Selecting a PET preform injection molding machine is not a question of buying the biggest tonnage you can afford. It is a question of matching four numbers — clamping force, shot weight, screw profile and hot runner cavity count — to one specific preform. Get those four right and the machine runs quietly for years. Get one wrong and you either cannot fill the parts, or you are paying for tonnage and barrel capacity you never use.
The most common error is choosing by preform cavity count alone. A 48-cavity preform mould on a machine that cannot plasticise enough PET per cycle will run short shots, and the answer is not to raise the temperature — that creates acetaldehyde and drops intrinsic viscosity. It is to size the machine properly the first time.
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 walks through the selection arithmetic in the order an engineer should do it.
Key Takeaways
- Clamping force is set by the preform’s projected area, not by its weight. Calculate it from the projected area of all cavities including the runner, then add a safety margin — do not guess from tonnage tables alone.
- Shot weight decides the barrel and screw, and the screw decides quality. A general-purpose screw creates too much shear for PET; the result is acetaldehyde and IV loss that shows up as weak bottles downstream.
- Cavity count is a balancing act with cooling and take-out. High cavity counts only pay off if the mould is thermally balanced and the robot can clear parts within the cycle.
Could Not Decide on Cavity Count?
Send the preform drawing and annual volume. We return two or three costed SW-P configurations with cycle time and output estimated.
1. Step One: Clamping Force from Projected Area
The preform is injection moulded with the cavity pressure acting on the projected area of the part and runner, which tries to push the mould open. Clamping force must exceed that force.
Required clamping force = Projected area (cm²) × Cavity pressure (kg/cm²) ÷ 1,000
Projected area covers the preform body and the runner that also sits in the parting plane. Then add a working margin — the mould must be able to hold the part closed on the first fill, not on the average fill.
In practice, buyers under-allow for the runner. A high-cavity preform mould with a long manifold feeds a projected area considerably larger than the sum of the finished preform sections, and that is where marginal machines fail.
2. Step Two: Shot Weight and the PET Screw
Shot weight is the total mass of PET needed per injection: preform weight × cavities, plus runner and gate allowance. Two rules apply.
Rule one: never use more than about 80% of rated shot capacity. Running a machine at its maximum shot weight means inconsistent melt, longer recovery and no reserve for process adjustment.
Rule two: the screw must be designed for PET, not for polyolefins. PET is shear-sensitive and hydrolytically sensitive. A dedicated PET screw profile with controlled compression, appropriate L/D and a corrosion-resistant barrel avoids the excessive shear that raises acetaldehyde and lowers intrinsic viscosity. Sailwin’s SW-P machines use a dedicated PET screw paired with far-infrared nano heating coils, which heat the barrel more evenly than conventional band heaters and reduce the hot spots that generate AA.
3. Step Three: Cavity Count, Cooling and Take-Out Together
| Preform programme | Typical cavity range | What selection really hinges on |
|---|---|---|
| Low volume, many formats | 8–16 | Fast mould change and short production runs, not maximum output |
| Standard water / CSD preform | 24–48 | Hot runner balance, cooling uniformity and cycle time repeatability |
| High-volume single SKU | 48–64 | Machine plasticising capacity, take-out robot speed and shut-height precision |
A 64-cavity valve-gate hot runner mould is a serious thermal engineering object. If the manifold is not balanced, moving from 24 to 64 cavities multiplies your quality problems instead of your output. Cavity count should be the last decision, once tonnage, shot weight and take-out speed are settled.
Get a Selection Report for Your Preform
Send the preform drawing, weight and annual volume. We will return tonnage, shot weight and cavity recommendations with the reasoning shown.
4. Step Four: Automation and Interface Checklist
- Euromap 67 interface — required if you intend to fit a take-out robot now or later. Sailwin SW-P machines include it as standard configuration.
- Servo-hydraulic drive — improves repeatability on the injection and clamp profile and reduces energy consumption against a fixed-displacement hydraulic system.
- Reinforced ejection — preform stripping needs more force and longer stroke than a typical injection part; check that the ejection specification is written for preforms specifically.
- Low-pressure mould protection — protects high-cavity preform moulds from damage caused by incomplete part removal, which is the most expensive common failure in preform production.
- Volumetric lubrication and intelligent controller — reduce manual intervention and make the machine’s condition visible to your maintenance team before a fault stops the cell.
5. Sailwin Case Study: Right-Sizing Instead of Over-Buying
- Beverage producer adding in-house preform production for water bottles
- Supplier had quoted a machine class above what the preform actually required
- Concern about acetaldehyde in the finished water and preform weight consistency
- Tonnage recalculated from projected area including the runner, removing the unnecessary tonnage step
- SW-P machine specified with dedicated PET screw and far-infrared nano heating zones
- Valve-gate hot runner selected for thermal balance at the target cavity count, with Euromap 67 for take-out automation
- Capital avoided by matching the machine to the preform rather than to a sales tier
- Preform weight consistency held within the agreed tolerance band, verified per cavity
- Melt quality controlled through screw design and even barrel heating rather than by running hotter
Scenario based on a Sailwin customer project; final configuration is confirmed against your preform drawing during engineering review.
Frequently Asked Questions
Get a Preform Machine Selection Report
Send your bottle drawing, container sample or target output. Our engineering team replies with a machine recommendation, mould assessment and factory-direct quotation within 24 hours.
Related Reading:
• PET Preform Injection Molding Machine — 14 models, 170–5500 kN
• PET Preform Injection Molding Process Guide
• Calculating Clamping Force and Shot Weight
• Valve Gate vs Self-Locking Preform Moulds
• Matched PET Blow Molding Machines
