The quotation for a 180 t press offers two screws, 40 mm and 45 mm, and the purchase team asks which box to tick. For thin-wall packaging, our answer is the 40 mm screw unless the shot is heavy. On the same injection unit, it gives about 27% more injection pressure, while the 45 mm screw gives about 27% more swept volume.
That trade is the whole decision. Choosing a screw diameter for thin-wall packaging means checking two numbers for your heaviest mould: the pressure it needs to fill, and the share of the barrel its shot will use.
Key Takeaways
- On the same unit, swept volume scales with the square of screw diameter and pressure with its inverse: (45 ÷ 40)² = 1.27, and (40 ÷ 45)² = 0.79.
- Thin-wall parts can need more than 30,000 psi (about 2,109 kg/cm²) of injection pressure, which favours the smaller screw.
- Aim for a shot of 20% to 65% of barrel capacity in general; for thin-wall work, a floor of 40% is a sensible starting point.
- The 45 mm screw earns its place on heavier shots with thicker walls, where volume matters more than peak pressure.
How Does Screw Diameter Change Pressure and Shot?
The hydraulic cylinder pushes with the same force whichever screw is fitted. Spread over a larger screw area, that force gives less melt pressure but moves more volume per millimetre of stroke. Going from 40 mm to 45 mm adds about 27% volume and removes about 21% pressure.
Plastics Technology columnist John Bozzelli describes this through the intensification ratio: the hydraulic ram area divided by the screw area. He reports ratios from 6:1 to 43:1 across machines, and notes the ratio changes with screw size on the same unit.
His example shows why it matters. A mould needing 23,000 psi (1,585 bar) could not fill on a machine limited to 20,000 psi (1,380 bar), but filled on one giving 29,200 psi (2,014 bar). The mould and resin were the same; only the available plastic pressure changed.
Injection rate moves the other way. At the same forward speed, the 45 mm screw pushes about 27% more melt per second, so it can fill a heavy shot faster. For a light thin-wall shot, though, the limit is pressure at the end of fill, not volume per second.
Same injection unit, two screws (calculated ratios; read actual figures from the data sheet)
| Property | 40 mm screw | 45 mm screw |
|---|---|---|
| Screw area | 12.57 cm² | 15.90 cm² (×1.27) |
| Swept volume per 100 mm of stroke | 126 cm³ | 159 cm³ (×1.27) |
| Maximum injection pressure | Base figure | About 79% of the 40 mm figure |
| Injection rate at the same forward speed | Base figure | About 127% of the 40 mm figure |
| Best suited to | Thin walls, long flow, lighter shots | Heavier shots, thicker walls, shorter flow |
Which Screw Diameter for Thin-Wall Packaging Gives the Right Shot Share?
The one that puts your shot between about 40% and 65% of the barrel. Bozzelli advises 20% to 65% for general moulding. Gary Tremblay's PlasticsToday article on thin-wall moulding asks for at least 40%, calling it the bare minimum for engineering resins.
Below the lower limit, melt sits in the barrel for several cycles and can degrade. Above 65%, melt quality suffers. We apply the 40% floor to thin-wall PP as well, as a starting margin, not a rule. Tremblay adds that a barrel too large for the shot may not give the pressure boost and allows degradation, which is the same warning from the thin-wall side.
Work in volume. Bozzelli notes that machine shot ratings are given in grams of polystyrene, and gives melt densities of 0.945 g/cm³ for PS and about 0.74 g/cm³ for PP. Convert the shot to melt volume, add 10% for check-valve leakage and melt uniformity, then compare it with the swept volume.
Run the check for your lightest thin-wall mould as well as your heaviest. A screw that suits the heavy job can leave the light one well below the floor, and that light mould is the one most likely to show black specks and colour streaks from long residence time.
When Is the 45 mm Screw the Better Choice?
When the heaviest shot would push a 40 mm barrel past 65% and the part does not need peak pressure. Thicker-walled tubs, buckets and reusable containers fit that pattern: more volume per shot, shorter flow ratios and lower fill pressure than thin-wall packaging.
Grade choice shifts the balance. Reliance lists Repol H200MA, made for thin-walled containers, at an MFR of 25 g/10 min, while its rigid packaging grade Repol H110MAS sits at 11. The stiffer-flowing grade needs more pressure on the same part, which pushes the choice toward 40 mm.
Bozzelli also warns against buying the biggest screw by default, since big screws tend to mean lower plastic pressure. If your mould list spans both thin-wall and heavier jobs, he notes that a second barrel ordered with a new machine can be low cost, and a skilled operator can swap one in under 30 minutes.
Our view: choose the screw for the hardest mould to fill, not the heaviest. A screw that cannot reach fill pressure has no workaround, while a shot near the top of the barrel can often be handled with a second barrel later.
What Should You Send Before Choosing the Screw?
Send the heaviest and the thinnest part on your mould list, with shot weight, wall thickness, flow length and resin grade. Ask for the swept volume and maximum injection pressure for each screw offered, then run the check below. See the 180 ton horizontal injection moulding machine page.
Frequently Asked Questions About Screw Diameter for Thin-Wall Packaging
Should I order both screws with the machine?
If your mould list has both thin-wall and heavier jobs, it is worth pricing. Bozzelli notes that a second barrel ordered with a new machine can be low cost and can be swapped in under 30 minutes by a skilled operator. Ordering later can mean a longer wait and a higher price.
Does a smaller screw take longer to recover?
It can. A smaller screw conveys less melt per revolution, so screw recovery for the same shot takes longer. On thin-wall work, cooling is short, and recovery must finish inside it. Check the recovery time on a trial; if it limits the cycle, a higher screw speed or a larger screw may be needed.
Why do thin-wall jobs quote injection speed in mm/s?
Fill time depends on how fast the screw moves forward. Tremblay's article gives velocities above 3 in/s (76 mm/s) and often 6 in/s (152 mm/s). At the same forward speed, a 45 mm screw delivers about 27% more melt per second than a 40 mm screw, so compare injection rate in cm³/s as well.
Can the 45 mm screw run thin-wall if I cut the cavities?
Fewer cavities make the shot smaller, which lowers the shot share of the 45 mm barrel further below the 40% floor. The pressure needed to fill each cavity stays the same, because it depends on wall and flow length. Cutting cavities therefore makes both problems worse, not better.
How do I convert a PS shot rating into PP grams?
Divide by the PS melt density and multiply by the PP melt density: 0.74 ÷ 0.945 is about 0.78. A rating of 300 g PS is about 235 g of PP. Then add the 10% allowance and keep the shot inside the target share, rather than running up to the converted maximum.
Related Articles
- What Can You Make on a 180 Ton Container Moulding Machine?
- Moulding Thin-Wall Food Containers on a 180 Ton Machine
- Dairy Cups and Small Tubs: Output Planning on a 180 Ton Machine
Sources & References
- Plastics Technology, J. Bozzelli: A Simpler Way to Calculate Shot Size vs. Barrel Capacity
- Plastics Technology, J. Bozzelli: Understanding Intensification Ratio
- PlasticsToday, G. Tremblay (Sabic Innovative Plastics): Never too thin
- Reliance Industries: Repol H200MA 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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