A 16-cavity mould making 24 g preforms on a 14 s cycle gives about 4,032 good preforms an hour at a 98% good-shot share, and uses about 99 kg of PET an hour. At 4 hours of drying, the dryer must hold about 395 kg of resin in process. Planning preform output means setting cavities, cycle time and drying capacity together, because the dryer has to keep pace with the mould.
This article shows how to turn cavities and cycle into output, what sets a preform cycle, how to size the dryer from resin use, and what else the plan should allow for.
Key Takeaways
- Output per hour = 3,600 ÷ cycle in seconds × cavities × good-shot share; resin use per hour follows from the shot weight.
- Dryer residence time is dryer capacity divided by throughput, and should be at least 4 hours, preferably 6.
- Drying air should have a dew point of minus 40 °C or lower and about 4 m³/h of airflow for each kg/h of PET.
- Plan on a cycle proven in a trial: on a 14 s cycle, each extra second costs nearly 7% of output.
What Goes Into Planning Preform Output?
Three numbers: cavities, cycle time and the share of good shots. Output per hour is 3,600 divided by the cycle in seconds, times cavities, times the good-shot share. Resin per hour is shots per hour times the shot weight, and that figure sizes the dryer.
Good preforms per hour by cavities and cycle time, at a 98% good-shot share (an assumption)
| Cavities | 12 s cycle | 14 s cycle | 16 s cycle |
|---|---|---|---|
| 8 | 2,352 | 2,016 | 1,764 |
| 12 | 3,528 | 3,024 | 2,646 |
| 16 | 4,704 | 4,032 | 3,528 |
The table shows the trade. Sixteen cavities on a 16 s cycle make the same 3,528 an hour as twelve on a 12 s cycle, but the larger mould costs more, needs a larger shot and may not fit the tie bars. Mould fit is covered in the article in this series on when 180 t is enough.
Turn hourly output into monthly output only with the hours you will really run. Mould changes, colour changes, start-ups and maintenance all come out of the month before the order book sees a preform.
What Sets the Cycle Time of a Preform?
Mainly the wall. A preform cycle is injection, hold, cooling, then mould opening, ejection and closing. Cooling follows the square of wall thickness, so the wall decides most of the cycle, while mould movement adds a fixed time that more cavities do not change.
The PETplanet Rapid Guide to Perfect Preforms gives an in-mould cooling time of about 1 to 3 s depending on the preform wall, with mould water at 7 to 10 °C, and an injection rate of about 10 to 12 g/s per cavity. Hold time and pressure must not be cut too far, or the body cools unevenly.
Why the wall matters more than the weight is explained in the heavy preforms article in this series. For planning, the point is simple: take the cycle from a trial on the real preform, not from a mould quotation.
Our view: plan output on the cycle you have proven, never the one you hope for. On a 14 s cycle, one extra second costs nearly 7% of output, which can turn a profitable order into a loss.
How Do You Size Drying Capacity for Planning Preform Output?
From resin use per hour and the residence time the resin needs. Alpek's data sheet for its Laser+ C bottle grade puts residence at dryer capacity divided by throughput, at least 4 hours and preferably 6. Multiply resin per hour by the hours to get the hopper size.
Air matters as much as the hopper. Alpek asks for a dew point no higher than minus 37 °C, preferably minus 40 °C or lower, and airflow of about 1 cfm for each lb/h of resin. Ottmar Brandau, in PETplanet Insider, gives the same as about 4 m³/h per kg/h.
Brandau also sets the target: no more than 50 ppm of water at the feed throat, which keeps the IV drop to an acceptable 0.03 to 0.04. Reliance's Relpet G5761 data sheet lists moisture when packed below 0.2%, which is 2,000 ppm, so even fresh bags need full drying.
Too much heat is a fault too. Alpek gives inlet air up to 171 °C, with 180 °C as an absolute maximum because of discolouration, and pellet temperature of 149 to 166 °C at the dryer exit.
What Should the Plan Allow for Besides Cycle Time?
Start-up, stoppages and changes. A dryer must hold a full residence of dried resin before the first good shot, so the first production hours start well before the machine does. Stoppages, mould changes and the first shots after each change also come out of the month.
Plan what happens when the machine stops. For long stoppages, Alpek says to drop the drying air to 116 °C or lower and keep dry air flowing. Resin left at full drying temperature for hours can yellow and lose IV before it ever reaches the barrel.
Hot-runner faults are the other planned loss. One blocked tip on a 16-cavity mould cuts output by one sixteenth, so record weight per cavity every shift and keep spare heaters, thermocouples and nozzle tips on site.
What Should You Send to Plan Your Preform Output?
Send the preform weight and wall, the cavity count you are considering, the monthly quantity your buyer needs and the hours you plan to run. With those, cavities, cycle, dryer and machine can be sized as one cell. See the 180 ton PET injection moulding machine page.
Frequently Asked Questions About Planning Preform Output
What good-shot share should I assume when planning output?
Use your own records from a similar mould and resin if you have them. If not, take the figure from a trial run long enough to settle, not from the first hour. Recheck it after the first month of production, because hot-runner faults and start-up losses show up over time.
How do I find the real drying time in my hopper?
Measure it. Brandau suggests placing a few colour pellets on top of the resin in the hopper and timing how long they take to reach the throat. A hopper run half full, or with uneven flow, can give much less residence than its rated size suggests.
What happens if the dryer is too small for the mould?
The resin reaches the barrel before it has dried for long enough, carrying more water than the 50 ppm target. The result is IV loss that cannot be seen in the preform, weaker bottles and higher acetaldehyde. Slowing the cycle to suit the dryer cuts output instead.
Should I add cavities or cut cycle time to raise output?
Compare the cost of each. More cavities raise output in proportion but need a new or larger mould, a bigger shot and room between the tie bars. Cutting cycle time costs less if cooling or hold settings have spare margin, but the preform wall sets a floor on it.
Why does chilled water temperature matter for preform output?
Preform moulds rely on cold water to freeze the wall quickly and keep it clear. The PETplanet guide gives 7 to 10 °C for mould cooling water. Water running warmer than planned lengthens cooling, and the cycle, on every shot, so check the chiller against the mould's flow rating.
Related Articles
- What PET Products Can You Make on a 180 Ton Machine?
- Heavy Preforms for Large-Volume Bottles on a 180 Ton Machine
- When the PET 180 Ton Is Enough and When to Move to 220 or 260 Ton
Sources & References
- Alpek Polyester: Laser+ C (F65A) Product Information (PDS-F65A 230601MO)
- PETplanet Insider, O. Brandau: Drying of PET: Bottles, Preforms and Closures, part 13 (Vol. 15, No. 08/14, p. 28)
- PETplanet Insider, R. Unterlander and O. Brandau: The Rapid Guide to Perfect Preforms, part 34: Wavy Body (Vol. 13, 11/2012)
- Reliance Industries: Relpet G5761 TDS (Aug 2022)
Technical Disclaimer
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