PET preform mould steel and cooling: what sets the cycle time
Cavity count and price are easy to compare. Cycle time is not, unless the quotation says which preform, which machine and which water produced it. The seconds come from the wall thickness, the cooling layout, the steel around that water, the hot runner and the press. This is how to read a cycle, and how to specify steel and cooling so the number still means something in your factory.
A cycle time is a set of conditions
Twelve seconds is not a catalogue speed. It is a target for one preform on one machine. Change the weight, the wall, the neck or the water temperature and the same mould takes a different time. A cycle with none of those conditions cannot be accepted, because the trial has nothing to match.
The two projects published here state the conditions beside the target. Use the table as an example of a complete line. Do not treat either number as the cycle for a different bottle.
| Item | 64-cavity case | 72-cavity case |
|---|---|---|
| Neck finish | 29/25 | 30/25 |
| Preform weight | 14 g, tolerance ±0.2 g | 14 g |
| Injection machine | Yizumi 320T | Yizumi 480T |
| Hot runner | Needle-valve | Needle-valve |
| Target cycle | 12 seconds | 15 seconds |
| Mold base steel | 4Cr13H | 4Cr13H |
| Core and cavity steel | S636 | S636 |
| Application | Mineral water | Mineral water |
The 72-cavity mould is not slower steel. It is a different neck, a different press and a different target, even though both preforms are 14 g mineral-water parts. Output arithmetic from cycle and cavity count is in the cavity guide. This page stays with the steel and the water.
Work out cavity count from output and cycle
Where the seconds go
A preform cycle is fill, hold, cooling, open, ejection and close. Cooling is the long part. The preform has to leave the core stiff, clear at the gate and stable at the neck. Opening early looks faster on a screen and comes back as oval necks, bent bodies and cloudy bases.
Fill and hold
Injection fills the cavity. Holding packs the base while the gate is still open. Close a valve pin too early, or let an open gate freeze too soon, and the base sinks or the weight drifts. Holding after the gate has already sealed only adds time. Those settings are tuned at the trial. They are not a reason to pick a steel grade.
Three cooling jobs
Heat leaves through the core, the cavity and the neck rings into water. In a sound stack the channels run close to the moulding surface and a technician can flush them. The cycle stretches when the channels sit far from the surface, or when too many cavities share one weak circuit and the last cavity gets warm water.
- The core cools the inside. A hot core tip leaves the thickest PET soft, and that heat sets the cycle.
- The cavity cools the body. The first cavity on a circuit should not be much colder than the last.
- The neck rings cool the finish. A hot neck goes out of round, and the cap line shows it even when the body looks fine.
The gate needs its own look. Too little cooling, or too much heat from the nozzle, and the base crystallises white or leaves a string. Cooling and hot-runner temperature are set together. The gate guide separates a cooling fault from a nozzle fault from wet resin.
Name the steel on each component
A preform mould is not one piece of steel. Plates, cores, cavities, neck rings and gate inserts do different work. "Imported steel", or one grade for the whole tool, is not a specification. Ask for a line against each component, and for the document you can check when the steel arrives.
Mold base
The plates locate the stacks and carry water for years. Preform tools run cold, so the plates sweat in a humid shop. That is why stainless plate steels are used. Both published cases record the mold base as 4Cr13H. That is the grade on those projects, not a hardness number and not a promise that every future mould copies it. If you need hardness, heat treatment or a mill certificate, put that in the order.
Cold plates sweat in a humid shop, which is common for buyers in Southeast Asia and India. Moulds from this Taizhou factory already run in Thailand, Vietnam, Indonesia, Malaysia and India, among other countries. Specify the steel for your shop anyway. A country name is not a grade.
Cores, cavities, neck rings and gate inserts
Cores and cavities touch the preform, so they need a polish a clear part will not show as pits, and they sit against hot PET and against water in the channels. Both published cases record S636. Do not treat that name as another brand unless a certificate says so. Ask for the grade and the supply condition, separately for cores and for cavities.
Neck rings form the thread and open every cycle. Gate inserts see the hottest PET and, on a valve-gate mould, the pin seat. Name them on the steel list, and hold them as spares, even when the steel family matches the cavities. A finish change such as PCO 1881 to PCO 1810, or 29/25 to 30/25, also means new rings and a fresh check of the water in those rings.
