Picture a plant that runs 10 L buckets well, then loads a 15 L mould. The rim flashes, and the handles crack in the first drop test. Both problems trace back to machine and material choice. A bucket and tub making machine needs enough clamp for the rim's projected area, enough shot for thick walls and enough opening stroke to clear a deep core, and the part needs a PP grade with real impact strength. Storage boxes add one more check: the lid and box must shrink alike.
None of these can be fixed later with process settings alone, so they belong in the machine decision.
Key Takeaways
- A bucket's projected area is its rim outline, so clamp grows with the square of the rim diameter. A rim 20% wider needs about 44% more clamp.
- Buckets carried full need impact copolymer PP. One Indian impact copolymer grade lists a notched Izod of 85 J/m against 30 J/m for a houseware homopolymer.
- Wall thickness drives cycle time, because cooling time rises with the square of the wall.
- Deep parts need opening stroke: the mould must open far enough to clear the core and still let the part come out.
What Clamp and Shot Does a Bucket and Tub Making Machine Need?
Clamp follows the rim outline, because that is the area melt pressure pushes against when the mould is full. Shot follows wall thickness and height. A wider rim raises clamp quickly, since area grows with the square of the diameter, while a taller bucket mainly adds shot weight.
Use the PP tonnage factor of 2 to 3 US t/in² and the method set out in the tonnage and shot weight article in this series. The table applies it to round rims.
Round rims: projected area and PP clamp (factor 2 to 3 US t/in²; 10% margin, a starting margin, not a rule)
| Rim diameter | Projected area | Clamp at 2 US t/in² plus 10% | Clamp at 3 US t/in² plus 10% |
|---|---|---|---|
| 250 mm | 491 cm² | 152 t | 228 t |
| 280 mm | 616 cm² | 190 t | 286 t |
| 300 mm | 707 cm² | 219 t | 328 t |
| 320 mm | 804 cm² | 249 t | 373 t |
| 350 mm | 962 cm² | 297 t | 447 t |
Rim size moves the answer more than capacity. Going from a 250 mm to a 300 mm rim is 20% wider but needs 44% more clamp, because 1.2 squared is 1.44. A taller bucket with the same rim needs little extra clamp, but more shot and more mould height.
Handles, lugs and lid seats add mould complexity and side actions. They sit on the side walls, so they add little projected area.
Which PP Grade Should You Use for Buckets, Tubs and Boxes?
Use an impact copolymer for buckets and tubs that are carried full or dropped, and a stiffer homopolymer for lids and boxes that need rigidity and gloss. Compare grades on notched Izod impact and flexural modulus from the datasheet, not on melt flow rate alone.
Two Reliance datasheets show the trade-off. Repol B120MA, an impact copolymer with an antistatic agent, lists a notched Izod of 85 J/m and a flexural modulus of 1,000 MPa. Repol H110MAS, a nucleated homopolymer for houseware, lists 30 J/m and 1,650 MPa. The copolymer is nearly three times as tough and about 40% less stiff.
Both grades sit at a melt flow rate of 11 to 12 g/10 min, so flow alone will not separate them. Both datasheets state compliance with IS 10910 for food contact, and the B120MA sheet adds IS 16738:2018, the positive list of constituents for PP and PE in contact with food.
Our view: a bucket that fails a drop test is a material problem first and a process problem second. Do not try to rescue brittle handles on a homopolymer by raising melt temperature.
Lower stiffness has a cost. A copolymer wall may need extra thickness to hold its shape when full, which adds shot weight and cooling time.
How Do Wall Thickness and Depth Change Cycle Time and Mould Fit?
Wall thickness sets the cooling time, and on thick household parts cooling can be the largest share of the cycle. Depth sets the opening stroke and mould height the machine must accept. Check both before cycle time or machine size is quoted, because process settings cannot fix either later.
RJG explains that cooling time depends most on wall thickness, which is squared in the cooling equation. In its trial on a 3.3 mm ABS part, about 85% of the cycle was cooling. A wall cut from 2.5 mm to 2.0 mm, 20% thinner, needs about 36% less cooling. RJG also suggests adding about 20% to a calculated cooling time as a safety factor.
Depth is a mould-fit question. A bucket stays on the core as the mould opens, so the stroke must cover the part height to clear the cavity, then roughly the part height again for the part to come off the core, plus clearance. Ask the toolmaker for this figure before choosing the machine.
Storage boxes with lids add a shrinkage check. If box and lid are moulded in different grades, colours or wall sections, they can shrink by different amounts and the lid will not seat. Mould both in one grade and keep wall sections alike where you can.
What Should You Send Before Choosing a Bucket or Tub Machine?
Send the drawing or sample, capacity, rim size, height, wall thickness, part weight, resin grade, cavities and target monthly output. Add the mould's outside size, shut height and the opening stroke the toolmaker needs. With those, clamp, shot, stroke and cycle can be checked together. See the household articles injection moulding machine range.
Frequently Asked Questions About Bucket and Tub Making Machines
Why do bucket rims flash before the rest of the part?
The rim sits at the parting line at the end of flow, where the mould is most likely to open if clamp is short or the mould plates flex. Check the clamp calculation against the rim outline first. If clamp is adequate, look at vents, parting-line wear and mould support before raising clamp further.
Can one machine mould both 10 L and 20 L buckets?
It can if the larger mould fits the platen, tie-bar spacing, shut height and opening stroke, and the clamp and shot cover the bigger bucket. Run the smaller mould at the clamp it needs, not at the machine maximum. The trade-off is running cost on the smaller job, so check the volumes of each.
Do storage box lids need a different grade from the box?
Not necessarily, and using the same grade makes the fit easier to hold. Different grades, colours or wall sections can shrink differently, so a lid that fits on day one may not fit a box from another resin lot. If the lid needs more flex, trial the fit across lots before production.
How thick should a bucket wall be?
There is no single figure, because it depends on capacity, load, grade and drop requirement. What the machine sees is the result: every extra 0.1 mm adds shot weight across the whole surface, and cooling time rises with the square of the wall, so a thicker wall costs cycle time as well as resin.
What output can a bucket mould give per hour?
Output is 3,600 divided by the cycle in seconds, times cavities, times the share of good shots. A single-cavity bucket at a 40 s cycle with 97% good shots gives about 87 per hour. Cycle depends heavily on wall thickness and cooling, so confirm it on a trial before planning capacity.
Related Articles
- Household Articles Moulding Machines: Tonnage and Shot Weight Guide
- Large Shot Weight Injection Moulding: Screw and Pump Selection
- Planning a Houseware Unit: Which Machine Sizes to Start With
Sources & References
- Plastics Engineering (SPE), A. Urbina: Mastering Clamping Force Calculation
- Reliance Industries: Repol B120MA technical data sheet (July 2024)
- Reliance Industries: Repol H110MAS technical data sheet (July 2024)
- RJG Inc.: How to Determine Injection Molding Cooling Time
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.
