Why Does a Mold Break?

Although no unnecessary force is applied to a mold when it is being assembled, when it is actually installed in an injection molding machine and the molding operation being conducted, it is subject to various external forces unlike those experienced during assembly.
For example, the mold clamping force when a mold is being clamped can be from several tons to several hundreds of tons, even several thousands of tons. It is necessary that the mold has enough strength to withstand that compression stress.
In addition, in order to completely fill the mold with molten plastic via the sprue and via the runner, it is necessary to apply pressure to the plastic and make it flow inside the mold. The reason for this is that since molten plastic is a fluid having viscosity, a sufficient pushing force is necessary to make it flow into the mold. The force of the pressure can be 1000 to 2000 kgf/cm2 near the sprue inlet, and even inside the cavity the force of the pressure is 200 to 600 kgf/cm2.
In addition, since the force of the pressure acts for a very short time which is normally not even 1 second, considerable shock is applied to the core pin and the walls of the cavity, and in some cases, this may cause buckling of the thin and long pin.
In this way, if we sequentially look at the process by which the parts of a mold break, we can find the corresponding causes. In order to make sure that a mold does not break, at the time of designing a mold, it is very important to make clear the basic environment of use (injection pressure, mold structure, acting stresses, etc.) in terms of numerical values, and to verify in advance the actual operation of the mold. This is because fatal damages can occur that cannot be covered by fine adjustments after the mold has been prepared if the mold preparation is done without carrying out the strength calculations of the basic structure and while defects are allowed to be present in the structure.
In addition, even when machining the parts of a mold or at the time of assembling and adjustment, it is very important to give considerations to machining after understanding the shapes of the parts, the surface quality, the accuracy of mating, etc. In the case of machining, although the minimum possible responsibilities can be said to have been carried out as long as the work has satisfied the dimensions, accuracy, and tolerances specified in the drawings, in order to make a more superior mold, it is desirable to understand the functions of all the parts of the mold, so as to advance one step further.
In order to prepare molds that do not break, it is very important that there is a balance between the basic concepts and the considerations in machining and assembly.