A plant head has a free 350 t machine and three enquiries: a round dustbin with a 360 mm rim, 480 mm tall; a slotted laundry basket of 450 × 350 mm; and a stool with a 330 mm seat. For large household articles moulding at this size, all three can pass the clamp check only near a PP factor of 2, and the dustbin only just. What decides the job after that is opening stroke, flow length against wall thickness, cooling time and ejection.
This article works through those checks for the biggest parts a 350 t machine can take, using the dustbin as the running example.
Key Takeaways
- At a PP factor of 2 with a 10% reserve, 350 t holds about 1,130 cm², so a 360 mm round bin sits close to the limit.
- A flow length more than 150 times the wall raises the risk of short shots and sinks, and pushes the tonnage factor up.
- Cooling time rises with the square of wall thickness, so a wall 20% thinner needs about 36% less cooling.
- Deep bins and tubs need generous ejection: more surface area, larger pins, or a stripper acting on the rim.
What Limits the Largest Articles a 350 t Machine Can Run?
Four things: projected area against clamp, part height against opening stroke, flow length against wall thickness, and cooling and ejection against cycle time. The first two decide whether the job can run at all; the last two decide whether it runs well, and at what cost.
The clamp limit is worked out in the applications article in this series: about 1,130 cm² of PP at a factor of 2 and about 750 cm² at 3. Opening stroke and daylight are covered in the toggle vs two platen article; as a rough guide, plan at least 2.5 times the part height as stroke.
Mould size is a fifth check, and it arrives early with big articles. Bin and basket moulds are wide and tall, with long cores and often a stripper plate, so ask the toolmaker for the outside dimensions and weight before you shortlist a machine.
How Long a Flow Path Can a Large Article Fill?
Compare flow length with wall thickness. Jeremy Williams of RJG gives an aspect ratio below 150:1 as low risk for sinks, short shots and dimensional problems. Large bins and baskets can exceed that, so they need well-placed gates, a higher-flow grade or a higher tonnage factor in the clamp check.
Flow length is the longest distance the melt travels from the gate to the last point it fills. On a centre-gated bin it runs across the base and up the wall to the rim. Williams' Plastics Technology article lists flow length, wall thickness, gate location, gate size and the number of gates among the factors that move the tonnage factor.
Flow length to wall ratio for centre-gated large articles (sizes and gate position assumed; flow length = gate to farthest base edge plus height)
| Article | Flow length | Ratio at 2.5 mm wall | Ratio at 3.0 mm wall |
|---|---|---|---|
| Bucket, 300 mm rim, 300 mm tall | 450 mm | 180:1 | 150:1 |
| Storage box, 380 × 280 mm, 250 mm deep | 486 mm | 194:1 | 162:1 |
| Laundry basket, 450 × 350 mm, 300 mm tall | 585 mm | 234:1 | 195:1 |
| Stool, 330 mm seat, 430 mm tall | 595 mm | 238:1 | 198:1 |
| Round dustbin, 360 mm rim, 480 mm tall | 660 mm | 264:1 | 220:1 |
That link matters at 350 t. A bin that passes at a factor of 2 may need 2.5 or more once a long, thin flow is allowed for, and the clamp check then fails. A second gate shortens the flow, but it also leaves a weld line, which matters on a rim that carries a load.
Our view: on the largest articles, fix the flow path before arguing about clamp. A gate move or a slightly thicker wall can cost far less than the next machine size up.
What Sets Cycle Time in Large Household Articles Moulding?
Cooling, which depends most on wall thickness and rises with its square. In an RJG trial on a 3.3 mm ABS part, about 85% of the cycle was spent cooling. On large articles, fill and movement times matter less than getting heat out of the thickest wall evenly.
RJG's guide to cooling time notes that only about 40% of the heat must be removed before the part can be ejected, and suggests adding about 20% to the calculated cooling time as a safety factor. A wall 20% thinner needs about 36% less cooling, because 0.8² is 0.64.
Thick spots set the timer. A rim bead, a handle boss or a heavy rib cools more slowly than the wall around it and can hold the whole part in the mould. Keep walls as even as the design allows, and cool the core of a deep part as well as the cavity, since a bin shrinks onto its core.
Output then follows from the cycle: 3,600 divided by the cycle in seconds, times cavities, times the share of good shots. Every second of cooling saved on a one-cavity bin shows up directly in parts per hour.
Which Ejection and PP Grade Suit Large Articles?
Use as much ejection area as the part allows, with larger pins at the deepest points or a stripper plate on the rim of bins and tubs. For the resin, impact copolymer PP suits articles that are dropped, kicked or sat on, such as bins, baskets and stools.
Peter Thal's ejection article in MoldMaking Technology advises placing ejection at the deepest areas of the part, erring toward more ejectors and choosing larger pin diameters. Strippers push on the part perimeter and cost the most, since they are fully custom, but a deep bin pulled off a long core is where that cost pays back.
For stools and bins, one example is Reliance's Repol B120MA, a medium-impact, antistatic copolymer whose TDS names furniture and industrial components among its uses. It lists 85 J/m notched Izod at 23 °C against a 1,000 MPa flexural modulus, trading some stiffness for toughness, and complies with IS 16738:2018, the Indian positive list of constituents for PP and PE in food contact.
What Should You Check Before Quoting a Large Article?
Gather the outline, height, nominal and thickest wall, gate position, part weight and resin grade for each article, plus the mould's outside size and weight. Run the clamp, stroke, flow ratio and shot checks together for 350 t. See the 350 ton horizontal injection moulding machine page.
Frequently Asked Questions About Large Household Articles Moulding
Can a 480 mm tall dustbin be moulded on a 350 t machine?
Only if the machine opens far enough. At 2.5 times the height, a 480 mm bin wants about 1,200 mm of opening stroke, plus daylight for the closed mould. Clamp is close at a PP factor of 2, so check stroke, mould height and daylight on the datasheet before anything else.
Should a large household article have more than one gate?
It can help when one gate gives a flow ratio well above 150:1. Extra gates shorten the flow and lower the pressure needed, but each meeting point leaves a weld line that is weaker than the wall around it. Keep weld lines away from rims, handles and hinge areas that carry load.
Why does a deep bin stick on the core?
Because PP shrinks onto the core as it cools, and a tall core gives a large gripping area. Adequate draft, a well-cooled core, ejection at the deepest points and a stripper or air blow at the rim all help. If sticking starts mid-run, check core cooling before adding ejection force.
Does a thicker wall make a large article stronger?
Stiffer, yes, but at a cost. Cooling rises with the square of wall thickness, so a 25% thicker wall needs about 56% more cooling, and the part also weighs more. Ribs can often add stiffness for less material and less cooling time than a thicker wall across the whole part.
How many large articles per hour can a 350 t machine make?
It depends on the cycle, which on large parts can be dominated by cooling. Output per hour is 3,600 divided by cycle time in seconds, times cavities, times the share of good shots. A one-cavity bin on a 60 s cycle with 98% good shots gives about 59 per hour; at 50 s, about 71.
Related Articles
- What Can You Make on a 350 Ton Household Moulding Machine?
- 350 Ton Toggle vs Two Platen Machine: Which Suits Your Mould?
- Shot Weights Near 1 kg: Screw and Barrel Choice on a 350 Ton Machine
Sources & References
- Plastics Technology, J. Williams (RJG): Clamp Tonnage: More Is Better...Right?
- RJG Inc.: How to Determine Injection Molding Cooling Time
- MoldMaking Technology, P. Thal (Thal Precision): Three Key Decisions for an Optimal Ejection System
- Reliance Industries: Repol B120MA 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.
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