The term “multi-component” here refers to the several different plastics which are  injected into the injection mould. For a two-component injection mould, which specialists refer to as a 2-C mould, this means that two different plastics are injected  one after another into the injection mould. This method is becoming more and more important. The demand for getting a  ready-to-use piece from an injection mould is steadily increasing. More and more  companies are upgrading their injection mould technology as well as machining  techniques. The areas of application for plastic parts are multiplying through this  method.

1 Material Pairings 

The plastic parts that are used for this method have different characteristics. For example they can be hard or soft. For a housing made of hard plastic, a soft  rubber sealing is injected in the second process step. For this pairing the hard and  the soft plastic must permanently bond.

Another common pairing is the same material but in different colours. For example, a common application is vehicle rear lights in several colours. For these types  of applications 3-C or 4-C moulds are not uncommon any more. In the brush head of an electric toothbrush different hard-hard pairings are processed. Here no bonding of the materials is needed but one of the plastics has  sliding properties so the brush can move.

2 Mould Technology 

The mould technology is a significant improvement compared to the mould types  described so far. We are talking about very complex and expensive moulds. 

2.1 Shifting Technology 

For these types of moulds the simplest method is shifting. For this procedure a  2-C mould has two cavities, and a multi-component mould has an even number of  cavities. The first material is injected in half of the cavities and the second material is  injected in the other half. The mould cavities are arranged in the injection mould  so the handling system or robot can shift the pieces from the first half to the second  half. The pieces are therefore evenly distributed, either linearly or in a circle. Our example is a mould in which a coaster is injected with hard/white–soft/blue  material pairing. In the first mould cavity the blank for the coaster made of hard  material is injection moulded. In this first injection, the area which is to be filled  with the soft material in the second injection process is left as a recess. All recessed  areas are connected together so that one feed point suffices for the soft material.  After the mould is opened, the semi-finished part from the first half is transported  to the second half where the coaster is finished. For every cycle there is one half with semi-finished and one half with finished  parts. The number of pieces that come out of the machine is therefore always half  of the number of cavities. Thus two cavities corresponds to one finished piece,  four cavities corresponds to two finished pieces, and so on. Two injection units are  required to inject all mould cavities simultaneously. The coaster is displayed in Figure 2.28, as it comes out of the mould as semifinished (after the first injection process) and as finished part.

The first, hard material comes into the mould from the right. At the same time, the  soft material is injected in the middle of the mould. The mould opens and the finished part is ejected on the left. The semi-finished part on the right is transported  to the left cavity by the handling system. In Figure 2.29 the top view of the moving half is presented. For this type of mould  the shifting takes place in the moving half.

In Figure 2.30 the whole mould—moving half and fixed half—is presented.

2.2 Rotary Table Technology 

A common technology for multi-component injection moulds is the mould with  rotary table. With this technique the semi-finished part is not shifted but stays  in the mould.

 In contrast, one half, in our example the moving half, rotates by 90°  on the injection moulding machine between the first and the second injection  processes. Depending on the size, complexity, and shape of the plastic part, the rotation can be big or small. For a large 2-C part the moving half rotates by  180°. The injection mould then has two cavities, one for the soft and one  for the hard material. For smaller 2-C parts there can be for example eight cavities in an injection  mould, four soft and four hard. The moving half rotates by only 45° between  the injection of the hard and soft material. 

Our example is a cover from a hard inner material with a circumferential seal from  a soft material on the outside. The tool is a 4-component injection mould with two  cavities each. In Figure 2.31 the front and back sides of the cover are displayed. Both materials  are injected via a hot runner with needle valve. This technology is described in  detail in Section 3.9.11.

In the moving half four identical cavities are incorporated in the four inserts. Two  different inserts are installed in the fixed half. The first two inserts have the  contour for semi-finished parts. The insert is formed so it covers the area where  the sealing is injected in the second step of the process. In both other inserts on  the fixed half the area of the sealing remains uncovered. In turn, the edges of the  semi-finished part are sealed with the insert. Thus, the soft material only comes in  contact with the semi-finished part where it is desired. In Figure 2.32 the differences between the two fixed halves are shown. For this  mould the materials are defined through different fixed halves. They have the sealing for both the semi-finished part and between the semi-finished part and the  seal.

Our injection mould example has an integrated rotary plate in the moving half. The  clamping plate remains screwed tight to the machine and the cavity plate is rotated  90° upwards and back via a small gear. Both mould halves must fit together exactly in both positions, 0° and 90°. The flat guiding units, which are mounted on  the side of the mould, assume this task. On the moving half there are eight fixtures  (four for every position) and four guiding rails are attached to the fixed half. In Figure 2.33 the entire moving half of our example injection mould is shown. It is  evident that all four cavities are the same.

In Figure 2.34 both mould halves are shown. Among other things, the eight fixtures and the four guiding rails are shown here. Clearly visible are also the two  different cavities for the hard and the soft material.

2.3 Sealing Slide Technology 

Moulds with sealing slides are another common type of multi-component injection  mould. The principle of this injection mould is that during the injection of the first material some areas are sealed for one or more materials by slides. After the first material reaches a certain solidity, one or all of the slides move back in the closed  mould, and the corresponding area is free for being filled with other materials. The  sealing slides do not move completely out of the area, but after the retraction remain  a part of the further geometry. The retraction of the slides is usually hydraulic. With this sealing slide technique, a plastic part comes from every cavity of the  injection mould. There is no shifting of the part or rotating of the injection mould.  The materials are injected into the mould one after another. Every time the injection mould opens, as many finished pieces come out of the mould as cavities it has.  A disadvantage can be that through the gradual filling of the injection mould the  cycle is longer. 

2.4 Further Technologies 

As for special designs, there are still further possibilities for multi-component injection moulds, whose explanation would take too long here. There are also injection  moulds which combine a number of different technologies. For further details see the literature referred to at the end of this chapter.