Start with one number: 284 cm². That is roughly the projected area a 220 t clamp can hold with a 10% reserve at an assumed 5 US tons/in². A 33 mm support ledge takes 8.55 cm², so clamp alone would allow about 33 cavities. For higher-cavity preform moulds on a 220 t machine, the real limits are injection rate, barrel size, mould width between the tie bars and cooling.
This article works through each of those limits and the order in which to check them.
Key Takeaways
- Clamp seldom sets the cavity count for narrow-neck preforms on 220 t; the clamp method is set out in the applications article in this series.
- Total injection rate grows with cavities, at about 10 to 12 g/s per cavity in the PETplanet Rapid Guide, so a 24-cavity mould needs roughly 240 to 288 g/s.
- The full shot must sit inside the barrel's healthy range, which for PET means checking melt volume, not grams of polystyrene.
- Cooling time follows wall thickness, so more cavities raise output only if the cooling circuit keeps every cavity at the same wall temperature.
What Limits Higher-Cavity Preform Moulds on 220 t?
Four things, in this order: total injection rate, shot size against the barrel, mould width between the tie bars, and cooling. Each grows with cavity count. Clamp grows too, but for narrow-neck preforms it stays well inside 220 t long after the other four have run out.
Mould width often binds first. The cavities must sit wholly inside the tie bars, because platens can pull in near the bars under clamp and wear the parting line. Rows of cavities add width fast, because each one carries its own core, neck splits and nozzle.
What grows as you add cavities to a preform mould
| Item | How it scales | What to check |
|---|---|---|
| Projected area and clamp | In line with cavities | Rarely the limit for narrow necks on 220 t |
| Injection rate | Cavities × about 10 to 12 g/s | The machine's rated injection rate in g/s of PET |
| Shot volume | Cavities × preform volume, plus about 10% | No more than 65% of the barrel's swept volume |
| Mould width | Rows and pitch of cavities | Fit inside the tie bars |
| Cooling water and hot runner zones | One circuit and one nozzle zone per cavity | Water flow and controller zones, covered in the hot runner article |
How Much Injection Rate Do More Cavities Need?
Roughly 10 to 12 g/s for every cavity, the rate the PETplanet Rapid Guide to Perfect Preforms gives when it discusses wavy preform bodies. Multiply by the cavity count to get the rate the machine must deliver. Falling short lengthens fill time and can leave cavities filling unevenly.
The same guide links a wavy body to uneven contact cooling. Alongside injection rate it lists hold pressure and time that are not too low, mould water at 7 to 10 °C, and water flow at the mould maker's rating. Each point gets harder to meet as cavities increase.
Check the units on the machine data. Injection rate is often quoted in cm³/s or in grams of polystyrene per second. Convert it to melt volume, then to grams of PET, before you compare it with the cavity count. A rate in PS grams reads lower than the same volume of PET, which weighs more per cm³.
Our view: do not order the highest cavity count the clamp allows. Order the count the injection unit can fill at the right rate and the cooling can freeze evenly, then add a second mould when demand grows.
How Big a Barrel Does a Full Preform Shot Need?
Big enough that the full shot uses no more than 65% of the barrel's swept volume, after adding about 10% for check-valve leakage and melt uniformity. That rule comes from Plastics Technology columnist John Bozzelli, who also advises working in volume because shot ratings are given in polystyrene.
For PET, melt density matters. PET melt is denser than polystyrene melt, which Bozzelli puts at 0.945 g/cm³, so a barrel rated in grams of polystyrene holds more PET by weight. Convert the preform weight to melt volume first, as the worked example below does.
The lower end matters as well. Bozzelli warns that shots below about 20% of barrel capacity lead to long residence and degradation. A mould with fewer cavities run on a barrel sized for many can drop under that floor. The screw options article in this series explains why that raises acetaldehyde in PET.
Does Cooling Scale With Cavity Count?
Cycle time does not grow with cavities, but cooling has to work equally in every cavity. RJG's guide to cooling time says wall thickness is squared in the cooling equation, so a cavity with a thicker wall or weaker water flow sets the cycle for the whole mould.
Resin grade can change from job to job on the same mould. Reliance lists separate Relpet grades for still water (G5761), aerated drinks (G5841) and hot fill (H7761), the last with higher neck and body crystallinity. Validate weight, clarity and IV loss on each grade before you lock the cycle.
On the shop floor, measure the water temperature at the mould inlet and outlet for each circuit. A large rise across one circuit means it is short of flow for the cavities it serves. Check this on the first trial, because a mould maker can still rebalance circuits before the mould is approved.
Lightweight water preforms add one more point. Taking grams out of a preform cuts the shot but not the cooling time, which follows the wall. The barrel then holds more shots at once, so the melt waits longer. The screw options article covers this in full.
What Should You Check Before Ordering a Higher-Cavity Mould?
Collect the preform drawing, weight, wall thickness, resin grade and the bottles per hour your buyer needs. Then check injection rate, shot volume, mould width and cooling for the cavity count you have in mind, before the mould maker quotes. See the 220 ton PET injection moulding machine page.
Frequently Asked Questions About Higher-Cavity Preform Moulds
Why not order the most cavities the clamp allows?
Because clamp is not what fails first. Injection rate, barrel size, tie-bar width and cooling all run out sooner on narrow-neck preforms. A mould with more cavities than the machine can fill and cool evenly gives weight spread, wavy bodies and rejects, and the output gain disappears in sorting.
Does doubling the cavities double the output?
Only if the cycle stays the same and the good-shot share holds. Fill takes longer if the injection rate is short, and one poorly cooled cavity can set the cycle for all. Run the output formula with your real cycle and reject rate, not the brochure figure.
How do I spot one bad cavity in a high-cavity mould?
Weigh preforms from every cavity on a regular schedule and compare them. A cavity that runs light, heavy or hazy points to a hot runner zone, a blocked water line or a worn core. Number the cavities on the mould so every preform can be traced.
Can I run fewer cavities by blocking some off?
Some moulds allow it, but the shot drops and may fall below a healthy share of the barrel. That lengthens residence and raises degradation. Ask the mould maker whether blocking is supported, and check the reduced shot against the barrel before you run it for long.
Is a higher-cavity mould worth it for short runs?
Rarely. Mould cost, changeover time and the minimum shot all rise with cavities. For many neck finishes and short runs, two moulds with fewer cavities can serve better. Higher-cavity moulds pay back on long runs of one preform for one buyer.
Related Articles
- What PET Products Can You Make on a 220 Ton Machine?
- Three Screw Options for PET on a 220 Ton Machine Explained
- Hot Runner Preform Moulds: What the Machine Must Provide
Sources & References
- PETplanet Insider, R. Unterlander and O. Brandau: The Rapid Guide to Perfect Preforms, part 34: Wavy Body
- Plastics Technology, J. Bozzelli: A Simpler Way to Calculate Shot Size vs. Barrel Capacity
- UL Prospector: Polyester: Relpet grade listing
- RJG Inc.: How to Determine Injection Molding Cooling Time
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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