PCO 1881, PCO 1810 and the lighter water finishes
We do not publish a shot life for a grade. Life depends on the resin, colourants, alignment, water and maintenance. A million-shot claim with none of those conditions is advertising, and it will not appear in our quotation.
What to ask about the cooling
You do not need to redraw the circuits. You do need to see that they exist as something you can pipe, flush and check.
A diagram before machining
Ask which circuits feed the cores, the cavities and the neck rings, and where the inlets sit. That sheet should travel with the assembly drawing. The acceptance checklist already names the cooling diagram as a shipment document. Ask for it at design review, not beside the crate.
Shared circuits and wall thickness
On a 48-, 64- or 72-cavity mould, one series circuit through too many cavities arrives warm at the far end. Parallel groups keep the shot closer. Weigh every cavity and measure the wall. A heavy corner and a light corner are a balance problem in the water, the hot runner, or both.
A lighter preform is not automatically faster. A thin body can hide a thick base or a heavy neck, and those sections decide when the part is stiff enough to eject. Send the wall, not only the gram weight. We will not invent a wall in order to print a shorter cycle.
Water quality and take-out cooling
Scale insulates a channel and cuts the flow, so a mould that met its target at trial can miss it later on hard water. Say whether the water is treated. Some lines also cool preforms in take-out tubes after the mould opens. We do not assume a cooling robot. If you have one, include it in the trial condition. If you do not, the mould has to release a stable preform by itself.
The mould still has to fit the press
Tie-bar spacing, mould height, opening stroke, shot weight and the water connections are the limits. A cooling layout that wants more flow than the machine can give will miss the quoted cycle. The cavity calculator checks cavity count against platen geometry for neck finishes we have drawings for. It does not estimate cycle time.
What to send so the quoted cycle is real
OSLON Precision Mold designs and machines these tools in Huangyan, Taizhou. The mould is trialled before shipment against what the order actually says. These are the items that make a cycle definable.
- Preform drawing with weight, wall thickness and neck finish. A sample helps where the drawing is incomplete.
- Resin grade, if chosen, and any colour or additive.
- Machine brand and model, or tie-bar distance, shot weight and the cooling connections you can offer.
- Whether a take-out robot cools the preforms.
- The water in plain language: treated chilled water, tower water, or a known scale problem.
- The output you need, so the cavity count can move if the cycle cannot.
- Any grade, certificate or hardness check your incoming inspection already uses.
Send the package to Frank Deng on the inquiry form. The reply can then name a cavity count, a steel list by component and a cycle target with its conditions. Missing facts stay as questions. They are not replaced with a typical value and called confirmed.
Questions buyers ask
Why is the 72-cavity cycle longer than the 64-cavity cycle?
They are different projects. The 64-cavity record is a 14 g, 29/25 preform on a Yizumi 320T with a 12-second target. The 72-cavity record is a 14 g, 30/25 preform on a Yizumi 480T with a 15-second target. Both use a needle-valve hot runner, 4Cr13H bases and S636 cores and cavities. Those three seconds are not a rule for the next mould.
Should I insist on one famous steel brand?
Insist on a named grade for each component, and on a document you can check against the steel you receive. The published cases use 4Cr13H and S636 because those moulds did. A brand name with no component list is not a specification.
Does better steel automatically make a faster cycle?
No. The cycle is mostly how fast heat leaves the preform. A respected grade with channels far from the surface will lose to the right grade with channels in the right place. Ask for the cooling diagram as well as the steel name.
What cooling fault shows up as a white gate?
PET that stays hot at the gate crystallises and looks milky. Check flow at the gate insert, nozzle temperature, and whether cooling time matches the thickness at the base. If only some cavities are white, compare those circuits and nozzles with the clear ones.
Can the cycle be guaranteed on my machine?
A target can be agreed for a defined preform, machine and trial, and the mould can be accepted against it before shipment. Your water, drying, take-out equipment and shop have to be inside that definition if you expect the same number on your floor. A cycle with no conditions is not a figure we will sign.