How to compute for injection machine tonnage:

1. Projected area or shadow area including runners and gates: A
2. Peak pressure: P
3. Clamping force = A x P
4. Include factor of safety
5. Choose the machine.

First, calculate the area, A, projected by the part that you will be molding. Include runners and gates and total number of cavities.

Find the pressure, P, that you will use in molding the part. There are multiple pressure in molding process and you have to identify the peak pressure.

Compute for the ideal clamping force, F:

F = P x A

Include factor of safety to prevent flashing on parting lines. This should be more than 1.

Select the machine tonnage that you will be using. The common practice is you have to choose the machine tonnage so that it is greater than the ideal clamping force that you have computed earlier.

For further reference, go to the thread.