injection machine tonnage
mold closed by an injection machine

Introduction: What is Injection Machine Tonnage?

Injection Machine Tonnage is the force applied by the machine to keep the mould closed during the injection process.

Injection moulding machines’ tonnage is typically measured in tons, and selecting a device with the right tonnage for your project is essential.

Tonnage also determines the size and complexity of the parts that the machine can produce.

Too much force can damage the mould, and too little pressure can result in a poor-quality product.

How to Calculate Injection Machine Tonnage

There are several ways to calculate the injection molding machine tonnage. we will give you a brief guide below.

1.Clamping force formula one

F = A * Ka

F – clamping force -tf

A – projected area – in2

Ka – area constant- tf/in2

Plastic TypeArea Constant
General plastics1.5-2 tf/in2 (0.233-0.31 kg/cm2)
Engineering Plastics3~4 tf/in2 (0.465-0.62 kg/cm2)
Fiber reinforced engineering plastics4~6 tf/in2 (0.62-0.93 kg/cm2)

This method is not accurate. It must be based on rich experience to know how to choose a more accurate constant. However, this method is both simple and convenient.

2.Clamping force formula two

F = A * K

F – clamping force – tf

A – projected area – in2

K – constant – 1.2 tf/in2

Plastic typeConstant
PE1.0-1.5 tf/in2 (0.155-0.233 kg/cm2) |
Soft PVC1.5-2.0 tf/in2 (0.233-0.31 kg/cm2)
PS/PP1.5-2.5 tf/in2 (0.233-0.387 kg/cm2)
PA2.0-2.5 tf/in2 (0.31-0.387 kg/cm2)
ABS2.0-3.0 tf/in2 (0.31-0.465 kg/cm2)
PPS3.0 tf/in2 (0.465 kg/cm2)
PC3.0-3.5 tf/in2 (0.465-0.542 kg/cm2)
Fiber reinforced PC4.0-6.0 tf/in2 (0.62-0.93 kg/cm2)

This method adds a safety factor and considers the differences between different whole materials and lists different constants, which is more reasonable than the empirical formula one. However, this method is not suitable for different structure parts using the same plastic.

 3.Clamping force formula three

F ≥A * P / 1000

F – clamping force – tf

A – projected area- cm2

P – mold cavity pressure- kgf/cm2

Plastic product typesMold cavity pressure
Easy to shape, even wall thickness daily use products24.5 Mpa
General civilian products29.4 Mpa
Products with mold cavity flow length ratio less than 5019.6-29.4 Mpa
Industrial products34.3 Mpa
Industrial products with high precision and complex shapes39.2 Mpa
Products with mold cavity flow length ratio greater than 5034.3~39.2 Mpa

The method also adds a safety factor, differs by product category, and considers the complexity of the part. However, the differences between different plastics are not considered and the accuracy of the application is not high.

4.Clamping force formula four

F = A * P * K ÷ K1

F-clamping force – tf

A -projected area- cm2

P – mold cavity pressure- kgf/cm2

K – viscosity coefficient

K1 – safety coefficient – 0.8-1

This method considers the differences between materials, structural, mold design. This method is more accurate. The Safety coefficient takes the value of 0.8-1.


Above four clamping force formulas can help you estimate the tonnage of an injection molding machine. However, these formulas are only estimates, and the actual tonnage may differ. If you need an accurate estimate, feel free to consult our engineers. We will recommend the most suitable machines for your projects.