Fit a 36 mm screw in place of a 40 mm screw on the same injection unit and the highest melt pressure rises by (40 ÷ 36)², about 1.23 times, while swept volume falls to about 81%. Fit a 45 mm screw instead and pressure drops to about 79% of the 40 mm figure, with about 27% more volume.
That is the whole trade of shot weight vs injection pressure. One hydraulic force is spread over a smaller or a larger screw. Choose the smaller screw when filling is hard, and the larger one when the shot is heavy and the part fills easily.
Key Takeaways
- On one injection unit, melt pressure scales with 1 ÷ D² and swept volume with D², so every screw option gives up one to gain the other.
- Plastics Technology columnist John Bozzelli puts the working range at 25 to 65% of barrel capacity, after adding about 10% to the calculated shot.
- Decide on pressure first for thin walls and long flows, then confirm that the shot sits inside the working range.
- Indian PP grades such as Repol H110MAS list a melt window of 200 to 250 °C, and an oversized barrel keeps that melt hot for more cycles.
How Does Screw Diameter Trade Shot Weight vs Injection Pressure?
The hydraulic cylinder pushes the screw with the same force whichever screw is fitted. Pressure is force divided by area, so a smaller screw gives more melt pressure and a larger screw gives more melt per millimetre of stroke. One unit cannot give the maximum of both.
Bozzelli calls this link the intensification ratio: hydraulic ram area divided by screw area. He notes that machines are sold with ratios from 6:1 to 43:1, and that the ratio changes each time a different screw goes on the same unit.
His example shows the stakes. A mould that needed about 23,000 psi, or 1,585 bar, would not fill on a machine giving 20,000 psi, about 1,380 bar. It filled on one giving 29,200 psi, about 2,014 bar. The mould and resin were the same; only the ratio differed.
So compare screws on plastic pressure, not on the oil pressure some screens display. Electric machines read plastic pressure directly, which Bozzelli describes as an effective ratio of 1:1.
Three screw sizes on one injection unit, relative to the 40 mm screw
| Point | 36 mm | 40 mm | 45 mm |
|---|---|---|---|
| Relative screw area | 0.81 | 1.00 | 1.27 |
| Relative melt pressure | 1.23 | 1.00 | 0.79 |
| Relative swept volume | 0.81 | 1.00 | 1.27 |
| Typical fit | Thin walls, long flows, viscous resins | General parts | Heavy shots, thick walls, easy-flow resins |
Which Parts Need the High-Pressure Screw?
Parts with thin walls, long flow paths, small gates or viscous resins need the smaller, high-pressure screw. Melt cools as it travels, so the pressure needed to finish filling rises with flow length. PC and glass-filled grades can sit here, while easy-flow PP often does not.
Thin-wall closures, connectors with long ribs and clear covers are typical examples. The longer the flow relative to the wall, the more of the available pressure is used just to reach the last corner of the cavity.
A short shot at the end of fill, with the machine already at its pressure limit, points to the screw. A hotter barrel can help, but only inside the resin's processing window, and some grades leave little room.
Our view: if pressure is borderline, take the smaller screw and trim the runner to bring the shot into range. Do not buy the larger screw for parts you may never run. Bozzelli warns that big screws usually mean lower plastic pressure, so the biggest option is not the safe one.
When Is the Larger Screw the Better Choice?
Choose the larger screw when your regular shots sit near the top of the smaller screw's capacity and the parts fill with pressure to spare. Thick walls, heavy single-cavity parts and easy-flow resins fit that pattern. The larger screw keeps such shots in the barrel's comfortable middle.
Bozzelli recommends 25 to 65% of barrel capacity. Above 65%, few feed flights remain to melt the granules, so unmelt, uneven melt and long recovery follow, and PP or PE can wear metal off the flights. Below about 20%, the melt sits too long and can degrade.
Work in volume. Shot ratings are usually given in grams of PS, and Bozzelli gives melt densities of 0.945 g/cm³ for PS and about 0.74 for PP. Add about 10% to the calculated shot volume for check-valve leakage and melt uniformity before comparing it with swept volume.
Residence time matters for Indian commodity grades too. Reliance lists Repol H110MAS, a nucleated PP homopolymer for rigid packaging, furniture and houseware, with a melt temperature of 200 to 250 °C. A barrel far too large for the shot holds that melt at temperature for many cycles.
If one screw cannot cover all your parts, Bozzelli suggests ordering a second barrel with the machine, when it can cost less, and notes a skilled operator can swap one in under 30 minutes.
What Data Settles the Screw Choice?
Send the part and runner weights, the resin grade, the thinnest wall and longest flow path, the cavity count, and the other moulds planned for the same machine. With those, shot volume and pressure can be checked against each screw option. See the 75 to 180 ton horizontal injection moulding machine page.
Frequently Asked Questions About Shot Weight vs Injection Pressure
What is the intensification ratio on a hydraulic machine?
It is the hydraulic ram area divided by the screw area. Multiply the oil pressure by this ratio to get the plastic pressure at the screw tip. Bozzelli notes ratios from 6:1 to 43:1 across machines, and the ratio changes whenever a different screw is fitted to the same unit.
Can a larger screw still fill a thin-wall part?
Sometimes, if the part fills and packs with pressure to spare. Check the plastic pressure reached at the end of fill during a trial. If it is close to the larger screw's limit, small changes in resin lot or melt temperature can cause short shots, and the smaller screw is the safer choice.
How can I check barrel usage on a job that is already running?
Divide the metering stroke by the screw's maximum stroke. On the same screw, that ratio equals the share of swept volume in use. A job metering 40 mm on a screw with 160 mm of stroke is using 25%, right at the bottom of the working range.
Why are shot weights quoted in grams of polystyrene?
It gives buyers one common basis for comparing machines. For another resin, divide the PS figure by the PS melt density of 0.945 g/cm³ to get volume, then multiply by your resin's melt density. A 200 g PS rating is about 212 cm³, or about 157 g of PP.
Does a larger screw plasticise faster?
It can deliver more melt per revolution, which helps recovery on heavy shots. Plasticising also depends on screw speed, drive power, screw design and resin. On small shots, a larger screw mainly adds residence time, so faster recovery only helps when the shot is large enough to use it.
Related Articles
- Injection Moulding Machine Tonnage Selection: Matching 75 to 180 Tons to Your Part
- Getting the Most From Multi-Cavity Moulds on 75 to 180 Ton Machines
- Injection Moulding Machine Upgrade: When to Move to a Container or Houseware Machine
Sources & References
- Plastics Technology, J. Bozzelli: Understanding Intensification Ratio
- Plastics Technology, J. Bozzelli: Revisiting Shot Size vs. Barrel Capacity
- Plastics Technology, J. Bozzelli: A Simpler Way to Calculate Shot Size vs. Barrel Capacity
- Reliance Industries Limited: Repol H110MAS 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.
Limitation of Liability: Pratishna Engineers makes no representation or warranty, express or implied, regarding the completeness, accuracy or suitability of the information for any particular application. To the extent permitted by applicable law, Pratishna Engineers shall not be liable for any loss, damage, cost or consequence arising from reliance on or use of the information contained in this document. Final machine selection, configuration and application suitability should be confirmed with Pratishna Engineers based on complete and accurate project requirements.
