Calculation of the Stiffness and Strength of the Plastic Injection Mold

--China Plastic Injection Mold Maker Lyter Engineering Ltd share

For quality plastic injection molds, when the plastic melt is filling the cavity, the pressure on the cavity gradually increases. When the cavity is completely filled, the pressure in the cavity reaches the maximum value. Then, as the plastic melt cools, the pressure gradually decreases, and the pressure in the cavity after the plastic is completely solidified drops to close to normal pressure. In this process, each pressure-bearing surface of the mold cavity (cavity sidewall, bottom plate or support plate) must have sufficient rigidity and strength to prevent the mold cavity from being deformed or damaged and cracked under the action of high pressure. Therefore, when designing the mold, the stiffness or strength of the cavity side wall, bottom plate or support plate must be calculated.

The strength of the mold refers to the plastic deformation or permanent deformation of the mold, that is, it is destroyed. The strength calculation of the mold is to check whether the tensile, shearing and bending stress of the mold during the working process exceeds the allowable limit stress.

Calculation of Sidewall Thickness and Bottom Plate Thickness of Combined Circular Cavity:

The force of the side wall and bottom plate of the combined circular cavity is shown as below,

1. Calculation formula for sidewall deformation of cavity sidewall thickness:

R—Outer diameter of round mold part(mm);
b1、b2—Cavity sidewall thickness(mm)；
r—Round mold part inner diameter(mm)；
E—The elastic modulus of the mold part material(MPa)，For Carbon steel, E = 2.1×105Mpa；
P—Plastic melt pressure in cavity(MPa)，generally 24.5~49 Mpa；
μ—Poisson's ratio，For Carbon steel μ=0.25；
fmax—Maximum deformation of cavity side walls, support plate or cavity’s bottom(mm)；
[f]—Allowable deformation(mm)；[f]=S×t， S is the plastic shrinkage(%)，t is the thickness of the product(mm).

3.  Side wall strength calculation formula:

b—Support plate or cavity bottom thickness(mm)；
b1、b2—Cavity sidewall thickness(mm)；
r—Round mold cavity inner diameter(mm)；
P—Plastic melt pressure in cavity(MPa)，generally 24.5~49 Mpa；
[σ]—Allowable stress of cavity material(MPa)；for 1.1730(S50C) carbon steel, [σ]=160MPa，for general mold and tool steel, [σ]=200MPa

b—Support plate or cavity bottom thickness(mm)；
r—Round mold inner diameter(mm)；
E—The elastic modulus of the cavity material(MPa)，for carbon steel, E = 2.1×105Mpa；
P—Plastic melt pressure in cavity(MPa)，generally 24.5~49 Mpa；
fmax—Maximum deformation of cavity side walls, support plate or cavity bottom(mm)；
[f]—Allowable deformation(mm)；[f]=S×t，S shrinkage of plastic resin(%)，t thickness of the part(mm).