Question regarding mold base size versus platen size

[nicholas]

There has been some debate within the company I work for regarding the relationship between the size of the platen and mold base. I am told the mold should cover at least 2/3 of the platen area to avoid the platen "flexing".

This does not make sense to me. Given your tie bars and location of toggles I would think you could go much smaller than that (also taking into account adjusting the clamp force).

Can someone provide some insight into this? Would be very much appreciated.

[molding]

Nick,

I just recently attended RJG Master Molding class and they also taught the 2/3 rule. A toggle press exerts more force on the outside corners of the B platen. A hydraulic press exerts more force in the center of the B platen. The most unsupported area on both toggle and hydraulic presses is the A side platen in the center where the locating hole is on both presses. You just have to remember that your platens try to wrap around your mold. The smaller surface area your mold takes up of the platen, the more "wrap" you will encounter. Maybe this picture will help explain it better.

[rickbatey]

Another often missed reason is if the mold is too small the calculated tonnage will be much higher due to the reduced surface area. If the clamp exerts 100 ton across the platten faces, a 2/3 mold will have 1/3 higher clamp force than the platten exerts. Smaller will mean even higher exerted clamping force.
Besides if you build your molds to a standard plate size for the press frame size, you can setup much faster and clamp the mold in with bolts through the platten for faster mold changes without spending lots of money to standardize the plant.
Rick