Special methods of plastic injection molding are explained starting from this lesson. Firstly, in this lesson we describe the two color injection molding method.

The two color injection molding method is a method of molding that is recently being called the "two material injection molding method" or the "different material injection molding method", etc. Two types of thermoplastic resin are successively injected into the mold by respectively different injection cylinders, thereby producing a product with two types of colors.

This is a molding technology that is being used widely in the preparation of key tops for high end desktop PCs, or the illuminated buttons of car navigation units, etc.
In general, it appears that very often two plastic resin of the same type such as PS plastic or ABS plastic are used. This is because there is very good adhesion between the two molded items. Although it is possible to produce molded products from two different types of plastics resin such as ABS and POM, the adhesion between them is not necessarily good. (There are different applications when the adhesion is good and when the adhesion is not good.)
In addition, recently there are some unique combinations that have been realized such as combinations of thermoplastic plastic material with a thermoplastic elastomer (rubber-like plastic resin). (Sports goods, etc.)

In order to practically use the two color injection molding method, usually, a special injection molding machine will be necessary. Such machines are being made by manufacturers of injection molding machines in Japan as well as in countries like Switzerland and Germany. The injection molding machine is equipped with two injection units, which respectively pour the molten material into the interior of the cavity of the mold via their respective sprues.
In the mold, the female part of the cavity is formed on the fixed side of the respective plastic material.
On the other hand, two male cores of the same shape are formed on the moving half, and the space between the male parts can be moved by a rotating mechanism or a sliding mechanism. (There are several types of patterns of this structure.)

In the two color injection molding method, since a beautiful multi-function molded product can be produced in one step, it is possible to produce a molded item with high value addition. It is also possible to have multiple cavities in a single shot in the case of molded items with small sizes.
However, the design of the molds requires knowledge about the design of wall thicknesses and know-how about the bonding between different plastic materials. Some techniques will be necessary regarding the temperature control of molds also.