(50 ÷ 45)² = 1.235. On the same injection unit, a 50 mm screw sweeps about 23% more volume than a 45 mm screw, but gives only about 81% of its injection pressure, because 1 ÷ 1.235 = 0.81. So choosing a 45 mm or 50 mm screw comes down to two questions: do your largest shots fit the smaller barrel, and do your hardest parts still fill at 81% of the pressure?
Household ranges make the choice harder because they mix large shells and baskets with small lids and knobs. This article shows how to check both ends of a range with your own part weights.
Key Takeaways
- On one injection unit, swept volume scales with the screw diameter squared and pressure with its inverse.
- Work in melt volume: PP melt is about 0.74 g/cm³ against 0.945 g/cm³ for PS, the usual basis of shot ratings.
- Keep each shot between about 20% and 65% of barrel capacity after adding 10% for leakage and uniformity.
- If a range has both very large and very small shots, a second barrel can cover both without compromise.
Why Does a Bigger Screw Give Less Pressure?
The hydraulic cylinder pushes with the same force whatever screw is fitted. A larger screw spreads that force over a larger area, so the pressure on the plastic falls. The ratio of ram area to screw area, called the intensification ratio, sets the pressure the melt sees.
John Bozzelli explains this in a Plastics Technology column. Intensification ratios across machines range from 6:1 to 43:1, and the ratio changes whenever the screw size changes. Electric machines read plastic pressure directly, so their ratio is effectively 1:1.
His example shows why it matters. A mould that needed 1,585 bar of plastic pressure could not fill on a machine giving 1,380 bar, but filled on one giving 2,014 bar. Long, thin flow paths are where a household part is most likely to hit that kind of limit.
Pressure demand rises with flow length and falls as walls get thicker. So the parts most at risk on the larger screw are thin trays, long handles and baskets with fine ribs. A thick bowl that fills easily on the 45 mm screw may never need the extra pressure, which is why the question is about specific moulds.
45 mm vs 50 mm screw on the same injection unit (relative values, calculated)
| Property | 45 mm screw | 50 mm screw |
|---|---|---|
| Screw area and swept volume per mm of stroke | 1.00 | 1.235 (23.5% more) |
| Maximum plastic pressure | 1.00 | 0.81 (19% less) |
| Largest shot inside the 65% limit | 1.00 | 1.235 |
| Smallest shot inside the 20% limit | 1.00 | 1.235 (small shots fall out sooner) |
| L/D if barrel length stays the same | Higher | Lower |
How Do You Size a PP Shot Against Each Barrel?
Convert the shot weight to melt volume using PP melt density, add 10% for check-valve leakage and melt uniformity, then compare with the barrel's swept volume. The shot should sit between about 20% and 65% of it. Shot ratings in PS grams overstate PP capacity.
In a second Plastics Technology column, Bozzelli gives melt densities of about 0.945 g/cm³ for PS and 0.74 g/cm³ for PP, and advises working in volume. Since shot ratings are usually given in grams of PS, a barrel holds only about 78% of its PS gram rating when filled with PP melt.
Below 20% of barrel capacity, he notes, the melt stays too long in the barrel and can degrade; above 65%, melt quality suffers. He also warns against buying the biggest screw on offer, since bigger screws usually mean lower plastic pressure.
Our view: size the screw on your smallest regular shot as carefully as your largest. A barrel that suits a basket but leaves lids at 15% of capacity will make good baskets and discoloured lids.
Which Household Shots Suit a 45 mm or 50 mm Screw?
Small multi-cavity items such as lids, knobs and spoons sit better on the 45 mm screw, which keeps small shots above 20% and gives more pressure. Large single shells, baskets and buckets suit the 50 mm screw. A range with both may need two barrels.
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. For a houseware range that mixes 20 g lids with 200 g shells, that can be cheaper than a compromise screw.
Resin choice can ease the pressure question. Reliance's Repol H200MA is a PP homopolymer with an MFR of 25 g/10 min, which its TDS lists for thin-walled containers, multi-cavity moulding and long flow paths. A higher-flow grade like this needs less pressure to fill, which can let a part run on the 50 mm screw.
Residence time is worth measuring, not calculating. In a third column, Bozzelli describes dropping a few granules of a different colour into the throat on cycle and counting shots until the colour clears. Swirls in the part during that test also show poor melt mixing.
Screw geometry changes with diameter too. If the barrel length stays the same, a larger screw has a lower L/D ratio, so it has fewer flights to melt and mix each shot. The same column describes a 20:1 general-purpose screw as having about 10 feed, 5 transition and 5 metering flights.
What Should You Send Before Choosing a Screw?
List every mould planned for the machine with shot weight including runner, resin grade, cavity count and longest flow path. Convert each shot to melt volume and check it against both screws. See the 150 ton horizontal injection moulding machine page.
Frequently Asked Questions About a 45 mm or 50 mm Screw
What does a shot weight in PS grams on a datasheet mean?
It is the largest shot the barrel can deliver if the resin were general-purpose polystyrene. PP is less dense in the melt, so the same barrel delivers less weight of PP. Divide by PS melt density to get volume, then multiply by PP melt density for a realistic PP figure.
Can I change the screw size after the machine is installed?
Yes, by fitting a different screw and barrel set on the same injection unit, if the maker offers one for that unit. Ordering the second set with the machine is usually simpler and can cost less. Changing later needs the pressure and shot settings re-checked for the new diameter.
Does a 50 mm screw recover faster than a 45 mm screw?
It moves more material per turn at the same screw speed, so it can recover a large shot in less time. On a small shot, that advantage disappears and the larger barrel holds melt longer. Recovery only matters if it finishes after cooling, so check it against your cooling time.
Is a higher-flow resin a substitute for injection pressure?
Partly. A higher MFR grade fills long, thin sections at lower pressure, but it may also have lower impact strength or different shrinkage. Compare the datasheets for the properties your part needs before switching grades to suit a screw. The part's function should decide the resin, not the screw.
What signs show that shots are too small for the barrel?
Black specks, yellowing or brown streaks in light-coloured parts, and a smell of degraded resin can all point to melt sitting too long. A colour-change test shows the real residence time. If the shot is under about 20% of barrel capacity, a smaller screw is the lasting fix.
Related Articles
- What Can You Make on a 150 Ton Household Moulding Machine?
- Casseroles and Thermoware Shells on a 150 Ton Machine
- Baskets and Trays: Flow, Venting and Cycle Time on a 150 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
- Plastics Technology, J. Bozzelli: Injection Molding: A Practical Approach to Calculating Residence Time
- 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.
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.
