A cream brand wants a heavy, glass-like PETG cosmetic jar with a 6 mm wall. The first trial parts come off a long cycle with a sink mark in the base, a few bubbles in the thick corner and splay near the gate. All three trace back to the same facts. Cooling time rises with the square of wall thickness, so a 6 mm wall needs about four times the cooling of a 3 mm one. Thick sections shrink most and need packing until the gate freezes. And PETG must be dried properly and moulded cool and slow to stay clear.
Key Takeaways
- Doubling the wall roughly quadruples cooling time, so wall thickness decides the jar's cycle and cost.
- Sink and voids come from thick sections shrinking after the gate freezes; gate into the thickest section and hold until the gate seals.
- Dry PETG at 65 to 70 °C for at least 6 hours with -30 °C dew point air, never hotter.
- Mould PETG at 250 to 270 °C melt and 15 to 40 °C mould, with slow injection to avoid splay.
Why does a thick-wall PETG cosmetic jar take so long to cool?
Heat has to leave through the wall, and cooling time rises with the square of wall thickness. A jar wall twice as thick needs about four times the cooling. In RJG's trial on a 3.3 mm ABS part, about 85% of the whole cycle was spent cooling.
RJG also notes that only about 40% of the heat must be removed before the part can be ejected, and it adds about 20% to the calculated cooling timer as a safety factor. EnviroTech's design guidance makes the same point from the other side: doubling wall thickness can need 3 to 4 times the cooling time.
On a jar, the slowest point is the thickest one, typically where the base meets the side wall. Calculate cooling there, not at the nominal wall. The mould matters too: a deep core needs its own cooling, through a bubbler or baffle reaching the core top, or the inside of the jar stays hot long after the outside has set.
Relative cooling time by wall thickness (3 mm = 1.0; scales with wall squared)
| Wall | Relative cooling | If 3 mm cools in 15 s |
|---|---|---|
| 3 mm | 1.0 | 15 s |
| 4 mm | 1.78 | 26.7 s |
| 5 mm | 2.78 | 41.7 s |
| 6 mm | 4.0 | 60 s |
| 8 mm | 7.1 | 106.7 s |
What causes sink and voids in thick PETG walls?
Thick sections cool last and keep shrinking after the outer skin sets. If the skin is weak, it pulls in as a sink mark; if the skin is strong, the core pulls apart into a void. Packing only helps while the gate stays open to feed more melt.
EnviroTech advises keeping walls under 6.4 mm to avoid shrink voids, especially far from the gate, and keeping wall thickness constant so uneven shrinkage does not build stress and warp. Where the wall must vary, it should thin progressively along the flow direction.
In practice that gives four controls. Gate into the thickest section so it can be packed. Make the gate and runner large; Eastman recommends full-round, large runners, large gates and generously radiused corners for its copolyesters. Hold pressure until the gate has frozen, confirmed by part weight against hold time. And where the look allows, core out the base or use a thinner wall with a glass-effect design rather than solid mass.
A flat view: if the design asks for a wall above about 6 mm, push back on the design before you buy a machine for it. No process setting fully rescues a jar that is thick for the sake of weight.
How should PETG be dried and processed for clear jars?
Dry PETG in a desiccant dryer at 65 to 70 °C for at least 6 hours, with air at a dew point of -30 °C or lower. Then mould at the lowest melt temperature that fills, normally 250 to 270 °C, with a cool mould of 15 to 40 °C.
Eastman's injection moulding dos and don'ts for its copolyesters add the details. Drying above 70 °C makes pellets stick and bridge, so you cannot shorten drying by raising temperature. Use slow screw speeds of 30 to 60 rpm, avoid high back pressure except with regrind or colour, and use slow injection, or a slow-moderate-slow profile, to reduce streaking and gate splay. Mould temperatures above 55 °C can make parts stick. Allow shrinkage of 0.003 to 0.005 mm/mm, and use small conventional vents of 0.03 to 0.13 mm.
Eastman also suggests planning the shot at no less than 50% of machine capacity, preferably 75% to 80%. That points to a smaller screw for heavy jars rather than the largest one, a choice covered in the PET screw options article in this series.
Cosmetic jars are rigid plastic packaging, and the PWM (Amendment) Rules notified in March 2026 set recycled-content targets for that category. Ask the brand owner how its targets apply to your jar before the material is fixed.
What should you send to plan PETG cosmetic jar production?
Send the jar and lid drawings with wall thickness at the base and side, jar weight, the finish required, the gate position if the mould exists, and the output target. A supplier can then estimate cooling, check shot as a share of barrel capacity, and suggest where wall can come down. See the 150 ton PET injection moulding machine page for machine details.
Frequently Asked Questions About PETG Cosmetic Jars
Is PETG the same as PET?
No. PETG is a glycol-modified copolyester that stays amorphous, so it moulds clear in thick sections without the crystallinity haze PET can show. It also dries at a much lower temperature, about 65 to 70 °C, against roughly 160 to 175 °C for bottle-grade PET. Do not dry PETG at PET settings, or the pellets will stick.
Why do PETG jars stick in the mould?
Sticking often points to a mould running too warm. Eastman advises avoiding mould temperatures above 55 °C for its copolyesters. If a warmer mould is needed for fill, it notes that a lubricant such as zinc stearate at about 0.1% can help without affecting clarity, while higher levels may cause yellowing.
Can I use regrind in PETG cosmetic jars?
Yes, within limits. Eastman notes that scrap which was dried and processed properly and is free of contamination may be blended with virgin material up to 25%. Regrind must be dried again before use. For clear premium jars, keep regrind lower and watch for colour shift or haze lot to lot.
What causes splay or streaks near the gate on PETG jars?
Splay often points to moisture or too fast an injection. Check the dryer dew point and drying time first, then keep the material sealed from the dryer to the hopper, as PETG picks up moisture quickly in open air. Use slow injection or a slow-moderate-slow profile, and the lowest melt temperature that fills.
What shrinkage should the mould maker allow for PETG?
Eastman gives 0.003 to 0.005 mm/mm for its copolyesters, depending on mould size and design. That is far lower than semi-crystalline plastics such as PP. Thick sections still shrink more than thin ones, so confirm critical dimensions such as lid fit on trial parts before the mould is hardened.
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?
- Wide-Mouth Jar Preforms on a 150 Ton Machine
Sources & References
- Eastman: Eastar copolyesters: Some dos and don'ts for injection molding (SP-TRS-5434, 3/18)
- RJG Inc.: How to Determine Injection Molding Cooling Time (Mar 2021)
- EnviroTech Molded Products: Molded Parts Design
- Polymerupdate, D. K. Jha: India eases plastic recycling regulations (8 Apr 2026)
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.
