Cavities by clamp at 150 t with a 10% reserve (ledge area basis)
| Ledge diameter | Area per cavity | Cavities at 400 bar guide | Cavities at 5 US t/in² |
|---|---|---|---|
| 48 mm | 18.10 cm² | 18 | 10 |
| 53 mm | 22.06 cm² | 15 | 8 |
| 63 mm | 31.17 cm² | 10 | 6 |
| 70 mm | 38.48 cm² | 8 | 5 |
| 80 mm | 50.27 cm² | 6 | 3 |
| 89 mm | 62.21 cm² | 5 | 3 |
The table shows why a wide-mouth jar preform changes the sums on a 150 t machine. Each cavity's ledge area grows with the square of its diameter, so an 80 mm jar neck loads the clamp almost six times as much as a 33 mm bottle neck. Expect a handful of jar cavities where a bottle mould would run dozens. The 10% reserve is a starting margin, not a rule, and the two columns bracket a 400 bar guide pressure and a cautious 5 US t/in² basis.
Key Takeaways
- Jar preforms are limited by clamp area per cavity far sooner than bottle preforms; 70 mm necks allow about 5 to 8 cavities at 150 t.
- Heavier jar shots need a larger barrel, so check the shot window as well as clamp force.
- Wide necks are prone to going out of round; neck cooling and hold pressure need extra care.
- Thicker jar walls lengthen cooling with the square of the wall, which sets the cycle.
How does a wide-mouth jar preform change machine sizing?
Three things move. Clamp force per cavity rises with the neck diameter squared, the shot per cavity rises because jar preforms are heavier, and the mould grows wider for the same cavity count. On a 150 t machine, clamp area and the tie-bar gap set the jar cavity count.
Take the clamp first. With a 10% reserve, 150 t gives about 334 cm² of total ledge area on a 400 bar guide and about 194 cm² on the cautious basis. A 70 mm ledge is 38.48 cm², so the range is 5 to 8 cavities. Which figure to trust depends on wall, flow length and the resin, so start cautious and let mould trials show whether more cavities hold.
Shot weight comes next. PET melt is about 1.15 g/cm³ against 0.945 for PS, and the smallest PS barrel rating for a PET shot works out at about 1.39 times the shot weight once you allow a 65% maximum and 10% for leakage. Picking the screw within that range is covered in the PET screw options article in this series.
For food jars, the Relpet listing on UL Prospector names G5801 for foods, beverages, water and FMCG containers such as cosmetics and hair oil, and H7761 where the jar is hot-filled. Dry-goods jars for pulses, spices or snacks are still food contact, so the FSSAI grade checks covered in the PET products article in this series apply to them as well.
Why do wide jar necks go out of round, and how do you stop it?
A neck goes out of round when it forms poorly or deforms after ejection while still warm. PETplanet Insider treats a neck inside-diameter range above 0.25 mm around the circumference as a fault. Wide necks have more circumference to distort, so neck cooling and hold matter more.
The same guide lists the checks. Mould water at 7 to 10 °C, with flow to the neck inserts and cores at the mould maker's rating. Hold pressure that is not too low, especially early in hold, and hold time long enough. Mould cooling of about 1 to 3 s depending on wall, robot cooling time maximised without exceeding the cycle, and melt at about 280 ± 10 °C.
Fit the closure check into incoming inspection. A jar neck that is round enough on the gauge but can oval after a day in a hot store is still a reject at the filler, so measure necks on preforms that have cooled and rested, not straight off the robot. Before the mould is cut, ask the closure supplier for the cap's tolerance on neck diameter, so the neck insert is designed to it rather than adjusted after trials.
How does wall thickness set the jar preform cycle?
Cooling time rises with the square of wall thickness, so the thickest wall in the jar preform sets the cycle. Going from a 3.2 mm wall to 4.0 mm adds about 56% to cooling, and 4.5 mm nearly doubles it. Weight on its own is a weaker guide.
RJG's cooling analysis puts wall thickness at the centre of the cooling equation, squared, and adds about 20% to the calculated cooling timer as a safety factor. PETplanet Insider makes the related point that cycle time follows wall, not weight: two preforms with the same wall, one twice as heavy, may differ by only 1 to 2 s.
Ejecting too hot shows up as surface marks. PETplanet Insider notes that blemishes from preforms touching each other can be amplified when they leave the mould above 60 °C surface temperature. On jar preforms, cutting cooling to gain cycle can cost more in rejects than it saves. Design the wall to the thinnest that blows the jar well, and let the cycle follow.
What should you send to plan a wide-mouth jar preform mould?
Send the preform drawing with neck finish, ledge diameter, wall thickness and weight, the jar's fill type (cold, hot or dry goods), the cavities you want and the output target. With those, a supplier can check clamp area, barrel window, tie-bar room and cooling together. The clamp, tie-bar and daylight figures to check against are on the 150 ton PET injection moulding machine page.
Frequently Asked Questions About Wide-Mouth Jar Preforms
What is a wide-mouth jar preform?
It is a PET preform with a large neck finish, used to blow jars for foods, dry goods, cosmetics and household products instead of narrow-neck bottles. The wide neck lets customers spoon or pour the contents. It needs more clamp area and usually a heavier shot per cavity than a bottle preform.
Can the same machine run jar and bottle preform moulds?
Yes, if each mould fits the tie-bar gap, mould height range and shot window. Jar moulds tend to use few cavities and heavy shots, bottle moulds many cavities and light shots, so one screw may sit near the edge of its window for one of them. Check both moulds against the spec sheet.
What causes flash on jar preforms?
Flash is melt seeping between mould parts, from too low a melt viscosity or not enough clamp. Preform guidance lists injection rate too fast (above about 10 to 12 g/s per cavity), injection pressure peaking at transfer, and hold pressure too high early in hold. Check these before adding clamp force.
Why does my jar preform show a wavy body?
A wavy body comes from uneven contact cooling. The checks are mould cooling time of about 1 to 3 s by wall, hold pressure and time high enough, mould water at 7 to 10 °C with full flow to the mould and robot tubes, injection rate about 10 to 12 g/s per cavity, and melt near 280 °C.
Why does the gate string on jar preforms?
Stringing means the gate is too hot, usually on moulds without valve gates or where valve stems are not closing. Nozzle tip heat matters most: set it low enough to freeze the gate nub but not so low that crystallinity forms. Keep cooling water at 7 to 10 °C and cooling time about 1 to 3 s.
Related Articles
- What PET Products Can You Make on a 150 Ton Machine?
- Three Screw Options for PET on a 150 Ton Machine: Which One Fits Your Preform?
- Thick-Wall PETG Cosmetic Jars: Cooling Time and Sink Control
Sources & References
- PETplanet Insider, R. Unterlander and O. Brandau: The Rapid Guide to Perfect Preforms, part 20: Neck Finish out of Round (Vol. 12, 05/2011)
- RJG Inc.: How to Determine Injection Molding Cooling Time (Mar 2021)
- PETplanet Insider, R. Unterlander and O. Brandau: The Rapid Guide to Perfect Preforms, part 29: Surface Blemishes (Vol. 13, 05/2012)
- UL Prospector: Polyester: Relpet grade listing
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.
