Torque for Clamping Bolts

Torque calculators are a great resource for the Scientific Molder as long as they take in effect the low strength of the cast platen.


We are trying to figure out what torque to use on our bolts when clamping in molds. I read that the bolt size and hole diameter make all the difference. If I send you those sizes, could you recommend those values for me?

My Response:

The proper way to determine max bolt torque is to use a calculator which allows you to select or enter strength of the platen material.

The grade of cast steel varies, but generally it has the strength of low grade cast steel around 30,000psi or 200 MPa. Since the integrity of most of these platen holes are generally suspect, it is safest to use a lower strength value such as 15,000-20,000psi or 120-160MPa.

Using a calculator such as the one below using a 5/8” steel bolt mating into a custom material with a yield strength of 20000, the recommended torque is 65ft-lb with a max of 86ft-lb – this falls in line with normal industry recommendations of using 50-60ft-lbs for 5/8” bolts (assuming you have at least 150% of the bolt diameter engaged).

Additional Thoughts:

Many people get confused because a 5/8″ bolt might be rated at hundreds of ft-lbs of torque, but this does not take in account the significantly weaker threads in the platen.

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