Injection moulding encapsulation

The main processing methods for addition polyimides are machining compression, transfer and injection moulding encapsulation, impregnation, stratification, filament winding, varnishing, sintering. 

As has already been stated in Chapter 5, the principal method used for the plastic encapsulation of electronic devices is transfer moulding with thermosetting epoxide resin based moulding compounds. A number of other thermosetting compounds are also occasionally used but until recently no thermoplastic materials capable of being injection moulded were suitable for the encapsulation of electronic components because of the high pressures required. One thermoplastic material that has been successfully developed for the packaging of electronic components is polyphenylene sulphide. 

There are a number of variations of the injection moulding process, one that is particularly applicable to recyclates is coinjection moulding. Co-injection moulding produces a sandwich like structure that can be used to encapsulate recyclate materials, covering them in a layer of virgin skin material. 

Figure 10.2 Encapsulation of core material by skin during co-injection moulding...

It was reported as far back as 1974 that two-phase flows such as those found in co-injection moulding are sensitive to differences in the rheological properties of the melts. Different rheological combinations produced different skin/core ratios. Later the simultaneous system driven by viscosity encapsulation phenomena was proposed as shown in Figure 10.11. 

LPM is primarily a one-step process to encapsulate, seal and protect electronic assemblies. The process is also referred to as over-moulding or hot-melt moulding. The electronic assembly is placed in an aluminium mould which is then closed and a thermoplastic compound injected into the mould cavity encapsulating the assembly. The moulds are cooled by chilled water circulating through the mould platens to approximately 20 °C. When the part cools, it is removed from the mould. Cycle... 

Also known as dual injection, sandwich moulding, 2K (or 3K). It can be recognised by the production of a defined skin and core structure. The core is fully encapsulated, to produce a sandwich like moulding. 

The trend towards multi-material moulding has been pushed by the growing importance of added value manufacture to give moulders commercial advantages over their rivals. An increase in the importance of material recycling has also aided the growth in co-injection technologies, where encapsulation enables recyclate materials to be used. 

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