Take a 5 litre PP bucket with a 200 mm rim, 190 mm tall, weighing 190 g with its sprue (assumed figures). With a 10% clamp reserve (a starting margin, not a rule), it needs about 97 to 146 t of clamp, about 282 cm³ of melt per shot and a barrel rated at about 411 g of PS or more. On a bucket making machine at 180 t, the clamp is comfortable; the shot decides.
This article sets out which bucket sizes fit 180 t, how to turn a bucket weight into a barrel size, what sets output and which PP grades suit buckets and pails.
Key Takeaways
- With a 10% reserve, round rims up to about 222 mm fit 180 t at either PP factor; up to about 272 mm only near a factor of 2.
- Shot ratings are usually given in PS, so convert through melt density: about 0.74 g/cm³ for PP against 0.945 for PS.
- Keep the shot plus a 10% allowance at or below about 65% of the barrel's swept volume.
- Cooling time rises with the square of the wall thickness, so the wall is the first lever on output.
Which Bucket Sizes Fit 180 t of Clamp?
Round rims up to about 222 mm fit at either end of the PP factor range, with a 10% reserve. Rims of about 230 to 270 mm fit only if trials confirm a lower factor; a 280 mm rim needs more than 180 t even at 2.
Clamp, shot and stroke for round PP buckets, one cavity (rims, heights and weights assumed; PP factor 2 to 3 US t/in²; 10% clamp reserve; shot at 65% of barrel with a 10% allowance; stroke at 2.5 × height)
| Nominal size (brimful) | Rim × height, weight | Clamp needed | Barrel needed (PS rating) | Opening stroke |
|---|---|---|---|---|
| 2 L (2.6 L) | 160 × 150 mm, 110 g | 62 to 93 t | 238 g | 375 mm |
| 5 L (5.1 L) | 200 × 190 mm, 190 g | 97 to 146 t | 411 g | 475 mm |
| 7 L (7.6 L) | 230 × 215 mm, 260 g | 129 to 193 t | 562 g | 538 mm |
| 10 L (10.9 L) | 260 × 240 mm, 330 g | 164 to 246 t | 713 g | 600 mm |
| 12 L (13.6 L) | 280 × 255 mm, 390 g | 190 to 286 t | 843 g | 638 mm |
The clamp figures use the projected area method explained in the applications article in this series. For a bucket, that area is the outer rim circle, since the wall and base sit inside it; handle lugs outside the rim add a little. The stroke column uses 2.5 times the bucket height, a sizing figure explained in the tubs and containers article in this series.
Read the table across, not only down. The 10 litre bucket fits the clamp if trials confirm a factor near 2.2, but its shot calls for a barrel rated at about 713 g of PS. For that size, the barrel, not the clamp, is the first thing to confirm.
How Much Shot Weight Does a Bucket Making Machine Need?
Enough swept volume that the bucket and sprue, plus a 10% allowance, use no more than about 65% of the barrel. Work in volume, not grams: divide the shot weight by PP's melt density of about 0.74 g/cm³, then compare it with the barrel's swept volume.
John Bozzelli lays out this method in Plastics Technology. Machine shot ratings are usually quoted in grams of PS, whose melt density is 0.945 g/cm³. Because PP melts at about 0.74, a barrel rated for a given weight of PS holds only about 78% of that weight in molten PP, before any allowance.
His working window runs from about 20% to 65% of barrel capacity. Below 20%, the melt sits too long and can degrade; above 65%, he links shots to poor melt uniformity and long recovery. He also notes that a second barrel ordered with a new machine can be low cost, which helps when a range spans small and large buckets.
Turned around, a barrel rated R g of PS gives a good PP shot of about 0.14 R to 0.46 R. A 500 g PS rating covers PP shots of roughly 71 to 232 g, which takes the 2 and 5 litre buckets in the table but not the 7 litre.
Our view: confirm the shot before the clamp on any bucket mould, and do it for the heaviest version your customers may ask for. A shot sitting right at 65% today leaves nothing for a wall that thickens during trials.
