In the design of molded article, quite often there will be a need for undercut shapes. (An undercut is a shape that cannot be taken out just by opening the parting surface. For example, a horizontal hole, a projection, a depression, etc.).

In the case of an undercut shape, it will be necessary to design the mold using mechanical engineering techniques so that the molded article can be taken out with a special mold structure such as a slide core or a slant pin, etc.

The design would be easy if the undercut can be handled using the simplest slide core structure, but sufficient knowledge of and expertise in mold design will be required if a complex undercut shape has to be produced.

When a mounting test is to be done by quickly obtaining a sample of the molded article with a trial mold, etc., the delivery time can become as long as two weeks if the development of a special undercut mechanism and mold preparation is to be made. In such situations, a method of handling undercuts called the "removable insert" is used which was devised by our seniors with some skillful techniques.

What is a "removable insert"?

• The core and the cavity that constitute the part of the molded article that becomes an undercut are divided, and then designed and fabricated.
• The removable inserts that are prepared by dividing the mold are taken out from the mold along with the molded product at the time of ejecting the molded article.
• The removable insert is removed from the molded article by a manual operation.
• The removable insert is installed in the mold manually at the time of the next molding.
• In other words, with a removable insert structure, it is not possible to carry out continuous injection molding, and the method is that of forming the undercut shape semi-automatically using manual operations additionally.
• There are also some cases in which the removable insert is pushed out from the mold by an ejector pin.
• In order to position the removable insert, a key structure or a knock pin structure is used in some cases.
• There are cases in which the molding is done after pre-heating the removable insert.
• There is also the method of molding which makes the semi-automatic molding more efficient by preparing two removable inserts, and inserting them alternately into the mold.
• In order to ensure that the direction of inserting the removable insert is not mistaken, there is also the technique of using a direction determining structure.
• The key point is to design the escape and the clearance so as to make it easy to install and remove the removable inserts.
• There is also the management aspect of provisionally preparing the trial mold with a removable insert structure, thereby obtaining time for developing a mold structure such as the slide core structure that can permit continuous molding.