Processing injection moulding

Admixing an additive-type flame-retardant to a polymer is not always easy. A plain blending of a granulated resin with the additive before the conventional processing (injection moulding, extrusion, etc.) does not provide appropriate homogenity in most cases unless special techniques (e.g. an internal mixer) are used beforehand. 

Surface finish can be a problem with composites, although there are many well-known examples of painted composite surfaces which are judged to be of Class A surface finish. However, the most consistently under-appreciated problem with fibre-reinforced composites is anisotropy, i.e. the directional dependence of mechanical and dimensional properties. It arises from the orientation of the fibres, and consequently is most pronounced when a component has been shaped by a high speed melt flow process injection moulding is the prime example. It is best to assume that anisotropy is always present in a composite, unless isotropy has been designed into the material either by the use of a random glass mat, or by a deliberate layering process in which the orientation in different layers is balanced out. 

T. Pfefferkom, Institute of Plastics Processing, Injection Moulding/PU Technology... 

Polymers owe much of their attractiveness to their ease of processing. In many important teclmiques, such as injection moulding, fibre spinning and film fonnation, polymers are processed in the melt, so that their flow behaviour is of paramount importance. Because of the viscoelastic properties of polymers, their flow behaviour is much more complex than that of Newtonian liquids for which the viscosity is the only essential parameter. In polymer melts, the recoverable shear compliance, which relates to the elastic forces, is used in addition to the viscosity in the description of flow [48]. 

Hiebcr, C, A. and Shen, S.F., 1980. A finite element/finite difference simulation of the injection-moulding filling process. J. Non-Newtonian Fluid Mech. 7, 1-32. 

M. E. Edwards, Chemical Reaction Engineering of Polymer Processing Reaction Injection Moulding Inst. Chem. Eng. Symp. Ser. 8(87), 783—796 (1984). 

Note The injection/compression moulding process capability maps 1, 2 and 3 are used for large parts with a major dimension greater than 50 mm typically and/or for large production volumes. Map 4 is for injection moulded parts that have a major dimension less than 150 mm and which are produced in small volumes. 

Deformation of a polymer melt—either thermoplastic or thermosetting. Processes operating in this way include extrusion, injection moulding and calendering, and form, in tonnage terms, the most important processing class. 

The principles of thermoplastic melt processing can perhaps best be illustrated by reference to Figure 8.1 illustrating extrusion, injection moulding, bottle blowing and calendering operations. In order to realise the full potential of the process it is necessary to consider the following factors ... 

The flow process in an injection mould is complicated by the fact that the mould cavity walls are below the freezing point of the polymer melt. In these circumstances the technologist is generally more concerned with the ability to fill the cavity rather than with the magnitude of the melt viscosity. In one analysis made of the injection moulding situation, Barrie showed that it was possible to calculate a mouldability index (p.) for a melt which was a function of the flow parameters K and the thermal diffusivity and the relevant processing temperatures (melt temperature and mould temperature) but which was independent of the geometry of the cavity and the flow pattern within the cavity. 

An important development of polymerisation casting is that of reaction injection moulding. Developed primarily for polyurethanes (and discussed further in Chapter 27), the process has also found some use with polyamides and with epoxide resins. 

Many articles, bottles and containers in particular, are made by blow moulding techniques of which there are many variations. In one typical process a hollow tube is extruded vertically downwards on to a spigot. Two mould halves close on to the extrudate (known in this context as the parison ) and air is blown through the spigot to inflate the parison so that it takes up the shape of the mould. As in injection moulding, polymers of low, intermediate and high density each find use according to the flexibility required of the finished product. 

The Phillips process for the manufacture of high-density polyethylene may be adapted to produce copolymers of ethylene with small amounts of propylene or but-l-ene and copolymers of this type have been available since 1958. These soon found application in blown containers and for injection moulding. Properties of two grades of such copolymers are compared with two grades of Phillips-type homopolymer in Table 11.11. 

Polynorbomene is also of interest to the plastics processor since by using a dry blending process for mixing and a modified injection moulding process for fabricating, the use of conventional rubber-processing equipment may be avoided. 

Converting the fluxed product into a suitable shape for further processing, e.g. granulating for injection moulding or extrusion. 

Due consideration of these principles has made it possible to process unplasticised PVC by all the standard melt processes, including injection moulding and bottle blowing, a state of affairs hardly conceivable in the 1950s. 

Paste injection moulding processes have also been developed. In one technique used for applying PVC soles to shoe uppers the paste is injected by gas under pressure into a hot mould, the last and shoe upper forming the top half of the mould. The paste gels in the mould, adheres to the upper in the presence of a suitable adhesive and is stripped hot from the mould. 

The polymer melts at 216°C and above this temperature shows better cohesion of the melt than PTFE. It may be processed by conventional thermoplastics processing methods at temperatures in the range 230-290°C. Because of the high melt viscosity high injection moulding pressures are required. 

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