---
title: "How to calculate the clamping force for injection molding"
url: "https://www.imspares.com/injection-molding-machine-parts/toggle/clamping-force/"
---

# How to calculate the clamping force for injection molding

How to calculate the clamping force (tonnage) a mold needs from projected area, cavity pressure factor and safety margin, with the formula and a worked example.

A clamping force calculation estimates how much force is needed to hold a mold shut against the melt pressure. The standard tonnage calculation formula is: **clamping force = projected area × cavity pressure factor × safety margin**. The projected area is the area of all the parts and runners seen in the direction the mold opens. The pressure factor is the average pressure expected in the cavity, which depends mainly on the material and the wall thickness. The margin covers what the estimate leaves out. The machine's rated clamping force must be higher than the result.

## The tonnage calculation formula

F = A × p × S

- **F** is the clamping force the mold needs.
- **A** is the total projected area: all cavities plus the runners in the parting plane.
- **p** is the cavity pressure factor for the material, wall thickness and flow length.
- **S** is the safety margin as a factor: a margin of x percent gives S = 1 + x/100.

In metric units, with A in square centimeters and p in bar, F in kN = A × p ÷ 100. To convert kN to metric tonnes-force, divide by 9.81. In US units, with A in square inches and p in tons per square inch, F comes out directly in US tons; one US ton-force is 8.90 kN and one square inch is 6.45 cm². A tonnage calculator does exactly this multiplication, and its answer is only as good as the pressure factor you give it.

## Step 1: the projected area

Look at the part from the direction in which the mold opens and take the area of its outline, as if it cast a shadow on the parting plane. It is not the surface area: the side walls of a deep box add nothing to it. For simple shapes, a rectangle is length × width and a circle is π × d² ÷ 4.

- Openings in the part that are shut off steel to steel, with no plastic in them, can be subtracted.
- Multiply by the number of cavities.
- Add the cold runners and gates that lie in the parting plane. The sprue runs in the opening direction and adds almost nothing.
- With a hot runner, count the parts and any cold sub-runners only: the melt in the hot manifold is contained inside the mold half and does not push the parting line open.
- Slides and lifters also feel the melt pressure. Their wedges or heel blocks hold them, and part of that force acts against the clamp, so the mold designer should include it.

## Step 2: the cavity pressure factor

The factor is the average pressure the melt exerts on the projected area during filling and packing. It is **not** the injection pressure set on the machine or shown on its display. The melt loses pressure along the runner, the gate and the flow path, and it starts to freeze from the walls, so the average pressure in the cavity is far lower than the pressure at the screw tip. Using the injection pressure would greatly overstate the force.

The factor rises with:

- **A stiffer-flowing material.** Easy-flowing grades need less pressure than viscous ones, and fillers such as glass fiber usually raise it.
- **Thinner walls.** The thinner the wall, the more pressure is needed to fill it.
- **A longer flow path.** The ratio of flow length to wall thickness is the key figure: the longer the flow relative to the wall, the higher the pressure.
- **The process.** Lower melt and mold temperatures, higher injection speeds and higher packing pressures all raise the cavity pressure.

Published rules of thumb give a factor per square inch or per square centimeter for each material, but they vary between sources and assume average walls and flow lengths, so treat them as a starting point only. For a real mold, take the factor from the material supplier's processing guide or technical service, from a mold flow simulation, which predicts the clamping force directly, or from the force used on a similar mold you already run. **Always confirm the figure with the material supplier**, especially for thin walls, long flow paths and filled materials.

## Step 3: the safety margin

The margin covers what the formula leaves out: variation between material lots and process settings, pressure that is higher near the gate than the average, a mold that does not load the platens evenly, and on a toggle machine a clamping force that changes as the mold warms up. Its size is a judgment, best agreed with the mold maker. Do not add so much that you choose a much larger machine than the mold needs.

## A worked clamping force calculation

The pressure factor and the margin below are assumed values, chosen only to show the arithmetic. They are not figures for any particular material.

1. Two cavities, each a flat cover 120 mm × 80 mm: 12 cm × 8 cm = 96 cm² each, so 192 cm².
2. Runners in the parting plane: 8 cm². Total projected area A = 200 cm².
3. Assumed cavity pressure factor p = 350 bar.
4. F = 200 × 350 ÷ 100 = 700 kN.
5. Assumed margin of 15 percent, so S = 1.15: F = 700 × 1.15 = 805 kN, about 82 tonnes-force.

This mold needs a machine with a rated clamping force above 805 kN. Check also that the mold fits between the tie bars and within the mold height range, and that the shot fits the barrel: see [tie bar distance](https://www.imspares.com/injection-molding-machine-parts/tie-bars/tie-bar-distance/), [mold height adjustment](https://www.imspares.com/injection-molding-machine-parts/tie-bars/mold-height-adjustment/) and [shot size](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/shot-size/).

## Setting the clamping force on a toggle machine

On a toggle machine the clamping force comes from the stretch of the tie bars when the links lock, and it is set by the mold height adjustment, not by the cylinder pressure. The [toggle mechanism guide](https://www.imspares.com/injection-molding-machine-parts/toggle/) explains how. Two consequences matter in practice:

- The force changes as the mold heats up and grows, so check it once the mold is at temperature.
- Wear in the [toggle pins](https://www.imspares.com/injection-molding-machine-parts/toggle/toggle-pins/) and [bushings](https://www.imspares.com/injection-molding-machine-parts/toggle/toggle-bushings/) lowers the force at the same setting.

Some machines measure the clamping force with sensors and display it. Where they do not, it can be checked by measuring the stretch of the tie bars; uneven stretch between the four bars points to an adjustment or wear problem. See the [tie bars guide](https://www.imspares.com/injection-molding-machine-parts/tie-bars/).

## Too little or too much clamping force

Too little force lets the mold open slightly under the melt pressure: the parts show **flash** at the parting line, their weight and dimensions vary, and the flash can damage the parting line.

Too much force does not make better parts. It loads the tie bars, toggle pins, bushings and platens harder on every cycle, and over time it can deform the parting line and close the vents, which leads to burns and short shots from trapped gas. See our article on [short shots](https://www.imspares.com/blog/short-shots-injection-molding-causes-fixes/).

On a running mold, a common practice is to lower the clamping force in steps while watching the part weight and the parting line, and to set it with a margin above the point where flash or a weight increase appears. Never exceed the machine's rated clamping force, or the force the mold is designed for.

## Questions

### How do you calculate the clamping force for injection molding?

Multiply the projected area of all cavities and runners by a cavity pressure factor for the material and wall thickness, then add a safety margin. In metric units, clamping force in kN equals the area in square centimeters times the pressure in bar, divided by 100.

### What is the projected area of a molded part?

It is the area of the part's outline seen in the direction the mold opens, like its shadow on the parting plane. It is not the surface area, and the depth of the part does not add to it.

### Can I use the injection pressure in the tonnage formula?

No. The injection pressure is far higher than the average pressure in the cavity, because the melt loses pressure along the runner, the gate and the flow path. Using it gives a much larger clamping force than the mold needs.

### How do I convert tons to kN?

One metric tonne-force is 9.81 kN, and one US (short) ton-force is 8.90 kN. To go from kN to metric tonnes, divide by 9.81.

### What happens if the clamping force is too high?

The parts do not get better, but the tie bars, toggle pins, bushings and platens carry more load on every cycle, and over time the mold's parting line and vents can be damaged. Use the lowest force that molds parts without flash.