What Output Can One Bucket Mould Give?
Output per hour is 3,600 divided by the cycle in seconds, times cavities and the share of good shots. Cooling can take most of a bucket cycle, and cooling time rises with the square of the wall thickness, so the wall sets the ceiling on output.
RJG's guide to cooling time explains that wall thickness is squared in the cooling equation. In its trial on an 8-cavity ABS tensile bar 3.3 mm thick, about 85% of the cycle was cooling. Only about 40% of the heat must leave before ejection, and RJG suggests adding about 20% to the calculated cooling time as a safety factor.
The square works in your favour on buckets. A wall 10% thinner needs about 19% less cooling (0.9² = 0.81) and uses less resin per bucket. Whether the thinner bucket still passes drop and stacking tests is a design question, so test it before chasing the cycle.
Compare options in kilograms per hour as well as buckets per hour. A heavier bucket adds both resin and cooling time, so similar hourly counts can hide quite different PP use.
Which PP Grades Suit Buckets and Pails?
Impact copolymer PP, because buckets are dropped, stacked and carried full. Repol C080MA and C120MN are listed for paint pails, and B120MA for general moulding. Where pails are sold as packaging, the recycled-content targets covered in the tubs and containers article in this series can also apply.
The Repol B120MA TDS describes a medium-impact, antistatic copolymer with an MFR of 12 g/10 min, flexural modulus of 1,000 MPa and notched Izod of 85 J/m at 23 °C. It lists compliance with IS 10910, IS 16738:2018 and FDA 21 CFR 177.1520, and gives the BIS designation IS 10951-3-MBJ-C.
A UL Prospector listing of Repol grades places C080MA, a high-impact antistatic grade, and C120MN, with a balance of impact and stiffness, against paint pails. That listing carries no property values, so ask Reliance for the current TDS of either grade before trials.
Impact and stiffness trade against each other across these grades. Run drop and stacking tests on the actual bucket before switching from one grade to another.
What Should You Send for a Bucket Check?
Send the drawing with rim, height, wall and part weight, the resin grade, the cavity plan and the heaviest variant you might make. Clamp, shot and output can then be checked together. See the 180 ton horizontal injection moulding machine page.
Frequently Asked Questions About Bucket Moulding at 180 t
Can a 180 t machine make 20 litre buckets?
Not on clamp alone. If a 20 litre pail has a 300 mm rim (an assumed size), it projects about 707 cm² and needs about 219 t at a PP factor of 2 with a 10% reserve. Its heavier shot and longer stroke add to the gap, so plan pails of that size on a larger machine.
Why does a bucket stick on the core?
PP shrinks onto the core as it cools, and a deep bucket has a large gripping area. Stripper rings that push on the rim spread the ejection load better than a few pins. Generous draft helps, and nucleated grades can be harder to strip, so check draft before changing grade.
How many buckets can one machine make in a day?
Work it from your own cycle and good-shot rate. As an illustration, a 40 s cycle with one cavity and 97% good shots gives about 87 buckets an hour, or about 1,920 in a 22-hour day. Mould changes, colour changes and start-up scrap reduce that, so plan with some slack.
Can I add recycled PP to non-food buckets?
Often, but test it first. Entec Polymers notes that recycled PP compounds vary more than virgin resin and gives a typical melt range of about 199 to 227 °C, a residence limit of 5 minutes and a preference for cold runners. Run drop tests on buckets made with your actual blend.
Does a bigger screw give more bucket output?
Not by itself. A bigger screw sweeps more volume but usually gives lower plastic pressure, and Bozzelli advises against buying the biggest screw by default. Output is set mainly by cooling, so a bigger screw helps only when the shot, not the cycle, is the limit.
Related Articles
- What Can You Make on a 180 Ton Household Moulding Machine?
- Storage Boxes With Lids: Matching Shrinkage on a 180 Ton Machine
- 180 Ton Household vs Container Moulding Machine for Tubs and Containers
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 B120MA Technical Data Sheet
- UL Prospector: Polypropylene (PP): REPOL 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.
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