Minimum Mold Sizing

Someone was preparing for some in-house training, and asked me this important question…
We are selecting a mold for training. I have a mold in house, however the size of the mold base is 8″ x 8″. I know the manual suggests a mold base minimum of 17″ x 17″ under full tonnage. Can this mold be used for training if it is not under full tonnage?
My Response
Although every situation is different… as a general rule, you do not want to use a mold which uses less than 2/3 of the space within the tie bars. For example, if the horizontal spacing between the tie bars is 18 inches and the vertical spacing between the tie bars is 12 inches… then the minimum mold size should be 12w x 8h.
If you go below this value, the force is will be concentrated on the center of the platen. The factor is not the amount of tonnage being used, but the centralization of force. Since the platens are cast, they do can be deformed very easily… a mold which is too small will quickly increase the concavity of the stationary platen, and possibly damage the movable platen as well.
Additional Thoughts
A bolster plate can also be added to the mold to provide additional support and distribute the force more evenly across the platen. This plate bolts to the clamping plate and extends beyond the mold base; thus increasing its effective size. In some cases, bolster plates can be as much as nine inches thick and can extend beyond the mold base by as much as twelve inches.

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