Sixteen cavities of a preform with a 38 mm neck ledge put about 181 cm² of projected area in front of the clamp. Sized with a 10% reserve (a starting margin, not a rule), that needs roughly 81 t on a 400 bar cavity-pressure guide, or about 140 t on a more cautious factor. A 23 g preform makes a 368 g shot, needing a barrel rated about 512 to 1,663 g of PS. Those two figures, clamp and barrel, are where reading PET preform machine specifications begins. The worked example near the end shows every step.
Key Takeaways
- Clamp force for preforms follows cavities × ledge area × a clamp factor. Show the buyer both a guide figure and a cautious figure.
- PET shots are heavier than the PS figure on a data sheet suggests: a barrel rated R g of PS holds about 1.22 × R g of PET melt.
- Keep each preform shot between 20% and 65% of barrel capacity. Size the barrel for the lightest high-volume preform, not the heaviest occasional one.
- Check plasticising rate in kg/h, injection rate, tie-bar gap, stroke, hot runner zones and power factor before price.
How much clamp force does a PET preform mould need?
Clamp force equals the projected area of all cavities times a clamp factor, plus a reserve. For preforms, the neck support ledge is taken as the largest diameter the clamp sees. A 400 bar cavity-pressure guide gives 0.408 t/cm²; a cautious 5 US tons/in² gives 0.703 t/cm².
The two factors differ a lot, and both come with caveats. The 400 bar figure is a general guide value for cavity pressure when no better data exist. The 5 US tons/in² figure is an assumption used here to stay on the safe side, not a rule. A 10% reserve on top is a starting margin, not a rule either. Mould trials and the mould maker can narrow the range.
Preform cavities by clamp at 260 t with a 10% reserve (calculated)
| Neck ledge diameter | Cavities at 400 bar guide | Cavities at 5 US tons/in² |
|---|---|---|
| 30 mm (7.07 cm²) | 81 | 47 |
| 33 mm (8.55 cm²) | 67 | 39 |
| 38 mm (11.34 cm²) | 51 | 29 |
| 48 mm (18.10 cm²) | 32 | 18 |
| 63 mm (31.17 cm²) | 18 | 10 |
Clamp is not the only limit on cavities. The mould must fit between the tie bars with its projected area fully inside them, and a rule of thumb asks the mould to cover at least 70% of the tie-bar gap so the platens do not bow. Opening stroke should be at least 2.5 times part height, which matters for long preforms. Too much clamp is not free: over-clamping can crush vents and damage the mould.
How do you read PET preform machine specifications for shot and barrel?
Shot capacity on a data sheet is usually given in grams of PS. PET melt is denser, about 1.15 g/cm³ against 0.945 for PS, so convert through volume. Then keep each shot between 20% and 65% of barrel capacity, with about 10% added for check-valve leakage.
Below about 20% of the barrel, the melt stays hot for too many cycles. In PET that means more acetaldehyde and lower IV. Above about 65%, melt uniformity suffers and unmelts appear. In practice, the smallest PS rating for a PET shot is about 1.39 × shot weight, and the largest is about shot weight ÷ 0.2213.
Our view: buy the barrel for your lightest high-volume preform, not your heaviest occasional one. If your range is wide, price a second screw and barrel with the machine; one ordered with a new machine can cost little, and a skilled operator can swap a barrel in under 30 minutes.
Which other specifications decide output and running cost?
After clamp and barrel, check plasticising rate in kg/h, injection rate in g/s, dry cycle time, hot runner zones and air, and the power drawn. These decide whether the machine holds the cycle all day and what each preform costs in electricity.
Plasticising rate must stay below the screw maker rating for PET, or unmelts appear. Injection rate should reach about 10 to 12 g/s per cavity; the comparison article in this series works through it for a 24-cavity mould. Count heater zones as nozzles plus manifold, sprue and spares.
Power has an Indian twist. In Maharashtra, MSEDCL bills HT and larger LT consumers on kVAh, so a poor power factor raises the bill directly. At a power factor of 0.85, every 100 kWh is billed as about 117.6 kVAh; at 0.95, about 105.3. Ask for the power factor at the machine, and plan correction if needed.
Dry cycle is worth a line on the quote. It is the time the clamp takes to close, build tonnage and open with no injection, and guidance puts it at about 1 to 2.5 s for thin-wall machines and 3 to 5 s otherwise. On a preform cycle of, say, 14 s, each second of clamp movement is a second the mould is not cooling or filling, so compare dry cycle across offers at the same stroke.
Base load matters too. On a standard hydraulic machine it can be 75% or more of total energy, while all-electric and hybrid machines sit nearer 10 to 20%. Over ten years, energy can cost more than the machine itself.
What should you send for a PET machine specification?
Send preform drawings or weights, neck ledge diameters, cavity targets, hourly output and resin grade. With those, clamp, barrel, injection rate and kg/h can be worked out before any model is discussed. See the PET injection moulding machine range for the next step.
Frequently Asked Questions About PET Machine Specifications
Why is the neck ledge used for preform clamp calculations?
The clamp resists melt pressure over the area it sees in the parting plane. On a preform mould, the support ledge below the threads is the widest part in that plane, so its circle is a simple and safe stand-in for projected area per cavity. Confirm the actual figure with your mould maker.
Which clamp factor should I use for PET preforms?
There is no single number. A 400 bar cavity-pressure guide gives about 0.408 t/cm², and a cautious 5 US tons/in² gives about 0.703 t/cm². Check both, then let mould trials or the mould maker settle where in that range your job sits.
Why does a data sheet give shot size in grams of PS?
Shot ratings are usually stated as the weight of polystyrene the screw can push in one stroke. Other resins have different melt densities, so convert through volume. PET melt at 1.15 g/cm³ weighs about 22% more than PS melt at 0.945 g/cm³ for the same stroke.
Does power factor matter for a preform machine in India?
Yes, where the bill is on kVAh. MSEDCL bills HT and larger LT consumers on kVAh, so a power factor of 0.85 turns every 100 kWh into about 117.6 kVAh billed. Capacitors or an APFC panel can bring it closer to 1. Use your latest bill for rates.
What mould dimensions should I compare with the machine?
Compare mould width and height with the tie-bar gap, mould thickness with the minimum and maximum shut height, and preform length with opening stroke. A rule of thumb asks the mould to cover at least 70% of the tie-bar gap, and opening stroke to be at least 2.5 times part height.
Related Articles
- What Is a PET Injection Moulding Machine?
- How Is a PET Preform Made?
- PET Injection Moulding Machine vs Standard Horizontal Machine
- Applications of PET Injection Moulding Machines
Sources & References
- SPE Plastics Engineering, A. Urbina: Mastering Clamping Force Calculation (Mar 2024)
- Plastics Technology, J. Bozzelli: A Simpler Way to Calculate Shot Size vs. Barrel Capacity (2017, updated 2022)
- MoldMaking Technology, B. Catoen: Machine Sizing and Specification for Optimal Mold Performance, Part 1 (Aug 2025)
- MSEDCL: kVAh Billing: Frequently Asked Questions
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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