Run trial preform moulds with one or two cavities on a small machine before you order a multi-cavity production mould. The trial proves the preform design, its weight, its IV loss and how it blows into a bottle, before the cost of production tooling is committed. A 75 t machine with a small barrel suits this, because a small trial shot still uses a healthy share of the barrel.
This article covers what a trial should prove, how to size the barrel for a small trial shot, how to check IV loss, and what changes when you scale up.
Key Takeaways
- A trial mould proves weight, neck dimensions, clarity, IV loss and blow behaviour before a multi-cavity mould is ordered.
- Keep the trial shot between about 25% and 65% of barrel capacity; below that range, melt uniformity suffers and the melt sits longer at temperature.
- The PETplanet Rapid Guide treats an IV drop of more than 0.04 dl/g from resin to preform as a fault.
- Blow the trial preforms into bottles and test the bottles, because a preform that looks good can still blow badly.
What Should Trial Preform Moulds Prove?
That the preform can be moulded clear, to weight and to the neck drawing, without losing too much IV, and that it blows into a bottle that passes its tests. Anything the trial does not prove becomes a risk in the production mould, where changes cost far more.
Weight and neck come first, because the closure must fit and the bottle maker has a target gram weight. Clarity follows: a white haze near the gate or in thick sections can mean the melt is crystallising as it cools too slowly, which points to wall thickness or cooling layout.
Blow behaviour is the test that is easiest to skip. The preform's wall distribution decides how it stretches. Our view: do not approve a preform design until trial preforms have been blown into bottles and those bottles have passed the top-load, drop or leak tests agreed with the buyer.
Checks to run on trial preform moulds
| Check | How | What to look for |
|---|---|---|
| Weight | Weigh several shots from each cavity | Spread between cavities and against the target weight |
| Neck dimensions | Gauge against the neck drawing | Thread, support ledge and sealing surface within tolerance |
| Clarity | Inspect against a light | No haze, streaks or bubbles |
| IV loss | Lab IV test on resin and preforms | A drop of no more than 0.04 dl/g |
| Acetaldehyde | Lab test on preforms | Compare with the resin's chip value and the buyer's limit |
| Blowing | Blow sample bottles | Even wall distribution and passing bottle tests |
How Do You Size the Barrel for a Small Trial Shot?
Keep the shot between about 25% and 65% of barrel capacity, the range Plastics Technology columnist John Bozzelli recommends. Below 25%, melt uniformity is often poor. A two-cavity trial shot on a large production barrel can fall under that floor, and PET is sensitive to it.
Bozzelli also advises working in volume, not grams, because machine shot ratings are usually given in polystyrene. He adds about 10% for check-valve leakage and melt uniformity before sizing the barrel. For PET, ask the machine supplier for the barrel's swept volume and work from the preform volume.
Residence time also comes from the cycle. A trial can run slower than production while settings are found, so the melt sits longer at temperature. The PETplanet Rapid Guide recommends keeping back pressure and screw speed as low as recovery within the cycle allows, with back pressure typically 15 to 20 bar.
How Do You Check IV Loss in Trial Preform Moulds?
Test the IV of the resin and of preforms from each cavity in a lab, then compare. The PETplanet Rapid Guide to Perfect Preforms treats a drop of more than 0.04 dl/g between resin and preform as a fault. The loss cannot be seen by eye.
The guide names hydrolytic or severe thermal degradation as the cause, and its first checks are on the dryer: a dew point above minus 40 °C must be corrected, and the resin must be dried below 50 ppm of water. Melt temperature, back pressure, screw speed and thermocouples come next.
Start from the resin certificate. Reliance's datasheet for Relpet G5761, a bottle grade for still water, gives an IV of 0.76 dl/g, plus or minus 0.02, and acetaldehyde of 1.0 ppm maximum in the chips. Reliance states that its Hazira PET test methods are NABL accredited, so ask your resin supplier for lot values rather than typical ones.
Dryer settings and dryer sizing are covered in the start-up article in this series.
What Changes When You Scale Up to a Production Mould?
Cavity balance, cooling and the hot runner. More cavities means more chances for one to fill differently, and production cycles push cooling harder than a trial. Recheck weight per cavity, IV loss and clarity on the first production shots, not only on the approved trial preforms.
Cooling is where trial and production differ most. RJG's guide to cooling time notes that wall thickness is squared in the cooling equation, so keep the production preform's wall the same as the trial's, or the cycle will not transfer. A wall 10% thicker needs about 21% more cooling.
Keep the trial mould after scale-up. It stays useful for new weights, resin trials and customer samples without stopping the production mould.
What Should You Send Before a Preform Trial?
Send the preform drawing, target weight, neck finish, resin grade with its lot IV, the bottle drawing and the bottle tests your buyer requires. With those, barrel size, trial settings and the checks above can be planned before the trial mould is cut. See the 75 ton PET injection moulding machine page.
Frequently Asked Questions About Trial Preform Moulds
How many cavities should a trial preform mould have?
One or two can be enough to prove the design and supply sample bottles. Two cavities also show whether the hot runner and cooling give the same weight in each cavity. Fewer cavities keep the shot small, so match the trial machine's barrel to that shot, as the worked example shows.
Can trial preforms be sent to customers as samples?
Yes, if they are made from the production resin grade with the same neck and wall as the production mould. Label them as trial samples with the resin lot and settings. Preforms from a mould with different cooling may blow differently, so recheck once the production mould is running.
How long should a preform trial run?
Long enough to reach a stable process, then to collect samples from a steady run, not from the first shots. A short run can hide dryer problems, because the first resin may have sat in the hopper longer than the running residence time. Run at least one full hopper turnover.
Do I need my own IV tester before starting trials?
Not on day one. An outside lab can test resin and preform IV for each trial. Once production starts, frequent checks become useful, and the PETplanet guide notes that IV loss cannot be seen in the preform, so a tester or melt viscometer, in house or outsourced, belongs in quality control.
What if trial preforms come out hazy?
Look first at where the haze sits. Haze near the gate or in the thickest wall can mean slow cooling, so check chiller flow and the wall design. General cloudiness can mean wet resin or melt problems, so check the dryer dew point and melt temperature next. Fix the cause before approval.
Related Articles
- What PET Products Can You Make on a 75 Ton Machine?
- Small Pharma and Cosmetic Bottle Preforms on a 75 Ton Machine
- Starting PET Preform Production With a 75 Ton Machine
Sources & References
- PETplanet Insider, R. Unterlander and O. Brandau: The Rapid Guide to Perfect Preforms (Vol. 11, No. 11/10, p. 24)
- Plastics Technology, J. Bozzelli: Revisiting Shot Size vs. Barrel Capacity
- Reliance Industries: Relpet G5761 bottle grade PET, typical properties
- 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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