Preform Cooling Time: Calculation and Practical Limits

On a PET preform cell, cooling is the largest single block of the cycle. Injection and plastification can be overlapped; mould open, take-out and mould close are measured in fractions of a second. Cooling is the one item that cannot be hidden behind another action, and it is the item that most factories adjust by feel — add a second here, take one away there — rather than by calculation.

The cost of guessing is asymmetric. Set cooling too short and you eject a preform that is still soft in the middle, which distorts, sticks, or arrives at the blow molding machine with an inconsistent wall. Set it too long and you pay for it on every cycle, on every cavity, for the life of the mould. On a 48-cavity mould running continuously, an extra half second of cooling per cycle is a permanent tax on output.

Sailwin builds the SW-P range of PET preform injection moulding machines — 14 models from 170 kN to 5,500 kN, with dedicated PET screws, far-infrared nano heating coils and support for valve-gate hot runners up to 64 cavities. This article gives the classical cooling time estimate, explains where it stops being reliable, and sets out the changes that shorten cooling time without trading away preform quality.

1. Why Cooling Dominates the Preform Cycle

A preform cycle can be divided into actions that cost time and actions that can run in parallel. Injection takes a defined number of seconds. Screw recovery can overlap with cooling, provided the machine has enough plasticising capacity and a dedicated PET screw. Mould open, take-out, and mould close are mechanical and fast. Cooling is the remaining block, and on most water preform programmes it is longer than everything else combined.

This is why preform cycle optimisation is mostly a cooling project. It is also why cavity count and cooling are inseparable decisions: adding cavities increases output per cycle but simultaneously increases the thermal load on the mould, and if the cooling circuits cannot remove that heat the extra cavities produce softer parts at the same cycle time rather than more good parts.

2. The Classical Cooling Time Estimate

The standard industry equation for cooling a flat section in a mould is:

Two things follow from the structure of the equation, and they are the two things worth remembering.

Wall thickness is squared. Thickness dominates everything else. This is why preform lightweighting projects cut cycle time as a side effect, and why a heavier preform for a carbonated application takes longer to cool even when the tooling is identical.

Temperature enters only logarithmically. Making the mould colder, or lowering the melt temperature, gives diminishing returns. Halving the temperature difference between the part and the mould does not halve the cooling time — it reduces it by a fraction of that. Flow and circuit design, which affect how effectively the mould wall is held at its target temperature, are the stronger levers.

The index values above follow directly from the squared thickness term; the logarithmic temperature term adds a smaller secondary effect. Use them to compare candidate preform designs, and substitute the actual resin diffusivity, melt temperature and ejection temperature into the full equation when you need a real number.

3. Where the Estimate Stops Being Reliable

The classical equation assumes a flat plate of uniform thickness, cooling equally from both faces, with no heat entering the part from anywhere else. Four real-world conditions break those assumptions, and each one usually adds time rather than removing it.

• The neck finish is much thicker than the wall. A preform neck can be several times the body wall thickness. It cools last, and because it must hold a precise thread and sealing surface, it also has the tightest dimensional requirement. The cycle follows the neck, not the average wall.
• The gate region is continuously reheated. A hot runner keeps hot melt in contact with the part until the gate freezes. The cooling estimate assumes the part is isolated from the melt the moment injection ends, which is not true at the gate.
• Ejection is a deformation limit, not just a temperature. The calculation asks for the average temperature at which the part can be ejected, but a preform is pushed off a long core. What matters is whether the surface layer is stiff enough to resist the ejection force without marking or ovality. That is a mechanical criterion as much as a thermal one.
• PET requires controlled crystallinity. Cooling too quickly can produce a preform with an unfavourable thermal history that shows up in the blow molding stage as uneven stretching. Faster is not automatically better.

If your calculated cooling time and your practical cooling time differ by more than about 20%, stop adjusting the cycle and look at the mould. A gap that size almost always means the mould is not at the temperature the calculation assumed — unbalanced circuits, a restricted feed, scaled channels or a controller with poor stability. Fix the cause before you spend the difference on extra cycle time every day.

4. Shortening Cooling Time Without Losing Quality

There are five practical levers, in the order they usually pay back.

5. Sailwin Case Study: One Slow Cavity Setting the Cycle

Scenario based on a Sailwin customer project; final configuration is confirmed against your preform drawing during engineering review.

6. Machine Features That Support Short, Stable Cycles

• PID temperature control to ±1°C on mould and barrel zones, which keeps the cooling calculation’s assumptions true in production.
• Dedicated PET screw with far-infrared nano heating coils, giving even barrel heating and a repeatable melt temperature rather than the hot spots that push operators to compensate with extra cooling.
• Valve-gate hot runners up to 64 cavities, with the balance that high-cavity preform production depends on. A well-balanced manifold lets cooling time be set by the preform rather than by the slowest cavity.
• Servo drive with energy savings up to 30% compared with a fixed-displacement hydraulic system, and better repeatability on the injection and clamp profiles that precede cooling.
• Euromap 67 robot interface as standard, so take-out can be automated and ejection does not become the limit on how short the cycle can be.
• PLC monitoring of 40+ parameters in real time, which turns a cooling drift into a trend rather than a quality escape.
• Low-pressure mould protection, which guards high-cavity preform moulds against the damage caused by incomplete part removal — the most expensive routine failure in preform production.

The SW-P range spans 170 kN to 5,500 kN across 14 models, with deliveries on a 30–45 day lead time (45–60 days for custom builds), full-load FAT testing before shipment, 3–7 days on-site installation and commissioning, common wear parts shipped within 48 hours, 7×24 remote support and a 2-year whole-machine warranty. Sailwin also supplies matched PET blow molding machines, so the preform and the bottle are engineered as one production system rather than as two separately sourced purchases.

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