Do a force balance normal to the mould split plane.

Your counsel is unfortunately wrong since the clamping force should be > the pressure force and consider ALL compliances ao the parts in the force loop. It is a simplified answer which could lead to problems.

there was already a quite long discussion on this subject and I am surprise dyou -who follow all comments- did not indicate it.

Feel free to indicate the thread for readers that have forgotten it and those, who like the original poster, are unaware of it.

<Vomit> and what about those that couldn't care less? <Splutter>

You need to calculate the square area of the part being molded at the parting line of the mold. Multiply by the amount of maximum injection pressure to get total force being exerted. Obviously, the clamping force must be greater. I'm not sure, but I believe that a factor of about 1.5 times is used to set tonnage.

Example: Part has an area of 10 square inches at the parting line with a maximum inject pressure 2,500 lbs / sq in. 2,500 x 10 = 25,000 x 1.5 = 37,500 /2,000 = 18.75 tons.

Example is for reference only: verify.