PET vs standard injection machine: where they differ
| Area | PET preform machine | Standard horizontal machine |
|---|---|---|
| Screw and barrel | Matched to preform weight to keep melt time short | Mixed-duty screw for PP, ABS and similar resins |
| Injection rate | High, about 10 to 12 g/s per cavity across many cavities | Set by the part; few cavities at once |
| Mould feed | Hot runner with a valve gate per cavity, many heater zones | Cold or hot runner, few zones |
| Mould cooling | Chilled water at 7 to 10 °C, high flow | Tower or chilled water as the part needs |
| After ejection | Robot or plate with cooled tubes | Parts drop or are picked out |
| Resin preparation | Desiccant dryer at -40 °C dew point | Hot-air dryer or none, by resin |
The table answers the PET vs standard injection machine question in short. Both are horizontal machines with a clamp and an injection unit. The PET machine is set up around one product: the preform. Its screw keeps melt time short, its injection unit fills many cavities fast, and it is fed by a dryer and followed by cooling the standard machine does not need for PP or ABS parts.
Key Takeaways
- A PET machine is a horizontal machine built around preforms: short melt time, high injection rate, many valve gates and fast cooling.
- A standard machine can mould PET only in limited cases, with a desiccant dryer, chilled water and a barrel not much bigger than the shot.
- Injection rate is the first gap. A 24-cavity mould at 10 to 12 g/s per cavity needs 240 to 288 g/s from the injection unit.
- Preform cycle time follows wall thickness, so post-mould cooling in robot tubes is where a PET cell gains output.
Can a standard horizontal machine mould PET preforms?
It can, within limits. With a desiccant dryer, chilled water, a hot runner controller with enough zones and a barrel close to the shot size, low-cavity preform jobs can run. Higher cavities, water bottle preforms and tight acetaldehyde limits expose the gaps quickly.
The barrel is the first limit. On a general machine, the screw is chosen to cover many resins and part sizes. Put a small preform shot on a large barrel and each gram of melt stays hot for many cycles. A rough guide: residence = barrel swept volume ÷ shot volume × cycle time, so halving the shot on the same barrel and cycle doubles residence.
Long residence in PET raises acetaldehyde and cuts IV; the article on what a PET injection moulding machine is in this series explains both. For a mixed-duty screw, shots below about 20% of barrel capacity already risk degradation in common resins, and PET is less forgiving.
The dryer is the second limit. A hot-air dryer heats room air, so it cannot dry PET below the moisture that air already carries, and humid Indian monsoon air carries plenty. PET needs a desiccant dryer that delivers air at -40 °C dew point or lower. A plant that runs only PP, PE or ABS may not own one, and it is a separate purchase from the machine.
Our view: for still water preforms, where taste limits are tightest, run PET on a machine set up for preforms. Trials on a general machine can suit samples or short runs of non-food containers.
Where does the PET vs standard injection machine gap show up first?
It shows first in injection rate and hot runner control. Preform moulds fill many cavities at roughly 10 to 12 g/s each, so the injection unit must deliver a high flow at once. Each cavity also needs a valve gate, its own heater zone and steady actuating air.
Injection rate is set by cavities, not by one big part. A 16-cavity mould needs 160 to 192 g/s; a 32-cavity mould needs 320 to 384 g/s. A machine sized for slow fills of thick parts may not reach that flow, and a slow fill lets the melt front cool unevenly across cavities.
Hot runner zones add up quickly. Count one zone per nozzle, plus manifold and sprue zones, plus spares. Valve gates typically run on 8 to 10 bar of air, so the air supply at the machine has to hold steady through every cycle.
Gate temperature also needs fine control. A nozzle tip set too hot leaves strings at the gate; set too cold, it freezes a white crystalline spot on the dome.
How do cooling and cycle time differ on a PET machine?
Preform cycle time is set by wall thickness, not weight. Two preforms with the same wall, one twice as heavy, can differ by only 1 to 2 s. So a PET cell gains output by cooling preforms outside the mould, in robot tubes, while the next shot runs.
Inside the mould, cooling lasts about 1 to 3 s with water at 7 to 10 °C. Cooled tubes on the take-out robot carry on cooling for one or more further cycles, which lets the mould open earlier without the preforms deforming or marking each other.
A standard machine with a simple pick-and-place arm or free-drop ejection has no such stage. The mould must hold the preform until it is stiff enough to fall, which lengthens the cycle. On a part like a PP lid that does not matter much. On a preform it costs output every hour.
Clamp force can matter less than these points, because preform cavities have small projected areas; the specifications article in this series works through clamp and shot sizing.
How should you decide between a PET machine and a standard machine?
Decide from the product. List preform weights, cavities, neck sizes and whether the contents are still water, carbonated drinks or non-food. Then check the injection rate and barrel size against each job. See the PET injection moulding machine range to compare.
Frequently Asked Questions About PET and Standard Injection Machines
Can I convert my standard injection moulding machine for PET?
Partly. A desiccant dryer, chiller, hot runner controller and suitable screw and barrel can be added, but injection rate, valve-gate air and post-mould cooling are harder to retrofit. Check the injection rate and barrel size against your preform before spending on a conversion.
Why is a large barrel a problem for PET?
A large barrel holds many shots of melt, so each gram stays hot for many cycles. PET held hot forms more acetaldehyde and loses IV. A rough guide is that halving the shot on the same barrel and cycle doubles the time the melt spends in the barrel.
How many hot runner zones does a preform mould need?
Count one zone per nozzle, then add manifold and sprue zones and a few spares. For example, 16 nozzles plus 4 manifold zones and 1 sprue zone gives 21 zones, and about 10% spares takes the controller to 24 zones.
Does a PET machine need more clamp force than a standard machine?
Not for the same mould size. Preform cavities have small projected areas, so clamp force is set by the number of cavities and the clamp factor used. Injection rate, shot size and cooling can limit a preform job before clamp force does, so check all four together.
Why do preform cells use robots with cooling tubes?
Preform cycle time is limited by cooling through the wall. Cooled tubes on a take-out robot let preforms keep cooling outside the mould, so the mould can open sooner. This is how a preform cell shortens its cycle without the preforms deforming or marking.
Related Articles
- What Is a PET Injection Moulding Machine?
- How Is a PET Preform Made?
- Important Specifications to Consider Before Buying a PET Injection Moulding Machine
- Applications of PET Injection Moulding Machines
Sources & References
- PETplanet Insider, R. Unterlander and O. Brandau: The Rapid Guide to Perfect Preforms, part 34: Wavy Body (11/2012)
- PETplanet Insider, O. Brandau: Bottles, Preforms and Closures extract, 3.6 Acetaldehyde in PET Bottles (03/2015)
- Plastics Technology, J. Bozzelli: A Simpler Way to Calculate Shot Size vs. Barrel Capacity (2017, updated 2022)
- PETplanet Insider, R. Unterlander and O. Brandau: The Rapid Guide to Perfect Preforms, part 28: Stringing (04/2012)
Technical Disclaimer
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