Container machine screw options are usually three screw diameters on one injection unit: a small screw for higher pressure and thin walls, a medium screw for general work, and a large screw for heavy shots. The same hydraulic force drives all three, so every gain in shot volume costs injection pressure.
The right screw is the one that puts your main shot between about 20% and 65% of barrel capacity while still giving the pressure and fill speed the mould needs. On a 220 t container machine, that choice can matter as much as the clamp.
Key Takeaways
- A screw 10% larger in diameter gives about 21% more swept volume and about 17% less maximum injection pressure on the same unit.
- Keep each shot at about 20% to 65% of barrel capacity; thin-wall work wants at least 40%.
- Where all three screws share one barrel length, the larger screws have a lower L/D ratio.
- A mix of heavy and light jobs can justify a second barrel ordered with the machine.
What Are Container Machine Screw Options and Why Offer Three?
They let one machine frame match different shot weights. The clamp and hydraulics stay the same; only the screw and barrel change. A smaller screw concentrates the hydraulic force on less area for more pressure, while a larger one sweeps more volume per stroke at lower pressure.
John Bozzelli's Plastics Technology column on intensification ratio explains the mechanism. The ratio of hydraulic ram area to screw area sets how much plastic pressure you get per unit of hydraulic pressure. Machines range from about 6:1 to 43:1, and the ratio changes whenever the screw size changes.
His example shows why it matters: a mould needing 23,000 psi (1,585 bar) would not fill on a machine giving 20,000 psi (1,380 bar), but filled on one giving 29,200 psi (2,014 bar). On the same injection unit, pressure scales with 1 ÷ D² and swept volume with D², which gives the pattern below.
How a larger screw on the same injection unit changes the numbers (calculated, relative to the small screw)
| Item | Small screw (D) | Medium screw (1.1 D) | Large screw (1.2 D) |
|---|---|---|---|
| Swept volume | 1.00 | 1.21 | 1.44 |
| Maximum injection pressure | 1.00 | 0.83 | 0.69 |
| Injection rate at the same ram speed | 1.00 | 1.21 | 1.44 |
| L/D if barrel length is the same and the small screw is 20:1 | 20:1 | 18.2:1 | 16.7:1 |
| Typical role | Thin walls, long flow, light shots | General containers and lids | Heavy shots, pails, multi-cavity boxes |
How Much of the Barrel Should Each Shot Use?
About 20% to 65% of the barrel's capacity, worked out in volume. Bozzelli warns that below about 20% the melt stays in the barrel too long and can degrade, and above 65% melt quality suffers. Size the screw so your main job sits comfortably inside that band.
Machine shot ratings are usually given in grams of polystyrene, so convert your part first. Bozzelli's method uses melt density, about 0.945 g/cm³ for PS and 0.74 g/cm³ for PP, adds 10% for check-valve leakage and melt uniformity, and then sizes the barrel so the shot is no more than 65%.
The lower limit matters as much as the upper one. A shot that uses a small part of the barrel sits in it for many cycles, and resin held hot too long can discolour or leave black specks. That is one reason a large screw can struggle on a light thin-wall job.
Which Screw Suits Thin-Wall Tubs and Lids?
Usually the small screw. Thin-wall parts need high pressure and fast fill, and a smaller barrel keeps the shot share high. Gary Tremblay's PlasticsToday article says the shot should be at least 40% of barrel size, and that too large a barrel may not give the pressure boost.
The same article puts thin-wall injection pressures typically above 30,000 psi, about 2,068 bar or 2,109 kg/cm², with injection velocities above 3 in/s (76 mm/s), many jobs at 6 in/s (152 mm/s), and fill times under one second. Thin-wall parts have flow length to wall ratios of about 100:1 to 200:1.
A larger screw delivers more cubic centimetres per second at the same ram speed, but less pressure. Check both against the mould, because a thin wall that needs pressure will not fill faster on a screw that cannot reach it. The IML article in this series covers labelled thin-wall tubs.
When Does the Large Screw or a Second Barrel Make Sense?
When your heaviest regular shot would exceed 65% of the smaller barrels. Heavy pails, multi-cavity storage boxes and thick-walled houseware fall here. If your mould list mixes heavy and light shots, a second barrel can cost less than forcing one screw to cover both.
Bozzelli notes that a second barrel ordered with a new machine can be low cost, and that a skilled operator can swap a barrel in under 30 minutes, even hot. He also cautions against buying the biggest screw by default, since big screws usually mean lower plastic pressure.
L/D is the other trade-off. If all three screws share one barrel length, the larger screws are shorter relative to their diameter, which leaves fewer flights to melt and mix. Ask what L/D each option has, especially for colour masterbatch or recycled feed.
For large articles, Reliance describes Repol B300MN, a PP impact copolymer with an MFR of 30 g/10 min, as suited to large parts with lower cycle times because of its high flow and fast crystallisation. A high-flow grade can ease fill on a large screw with less pressure.
Our view: pick the screw for the mould that will run the most hours, then check the others against it. A screw chosen for an occasional heavy job will run daily thin-wall work at a low shot share and lower pressure.
What Should You Send to Choose the Right Screw?
List every mould you expect to run, with shot weight, resin, wall thickness and cavity count, and mark the one that will run the most hours. Shot share and pressure can then be checked against each screw. See the 220 ton horizontal injection moulding machine page.
Frequently Asked Questions About Screw Options
Can I change screws on a container machine after purchase?
Often, if other screw sizes are offered for that injection unit. The barrel, screw, nozzle and heater bands change together, and pressure and speed settings must be re-checked. Ordering the second barrel with the machine can cost less than adding it later, and a skilled operator can swap it quickly.
Does a larger screw make the cycle faster?
Not by itself. It injects more volume per second at the same ram speed, which helps large shots fill, but it lowers maximum pressure. Container cycle time is often dominated by cooling, which depends on wall thickness rather than screw size, as the ice cream containers article in this series explains.
Why is my shot weight lower than the machine's rated shot?
Ratings are usually in grams of polystyrene. PP has a lower melt density, about 0.74 g/cm³ against 0.945 for PS, so the same barrel holds roughly 22% less PP by weight. Work in volume, then convert using your resin's melt density, and add 10% before comparing.
What screw design suits PP and HDPE containers?
A general-purpose screw with a non-return valve suits much PP and HDPE container work. If you run high-flow thin-wall grades, colour masterbatch or recycled feed, discuss compression ratio, metering depth and a mixing section with the screw supplier, because those jobs can need better melting and mixing.
How do I know if my screw is too big for a job?
Signs include a shot using less than about 20% of the barrel, discolouration or black specks from long residence, and trouble reaching the pressure the mould needs. Calculate shot share in volume. If it is low on your main job, a smaller screw or barrel is a better fix than lower temperatures.
Related Articles
- What Can You Make on a 220 Ton Container Moulding Machine?
- IML Tubs and Lids on a 220 Ton Machine
- Ice Cream Containers: Mould Height and Cycle Time on a 220 Ton Machine
Sources & References
- Plastics Technology, J. Bozzelli: Understanding Intensification Ratio
- Plastics Technology, J. Bozzelli: A Simpler Way to Calculate Shot Size vs. Barrel Capacity
- PlasticsToday, G. Tremblay (Sabic Innovative Plastics): Never too thin
- Reliance Industries: Repol B300MN Technical Data Sheet (July 2024)
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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