How to Calculate Injection Molding Machine Tonnage

What size injection molding machine do I need?

Normally, most plastic injection engineers will rough calcualate the machine`s tonnage with the product`s surface multiply by a index number.


If you are using square inch as the surface, then you can make it 2.5 times (higher flow materials like PS, PE and PP) to 4 times (lower flow materials like PA, PC and PSU) as the index number.


While your are using square cm as the surface, then you should make it 0.48 times (higher flow materials like PS, PE and PP) to 0.72 times (lower flow materials like PA, PC and PSU) as the index number.


What is the formula to calculate tonnage?

Empirical Formula 1

Clamping force (T) = Clamping Force Constant Kp * Product Projected Area S (CM * CM) * Safety Factor(1+10%)


To properly calculate necessary clamp tonnage, you must first know the projected area of the molded part/s plus any runners.

Area can be calculated by multiplying length times width, A = L x W or A=πr2.


Kp experience value for square cm:

PS/PE/PP – 0.32;

ABS – 0.30~0.48;

PA – 0.64~0.72;

POM – 0.64~0.72;

Add glass fiber – 0.64~0.72;

Other engineering plastics – 0.64~0.8;

For example, a product has a projected area of 410 cm2 and material is PE, calculate clamping force.

Calculated by above formula: P = Kp * S = 0.32 * 410*1.1 = 141 (T), 150 ton injection molding machine should be selected.


Kp experience value for square Inch:


Most injection molders are make it 2.5 times(higher flow materials like PS, PE and PP) or 4 times (lower flow materials like PA, PC and PSU)

the surface square inches of the part and an additional 10% as a safety factor.


If you have a part that is 120 square inches, you would need a injection machine with 300 tons clamping tonnage for injection molding.


After added 10% safety factor, the required press size would be 330 tons of clamping force.Then you will need a 330 ton injection molding machine for your project.


Empirical Formula 2

Clamping force (T) = Material Molding Pressure * Product Projected Area S (CM * CM) * Safety Factor(1+10%)

350bar*S(cm^2)/1000 * (1+10%)

As above, 350*410/1000*1.1=160T, choose 160T injection molding machine.


Accurate Calculation Formula 3


Two important factors in calculating clamping force:

1. Projected area (S) is the largest area viewed along mold opening and closing.

2. Determination of cavity pressure (P)

Cavity pressure is affected by following factors