Put a 95 g PP shot on 35, 38 and 40 mm screws sharing one injection unit, and assume the 35 mm screw sweeps 190 cm³. The shot then fills about 74%, 63% and 57% of each barrel, while melt pressure falls to about 85% and 77% of the smallest screw's. Choosing an injection moulding screw comes down to that trade: pressure against shot volume.
The 190 cm³ figure is an illustration, not a rating. Check your machine's data sheet for the real swept volume and pressure of each screw.
Key Takeaways
- On one injection unit, a 38 mm screw gives about 18% more swept volume and 15% less melt pressure than a 35 mm screw; a 40 mm screw gives about 31% more and 23% less.
- Plastics Technology columnist John Bozzelli recommends using 25 to 65% of barrel capacity for each shot, and warns that shots below about 20% risk degradation.
- Shot ratings are quoted in polystyrene. Convert your resin by melt volume: PS melts at about 0.945 g/cm³ and PP at about 0.74.
- Where the barrel length stays the same, a larger screw also has a lower L/D, which leaves less length for feeding and melting.
How Do Three Screws on One Unit Differ?
The hydraulic cylinder pushes with the same force whatever screw is fitted, so melt pressure falls as screw area rises, and swept volume rises with that same area. Each step up in diameter buys shot volume and gives up pressure, in proportion to the square of the diameter ratio.
Three screws on the same injection unit (35 mm = 1.00; flighted length assumed 700 mm)
| Point | 35 mm | 38 mm | 40 mm |
|---|---|---|---|
| Relative screw area and swept volume | 1.00 | 1.18 | 1.31 |
| Relative melt pressure | 100% | About 85% | About 77% |
| L/D at 700 mm flighted length | 20:1 | 18.4:1 | 17.5:1 |
| Suits | Thin walls, long flow, small shots | Mixed work in the middle | Heavy shots, thick walls, easy-flow grades |
Bozzelli explains the pressure side as the intensification ratio: hydraulic ram area divided by screw area. He notes machines are sold with ratios from 6:1 to 43:1, and that the ratio changes with every screw fitted to the same unit. In his example, a mould needing about 1,585 bar would not fill on a machine giving 1,380 bar.
The L/D row is easy to miss. If all three screws share one barrel length, 700 mm of flights gives 20:1 at 35 mm but 17.5:1 at 40 mm. Bozzelli counts about 10 feed flights on a 20:1 screw; a lower ratio leaves fewer diameters of length to convey and melt, which matters most on large PP shots.
What Matters Most When Choosing an Injection Moulding Screw?
The shot you run most often matters most. Bozzelli recommends keeping each shot between 25 and 65% of barrel capacity. Below about 20%, melt sits too long and can degrade; above 65%, you risk unmelt, uneven melt and slow recovery. Check every regular job against all three screws.
The reasons are physical. He puts the share of melting energy that should come from screw shear at about 80%, and a tiny shot gets too few screw turns to melt evenly. A shot near the top of the barrel leaves granules too little flight length to melt, and PP or PE can wear metal off the flights.
Bozzelli also warns against buying the biggest screw available, because big screws usually mean lower plastic pressure. The larger screw looks like insurance for future moulds, but every small job you run on it sits low in the barrel.
To compare screws on paper, add 10% to the calculated shot volume for check-valve leakage and melt uniformity, as he suggests, then divide by each screw's swept volume. The screw that keeps most of your shots nearest the middle of the range is usually the one to choose.
How Do Melt Density and Resin Grade Change the Choice?
Barrel ratings are quoted in grams of polystyrene, but your resin melts at a different density. Bozzelli gives about 0.945 g/cm³ for PS melt and about 0.74 for PP, so 100 g of PP needs about 28% more barrel volume than the same weight of PS.
Semi-crystalline resins also need more heat. Bozzelli puts PP at about 300 BTU per pound to melt against about 150 for ABS, so a large PP shot near the top of a short screw is the riskiest case in this comparison.
Grade choice can move the pressure side. Reliance lists its Repol B300MN impact copolymer at a melt flow rate of 30 g/10 min and describes it as suited to large articles with lower cycle times. Its Repol B120MA lists 12 g/10 min. Within the same polymer family, a higher-flow grade fills at lower pressure, which can let the larger screw handle a job that a stiffer grade would push onto the smaller one.
Compare melt flow only within one resin family. A PC grade with a high MFI still moulds as a viscous resin.
When Does a Second Barrel Make Sense?
When your regular shots fall on both sides of one screw's working range, a second screw and barrel can beat forcing every job onto one. Bozzelli notes 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: list your three or four most frequent shots before you choose. If one screw keeps nearly all of them inside 25 to 65%, buy that screw and accept a slower process on the odd outlier. If the list splits into a small-shot group and a large-shot group, plan the second barrel at purchase, not after the first quality complaint.
The middle screw is a sensible default when you cannot predict the job mix, because it gives up the least in either direction. It is a default, not an answer: the worked example below shows a shot where the largest screw fits better.
What Should You Send to Choose Between the Three Screws?
Send part and runner weights, the resin grade, wall thickness and flow length, the cavity count and the other moulds you plan to run on the same machine. With those, shot volume and pressure can be checked against each screw before you decide. See the 150 ton horizontal injection moulding machine page.
Frequently Asked Questions About Choosing an Injection Moulding Screw
Why does a larger screw have a lower L/D on the same machine?
L/D is flighted length divided by diameter. When a maker offers several screws for one injection unit, the barrel length often stays the same, so a larger diameter gives a lower ratio. With 700 mm of flights, a 35 mm screw is 20:1 and a 40 mm screw is 17.5:1, leaving less length for melting.
Can a smaller screw cure short shots on a thin part?
It can, if the short shot comes from running out of pressure. A smaller screw on the same unit raises the melt pressure available, by about 31% going from 40 mm to 35 mm. If the cause is a cold mould, poor venting or a frozen gate, a smaller screw will not help.
How do I turn my part weight into a shot volume?
Add the weights of all parts and the cold runner, then divide by the melt density of your resin, not its room-temperature density. Bozzelli gives about 0.74 g/cm³ for PP melt. Add about 10% for check-valve leakage, then compare the result with each screw's swept volume.
Do resin suppliers give their own shot size limits?
Some do, and they differ from the general 25 to 65% range. Covestro lists a shot-to-cylinder size of 30 to 70% for Makrolon 6555, and INEOS suggests 50 to 75% for Lustran ABS 248. Where the supplier gives a figure for your grade, check your shot against it as well.
Does a larger screw always recover faster?
Not always. A larger screw delivers more melt per turn, which helps on heavy shots, but recovery also depends on screw speed, back pressure, the drive and the resin. On a small shot, the larger screw adds residence time without saving cycle time, because recovery was not the limit in the first place.
Related Articles
- What Can You Make on a 150 Ton Injection Moulding Machine?
- Moulding Junction Boxes and Housings on a 150 Ton Machine
- 150 Ton Machine for Small Containers and Mugs
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 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.
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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