Swap a 32 mm screw for a 35 mm screw on the same injection unit and the screw's area grows by (35 ÷ 32)², or about 1.20 times. Every millimetre of stroke then delivers about 20% more melt, while the highest melt pressure falls to about 84% of the smaller screw's. That is the whole trade in injection moulding screw diameter: the smaller screw for pressure, the larger for shot volume.
The figures assume both screws share the same hydraulic cylinder and stroke. If your machine's options differ, check the rated pressure and swept volume of each before deciding.
Key Takeaways
- Melt pressure falls with screw area and shot volume rises with it, so a 35 mm screw gives about 20% more volume and about 16% less pressure than a 32 mm screw on the same unit.
- Plastics Technology columnist John Bozzelli recommends using 25 to 65% of barrel capacity for each shot.
- Work in volume, not grams: PP melts at about 0.74 g/cm³ against 0.945 for PS, the resin that shot ratings are usually quoted in, Bozzelli notes.
- Choose for the shot you run every day, not for the biggest mould you might buy one day.
How Does Injection Moulding Screw Diameter Change Pressure?
The hydraulic cylinder pushes with the same force whatever screw is fitted. Pressure is force divided by area, so a larger screw spreads that force over more area and gives lower melt pressure. On the same unit, a 35 mm screw delivers about 84% of a 32 mm screw's pressure.
Bozzelli explains this as the intensification ratio: the hydraulic ram's area divided by the screw's area. He notes that machines are sold with ratios from 6:1 to 43:1, and that the ratio changes with every screw size fitted to the same unit.
His example shows why it matters. A mould needed 23,000 psi, about 1,585 bar, to fill. One machine could give 20,000 psi and could not fill it; another could give 29,200 psi and had pressure to spare. Same mould, different intensification ratio.
Electric machines read plastic pressure directly, and Bozzelli notes they work at an effective ratio of 1:1, so the conversion step disappears there. On hydraulic machines it is an easy step to skip when a mould moves between presses.
So compare screws on plastic pressure, not on the hydraulic pressure shown on some machine screens. Thin walls and long flow paths are where the extra pressure of the smaller screw earns its place.
Smaller screw vs larger screw on the same injection unit
| Point | 32 mm screw | 35 mm screw |
|---|---|---|
| Relative screw area | 1.00 | About 1.20 ((35 ÷ 32)²) |
| Melt pressure available | Higher | About 84% of the 32 mm screw |
| Melt delivered per mm of stroke | Lower | About 20% more |
| Suits | Thin walls, long flow paths, smaller shots | Heavier shots, thicker walls, easy-flow resins |
| Risk when mismatched | Shot above 65% of capacity: unmelt and slow recovery | Shot below 25% of capacity: long residence and uneven melt |
How Much of the Barrel Should Each Shot Use?
Bozzelli recommends using 25 to 65% of barrel capacity. Shots below about 20% sit in the barrel too long and can degrade, while shots above 65% risk unmelted granules, uneven melt and long screw recovery. Each screw option has its own capacity, so check the shot against both.
Above 65%, he explains, a 20:1 screw has only a few feed flights left to pick up and compress granules, and semi-crystalline PP or PE can wear metal off the flights. Below 25%, the screw turns too few revolutions per shot to melt the granules by shear, and he puts the shear share of melting energy at about 80%.
You can check an existing job without any weighing. On the same screw, metering stroke divided by maximum stroke gives the share of barrel capacity in use. A job metering 30 mm on a screw with 140 mm of stroke is using about 21%, which is below the working range.
He also adds about 10% to the calculated shot volume, to allow for check-valve leakage and melt uniformity, before comparing it with the barrel. The worked example below uses that allowance.
Why Do Melt Density and Material Change the Choice?
Bozzelli notes barrel ratings are usually quoted in grams of polystyrene, but your shot needs its own melt volume. Bozzelli gives melt densities of 0.945 g/cm³ for PS and about 0.74 for PP. The same 100 g of PP therefore needs about 28% more barrel volume than PS.
Semi-crystalline resins add another twist. Bozzelli notes that PP needs about twice the energy of amorphous ABS to melt, roughly 300 against 150 BTU per pound, so a large PP shot near the top of the barrel is the riskier case.
