Cooling and Cycle Time in Vacuum Forming

Where the cycle time actually goes

People new to thermoforming assume the forming step is the slow part. It is not. Forming takes two to ten seconds. Cooling takes two to four times as long, and it is the only part of the cycle that cannot be shortened by doing it faster.

Table 1 — Cycle breakdown for thin gauge. On heavy gauge the cooling share climbs towards 70%: heat enters the sheet quickly but has to leave it through the thickness.

Cooling is the constraint because the part has to be stiff enough to demould without distorting. That is a material property, and the time to reach it is set by heat transfer — which follows a relationship you can calculate.

The relationship that governs everything

Cooling time in thermoforming follows the same square law as injection moulding: it scales with the square of the thickness. Double the sheet thickness and the cooling time roughly quadruples.

For a sheet cooled from both faces, the time to reach a given average temperature is approximately:

t ≈ (s² / (π² · α)) · ln( (8/π²) · (T_sheet − T_mould) / (T_demould − T_mould) )

Where s is thickness, α is thermal diffusivity (about 0.10 mm²/s for most plastics), and the temperatures are sheet, mould and demould.

Table 2 — Cooling time by sheet thickness. The gap between theory and practice is the tool interface: air gaps, contact quality and mould temperature control.

The gap between the theoretical and realistic numbers is where the money is. A 2 mm part that calculates at 3.6 s and runs at 25 s has 20 seconds of avoidable time in it — usually from poor contact between sheet and mould, or from a mould that is running too hot.

The four levers you actually have

Table 3 — The levers. Air assist is the biggest single gain available on an existing tool, and the cheapest to retrofit.

Air assist works by blowing ambient or chilled air onto the exposed face of the part while it is still on the tool. It roughly doubles the heat transfer coefficient on that face, which cuts cooling time by 25–40%. A simple manifold of nozzles is enough; it does not need to be a precision system.

The limit is distortion. Blowing air on a part that is still soft will push it out of shape, particularly on thin walls. The usual approach is to delay the air until the part has set enough to hold its shape, then blow hard.

Mould temperature: the quality constraint

Table 4 — Mould temperature trade-off. Running a cold mould to save cycle time costs surface quality and dimensional stability, which usually costs more downstream.

Cold moulds are the most common false economy in thermoforming. A mould at 25 °C will cut 30% off the cooling time and produce parts with chill marks, high residual stress and a surface that will not hold a texture. The customer sees the surface; nobody sees the cycle time.

Heavy gauge: where the cycle becomes a different problem

Above about 4 mm the cooling time dominates so completely that the process economics change. A 6 mm part might have a 90-second cooling time against a 30-second heating time, which means the machine is idle for two-thirds of the cycle unless it is designed for it.

Table 5 — Handling long cooling times on heavy gauge. The carousel and the cooling fixture are the two that actually work; the rest just accept the loss.

The cooling fixture is the cheapest of these. The part comes off the tool while still warm and slightly flexible, is transferred to a shaped jig that holds it while it cools, and the tool is free for the next cycle. The jig costs a fraction of a second tool and it can cut effective cycle time by 40%.

Measuring and improving

Table 6 — What to measure to improve the cycle. The water ΔT is the fastest diagnostic: a high ΔT means the mould is not transferring heat as designed.

Part temperature at demould is the number that tells you whether the cooling time is right. If the part comes off at 70 °C when the resin needs to be below 60 °C, it will distort after demould and the cycle is being pushed too hard. If it comes off at 45 °C, there is cooling time to give back.