 13 seconds - a 0.5 mm change costing about four seconds per shot.
Why wall thickness dominates cycle time
Cooling time scales with the square of wall thickness, not linearly: doubling the wall roughly quadruples the time. A part with 1.5 mm walls and one 4 mm boss is governed by the boss, so uniform walls matter more than a thin average. Semi-crystalline resins such as PP and PA6 also release latent heat as they crystallise, which lengthens cooling and calls for higher mould temperatures to reach proper crystallinity. A typical cycle decomposes into mould close (1-2 s), fill (0.3-1.5 s), pack and hold (1-5 s), cooling (60-80%) and open plus eject (1-3 s).
| Resin | Wall (mm) | Mould temp | Approx. cooling time | Thermal diffusivity (mm²/s) |
|---|---|---|---|---|
| ABS | 2.0 | 50°C | 8-10 s | 0.09-0.12 |
| ABS | 3.0 | 50°C | 18-22 s | 0.09-0.12 |
| PP | 2.0 | 40°C | 10-13 s | 0.08-0.09 |
| PP | 3.0 | 40°C | 22-28 s | 0.08-0.09 |
| PA6 | 2.0 | 80°C | 7-9 s | 0.12-0.13 |
| PA6 | 3.0 | 80°C | 16-20 s | 0.12-0.13 |
| PC | 2.0 | 90°C | 12-15 s | 0.10-0.11 |
| PC | 3.0 | 90°C | 26-32 s | 0.10-0.11 |
What the formula leaves out
The closed-form estimate assumes a flat, uniform wall at constant mould temperature. Real parts carry ribs, bosses and thickness transitions; runners add heat that must also leave the steel; and cooling-circuit layout sets how fast heat escapes. Mould temperature cuts both ways: lower temperatures shorten cooling but risk flow marks, poor crystallinity in PA6 below about 60°C, and residual stress that surfaces later as warpage - covered in our part warpage guide.
Levers that actually shorten cooling
- Design: uniform walls, ribs at 50-70% of nominal wall thickness, cored-out thick sections, and internal radii of at least half the wall thickness.
- Tooling: optimised channel placement, bubbler and baffle designs in deep cores, and conformal cooling that follows the part contour - typically 20-40% faster on thick or complex parts.
- Process: hold time set to gate freeze-off rather than habit, and mould temperature as low as the resin's quality window allows.
What to confirm in an RFQ
Ask the moulder to quote cycle time with the cooling design stated: channel diameters and positions, the number of temperature-control zones, and whether the quote assumes a standard drilled layout or conformal channels. A quoted cycle that beats the benchmarks by a wide margin usually means a thinner wall, higher regrind or a hot eject - our cycle time reduction guide and injection molding process guide cover the trade-offs. Custom moulding programmes are listed under custom molding.