Indian grades follow the same logic. Reliance lists its Repol H110MAS, a nucleated PP homopolymer for rigid packaging and houseware, with a melt flow rate of 11 g/10 min, a density of 0.90 g/cc and a melt temperature of 200 to 250 °C. Long residence in an oversized barrel is one way the melt can drift outside that window.
Which Screw Suits Recycled Content and Mixed Jobs?
If your jobs vary, choose the screw that keeps most of your regular shots between 25 and 65% of its capacity. Recycled resin can vary from lot to lot, so a shot sitting near the middle of that range leaves room to adjust without leaving the working window.
A quick way to decide: list your three or four most frequent shots in cm³, including runners and the 10% allowance, divide each by both swept volumes, and see which screw keeps the most of them inside 25 to 65%. The answer can be clearer on paper than in a sales discussion.
This can matter more for rigid packaging in India. Polymerupdate reports that under the Plastic Waste Management Rules as amended on 31 March 2026, rigid plastic packaging must contain at least 30% recycled material for 2025-26, rising to 60% by 2028-29, with exemptions where food and similar regulations restrict recycled content.
Our position: choose for the shot you run every day, not for the biggest mould you might buy. Bozzelli warns that going with the biggest screw possible costs more over time and that big screws usually mean lower plastic pressure. Where one screw cannot cover all your parts, he suggests ordering a second barrel with the machine, and notes a skilled operator can swap one in under 30 minutes.
What Should You Send to Choose the Right Screw?
Send part and runner weights, the resin grade, wall thickness and flow length, the number of cavities, and the other moulds you plan to run on the same machine. With those, shot volume and pressure can be checked against each screw option before you decide. See the 100 ton horizontal injection moulding machine page.
Frequently Asked Questions About Injection Moulding Screw Diameter
Does a bigger screw make the machine faster?
Not by itself. A larger screw delivers more melt per millimetre of stroke and can recover a heavy shot in fewer turns, but injection speed and recovery also depend on the drive, screw speed and resin. On a small shot, a larger screw can slow you down by adding residence time and uneven melt.
How do I convert a PS shot rating to PP?
Divide the rated PS grams by the PS melt density, 0.945 g/cm³, to get swept volume. Then multiply by the PP melt density, about 0.74 g/cm³. A 150 g PS rating becomes about 159 cm³, or about 117 g of PP. Then apply the 25 to 65% working range to that figure.
Can I change screws after buying the machine?
Often, if a second screw and barrel are offered for that injection unit. Bozzelli suggests ordering the second barrel with the new machine when one screw cannot cover all your parts, because the price can be lower then. With a skilled operator, he says, a barrel can be swapped in under 30 minutes, even hot.
Why does the pressure on my screen not match the mould?
On a hydraulic machine the screen may show oil pressure. Plastic pressure at the nozzle is that value times the intensification ratio, which changes with screw size. Pressure then drops through the runner and gate before the cavity. Compare screws, and copy processes between machines, using plastic pressure only.
Is a shot below 25% of the barrel always a problem?
It is a warning, not a fixed rule. Bozzelli finds melt uniformity is often poor below 25% because the screw turns only a few revolutions per shot, and residence time grows below about 20%. Heat-sensitive resins are the bigger worry. If a small job must run there, watch for discolouration and part weight drift.
Related Articles
- What Can You Make on a 100 Ton Injection Moulding Machine?
- 100 Ton Machine for Switch Plates and Electrical Accessories
- 100 Ton Injection Moulding Machine Running Costs: Power, Cycle and Output
Sources & References
- Plastics Technology, J. Bozzelli: Revisiting Shot Size vs. Barrel Capacity
- Plastics Technology, J. Bozzelli: A Simpler Way to Calculate Shot Size vs. Barrel Capacity
- Plastics Technology, J. Bozzelli: Understanding Intensification Ratio
- Reliance Industries Limited: Repol H110MAS technical data sheet (July 2024)
- Polymerupdate, D. K. Jha: India eases plastic recycling regulations (8 April 2026)
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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