How are the external dimensions of the cavity (nest in the fixed side) determined? In most cases, the reality is that the dimensions of a similar past mold are used as a reference, or the dimensions are determined by experience and intuition. If the correct procedure for determining the external dimensions is known, it is possible to avoid the danger of accidents of the mold breaking due to the pressure of the plastic, and also to avoid the wastage of preparing an unnecessarily strong and large mold.

The correct procedure for determining the dimensions is explained below.

Step 1: Calculating the minimum wall thickness

A cavity is formed by carving a concave shape inside a block of steel material. Unless the thickness 'h' of the wall between the carved complementary shape of the molded product and the external shape of the steel material has a certain thickness, the mold may break or may become greatly deformed due to the filling pressure of the plastic. It is possible to obtain the recommended value of this thickness by theoretical calculations by applying the equations of the field of strength of materials.

The appropriate equation should be selected since the equation to be used differs depending on - (1) the external shape of the cavity (cubical or cylindrical), and (2) the structure of the cavity (unified or separated).

The data to be substituted in the equation are determined considering the molding conditions, and the type of steel material, etc. The technique of a professional is also to assume a variety of cases such as the case of bad preconditions for calculation, a case of the best preconditions, etc., and to make a comparison of the results of the calculations. The minimum wall thickness based on theory is determined by taking into account a margin of safety for the value of h obtained by the calculations.

Step 2: Cavity

If the external shape of the cavity is determined using the value of h obtained by calculations, at the time of fixing the cavity to the mold plate, etc., it may some times not be possible to obtain sufficient dimensions of flanges or to obtain sufficient space for drilling screw holes. In such cases, dimensions should be determined so that any one of these can be placed, and the final cavity dimensions are determined using even integer numbers that are round numbers (for example, 50 mm, 80 mm, etc.).