A 10 litre paint pail with a 250 mm rim (an assumed size) needs only about 152 to 228 t of clamp at PP factors of 2 to 3 US t/in² with a 10% reserve, but about 625 mm of opening stroke by a common sizing rule. On a paint bucket making machine at 320 t, stroke, mould height and shot can run out before clamp does.
This article covers clamp and stroke by pail size, resin choice, the effect of lids, handles and tamper bands on the mould, and the recycled-content rules that now apply to rigid plastic packaging in India.
Key Takeaways
- Pail rims are modest in area, so clamp rarely decides; a single 20 litre pail with a 300 mm rim needs about 219 to 328 t.
- Opening stroke of about 2.5 times the pail height is a common sizing rule, so a 250 mm tall pail needs about 625 mm.
- Reliance lists PP impact copolymers Repol C080MA and C120MN for paint pails.
- Rigid plastic packaging must contain at least 30% recycled material in 2025-26, rising to 60% by 2028-29.
What Clamp and Stroke Does a Paint Bucket Making Machine Need?
Clamp comes from the rim area and stroke from the height. Pail rims are modest, so 320 t covers one 20 litre pail at the lower PP factor. Height is the harder limit: by a common sizing rule, the mould must open about 2.5 times the pail height.
Clamp and opening stroke per cavity for round pails (rim and height are assumed sizes; PP factor 2 to 3 US t/in²; 10% reserve, a starting margin, not a rule; stroke at 2.5 × height)
| Pail | Rim diameter | Height | Clamp per cavity | Opening stroke |
|---|---|---|---|---|
| 1 litre | 120 mm | 110 mm | 35 to 53 t | 275 mm |
| 5 litre | 200 mm | 200 mm | 97 to 146 t | 500 mm |
| 10 litre | 250 mm | 250 mm | 152 to 228 t | 625 mm |
| 20 litre | 300 mm | 350 mm | 219 to 328 t | 875 mm |
The 2.5 times rule comes from Bruce Catoen's MoldMaking Technology guide to machine sizing. The large IML containers article in this series explains it in full, along with daylight and ejector stroke. The clamp figures follow the method in the applications article, with PP at 2 to 3 US t/in² as given by SPE Plastics Engineering.
A pail mould is also thick, because the core and cavity are both about as tall as the pail. Check the closed mould against the machine's maximum mould height first. A mould that will not fit cannot run at any clamp.
Which Resins Suit Paint Pails?
PP impact copolymers and HDPE. Pails are dropped, stacked full and opened many times, so toughness matters as much as stiffness. Impact copolymers add a rubbery phase to PP, and HDPE stays tough in the cold, which suits pails stored in unheated sheds or moved in winter.
Reliance lists two Repol impact copolymers with paint pails among their applications: C080MA, for high impact strength, and C120MN, for a balance of impact and stiffness. Both carry an antistatic agent, and their BIS designation codes are IS 10951-3-MBJ-B and IS 10951-3-MBJ-C. The listing appears in the UL Prospector materials database.
Check the pail against the paint it will hold. Solvent-based paints can attack or swell some plastics, so ask the paint maker for compatibility data and run a fill-and-store trial before releasing a new resin or a new recycled share.
How Do Lids, Handles and Tamper Bands Shape the Mould?
They add actions, undercuts and ejection work. Handle lugs on the pail wall, tamper-evident bands on the lid and snap beads on the rim need side cores, collapsing features or careful stripping. Each adds mould height, cost and sometimes cycle time, so settle them early.
Lids are flat, so they seldom limit stroke, but their tamper band and sealing bead must strip cleanly without tearing. That is a mould design question as much as a machine one. Involve the mould maker when you choose the band geometry and the resin, because the two work together.
Handles can be wire or moulded plastic. A moulded handle is a separate, smaller part, which suits a smaller machine than the pail. Keeping it off the 320 t machine frees hours for pails and lids. Ejector stroke for deep pails is covered in the large IML containers article in this series.
Do Recycled-Content Rules Apply to Paint Pails?
Yes, where the pail is rigid plastic packaging. The PWM (Amendment) Rules notified on 31 March 2026, as reported by Polymerupdate, require at least 30% recycled material in rigid packaging for 2025-26, rising to 60% by 2028-29. Exemptions are tied to food-contact rules such as FSSAI's.
The same report notes reuse targets for rigid packaging, a carry-forward of up to three years for 2025-26 shortfalls, and penalties from ₹10,000 to ₹15,00,000. Check the notified text for your current obligations before planning resin purchases.
Recycled PP can vary in melt flow and colour from lot to lot. Allow for that in the process window, and test drop strength and stacking at each recycled share before committing a customer order.
Our view: run trials at the 60% level now, not at 30%. A mould and process proven at the higher share will not need rework when the target rises, and the drop-test data will already be on file.
What Should You Send for a Pail Machine Check?
Send the pail and lid drawings with rim, height, wall, part weight, resin and planned recycled share, plus the closed mould thickness if the mould exists. Clamp, stroke, mould height and shot can then be checked together. See the 320 ton horizontal injection moulding machine page.
Frequently Asked Questions About Paint Pail Moulding
Can one machine mould both pails and their lids?
Yes, with separate moulds. A lid needs about the same clamp per cavity as its pail, because the rim outline matches, but far less stroke and shot. Running both on one machine means mould changes, so weigh changeover time against the output you need for each part.
Why do paint pails crack when dropped?
Common causes are a resin with too little impact strength for the pail size, cold storage, weld lines near the handle lugs, moulded-in stress from fast filling or a cold mould, and thin spots from core shift. Check the resin grade first, then gate position, wall uniformity and mould temperature.
How do I stop pails from wedging in the stack?
Stacking depends on draft angle, a stacking ring or lugs that stop pails nesting too deeply, and wall stiffness. Too little draft or a soft wall lets pails wedge together. Settle the stacking feature in the design, because adding it later means re-cutting the mould.
Can I use regrind in paint pails?
In most cases, within limits your customer accepts. Regrind from the same grade and colour, mixed at a fixed share, keeps melt flow steady. Repeat drop and stacking tests whenever the share changes, and keep regrind clean and dry, because contamination shows up as weak spots and specks.
What wall thickness suits a paint pail?
There is no single figure; it depends on pail size, resin, fill weight and stacking load. Thicker walls add stiffness and drop strength but also weight and cooling time. Start from the customer's drop and stacking requirements, then confirm the wall with trial pails before cutting production steel.
Related Articles
- What Can You Make on a 320 Ton Container Moulding Machine?
- When to Move From a 320 Ton Toggle Machine to a Two Platen Machine
- Large IML Containers: Daylight and Ejector Stroke on a 320 Ton Machine
Sources & References
- MoldMaking Technology, B. Catoen: Machine Sizing and Specification for Optimal Mold Performance, Part 1
- SPE Plastics Engineering, A. Urbina: Mastering Clamping Force Calculation
- UL Prospector (Reliance Industries data): Polypropylene (PP): REPOL grade listing
- Polymerupdate, D. K. Jha: India eases plastic recycling regulations
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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