Take a 10 litre PP bucket with a 240 mm rim, 260 mm tall, weighing 330 g (assumed figures). It needs about 140 to 210 t of clamp, about 491 cm³ of melt per shot including a 10% allowance, and about 650 mm of opening stroke. On a plastic tub making machine at 220 t, the clamp fits; shot and stroke decide the rest.
This article works through clamp by rim size, shot and barrel sizing, PP grades for larger buckets and tubs, and what sets their cycle time.
Key Takeaways
- At PP factors of 2 to 3 US t/in², rims up to about 245 mm fit 220 t at either factor; up to about 300 mm only near 2.
- Keep the shot, with a 10% allowance, between about 20% and 65% of the barrel's swept volume.
- Shot ratings are usually given in PS, so convert using melt densities of about 0.74 g/cm³ for PP and 0.945 for PS.
- Cooling time rises with the square of wall thickness, so a 25% thicker wall needs about 56% more cooling.
How Large a Bucket or Tub Fits 220 t of Clamp?
Round rims up to about 245 mm fit at either end of the PP range, with a 10% reserve. Rims of 260 to 300 mm fit only if trials confirm a factor near 2. Rectangular tubs follow the same rule: length times width against about 474 to 711 cm².
Clamp and opening stroke for round buckets and tubs, one cavity (rims and heights are assumed; PP factor 2 to 3 US t/in²; 10% reserve, a starting margin, not a rule; stroke at 2.5 × height)
| Rim diameter | Height | Projected area | Clamp needed | Opening stroke |
|---|---|---|---|---|
| 200 mm | 200 mm | 314 cm² | 97 to 146 t | 500 mm |
| 240 mm | 260 mm | 452 cm² | 140 to 210 t | 650 mm |
| 260 mm | 280 mm | 531 cm² | 164 to 246 t | 700 mm |
| 280 mm | 300 mm | 616 cm² | 191 to 286 t | 750 mm |
| 300 mm | 320 mm | 707 cm² | 219 to 328 t | 800 mm |
The clamp figures use the projected area method set out in the applications article in this series. For a bucket, the projected area is the outer rim circle, since the walls and base sit inside it. Handle lugs that stand outside the rim add a little area and belong in the sum.
Stroke can be the tighter limit. The 2.5 times height figure comes from a published machine sizing guide, and the deep moulds article in this series explains it alongside daylight and ejector stroke. Check both before you assume a tall bucket suits 220 t.
How Big a Shot Does a Plastic Tub Making Machine Need?
Work in volume. Divide the part and runner weight by the melt density of PP, add 10%, and keep the result between about 20% and 65% of the barrel's swept volume. Machine ratings in grams usually refer to PS, so convert before you compare.
John Bozzelli sets out this method in Plastics Technology. He gives melt densities of 0.945 g/cm³ for PS and about 0.74 g/cm³ for PP, against 1.04 and 0.90 at room temperature. That gap is why a barrel rated for a given weight of PS holds noticeably less weight of PP.
For the 10 litre bucket, 330 g ÷ 0.74 = 446 cm³ of melt, or 491 cm³ with the allowance. At 65%, the barrel must sweep at least 755 cm³, about 713 g on a PS rating. At 20%, it can sweep up to about 2,450 cm³, so an oversized barrel is not the risk for this bucket.
Bozzelli links shots above 65% to melt-quality problems and shots below 20% to long residence and degradation. He also notes that big screws usually mean lower plastic pressure, so a larger screw trades pressure for volume, which matters when long, thin bucket walls need pressure to fill.
Our view: size the barrel so your heaviest tub sits comfortably below 65%, not on it. A shot that only just fits leaves nothing for a wall that thickens during mould trials or a customer who asks for a heavier bucket.
Which PP Grades Suit Larger Buckets and Tubs?
Impact copolymer PP suits buckets and tubs that are dropped, stacked or carried full. Higher-flow grades help fill large parts at lower pressure and can shorten the cycle. Where the tub holds food, choose a grade listed against IS 10910 and IS 16738:2018.
