Injection Molding Press Selection Worksheet: Clamp Tonnage and Shot Utilization
Putting a mold in the wrong press causes flash, short shots or degraded resin, and none of it shows up in the quote. This worksheet checks clamp force and shot size for each mold and press pair in one table, so planners and estimators can screen the whole book of tools at once.
| Part and material | Projected area per cavity (sq in) | Cavities | Runner projected area (sq in) | Cavity pressure factor (tons/sq in) | Required clamp (with 10% margin) (tons) | Press clamp rating (tons) | Clamp utilization | Part weight (g) | Runner per shot (g) | Shot weight (g) | Press shot capacity (as PS) (g) | Material density factor | Shot utilization | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
154 | 102.7% | 420 | 97.7% | |||||||||||
168.96 | 67.6% | 57.6 | 10.7% | |||||||||||
178.2 | 81% | 174 | 20.5% | |||||||||||
138.6 | 84% | 124 | 20.6% | |||||||||||
113.96 | 76% | 69 | 26.1% | |||||||||||
90.64 | 82.4% | 41.2 | 27.5% |
Required clamp = (projected area x cavities + runner area) x cavity pressure factor x 1.1. Shot capacity is converted from the machine's polystyrene rating by the density factor. Treat results as a screening check and confirm against your press manufacturer data and a molding trial.
Mold to press compatibility in your ERP
Optional: we can outline how to hold compatible presses per mold in your ERP routing so planners cannot schedule an overloaded pairing.
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How to use it
Add one row per mold and press combination. Enter projected area per cavity, cavity count and runner area from the mold drawing, then a cavity pressure factor that matches the flow length and wall thickness. Enter the press clamp rating, the part and runner weights, the machine shot capacity in grams of polystyrene and a density factor for your resin. The sheet returns required tons, clamp utilization, shot weight and shot utilization, and totals the table for Excel export.
The method
Clamp force must exceed the force that injection pressure creates over the projected area: area times cavity pressure. Shops use a pressure factor in tons per square inch of 2 to 3 for easy flow materials, and up to 4 to 5 for thin wall, long flow or filled resins. A 10 percent margin covers pressure variation. Shot utilization compares the shot weight with machine capacity corrected for resin density. A shot below about 20 percent of capacity risks thermal degradation from long barrel residence time. Above about 80 percent there is little room for process adjustment.
- Required tons = (area x cavities + runner area) x factor x 1.1
- Shot utilization = shot weight / (PS capacity x density factor)
- Clamp utilization above 100 percent means flash risk
Reading the examples
The default table shows common outcomes. The housing cover needs about 154 tons and is on a 150 ton press, so it is overloaded. The closure cap runs well within clamp but uses less than 10 percent of the barrel, which argues for a smaller press. Rows near 100 percent utilization need a bigger press or fewer cavities, and rows far below need a smaller one, freeing the large press for work that needs it.
ERP and scheduling notes
Store the compatible presses on the routing or the mold master so MES or the planning board never schedules a mold on an unsuitable machine. Also check tie-bar spacing, daylight and mold height, which a pure tonnage check does not cover. Revisit the pressure factor after the first trial, using the actual peak cavity pressure if you have a sensor.
Frequently Asked Questions
What cavity pressure factor should I use?
Many shops use 2 to 3 tons per square inch for general purpose parts and 4 to 5 for thin wall or long flow path parts. Better still, use measured peak cavity pressure from a trial or a similar tool. The factor has a large effect on the answer.
Why convert shot capacity with a density factor?
Press makers rate shot capacity in grams of general purpose polystyrene, which has a density of about 1.05. Denser resins fit more mass in the same barrel volume and lighter ones fit less, so the factor is resin density divided by 1.05.
Is 100 percent clamp utilization acceptable?
It is risky. Pressure varies with viscosity, temperature and fill speed, and a mold running exactly at rated tonnage often flashes. Most processors plan for 60 to 90 percent and use the sheet to catch the rows that exceed 100.
Does the sheet check hydraulic versus electric presses?
No. It checks force and volume only. Electric and hybrid presses may have different usable shot sizes and injection rates, so confirm with the machine specification for critical or high speed jobs.
Netray can connect this check to your ERP routing data so that mold and press compatibility is validated when a work order is created, not discovered at the press.
Related Tools
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