Clamping Force

What Is Clamping Force in Injection Molding?

When plastic is injected, the pressure inside the cavity acts on every square inch of the part's footprint and tries to force the mold halves apart at the parting line. The machine's clamping unit, hydraulic or toggle, has to hold them together against that force until the gate freezes.

Clamping force is also called clamp tonnage, and it is how injection molding machines are sized: a "100-ton press" can exert 100 tons of clamping force. Machine tonnage is a practical limit on the parts a press can run, together with the shot size its injection unit can deliver.

Too little clamping force lets the mold open very slightly under pressure. Molten plastic escapes into the gap and forms flash along the parting line and around ejector pins. Too much is not free either: it ties up a larger press than the job needs.

How Do You Calculate Clamping Force?

The standard estimate is:

Clamping force = projected area × cavity pressure

Projected area is the area of the part's shadow when you look at it along the direction the mold opens, not its total surface area. It includes every cavity in the mold and, in a cold-runner mold, the runners too.

The 4–5 tons/in² figure is a starting point for most plastics, not a guarantee. Mold-flow simulation or the material supplier's data sheet gives a better number for demanding parts.

How to Use Clamping Force When Designing a Part

• Measure projected area early. Large, flat parts such as covers and panels drive tonnage more than tall, narrow ones.
• Remember the cavities multiply. Doubling the cavity count roughly doubles the clamping force needed, which can push a job into a larger press.
• Treat flash as a clue. Flash along the parting line can come from too little clamping force, too much injection pressure or worn mold faces; checking the tonnage calculation is one of the first steps.
• Keep walls reasonable. Very thin walls over long flow lengths raise the pressure needed to fill, and with it the tonnage.

Frequently Asked Questions

What is clamping force in injection molding? It is the force that holds the two halves of an injection mold closed while plastic is injected. It has to be greater than the force the cavity pressure exerts across the part's projected area, or the mold opens slightly and the part flashes.

What does injection molding tonnage mean? Tonnage is the clamping force a molding machine can exert, and it is how presses are rated. A 100-ton machine can hold a mold shut with 100 tons of force. A common estimate of the tonnage a part needs is 4–5 tons per square inch of projected area.

What happens if clamping force is too low? The injection pressure forces the mold halves apart very slightly, and molten plastic escapes into the gap. The result is flash: thin fins of excess plastic along the parting line or around ejector pins.