Moulding sequential

The sequential hypothesis assumes that the substrate has a more direct influence on the shape of the enzyme. In the absence of substrate the enzyme would exist more or less entirely in the T-form which has a very poor fit for the substrate. As the substrate enters the active site, as usual by purely random collision, the enzyme protein moulds itself around the substrate to produce a good fit. This is known as inducedfit. The process of induced fit has resulted in the subunit to which the substrate has bound being converted to the R-conformation of the enzyme. 

Although manual moulding is still in use, machine moulding is currently the most widespread. Two sequential operations must be performed by a moulding machine the first is ramming the sand, followed by separating the pattern from the compacted sand. The most commonly used processes proceed from the working principles described hereafter. 

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... 

HR make it possible to design moulds of, for example, the modular type for thin-walled packings, bottle preforms, and so forth, and also moulds for sequential moulding, moulds for in-mould decoration and lamination, and so on. 

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. 

Mould filling control. An HR system with hydraulically or pneumatically closed nozzles has given rise to a new potential for controlling the mould filling process. These techniques are known as sequential and cascade moulding (see Chapter 8.1). 

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. 

Sequential moulding is based on mutually independent opening and closing of the nozzles in the mould cavity in such a way as to allow the weld lines to be shifted until they reach an optimum position. This technique is used for products of large area and with numerous apertures which cause weld lines to appear. The use of simulation clearly provides a good starting point for optimisation of nozzle 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. 

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

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. 

Figure 5.9 Coolant expansion tank for Jaguar car, blow-moulded in propylene-ethylene sequential copolymer. By permission of RB Blowmoulders Ltd.

In the most simple single-station machine the heating and cooling processes must take place sequentially so that their utilisation must be quite low, whilst in the more complex machines the heating and cooling process is continuous, the moulds being moved between stations automatically. 

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...

Keeping the cycle time the same due to sequential moulding whereby the three cavities are each moulding at any one cycle... 

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