A 900 g PP shot is about 1,216 cm³ of melt. Add 10% and keep it at or below 65% of the barrel, and you need a swept volume of about 2,060 cm³. That number, not the clamp, decides the screw. Large shot weight injection moulding on 120 to 350 ton machines comes down to three choices: a screw diameter that sweeps the volume without starving pressure, a stroke inside the working window, and pump flow fast enough to fill the part before it freezes.
The figures below are generic and calculated, so you can repeat them for your own part.
Key Takeaways
- Work in volume. PP melt is about 0.74 g/cm³, so a PP shot uses more barrel than its gram figure suggests.
- Keep the shot between about 25% and 65% of barrel capacity. Below that, the melt can be uneven or degrade; above it, unmelt and long recovery appear.
- A bigger screw sweeps more volume but gives less plastic pressure. On one injection unit, pressure falls with the square of the diameter.
- Pump flow sets fill speed. Oil flow is melt flow times the intensification ratio, so a large shot filled fast needs a lot of oil.
How Do You Choose Screw Diameter for Large Shot Weight Injection Moulding?
Choose the smallest screw that sweeps your shot volume, plus 10%, at or below 65% of its capacity, then check it still gives the plastic pressure the part needs. A larger diameter adds volume but cuts pressure, so the right screw is a balance, not the biggest option.
John Bozzelli's Plastics Technology columns set the working window at about 25% to 65% of barrel capacity. Below 25%, the melt sees too few screw turns and can be uneven, and very small shots sit in the barrel long enough to degrade. Above 65%, unmelt and long recovery appear, and PP or PE can grind metal off the flights at large shots.
Generic screw diameters on one injection unit: volume and relative plastic pressure (calculated)
| Screw diameter | Screw area | Swept volume per 100 mm stroke | PP melt per 100 mm stroke | Plastic pressure vs 70 mm |
|---|---|---|---|---|
| 60 mm | 28.3 cm² | 283 cm³ | 209 g | 136% |
| 70 mm | 38.5 cm² | 385 cm³ | 285 g | 100% |
| 80 mm | 50.3 cm² | 503 cm³ | 372 g | 77% |
| 90 mm | 63.6 cm² | 636 cm³ | 471 g | 60% |
| 100 mm | 78.5 cm² | 785 cm³ | 581 g | 49% |
Going from 70 mm to 80 mm adds about 31% volume but leaves about 77% of the pressure, because (70/80)² is 0.77. The PS-to-PP conversion behind the volume figures is explained in the tonnage and shot weight article in this series.
Our view is flat: on large PP shots, pick the screw for the shot first, then confirm pressure. A screw chosen for pressure alone can end up running near the top of its barrel, and melt quality pays for it.
If one machine must run both small and large shots, ask about a second barrel. Bozzelli 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.
What Does Intensification Ratio Mean for Injection Pressure?
Intensification ratio is the hydraulic ram area divided by the screw area. Plastic pressure equals hydraulic pressure times that ratio. Fitting a larger screw on the same ram lowers the ratio, so the same hydraulic pressure gives less pressure on the melt at the screw tip.
Bozzelli reports ratios from 6:1 to 43:1 across machines. In one case he describes, a mould needing about 1,585 bar of plastic pressure would not fill on a machine giving 1,380 bar, yet filled on one giving 2,014 bar.
Compare screws on plastic pressure, not on the hydraulic gauge. At a 10:1 ratio, a gauge reading of 140 bar means about 1,400 bar on the melt, and a larger screw on the same unit lowers the ratio and that figure with it.
Thick household parts with short flow paths can need less pressure than thin-wall packaging, which is why a larger screw can work on buckets and tubs. Long, thin flow paths are the exception.
How Much Pump Flow Does a Large Shot Need?
Pump flow sets how fast the screw moves forward. Melt flow is screw area times injection speed, and oil flow is that melt flow times the intensification ratio. A large shot filled in a few seconds can need several hundred litres of oil per minute during injection.
Hydraulic power is pressure in bar times flow in L/min, divided by 600, which gives kW before pump and motor losses. At 140 bar and 300 L/min that is 70 kW, needed only for the seconds injection lasts.
The pump also drives clamp moves, ejection and screw recovery. A large shot means a long recovery, and if recovery runs longer than cooling, the cycle waits for the screw. Check recovery time against cooling time on every large shot.
How Does Pump Sizing Show Up on an Indian Power Bill?
A larger pump motor raises the connected load, and under kVAh billing a poor power factor raises the units you pay for. In Maharashtra, MERC's order provided for kVAh billing of HT consumers and LT consumers above 20 kW from 1 April 2020, so both count.
MSEDCL's FAQ explains that demand is charged on kVA and energy on kVAh. At a power factor of 0.85, 15% of the power drawn does no useful work, and MSEDCL recommends capacitors or APFC panels. Under kVAh billing, that share shows up on the bill.
Size the pump for the fill your largest regular job needs, not for a shot you might run once a year. A pump sized for a 1 kg shot but running 400 g jobs carries the larger motor's losses on every cycle.
What Should You Send for a Screw and Pump Check?
Send the part weight with runner, resin grade, wall thickness and longest flow path, the fill time you get today if the mould already runs, and the plastic pressure it needs. With those, swept volume, screw diameter, intensification ratio and pump flow can be worked out first. See the household articles injection moulding machine range.
Frequently Asked Questions About Large Shot Weight Injection Moulding
What happens if the shot uses more than 65% of the barrel?
The screw has fewer turns of feed and compression to melt each shot, so unmelted pellets, colour streaks and uneven melt can appear, and recovery takes longer. With PP and PE at large shots, Bozzelli also warns of metal being ground off the flights. Weigh a larger screw or barrel against the scrap it would save.
Why does a bigger screw give less injection pressure?
The hydraulic ram pushes with the same force whatever screw is fitted. Spread that force over a larger screw face and the pressure on the melt drops. On one injection unit, pressure falls with the square of the diameter, so going from 70 mm to 80 mm leaves about 77% of the original pressure.
Does an accumulator help with large shots?
It can. An accumulator stores oil under pressure during the cycle and releases it during injection, so a smaller pump can deliver a high flow for a few seconds. It suits jobs that need a fast fill on a large shot. It adds parts to maintain, so weigh it against a larger pump.
How long should screw recovery take compared with cooling?
Recovery should finish before cooling ends, with some time to spare, so the screw never holds up mould opening. If recovery runs longer than cooling on a large shot, check screw speed and back pressure first, then whether the shot sits too high in the barrel. A larger screw can recover the same volume faster.
Is a servo pump worth it on a large machine?
It can be, where the machine spends long periods in cooling and hold, because a servo drive slows the pump when little flow is needed. Measure the existing machine's energy per kilogram on your own jobs before deciding, since the saving depends on how much of each cycle needs full flow.
Related Articles
- Household Articles Moulding Machines: Tonnage and Shot Weight Guide
- Bucket and Tub Making Machine Guide: Buckets, Tubs and Storage Boxes
- Planning a Houseware Unit: Which Machine Sizes to Start With
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
- Maharashtra State Electricity Distribution Co. Ltd (MSEDCL): kVAh Billing: Frequently Asked Questions
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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