A plastic injection mold is subjected to high internal pressure at the time of filling the molten plastic, and also, it is subjected to high compression stress at the time of clamping the mold. In addition, if the mold becomes large, it can also be subjected to a bending stress due to its own weight. In order to make sure that the mold does not get deformed or broken due to the external stresses or stresses due to its own weight, it is necessary to strengthen the rigidity of the mold.

Here, let us understand from the basics what rigidity is.

Rigidity is the resistance to deformation when subjected to a load. The modulus of longitudinal elasticity E and the modulus of transverse elasticity G of the material affect the rigidity. A material for which the value of E or G is large can be said to have a high rigidity. In other words, it exhibits strong resistance to bending or twisting. In more easy to understand terms, the material is difficult to bend, and also has a very small deflection.

For example, while the value of E for SCM440 series pre-hardened steel is 203  104 (kgf/cm2) but the value of E for SKD11 (cold rolled die steel) is 210  104 (kgf/cm2), it can be said that SKD11 is more rigid.

Explaining in more detailed terms, rigidity can be "bending rigidity" or "twisting rigidity". "Bending rigidity" is particularly more important in the case of the molds for plastic injection molding.

Bending rigidity (flexural rigidity) indicates the resistance to bending due to a bending load. In general, the bending rigidity is expressed by "E  I". (I is moment of inertia of area.For details, see the previous course.) In order to increase the bending rigidity, it is necessary to make the product E  I large. In other words, selecting a material with a large value of the modulus of longitudinal elasticity E and also adopting a cross-sectional shape that makes the moment of inertia of area I large results in making the bending rigidity high. If the structure has a high bending rigidity, even the deflection becomes small and it is also possible to resist breakage due to bending deformations.