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What size press does my mould need?

Estimate the press size by multiplying the total projected area (every cavity plus the runner) by a tonnage factor; RJG says processing guides commonly call for 3 to 5 tons per square inch and that it teaches 3 as a safe figure.

That gives clamp force, which is how presses are rated. But clamp force is only one of three checks. The mould must also physically fit the machine, and the shot must suit the machine's barrel. This article walks through all three so you can read a moulder's quote with some understanding of why it names a particular press.

What clamp tonnage is

During injection, molten plastic pushes the two halves of the mould apart. The press's clamp must hold them shut with more force than that. Crescent Industries notes that presses are classified by their tonnage or clamping force, and that they vary from 5 tons to 4,000 tons. RJG gives a similar picture: small machines may be only 25 tons, and they go up to 4,000 tons and beyond.

Published tonnage factors

Sources agree on the method but quote different ranges for the factor:

Plastics Engineering gives 2 to 3 tons/in² for a polypropylene example and notes that polyethylene needs lower factors than polycarbonate. RJG adds that thinner-walled parts generally need more tonnage than thicker ones. Neither source publishes a full table by resin, so ask your resin supplier or moulder for the factor they use for your grade.

These sources work in tons and square inches. For metric drawings, 1 in is 25.4 mm.

How to estimate it, step by step

1. Look at the part from the direction the mould opens. The projected area is the area of that outline, not the surface area.
2. Multiply by the number of cavities.
3. Add the projected area of the runner. RJG's formula is: total projected area = (part area × number of cavities) + runner area.
4. Multiply the total by a tonnage factor from the table above.
5. Look for a press whose rated clamp covers that figure, then run the fit and shot checks below.

Worked examples

RJG's own example: each part is 5 × 5 in, so 25 in²; there are four cavities and the runner adds 10 in². Total projected area is (25 × 4) + 10 = 110 in². At its factor of 3 tons/in², the mould needs 330 tons.

A second example using the same method, for a flat part 4 × 6 in (about 102 × 152 mm) in a two-cavity mould:

The gap between 144 and 240 tons shows why the factor matters: one choice could put the job on a smaller, cheaper press.

Why more tonnage is not safer

It is tempting to set the clamp to the press maximum. RJG and Plastics Technology both warn against it:

• Vents crushed. RJG says vents are only around 0.001 to 0.0015 in deep, and too much clamp force can pinch them shut, trapping air that heats up and burns the plastic.
• Mould wear. RJG reports moulds whose parting lines were so worn by excess tonnage that they flashed once the clamp was set back to a normal level.
• Damage. Plastics Technology lists crushed vents, rolled parting lines, broken inserts and cracked cavity blocks, and even damaged platens and machine frames, and describes a mould clamped at four times the required tonnage in which the cavity block separated.

RJG recommends a clamp force optimisation study on the press to find what the mould actually needs.

Check two: will the shot suit the barrel?

The shot (all parts plus runner) must suit the machine's injection unit. Published guidance differs:

RJG explains that a very small share of the barrel leaves material in it longer, and that a very high share may not give the polymer enough time to melt properly. A large clamp with a large barrel can be a poor match for a small part.

Check three: will the mould fit the press?

A mould must fit between the tie bars and on the platens, open far enough to eject parts, and match the ejector layout. RJG notes that machine makers commonly suggest using about 66 % of the platen, and that some moulders end up buying a larger machine purely for tie bar spacing. Your moulder checks these dimensions against the machine's data sheet.

Why the press size affects price

Crescent Industries notes that moulding cost is driven by the machine's hourly rate and the number of cavities, and that the hourly rate depends on the size of the press. A mould that needs a bigger press than necessary costs more for every part it makes.

This is an estimate for planning. The moulder should confirm tonnage, shot size and mould fit from the actual resin, mould design and a fill analysis.

Questions buyers ask

Should I tell the moulder what press to use?

Usually not. Give them the part, resin, cavity count and volumes, and ask which press they plan to use and why. Your own estimate is useful as a sense check on their answer.

Does adding cavities always need a bigger press?

Clamp force scales with total projected area, so each extra cavity adds its share, as RJG's formula shows. More cavities also mean a bigger shot and a larger mould, so the shot and fit checks change too.

Why does a quote name a much bigger press than my estimate?

The mould may be too large for the tie bar spacing of a smaller machine, or the shot may need a larger barrel. Ask which check drove the choice; if it was tonnage alone, ask what factor they used.

What about very thin parts?

RJG says thinner walls generally need more tonnage. Use the upper end of the factor range for planning and ask the moulder for a fill analysis.

Sources

• RJG, Why Machine Tonnage Matters and How to Set It Correctly: https://rjginc.com/why-machine-tonnage-matters-and-how-to-set-it-correctly/
• RJG, Right Mold, Right Machine: https://rjginc.com/right-mold-right-machine/
• Plastics Technology, Clamp Tonnage: More Is Better...Right?: https://ptonline.com/articles/clamp-tonnage-more-is-betterright
• Plastics Technology, Injection Molding: Are You Using More Than 65% Of Your Barrel Capacity?: https://www.ptonline.com/articles/are-you-using-more-than-65-of-your-barrel-capacity
• Plastics Engineering (SPE), Mastering Clamping Force Calculation: https://www.plasticsengineering.org/2024/03/clamping-force-decoding-by-key-strategic-ways-003900
• Crescent Industries, The Impact of Injection Mold Classification on Cost: https://www.crescentind.com/blog/the-impact-of-injection-mold-classification-on-cost