In this course, we explain the basic structure of a mold when a hole is to be prepared in a molded product. In order to prepare a hole in the molded product, it is necessary to form a part in to which plastic does not flow using the cavity or a core pin. An example of the basic structure for this is shown in [Fig. 1].

A. Touching structure

This is the most basic structure. A hole is formed by providing a projection from one side making it touch against the plane surface on the other side. A know-how, regarding the surface on the projection side that is touched, is, while preparing the core pins, to make them longer than the reference height by about 0.005 to 0.03 mm as the "compression margin". If this is done, it will be difficult for flash to form on the touching surface, and sharp edges can be obtained in the periphery of the hole in the molded product. A drawback to this structure is that, when the core pin is thin and long, the core pin gets deformed due to the filling pressure of the plastic thereby causing the likelihood of shifts in the hole position, or a bend in the hole. In addition, in some cases the core pin may bend and break due to the filling pressure.

B. Socket structure

In this structure, the tip of the projection provided from one side is made to engage in a hole provided on the other side thereby forming a hole. If this structure is used, since the core pin will have the structure of a beam that is supported at both ends, the capacity to resist the pressure of the plastic becomes better than that of the touching structure and has the effect of preventing bending and breaking of the core pin or shift in the hole position. A taper is provided on the side periphery of the tip of the projection and the mating hole so that they can mate smoothly. A drawback to this structure is that the cost of preparing the mold will be higher than that of the touching structure.

C. Structure for butting in the middle

In this structure, projections are provided from both sides so that they butt against each other in the middle. In this structure, since the total length of the core pins can be made shorter, it is possible to reduce the likelihood of breaking the core pin. A drawback to this structure is that a parting line is generated in the middle of the hole in the molded product.

D. Shut off structure

In this structure, projections are provided from both sides, and their angles are matched in the middle.

E. Structure for socket in the middle

This structure is intermediate between the structures of B and C.