PET Preform Cycle Time Optimisation

PET preform cycle time is the first number a project is judged on and the last one that is properly understood. A quotation states a cycle, the plant measures a different cycle, and the gap is then argued about rather than diagnosed. In most cases nothing is broken — the two numbers simply describe different things, because a cycle quoted for a mould is not the same as a cycle achieved on a warm machine running a full shift.

Chasing the number the wrong way is what costs money. Cutting cooling time to hit a target produces preforms that look acceptable and warp in the blow moulder. Raising melt temperature to shorten plasticising produces acetaldehyde issues in still water bottles. Reducing hold pressure to eject earlier produces short shots and sink marks that only appear after the preform has cooled. Each of those buys a fraction of a second and pays for it in rejected bottles downstream, where the value added is far higher than the preform itself.

Sailwin builds the SW-P series of PET preform injection molding machines — 14 models from 170 kN to 5500 kN — with dedicated PET screws, far-infrared nano heating coils, hot runners supporting up to 64 valve-gate cavities and EUROMAP 67 robot interfaces, manufactured under ISO 9001:2015 with CE marking. This guide breaks the cycle into its real segments, identifies which one usually constrains it, and lists the changes that shorten the cycle without moving part quality.

1. Breaking the Cycle Into Its Real Segments

A PET preform cycle contains six segments that matter. Injection fills the cavity. Hold and packing compensate for shrinkage while the gate is still open. Cooling brings the part down to a temperature at which it can be ejected without deformation. Plasticising prepares the next shot. Mould open and close moves the platens. Take-out removes the parts and, in a well-designed cell, happens inside the mould-open window rather than after it.

The practical point is that several of these overlap. Plasticising normally runs during cooling. Take-out can run during mould movement. If those overlaps are not being used, the machine is holding still while it could be working, and no amount of tinkering with temperature or pressure will recover that time because the loss is in the sequence, not in the process.

Relative weight describes which segment usually limits a preform cycle. It should be measured on your own tool, resin and mould temperature set-up, not assumed from a table.

Before changing a process setting, check whether the time you are trying to remove is actually spent on the process. Sequence losses are free to fix; process losses are not.

2. Where the Constraint Usually Sits

In most preform tools the constraint is cooling, and cooling is a function of wall thickness and heat removal. Wall thickness is fixed by the preform design and the blow molding stretch ratio it has to serve, so it is rarely available as a lever on the injection side. That leaves the cooling circuit and the mould temperature control as the places where real time can be recovered.

Mould temperature control is where the discipline matters. Sailwin machines use PID control holding approximately ±1 °C, and that stability is what allows a cooling time to be set close to the true minimum rather than padded with a safety margin against drift. A mould that swings several degrees cannot be run at the minimum, because the cycle that works on a warm afternoon fails on a cold night shift.

• If plasticising runs longer than cooling, the screw is the constraint. A dedicated PET screw and an appropriate barrel temperature profile address that; simply speeding the screw up trades cycle time for unmelted or overheated resin.
• If cavity-to-cavity weight varies, the hot runner is the constraint. Up to 64 valve-gate cavities can be balanced, but balance has to be verified by weighing parts from each cavity rather than assumed from the drawing.
• If the cycle is regular but slower than the quotation, check the dry cycle first. Mould open and close is a fixed cost per cycle and is usually quoted from the machine specification rather than measured on the installed cell.
• If the cycle varies between shifts, look at the utilities. Chilled water temperature, flow rate and pressure stability decide whether the minimum cooling time is repeatable. The PLC monitors 40+ parameters in real time, which turns that question into a data check rather than an argument.

3. Changes That Shorten the Cycle Without Touching Part Quality

Work through this list before touching a process window. Everything here recovers time that is currently being lost rather than time that is currently protecting the part, so none of it carries a quality risk.

1. Confirm plasticising is fully overlapped with cooling. If the screw finishes before the mould is ready, part of that recovery time is wasted; if it finishes after, it is adding directly to the cycle.
2. Move take-out inside the mould-open window. With the EUROMAP 67 robot interface the take-out sequence runs on the machine’s own timing rather than waiting for a signal, which removes a handshake delay from every cycle.
3. Stop holding after gate seal. Holding pressure beyond the point at which the gate freezes adds time without adding material to the part. Establishing gate seal by part weight is a one-off experiment that pays back permanently.
4. Verify hot runner balance across all cavities. An unbalanced 64-cavity hot runner forces the cycle to be set by the slowest cavity, so every good cavity is running slower than it needs to.
5. Stabilise mould temperature instead of padding cooling time. PID control at approximately ±1 °C allows a cooling time close to the true minimum, and the time saved is repeatable rather than a lucky shift.
6. Stop the cycle for mould changes, not the shift. Sailwin mould changes are designed to complete in under 30 minutes, and the recipe can be recalled from the PLC rather than re-established by trial.

Servo-driven machines add a further gain: power consumption can be up to 30% lower than on fixed-speed hydraulic equivalents, because the pump only delivers what the current segment requires. That is an operating cost improvement rather than a cycle time one, but it is the reason a comparison based on cycle time alone is incomplete.

4. Case Study: A High-Cavity Preform Tool Running Above Its Quoted Cycle

A preform producer had installed a high-cavity tool that consistently ran slower than the quoted cycle, and the first response had been to shorten cooling time and raise melt temperature.

Scenario based on a Sailwin customer project; site-specific figures available on request during engineering review.

Frequently Asked Questions