A PET injection moulding machine, usually called a PET preform moulding machine, is an injection moulding machine built to turn dried PET resin into preforms: the thick, test-tube shaped parts that are later reheated and stretch blow moulded into bottles and jars. It looks like any horizontal machine from the outside. The differences sit in the screw, the injection speed, the hot runner and valve-gate controls, the cooling, and the dryer and post-mould cooling around it. Those changes exist because PET picks up moisture, breaks down when it stays hot too long, and turns hazy if it cools slowly.
Key Takeaways
- A PET preform moulding machine makes preforms, not bottles. The neck is moulded to its final size; the body is stretched later in a blow moulding machine.
- PET is hygroscopic. Moisture and long melt time cut the viscosity (IV) of the resin and raise acetaldehyde, and neither defect can be seen on the preform.
- A preform cell is a system: dryer, machine, hot runner mould, chiller and post-mould cooling. A weak link in any one shows up as preform defects.
- Size the cell from your target output and preform weight first. The plasticising rate in kg/h follows from cavities, cycle time and shot weight.
What does a PET preform moulding machine actually make?
It makes preforms. Each preform has a fully formed neck finish with threads and a support ledge, plus a short, thick body. The neck never changes size again. The body is reheated and stretched, lengthwise and outward, into the bottle in a separate blow moulding step.
This split is called two-stage production. The preform plant and the blowing plant can sit in different cities, so preforms can be moulded in one place and shipped to fillers near their market. A preform is far denser to transport than an empty bottle.
Because the neck is final, its dimensions decide whether the cap seals. Neck inserts and cores need full cooling water flow, and hold pressure must not be too low, or the neck goes out of round. A neck whose inside diameter varies by more than 0.25 mm around the circumference counts as a fault.
The body, in turn, decides how the bottle blows. Wall thickness, weight and clarity have to repeat cavity to cavity and shot to shot, or the blow moulder sees uneven bottles that it cannot correct with its own settings.
Why can PET not be moulded like PP or ABS?
PET absorbs moisture from air, and wet PET loses molecular weight in the barrel. It also forms acetaldehyde whenever it is held hot, and it must cool fast to stay clear. A machine for PP or ABS is not built around these three limits, so results suffer.
The first limit is viscosity loss. Water in the melt breaks the polymer chains, and the drop in intrinsic viscosity (IV) from resin to preform is the measure. A drop above 0.04 dl/g is treated as a fault, and it needs a lab IV tester to find. Drying is covered step by step in the preform process article in this series.
Acetaldehyde (AA) is the second limit. It is a by-product of heating PET, and it rises linearly with the time the melt spends in the barrel and exponentially with melt temperature. Still water carries its taste most clearly; the sugar in carbonated drinks and juices masks it. Screw and barrel wear raise shear and AA, so some preform makers track AA to spot wear early.
AA is also why an oversized injection unit hurts. A lightweight preform on a large screw sits in the barrel for many cycles, and the extra residence time shows up as AA. Match the extruder to the preform weight, not to the biggest job you might run some day.
The third limit is clarity. PET cooled slowly from the melt starts to crystallise and turns white or hazy. Mould cooling water at 7 to 10 °C, a mould closed time of about 1 to 3 s set by wall thickness, and further cooling after ejection keep the preform amorphous and clear.
Which parts make up a complete PET preform moulding machine cell?
A working cell has five parts: a desiccant dryer, the injection moulding machine, a hot runner mould with valve gates, a chiller for 7 to 10 °C water, and post-mould cooling, often a take-out robot with cooled tubes. Output and preform quality depend on all five together.
