How to improve the rigidity of the injection mold

Injection molds are subject to high internal pressures when filling with molten resin and high compressive stress when closing the mold. In addition, large molds are subject to bending stresses due to their own weight.

Therefore, the rigidity of the mold must be increased to prevent deformation and breakage due to external or self-weight stress.

Here, I hope to return to the origin and re-understand rigidity.

The so-called rigidity refers to the deformation resistance against the load. Stiffness depends on the elastic modulus E and shear modulus G of the material.

The higher the E or G value of the material, the higher the stiffness. In other words, it exhibits strong resistance to bending and twisting. To put it more succinctly and understandably, this material will be difficult to bend, and the deflection will be very small.

For example, the E value of SCM440 series pre-hardened steel is 203×104 (kgf/cm2), while the E value of SKD11 (cold work die steel) is 210×104 (kgf/cm2), so it can be said that SKD11 has higher rigidity.

If explained in further detail, rigidity can be divided into two categories: "flexural stiffness" and "torsional stiffness". For injection molds, "flexural stiffness" is particularly important.

The so-called flexural rigidity refers to the deformation resistance against bending loads. Bending stiffness is usually denoted "ExI".

(I is the moment of inertia of the section. Please refer to the previous lecture for more information)

In order to increase the flexural rigidity, it is necessary to increase the product of E×I. That is to say, selecting a material with a larger elastic modulus E and using a cross-sectional shape with a larger area moment of inertia I can improve the "flexural rigidity".

If the structure has a high "bending stiffness", it will deflect less and be able to resist failure due to bending deformation.