How is clamping force calculated? Step by step, with an example

The first step in choosing the right machine is clamping force. Choose too little and you get flash; choose too much and it means wasted energy and expensive machine hours.

Basic formula

Required clamping force = total projected area × average cavity pressure

• Projected area: The shadow of the part on a plane perpendicular to the mold-opening direction. If there is more than one cavity, add them all up; if there's a cold runner, its projected area is added too.
• Cavity pressure: The average pressure inside the cavity. It varies with material, wall thickness and flow length. It is high in thin, long flow paths.

Convert units carefully: if area is in cm² and pressure is in kgf/cm², the result is in kgf; divide by 1000 to convert to tons.

Example

Consider a round-base container: base diameter 135 mm, single cavity, hot runner.

1. Projected area = π/4 × 13.5² ≈ 143 cm²
2. Assume a cavity pressure of 400 kgf/cm² (around 390 bar).
3. Opening force = 143 × 400 = 57,200 kgf ≈ 57 tons
4. Add a safety margin (roughly 10–20%): approximately 63–69 tons

For this part, 80 tons of clamping force is enough; 50 tons is definitely not.

Common mistakes

• Assuming cavity pressure is constant. In a thin-walled part with a long flow path, cavity pressure can double. A single "rule-of-thumb value" doesn't fit every job.
• Forgetting the cold runner. In multi-cavity molds the runner's projected area takes up a significant share.
• Looking only at clamping force. Shot volume (usually preferred at 20–80% of machine capacity), screw diameter and tie-bar spacing also determine the choice.
• Expecting the same pressure from an old mold. A worn parting line can produce flash at the same clamping force.

A practical tip

After calculating the clamping force, if no flash appears at the mold's parting line during the trial run, don't use more tonnage than necessary: excess tonnage fatigues the mold and the platens. It's good practice to keep the clamping force just slightly above the lowest value that produces no flash.

Note: The numbers in this article are examples and approximate. Measuring actual cavity pressure with an in-mold sensor is the most reliable method.

The calculation tool carries out these steps based on part geometry, number of cavities and material, and shows the result together with the recommended standard machine tonnage.