Choosing a Compression Moulding Press: Tonnage, Platen Size, Daylight and More

A compression moulding press is a hydraulic (or sometimes mechanical or servo) press that closes a heated mould on a charge of material and holds pressure while the part cures. Choosing one means matching five things to your parts and moulds: tonnage (clamping force), platen size, daylight and stroke, closing speed control, and ejection and heating provisions. Get these right and moulds run smoothly; get them wrong and even a perfect mould will make flash, short fills or warped parts.

At Prime Tools we design every mould around the customer’s press, and every mould is tried on a press before dispatch. This guide explains what to check. We deliberately do not give a one-size tonnage formula: every moulder and material supplier uses different pressure factors, safety margins and standards, so the final figure should come from your material datasheet and your toolmaker’s DFM review.

1. Tonnage (clamping force)

Tonnage is the maximum force the press can apply. It must be enough to make the material flow, fill the cavity and stay closed against the moulding pressure without the mould opening and flashing.

What drives the tonnage you need:

• Projected area of the part (and the shear edge) seen from the press direction. Bigger area needs more force.
• Material and grade: SMC, DMC, phenolic and melamine need different moulding pressures; each supplier gives a recommended range.
• Part depth and flow length: deep parts and long flow paths need more pressure.
• Your standards and margin: most moulders add a margin so the press is not run at its limit.

This is why we did not build a tonnage calculator for our downloads page: the right number is specific to your material and your company’s standards. In our DFM review we check your part area against your press and tell you if it is comfortable, tight or too small.

2. Platen size

The platens are the flat plates the mould is clamped to. The mould’s footprint must fit within them, with room for clamps, heater connections, thermocouple wiring and lifting. Check:

• Platen length and width (and any tie-rod or column spacing on four-column presses).
• Mounting pattern: T-slots or tapped holes.
• Whether the mould will be centred so the load is even.

Large SMC enclosure moulds often need big platens even when the tonnage is moderate.

3. Daylight, stroke and shut height

• Daylight is the maximum opening between the platens.
• Stroke is how far the moving platen travels.
• Shut height is the mould height when closed.

The press must open enough so that, with the mould installed, the operator can load the charge and remove the part safely, and ejectors can push parts clear. Deep parts like boxes and battery enclosures need more opening. A common mistake is a mould that fits closed but does not leave enough opening for loading.

4. Speed and pressure control

Thermoset moulding needs control at each stage:

• Fast approach to save cycle time.
• Slow pressing speed when the mould touches the charge, so air escapes and fibres are not damaged.
• Pressure build-up and hold during cure, with stable pressure.
• Breathing (degassing) for some materials like phenolic: a short opening to release gases.
• Parallelism control on large presses so the mould closes evenly.

Modern presses with proportional hydraulics or servo control give repeatable cycles, which means consistent parts.

5. Ejection

Most compression moulds use the press’s ejection system (hydraulic ejector cylinders, sometimes top and bottom) to push the part out. Check the ejector position, stroke and force so the mould’s ejector plate can be connected correctly.

6. Heating

The mould is usually heated by its own cartridge heaters with temperature controllers, or by heated platens (electric, steam or oil). Check:

• Electrical supply and controller zones for mould heaters.
• Whether the platens are heated or need insulation boards.
• Thermocouple types and connectors.

7. Safety and automation

Look for light curtains, two-hand controls, guards, emergency stops and safety valves. For volume production, consider charge loading aids and part removal automation.

Compression moulding press checklist to share with your toolmaker

When you request a quote for a new compression mould, send:

Our free RFQ checklist for new compression moulds includes all of these fields.

New press or existing press?

If you already own presses, design the mould for them. If you are buying a press for a new product, decide the part and mould concept first, then buy the press. A DFM review will give you the projected area, mould size and opening needed, so you buy the right press once.

Frequently asked questions