Co-injection moulding

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. 

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The co-injection moulding of PVC-U with other thermoplastics (glass fibre reinforced PVC, polypropylene, ABS and polycarbonate), was investigated using the mono-sandwich process and the properties determined. Polypropylene was the only polymer not to exhibit good adhesion. The mechanical properties of the other samples were intermediate between those of the constituent polymers (104). 

Figure 6.10 Skin/core configuration of co-injection moulding...

A co-injection moulding machine has two injection barrels feeding into a common die. A machine is shown in Figure 6.9. The recyclate materials that make up the core can be contaminated or mixed plastics whilst the surface skin, which can be seen, is made up of 100% virgin material. The process works better if the core is more viscous than the skin. Figure 6.10 shows what the skin-core configuration would look like, if the skin were... 

Co-injection moulding Complex Injection Closed Single layers Low... 

This is simultaneous injection of different materials through different gates as opposed to through the same gate as in co-injection moulding. 

This method describes a process whereby plastics are injected into one mould dining a single injection cycle. The most common process in this category is co-injection moulding. Other less common variants are bi-injection and intermittent techniques. 

Co-injection moulding offers numerous possibilities in terms of a variety of material eombinations, some of which are shown in Table 10.2. 

The process does have limitations. There is a need for the skin and core materials to be compatible with each other in terms of adhesion and shrinkage. Adhesion of the layers is necessary to prevent the core material becoming detached from the skin especially if the moulding is likely to be exposed to mechanical loads. Therefore materials must be compatible or a suitable compatibiliser used in the core component. The use of compatibilisers in the core component of co-injection moulding was developed and patented by the Rover Group in collaboration with University of Warwick [1]. Researchers from Warwick have also developed and reported methods to mechanically interlock immiscible materials for co-injection moulding but these are currently in the early development stages [2]. 

In co-injection moulding two compatible melts are injected either sequentially or simultaneously into the mould thus forming a layered structure. The melt injected first forms the skin, whilst the melt injected afterwards forms the core. 

There are a number of variations of the co-injection moulding process which have been developed. They can be split into two types, sequential injection and simultaneous injection. These methods will now be introduced and the advantages and disadvantages of both discussed. 

Part Design and Tooling Requirements for Co-Injection Moulding... 

Rheology and Mould Filling Why and How Co-Injection Moulding Works... 

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. 

Figure 10.13 Co-injection mouldings produced with non-laminar flow (left upper and lower), with laminar flow (right upper) and melt fingering (right lower)...

Co-Injection Moulding Application Case Studies Preforms for blow moulding... 

Co-injection moulding provides processing routes for obtaining property combinations that are, in general, not possible with conventional injection moulding. However, there are a number of factors that have tended to limit the commercial take up of co-injection moulding technologies. 

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