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1、Multi-material technologyThe injection molding of parts with different material/color combinations has been known for many years. Today the utilization of different material combinations is increased, figure 1:Figure 1: Different material/color combinationsThe combination of two or more materials is
2、 achieved by a number of techniques. These may be characterized as mould-controlled or machine-controlled processes.Mould-controlled processesOvermolding 32Two or more materials are injected into different cavities or into two or more moulds which must be opened and closed between each injection ste
3、p. The principle is shown in figure 2.41 Figure 2: Two or more moulds with a rotating systemThe mould is mounted on a rotary table fitted to the machine. Material 1, 2 and 3 are injected into the moulds A, B and C. After the mould opens, the molding stays in the moving half and is rotating together
4、with the mould-half to line up with cavity ABC into which the fourth material is injected by injection unit 4. The mould-halves on the moving platen are identical, while those on the fixed platen are different. Each time the mould opens a finished component is ejected from cavity ABC. For an example
5、 see figure 3. N9Figure 3: Example of overmoldingMulti-component moldingThis a another mould-controlled process. In contrast to overmolding, a single mould or cavity is used which has moving inserts which block off one or more parts of the cavity at the start of the injection process. The first poly
6、mer is injected into the mould, only filling the first part of the cavity with material. A few seconds after injection of the first material, cores or moving inserts in the mould are activated into which the second polymer is then injected. The principle is shown in figure 4.4A4BFigure 4A: Injection
7、 of material 1 into the first part of cavity, second part of cavity blocked by moving slide.Figure 3B: Injection of material 2 into second part of cavity.Limits to the possible applications of this process are only set by the geometry of the product. Examples of multi-component injection molding are
8、 shown in figure 5.Figure 5: Examples of composite injection moldingMachine-controlled processesSandwich injection moldingIn contrast to the techniques discussed above, sandwich injection molding is a machine-controlled process. The mould used is a conventional injection molding tool. Figure 6 show
9、how the process starts with injection of the skin component and is completed after a simultaneous phase with the core component. A small amount of skin can be injected afterwards for the purpose of sealing the gate. Figure 6: Filling sequence for sandwich injection moldingThis process can be used wi
10、th two materials of the same or different types, the following combinations are possible: Compact outer skin/foamed coreThere are many applications of this process using the combination of a compact skin and a foamed core. Parts produced in this way have the recognized advantages of structural foam,
11、 but still have a surface quality which may be compared with that achieved by conventional injection molding. For moldings with a wall thickness of 4mm or more a foamed core component is preferred in order to prevent sinking and ribbing witness. In this case injection can be done without holding pre
12、ssure. Compact outer skin/compact coreIn the wall thickness range from 1-4mm both components should preferably be compact. In this case sink marks are prevented by the conventional application of holding pressure.Applications for sandwich molding are shown in figure 7.Figure 7: Example of a sandwich
13、ed molded part Material combination for mould controlled processesIn multi-material molding a combination of materials is used which depending on the application, either possess a good or no adhesion at all towards each other. To obtain good adhesion, attention should be paid not only to the choice
14、of materials but also to the order of their injection. If the polymer with the lower processing temperature is injected first, followed by one at a higher temperature, better adhesion is achieved than with the reverse order. Good adhesion is especially desirable for soft-rigid combinations.Material
15、combination for machine controlled processesBy using sandwich injection molding it is possible to produce molding from different materials. The following criteria should be observed when selecting suitable materials: The same or similar shrinkage of both materials Good adhesion between the materials Adhesion is influenced by component design, wall thickness of individual layers and sequence of molding.Table 1: Typical material adhesion combinations
