SEQUENTIAL INJECTION MOULDING

A shut-off nozzle is the only type of nozzle that allows HR to be used for injection moulding of structural foam, and for sequential and cascade moulding. 

Two special injection moulding methods have been developed to get around this problem, both based on HR systems with multiple gating sequential moulding and cascade moulding. Both methods require the use of shut-off nozzles with independent control. 

Another example of the use of sequential injection is a multi-cavity mould for products of varying size, e.g., a box and a cover. The HR system in this case would require rheological balancing, or an alternative would be sequential filling, i.e., successive filling of cavities by nozzles opening one after another. 

One restriction on the use of sequential and cascade injection moulding is the need to use expensive shut-off nozzles and an additional control system. 

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. 

In smnmary, sequential injection techniques provide a cheap and useful method for producing sandwich mouldings. It is a technique more suited to simple geometries since the main problem with sequential injection is the lack of control in the skin/core distribution. With the melt stream injection of both skin and core controlled together and with just one velocity profile, skin thickness cannot be adjusted in various parts of the moulding. To overcome these problems, the simultaneous method was developed. Here skin and core velocities can be controlled separately giving enhanced control. 

Figure 10.5 Comparison of mould filling of simultaneous (left) and sequential (right) injection moulding...

Multi-colour moulding. This concept was devised for the automotive industry, specifically for rear light lens clusters in PMMA (polymethyl methacrylate). It involves a rotary mould assembly, with the different colours being injected sequentially from separate cylinders as the mould rotates. 

Sandwich moulding. This is a way of combining the best features of two materials by injecting them sequentially through the same sprue, so that... 

Figure 8.2 Sequential moulding of a box with flexible hinge provides freedom in locating the weld line. 1,2- injection points...

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