Reliance's Repol B300MN is one example of a high-flow impact copolymer. Its TDS describes high flow and fast crystallisation that suit large articles with lower cycle times, with an MFR of 30 g/10 min, flexural modulus of 1,600 MPa and notched Izod of 65 J/m.
The same TDS lists compliance with IS 10910 and IS 16738:2018, the Indian positive list of constituents for PP and PE in food contact, and FDA 21 CFR 177.1520. Its BIS designation is IS 10951-3-MB-D. Ask your supplier for the current TDS and compliance certificate before releasing a food tub.
Higher flow is not free. A grade at 30 g/10 min fills more easily than one around 12, but stiffness and toughness balance differently, so run drop and stacking tests on the actual bucket before switching.
What Sets the Cycle Time of a Large Tub?
Cooling, which depends most on wall thickness and rises with its square. In one RJG trial, about 85% of the cycle was cooling. A tub with a 25% thicker wall needs about 56% more cooling time, so wall thickness is the first lever on output.
RJG's guide to cooling time also notes that only about 40% of the heat must leave the part before it can be ejected, and suggests adding about 20% to the calculated cooling timer as a safety factor. Even cooling of the base and rim lets you trim that margin once trials confirm it.
Output per hour is 3,600 divided by the cycle in seconds, times cavities and the share of good shots. A heavier tub adds cooling time and resin per part together, so compare designs on kilograms per hour as well as parts per hour.
What Should You Send for a Bucket or Tub Check?
Send the drawing with rim, height, wall thickness and part weight, the resin grade, and any handle or lid parts. Clamp, shot and stroke can then be checked together for 220 t. See the 220 ton horizontal injection moulding machine page.
Frequently Asked Questions About Larger Buckets and Tubs
How many buckets per hour can a 220 t machine make?
It depends on the cycle, which depends mainly on wall thickness and cooling. As an illustration, one cavity on a 45 s cycle with 97% good shots gives about 78 parts per hour. Work from your own wall thickness and a trial cycle rather than a quoted figure for a different bucket.
Can HDPE replace PP for buckets?
It can, where a bucket must stay tough in the cold or hold a chemical that PP does not resist. HDPE has its own shrinkage and tonnage factor, so the mould and the clamp check must be made for HDPE rather than carried over from a PP design. Confirm chemical resistance with your resin supplier.
Why do bucket rims go oval?
Uneven cooling between core and cavity, a single side gate, or ejecting too hot can pull a rim out of round. A centre gate in the base, balanced core cooling and enough cooling time before ejection keep the rim circular. Stacking warm buckets can also distort them, so let them cool first.
Should moulded bucket handles run on the same machine?
A moulded handle is a small, separate part. Running it on the 220 t machine ties up hours better spent on buckets, and a small shot on a large barrel stays in the heat too long. A smaller machine with a multi-cavity handle mould can suit the handles better.
Does a thicker wall make a bucket stronger?
It adds stiffness and drop strength, but cooling time rises with the square of the wall, so 3.0 mm instead of 2.5 mm needs about 44% more cooling. Ribs, a stiffer rim bead or a tougher grade can add strength with less cycle penalty. Test drop and stacking on trial buckets.
Related Articles
- What Can You Make on a 220 Ton Household Moulding Machine?
- Deep Houseware Moulds: Daylight and Ejector Stroke on a 220 Ton Machine
- Using Recycled PP on a 220 Ton Household Machine
Sources & References
- Plastics Technology, J. Bozzelli: A Simpler Way to Calculate Shot Size vs. Barrel Capacity
- RJG Inc.: How to Determine Injection Molding Cooling Time
- Reliance Industries: Repol B300MN Technical Data Sheet
- MoldMaking Technology, B. Catoen: Machine Sizing and Specification for Optimal Mold Performance, Part 1
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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