Parts of a PET preform moulding cell and what to check
| Part | Job in the cell | What to check |
|---|---|---|
| Desiccant dryer | Brings resin moisture down before the throat | Dew point, hopper size against kg/h, airflow |
| Injection unit | Melts and injects PET | Plasticising rate in kg/h, shot size against preform weight |
| Hot runner mould | Feeds each cavity through a valve gate | Heater zones, valve-gate air at 8 to 10 bar |
| Chiller and water circuit | Cools cavities, cores and neck inserts | Water at 7 to 10 °C, flow at the mould maker rating |
| Post-mould cooling | Keeps cooling preforms after ejection | Cooling time available without stretching the cycle |
Post-mould cooling matters more than buyers expect. Preforms ejected with a surface temperature above 60 °C mark each other during handling, leaving scratches and smudges best seen under polarised light. Cooling preforms in robot tubes lets the mould open sooner while the preform finishes cooling outside it.
Our view: in a preform cell, the dryer decides preform quality more often than the machine does. If you are chasing haze or IV loss at the barrel heaters, check dew point and hopper residence time first.
What should you work out before asking for a PET machine quote?
Start with three numbers: preforms per hour, preform weight and neck size. From these, cavities, cycle time and plasticising rate follow, and the clamp and shot size follow from those. Send the preform drawing or a sample if you have one. See the PET injection moulding machine range for the next step.
Frequently Asked Questions About PET Preform Moulding Machines
Is a PET preform moulding machine the same as a blow moulding machine?
No. The injection moulding machine makes the preform, with its neck finished to final size. A separate stretch blow moulding machine reheats the preform body and blows it into the bottle. In two-stage production the two steps can run in different plants, with preforms shipped between them.
What is intrinsic viscosity (IV) in PET?
IV is a lab measure of PET molecular weight, given in dl/g. Bottle-grade resins are supplied at a stated IV. Moisture and heat during moulding lower it. A drop of more than 0.04 dl/g from resin to preform is treated as a fault and needs an IV tester or melt viscometer to detect.
Why does acetaldehyde matter in water bottles?
Acetaldehyde forms when PET is heated, and it can pass a taste to the contents. Still water shows it most because nothing masks it, while sugar in carbonated drinks and juices hides small amounts. Long barrel residence and high melt temperature both raise acetaldehyde in the preform.
Why do PET preforms turn white or hazy?
Haze usually means the PET started to crystallise because it cooled too slowly, or that unmelted pellets passed through. Cold mould water at 7 to 10 °C with full flow, enough cooling time for the wall thickness, and correct drying and melt temperature keep preforms clear.
Does a preform cell need a robot?
Not always, but post-mould cooling helps. Preforms ejected above about 60 °C surface temperature can scratch and smudge each other. A take-out robot with cooled tubes lets the mould open earlier while preforms keep cooling, which can shorten the cycle without raising handling marks.
Related Articles
- How Is a PET Preform Made?
- PET Injection Moulding Machine vs Standard Horizontal Machine
- Important Specifications to Consider Before Buying a PET Injection Moulding Machine
- Applications of PET Injection Moulding Machines
Sources & References
- PETplanet Insider, O. Brandau: Bottles, Preforms and Closures extract, 3.6 Acetaldehyde in PET Bottles (03/2015)
- PETplanet Insider, R. Unterlander and O. Brandau: The Rapid Guide to Perfect Preforms, 16. IV Loss (11/2010)
- PETplanet Insider, R. Unterlander and O. Brandau: The Rapid Guide to Perfect Preforms, part 29: Surface Blemishes (05/2012)
- PETplanet Insider, R. Unterlander and O. Brandau: The Rapid Guide to Perfect Preforms, part 20: Neck Finish out of Round (05/2011)
Technical Disclaimer
Disclaimer: The information provided in this document is for general informational and educational purposes only. It is based on technical references and industry information available at the time of preparation and may be subject to change. It does not constitute engineering, technical, commercial or professional advice, nor does it constitute a quotation, specification, warranty or contractual commitment by Pratishna Engineers.
Actual injection moulding machine and process requirements vary according to the component design, material, mould, production volume, machine configuration and operating conditions. The information, examples, calculations and recommendations provided should be independently evaluated and verified for the specific application before making any equipment, process or production decision. Information obtained from third-party sources remains subject to the accuracy and limitations of those sources.
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