A paint maker asks for 10 litre pails on the new 260 t press. The clamp sum looks fine, the mould arrives, and then the pail will not clear the core when the mould opens. For plastic pail moulding, clamp is only the first of four checks: clamp, opening stroke, shot size and grade.
A 260 t machine can run a 5 litre pail comfortably and a 10 litre pail where the grade flows easily. This article takes each check in turn, with a 10 litre worked example at the end.
Key Takeaways
- A 10 litre pail with a 290 mm rim needs about 204 t at a tonnage factor of 2 and about 255 t at 2.5, so 260 t holds it only near the easy end.
- Published guidance asks for an opening stroke of at least 2.5 times the part height, which for a 250 mm pail is 625 mm.
- Size the barrel so the shot uses no more than 65% of it, after adding 10% to the melt volume for check-valve leakage.
- PP impact copolymers trade some stiffness for drop strength; one Indian copolymer grade lists notched Izod of 85 J/m at a flexural modulus of 1,000 MPa.
Which Pail and Tub Sizes Fit 260 t by Clamp?
A single-cavity 5 litre pail fits at any conventional factor up to about 4. A 10 litre pail fits only if a fill study shows a factor of about 2.5 or less. A 20 litre pail does not fit, even at a factor of 2.
Clamp needed per cavity for pails and large tubs, with a 10% reserve (calculated; rim sizes are assumptions)
| Item | Rim | Area | At factor 2 / 3 | Highest factor at 260 t |
|---|---|---|---|---|
| 5 kg grease tub | 200 mm round | 314.2 cm² | 97.1 t / 145.8 t | about 5.4 |
| 5 litre pail | 230 mm round | 415.5 cm² | 128.4 t / 192.9 t | about 4.0 |
| 10 litre pail | 290 mm round | 660.5 cm² | 204.2 t / 306.6 t | about 2.5 |
| 20 litre pail | 330 mm round | 855.3 cm² | 264.4 t / 397.0 t | below 2, does not fit |
The clamp method and the factors behind these figures are set out in the container applications article in this series. For pails, the rim outline is the projected area, and a hot runner keeps the runner out of the sum.
How Much Opening Stroke Does Plastic Pail Moulding Need?
At least 2.5 times the pail height, by one published rule. A 250 mm pail then needs about 625 mm of stroke so the part clears the core and drops or is picked out. The machine must also open that far at the mould's actual shut height.
Bob Catoen's MoldMaking Technology article on machine sizing gives that rule and explains the difference between clamp types. On a toggle clamp, maximum daylight equals maximum shut height plus stroke, so full stroke is available at every shut height. On a direct hydraulic clamp, maximum daylight is minimum shut height plus maximum stroke, so a tall mould eats into the stroke.
Pail moulds are tall, because the core is as long as the pail is deep. Check the mould's shut height against both the minimum and maximum the machine allows, then confirm that the stroke left over still clears the pail and the handle lugs.
The same article asks you to check mould weight for both platens. A heavy pail mould hanging on the moving platen loads the guides and tie bars, and that load grows with every year of running.
How Big a Barrel Does a Heavy Pail Shot Need?
Big enough that the shot uses no more than about 65% of it, and small enough that it uses at least 20%. Work in volume, not grams, because machine shot ratings are usually given in grams of polystyrene and PP is much lighter in the melt.
John Bozzelli's Plastics Technology method uses melt densities of 0.945 g/cm³ for PS and about 0.74 g/cm³ for PP. Convert the pail weight to melt volume, add 10% for check-valve leakage and melt uniformity, then divide by 0.65 to find the smallest barrel that keeps melt quality sound.
He also warns against buying the biggest screw by default. A large screw usually means lower plastic pressure, and shots below 20% of the barrel sit too long and degrade. If one press must run both pails and small tubs, he notes that a second barrel ordered with a new machine can be low cost.
Which PP Grade Suits Pails and Large Tubs?
An impact copolymer where the pail must survive drops, and a higher-flow grade where the wall is long and thin. Indian resin makers publish both kinds. The choice trades stiffness against toughness, so set it from the drop test your customer runs, not from habit.
Reliance lists Repol B120MA, a medium-impact PP copolymer with an MFR of 12 g/10 min, notched Izod of 85 J/m and a flexural modulus of 1,000 MPa. Repol B300MN, a high-crystalline copolymer, has an MFR of 30 g/10 min, notched Izod of 65 J/m and a flexural modulus of 1,600 MPa. Reliance says its high flow and fast crystallisation suit large articles with lower cycle times.
Our view: on a 10 litre pail near the clamp limit, the higher-flow grade is often what makes the job fit, because easier flow lowers the tonnage factor. Confirm drop strength at the handle lugs before you switch, since the lugs carry the whole filled weight.
What Should You Send Before Ordering a Pail Mould?
Send the pail drawing with rim, height and wall thickness, the part weight, the handle and lid design, the resin grade and the drop test your customer applies. With these, clamp, stroke, shut height and barrel size can be checked before steel is cut. See the 260 ton horizontal injection moulding machine page.
Frequently Asked Questions About Plastic Pail Moulding
Why do pail rims warp or go oval?
Rims are often the thickest and last-filled section, so they shrink and cool unevenly. Uneven core cooling makes it worse, because the pail grips a hot core and releases it unevenly. Check core cooling first, then gate position and pack time, before lengthening the cycle to hide the problem.
Should pail handles be moulded in place or fitted later?
Pail handles can be steel wire or separate plastic parts fitted after moulding into lugs on the side wall. The lugs need side actions or collapsing cores in the mould. Moulding the handle in place needs a more complex mould, so compare that cost with a fitting station on the line.
Can I run pails and pail lids on the same 260 t press?
Yes, in campaigns. A lid has the pail's rim outline, so it needs similar clamp per cavity, but it is shallow and needs far less stroke. Plan stock levels so that lid campaigns fit between pail runs, and budget the mould change time in your effective hours.
How does wall thickness affect pail cycle time?
Cooling time rises with the square of wall thickness, so a modest increase in wall costs a disproportionate share of cycle. A pail with a thick rim and base cools at the speed of those sections, not the side wall. Keep wall sections as even as the design allows.
Is a direct hydraulic or toggle clamp better for pails?
Both are used. On a toggle, full stroke is available at every shut height, which suits tall moulds. On a direct hydraulic clamp, a tall mould reduces the stroke left over, so check the remaining stroke against 2.5 times the pail height before you choose.
Related Articles
- What Can You Make on a 260 Ton Container Moulding Machine?
- High-Cavity Thin-Wall Moulds on a 260 Ton Machine
- Sweet Boxes and Food Trays: Planning Output on a 260 Ton Machine
Sources & References
- MoldMaking Technology, B. Catoen: Machine Sizing and Specification for Optimal Mold Performance, Part 1
- Plastics Technology, J. Bozzelli: A Simpler Way to Calculate Shot Size vs. Barrel Capacity
- Reliance Industries: Repol B120MA Technical Data Sheet
- Reliance Industries: Repol B300MN Technical Data Sheet
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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