How many cavities does your PET preform mould need?
Cavity count decides how many preforms you make per hour, which machine you need and how much the mould costs. Choose too few and you buy a second mould within two years. Choose too many and the mould will not fit your machine, or the machine cannot fill it. Here is how we work it out with customers.
Start with the output you need
Hourly output is the cavity count multiplied by the number of shots per hour. A 15-second cycle gives 240 shots an hour, so every extra cavity adds 240 preforms an hour.
Preforms per hour = cavities × 3,600 ÷ cycle time in seconds.
| Mould | Machine | Cycle | Preforms per hour | Per 24-hour day |
|---|---|---|---|---|
| 64 cavity | Yizumi 320T | 12 s | 19,200 | 460,800 |
| 72 cavity | Yizumi 480T | 15 s | 17,280 | 414,720 |
Plan on less than the theoretical figure. Colour changes, maintenance and start-up time all take hours out of the year, so size the mould for your real demand plus a margin, not for a perfect 24-hour day.
Check the machine can fill the mould
The injection unit has to deliver the whole shot every cycle. Multiply the preform weight by the cavity count: 72 cavities of 14 g preforms is 1,008 g of PET per shot. The machine's PET shot weight must cover that with a working margin, because running a screw at the very top of its stroke hurts melt quality and cycle stability.
Plasticising rate matters as much as shot size. A screw that can hold 1,000 g but only melt it slowly will stretch your cycle and erase the gain from the extra cavities.
Check the mould fits between the tie bars
Each cavity needs space for its neck ring, cooling and hot runner nozzle, so a higher cavity count makes a bigger mould. The pitch, the centre-to-centre distance between cavities, depends on the neck finish. Below are typical layouts for neck finishes up to 30 mm from our cavity calculator.
| Cavities | Arrangement | Pitch | Mould size |
|---|---|---|---|
| 16 | 2 × 8 | 60 × 160 mm | 750 × 440 mm |
| 24 | 4 × 6 | 60 × 140 mm | 780 × 555 mm |
| 32 | 4 × 8 | 55 × 130 mm | 810 × 520 mm |
| 48 | 4 × 12 | 50 × 130 mm | 930 × 560 mm |
| 64 | 4 × 16 | 50 × 130 mm | 1,000 × 650 mm |
| 72 | 6 × 12 | 50 × 130 mm | 1,120 × 690 mm |
| 96 | 6 × 16 | 50 × 130 mm | 1,260 × 850 mm |
Compare the mould size with the distance between your machine's tie bars and the maximum mould height and opening stroke. Larger neck finishes such as 38 mm or 48 mm need a wider pitch, so the same cavity count gives a bigger mould.
Weigh cost against flexibility
- A higher cavity count lowers the cost per preform but raises the mould price, and one damaged cavity or nozzle affects a larger share of output.
- Two medium moulds on two machines give you redundancy and let you run two preform weights at once.
- One high-cavity mould on a large machine uses less floor space and fewer operators for the same output.
There is no single right answer. It depends on how many preform designs you run, how often you change them and how much downtime your customers will accept.
What to send us for a recommendation
- Preform drawing or sample, with weight and neck finish.
- Injection machine brand and model, or the tie-bar distance and shot weight.
- Target output per hour or per day.
- Resin grade, if already chosen.