1,000 g ÷ 0.945 g/cm³ = 1,058 cm³. That is the swept volume behind a barrel rated at 1 kg of PS. Fill it with PP melt at about 0.74 g/cm³, allow 10% for check-valve leakage and keep the shot within 65% of the barrel, and the largest good PP shot is about 463 g, not 1 kg. A large shot weight screw for a true 1 kg PP shot needs roughly twice that swept volume.
That gap is why heavy household and industrial shots need care on a 350 t machine. This article covers how to size them, what happens above the limit, and how screw diameter trades volume for pressure.
Key Takeaways
- Shot ratings in grams usually refer to PS; a barrel holds only about 78% of that weight in PP melt.
- Keep each shot, plus 10%, between about 20% and 65% of swept volume; above 65%, melt quality and recovery suffer.
- PP needs about twice the melting energy per pound of ABS, which makes large PP shots harder work for the screw.
- On one injection unit, a larger screw sweeps more volume but gives proportionally less injection pressure.
How Much PP Does a Barrel Rated Near 1 kg Really Hold?
About 463 g per good shot, on Bozzelli's limits. A 1,000 g PS rating means about 1,058 cm³ of swept volume. At a PP melt density of 0.74 g/cm³, with 10% added and the shot capped at 65%, that volume supports a PP shot of roughly 460 g.
John Bozzelli's Plastics Technology column on shot size gives melt densities of 0.945 g/cm³ for PS and about 0.74 for PP, against room-temperature densities of 1.04 and 0.90. He advises working in volume, adding 10% for check-valve leakage and melt uniformity, and sizing the barrel so the shot uses no more than 65% of it.
Good PP shot window for barrels of different PS ratings (calculated: PS melt 0.945 g/cm³, PP melt 0.74 g/cm³, 10% added, shot 20% to 65% of swept volume)
| Barrel rating (g PS) | Swept volume | Largest good PP shot | Smallest good PP shot |
|---|---|---|---|
| 800 g | 847 cm³ | 370 g | 114 g |
| 1,000 g | 1,058 cm³ | 463 g | 142 g |
| 1,200 g | 1,270 cm³ | 555 g | 171 g |
| 1,500 g | 1,587 cm³ | 694 g | 214 g |
| 2,000 g | 2,116 cm³ | 925 g | 285 g |
Read the rating as a ceiling, not a target. A barrel rated at 1,500 g of PS still keeps PP shots below about 700 g inside the 65% limit, and a true 1 kg PP shot points to a rating above about 2,160 g.
What Happens When a Large Shot Runs Above 65%?
Melt quality and recovery suffer. In another Plastics Technology column, Bozzelli links shots above 65% of barrel capacity to unmelted pellets, poor melt uniformity and long recovery, and notes that PP and PE at large shots can even grind metal off the screw flights.
The reason is energy. Bozzelli writes that about 80% of melting energy should come from screw shear, and that PP needs about 300 BTU per pound to melt against about 150 for ABS. A large PP shot asks the screw to put twice the energy per kilogram into the melt within the same recovery time.
A shot near the top of the barrel also gives the pellets fewer screw turns to melt, so unmelt and colour streaks tend to appear first, and part weight starts to drift from shot to shot.
Our view: never size a screw for heavy shots from the PS gram rating on a quotation. Convert your heaviest shot to PP melt volume, add 10% and divide by 0.65; a barrel smaller than that figure will run the part, but not run it well.
How Do You Choose a Large Shot Weight Screw Diameter?
Choose the smallest diameter whose swept volume keeps your heaviest shot within 65%, then confirm it gives enough pressure for your hardest mould. On the same injection unit, swept volume rises with the square of screw diameter and injection pressure falls by the same ratio.
Bozzelli's column on intensification ratio explains the pressure side. The ratio is hydraulic ram area divided by screw area, ranges from 6:1 to 43:1 across machines, and changes whenever the screw size changes. In his example, a mould needing 1,585 bar of plastic pressure would not fill at 1,380 bar but filled at 2,014 bar.
For two screws of diameters D1 and D2 on one unit, volume scales by (D2 ÷ D1)² and pressure by the inverse. A screw 8% larger in diameter sweeps about 17% more volume and gives about 86% of the pressure. Thick, short-flow parts may accept that trade; long, thin flows may not.
Resin flow can widen the choice. Reliance's Repol B300MN is a high-crystalline PP impact copolymer with an MFR of 30 g/10 min, which its TDS describes as suited to large articles with lower cycle times. It lists IS 10910 compliance and the BIS designation IS 10951-3-MB-D.
What About Small Shots on the Same Barrel?
They can degrade. Bozzelli warns that shots below about 20% of barrel capacity stay in the barrel too long. A barrel sized for a 900 g PP shot leaves anything under about 280 g at risk, so a range mixing heavy and light parts may need a second, smaller barrel.
He notes that a second barrel ordered with a new machine can be low cost, and that a skilled operator can change a barrel in under 30 minutes, even hot. For a plant that runs 900 g bins and 200 g lids on one machine, two barrels can serve both without a compromise screw.
Bozzelli also cautions against buying the biggest screw on offer, since bigger screws usually mean lower plastic pressure. Size for the shot list you will run most, not for the single heaviest part you might quote one day.
What Should You List Before Choosing a Screw and Barrel?
List every mould planned for the machine with part and runner weight, resin grade, cavities and longest flow path. Convert each shot to melt volume, add 10%, and check it against the 20% to 65% window for each screw option. See the 350 ton horizontal injection moulding machine page.
Frequently Asked Questions About a Large Shot Weight Screw
Why are shot weights on datasheets given in PS grams?
Because PS is the usual reference resin for rating barrels; Bozzelli notes ratings are normally given in grams or ounces of PS. PP melt is less dense, so the same barrel holds only about 78% of the rated weight in PP. Convert to volume before comparing your part with any machine.
Can I run a large shot near 100% of the barrel to avoid a bigger machine?
It is not advisable. Above about 65%, Bozzelli links shots to unmelt, poor uniformity and long recovery, and for PP or PE to flight wear. Parts may fill but vary in colour, weight and strength. Fewer cavities or a larger barrel is the steadier fix than running at the limit.
How do I know if screw recovery is too slow for a large shot?
Compare recovery time with cooling time on the cycle display. If the screw is still turning when the mould is ready to open, recovery is setting the cycle. Large PP shots need about twice the melting energy per pound of ABS, so raising screw speed or back pressure can overheat the melt.
Does a screw wear faster with big PP shots?
It can, when shots run above the window. Bozzelli notes that PP or PE at shots above 65% of barrel capacity can grind metal off the flights. A screw sized so that shots sit between about 20% and 65% works within its design, which protects both the flights and the check valve.
Is any 350 t machine's barrel enough for a 1 kg PP shot?
Check its swept volume, not its tonnage. Clamp and injection unit are sized separately, and machines of one tonnage can carry different screws. For a 1 kg PP shot within 65%, look for about 2,290 cm³ of swept volume, a PS rating above about 2,160 g, or plan a larger injection unit.
Related Articles
- What Can You Make on a 350 Ton Household Moulding Machine?
- Moulding the Largest Household Articles on a 350 Ton Machine
- 350 Ton Toggle vs Two Platen Machine: Which Suits Your Mould?
Sources & References
- Plastics Technology, J. Bozzelli: A Simpler Way to Calculate Shot Size vs. Barrel Capacity
- Plastics Technology, J. Bozzelli: Revisiting Shot Size vs. Barrel Capacity
- Plastics Technology, J. Bozzelli: Understanding Intensification Ratio
- 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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