The section modulus of mold components which is very important for predicting the bending stress is explained here.

At the time of calculating the strength against deformation or bending of a mold for plastic injection molding, the term "section modulus" appears very frequently. Let us understand how the "section modulus " is similar to the "moment of inertia of area" so as to carry out the mechanical calculations while understanding it more accurately.

The "section modulus " is a numerical value that is determined by the cross-sectional shape of the part. In that sense, it is similar to "moment of inertia of area ".

The "section modulus " varies depending only on the cross-sectional shape of the part. Therefore, it has no relationship whatsoever with the material of the part. For example, if the cross-sectional shape is the same, the value of the "section modulus " will be the same irrespective of whether the material is non-heat treated steel, tempered steel, or even wood. The definition of section modulus according to mechanics is as follows. A "section modulus is the value of the "moment of inertia of area" related to the neutral axis of the cross-section of a beam multiplied by the distance from the neutral axis to the outer surface". Therefore, the relationship between the section modulus Z and the moment of inertia of area I is as expressed by the following equation.

The symbol Z is used customarily for the section modulus. In general, as the section modulus becomes larger, the strength against bending also becomes larger. Regarding bending, the maximum bending stress σ acting on the outer surface of the part can be calculated using the following equation.

If the cross-sectional shape is rectangular or circular, the basic equation for calculation becomes clear as is shown in Table 1.