A 32-cavity mould for preforms with a 33 mm support ledge has about 274 cm² of projected area. At a 400 bar guide pressure it needs about 123 t with a 10% reserve (a starting margin, not a rule); on a cautious 5 US t/in² basis, about 212 t. Both fit 260 t. For high-cavity preform moulds, the tie-bar gap, mould weight, shot and injection rate usually decide the cavity count before clamp does.
This article shows how many cavities 260 t allows by clamp, the checks that limit a large preform mould first, and what one large mould means for running cost against two smaller machines.
Key Takeaways
- Clamp equals area per cavity times cavities times cavity pressure; for a preform, the area per cavity is the circle of its widest diameter, usually the support ledge.
- At 260 t with a 10% reserve, clamp allows about 67 cavities of a 33 mm ledge at 400 bar, or 39 on a cautious 5 US t/in² basis.
- At 10 to 12 g/s per cavity, a 32-cavity mould needs a total injection rate of about 320 to 384 g/s from the injection unit.
- In Maharashtra, MSEDCL bills demand on kVA, so a larger machine's motor and heaters can raise the demand charge as well as the energy bill.
How Many Cavities Does 260 t Allow by Clamp?
Divide the usable clamp by the area per cavity and the cavity pressure. With a 10% reserve, 260 t gives 236 t. At 400 bar that covers about 579 cm²; at 5 US t/in², about 336 cm². A 33 mm ledge is 8.55 cm², so 67 or 39 cavities.
The 400 bar figure comes from SPE's Plastics Engineering magazine, as a guide value where you have no data for the resin. The same article gives tonnage factors of 2 to 8 US tons/in² depending on the material. The 5 US t/in² row is an assumption chosen inside that range for a cautious check, not a PET value.
On a hot-runner mould only the cavities count. A cold runner would add its own area, but preform moulds are built as hot-runner tools, so the sum is the cavity count times the circle of the ledge.
Cavities by clamp at 260 t with a 10% reserve (236.4 t usable), hot-runner preform mould
| Ledge diameter | Area per cavity | Cavities at 400 bar (0.408 t/cm²) | Cavities at 5 US t/in² (0.703 t/cm²) |
|---|---|---|---|
| 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 |
Our view: treat the 400 bar column as the ceiling and the cautious column as the plan, until the mould maker gives you a cavity pressure for your preform. A mould built to the ceiling has no room left for a stiffer melt or a worn check valve.
What Limits High-Cavity Preform Moulds Before Clamp Does?
Often the tie-bar gap, then mould weight and stroke. Cavities sit in rows, each with a core, neck splits and a hot-runner nozzle, so a high-cavity mould grows wide and heavy. Its cavities must sit inside the tie bars, and both platens must carry its weight.
Catoen's machine sizing guide in MoldMaking Technology explains why the tie bars matter: platens can pull in near the bars and wear the parting line, so the parts' projected area must sit wholly inside them. He puts the opening stroke at least 2.5 times the part height and asks for the maximum mould weight to be checked for both platens.
Under-clamping shows as flash. The PETplanet Rapid Guide to Perfect Preforms, by Rick Unterlander and Ottmar Brandau, gives too low a melt viscosity or insufficient clamping tonnage as the causes of preform flash. Flash on one or two cavities tends to point to the tool; flash across the mould points to clamp or pressure settings.
Can the Shot and Injection Rate Keep Up?
They must, because every cavity fills from one shot. Total shot is cavities times preform weight, and the Rapid Guide gives an injection rate of about 10 to 12 g/s per cavity. A 32-cavity mould therefore needs about 320 to 384 g/s from the injection unit.
In volume, at a PET melt density of about 1.15 g/cm³, that is about 278 to 334 cm³/s of melt. Ask the machine supplier for the injection rate at the screw you will use, since a larger screw moves more melt per stroke but at a lower plastic pressure.
The shot itself must sit inside the barrel window set out in the heavy preforms article in this series. A 32-cavity mould of 28 g preforms shoots 896 g, so its barrel needs a rating of at least about 1,245 g PS.
Is One High-Cavity Mould Better Than Two Smaller Machines?
It can be for a long, stable run: one machine, one dryer, one chiller and fewer operators per preform. The risk is concentration. If the mould or machine stops, all the output stops, so keep spare heaters, thermocouples and nozzle tips for the hot runner on site.
Power billing is part of the comparison. In Maharashtra, MSEDCL bills HT consumers and LT consumers above 20 kW for demand on kVA and energy on kVAh. A larger machine's pump motor and heaters can lift the maximum demand you pay for.
MSEDCL also notes that at a power factor of 0.85, 15% of power does no useful work, and recommends capacitors or APFC panels. Check the power factor of the whole cell, not the machine alone, before comparing one large machine with two smaller ones.
What Should You Send to Size a Machine for a High-Cavity Mould?
Send the preform drawing with weight and ledge diameter, the cavity count, and the mould maker's width, height, thickness, weight and required stroke. Add the target cycle and monthly volume. With those, clamp, tie-bar fit, shot and injection rate can be checked at 260 t. See the 260 ton PET injection moulding machine page.
Frequently Asked Questions About High-Cavity Preform Moulds
Should I fill every cavity the clamp allows?
No. The clamp count is a ceiling, and the tie bars, mould weight, shot and injection rate tend to set a lower number. Plan the cavity count with the mould maker from all five checks, and keep a reserve in case the real cavity pressure is higher than the guide figure.
What happens if one cavity of a high-cavity mould is blocked?
A hot-runner preform mould can often keep running with a faulty cavity shut off, at lower output. Check with the mould maker how the hot runner and the take-out handle a closed cavity, and recalculate hourly output and resin use with the cavity count you actually run.
Does a high-cavity mould need a faster dry cycle?
It helps, because mould movement is part of every shot. Catoen gives dry cycles of 1 to 2.5 s for thin-wall machines and 3 to 5 s otherwise. On a 12 s preform cycle, taken as an example, saving 1 s of movement raises output by about 9%.
How do I check a preform mould's weight against the machine?
Ask the mould maker for the total weight and the split between the fixed and moving halves, then compare both with the machine's rated mould weight. Catoen's guide says to check the maximum mould weight for both platens. Plan the crane and lifting gear for mould changes at the same time.
Why does flash appear only after the mould warms up?
Steel and hot-runner manifolds expand as the mould reaches running temperature, which can change how the parting line closes. Flash that appears after warm-up can point to settings, venting or tool fit rather than clamp alone. Check the mould maker's warm-up instructions before raising tonnage.
Related Articles
- What PET Products Can You Make on a 260 Ton Machine?
- Heavy Large-Volume Preforms on a 260 Ton Machine
- Post-Mould Cooling and Handling for High-Output Preform Production
Sources & References
- SPE Plastics Engineering, A. Urbina: Mastering Clamping Force Calculation
- MoldMaking Technology, B. Catoen: Machine Sizing and Specification for Optimal Mold Performance, Part 1
- PETplanet Insider, R. Unterlander and O. Brandau: The Rapid Guide to Perfect Preforms, 10. Flash (Vol. 11, 04/2010)
- Maharashtra State Electricity Distribution Co. (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.
Limitation of Liability: Pratishna Engineers makes no representation or warranty, express or implied, regarding the completeness, accuracy or suitability of the information for any particular application. To the extent permitted by applicable law, Pratishna Engineers shall not be liable for any loss, damage, cost or consequence arising from reliance on or use of the information contained in this document. Final machine selection, configuration and application suitability should be confirmed with Pratishna Engineers based on complete and accurate project requirements.
