Injection moulds

PoJy (3,3-bis (chloromethyl) oxelane), chlorinated polyether (R CHiCl) is widely used for injection moulding and coaling. 

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. 

MaranylNylon Compounds, Injection Moulding, N102,11th ed., Du Pont de Numours Int. SA, Geneva, Switzedand, 1987. 

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

When metals are rolled or forged, or drawn to wire, or when polymers are injection-moulded or pressed or drawn, energy is absorbed. The work done on a material to change its shape permanently is called the plastic work- its value, per unit volume, is the area of the cross-hatched region shown in Fig. 8.9 it may easily be found (if the stress-strain curve is known) for any amount of permanent plastic deformation, e. Plastic work is important in metal- and polymer-forming operations because it determines the forces that the rolls, or press, or moulding machine must exert on the material. 

Demonstrations (a) Give four injection-moulded close-packed planes to each student to allow personal building of f.c.c. and c.p.h. (b) Atomix atomic model on overhead projector to show atom packing (Emotion Productions Inc., 4825 Sainte Catherine O, Montreal 215PQ, Canada) or ball bearings on overhead projector. 

When you have to estimate how a change of temperature changes the viscosity of a polymer (in calculating forces for injection moulding, for instance), this is the equation to use. 

Fig. 24.3. (a) Extrusion polymer granules ore heated, mixed and compressed by the screw which forces the now molten polymer out through a die. (b) Injection moulding is extrusion into a mould. If the moulding is cooled with the pressure on, good precision and detail ore obtained. 

In injection moulding, polymer granules are compressed by a ram or screw, heated until molten and squirted into a cold, split-mould under pressure (Fig. 24.3b). The moulded polymer is cooled below T, the mould opens and the product pops out. Excess polymer is injected to compensate for contraction in the mould. The molecules are oriented... 

Both thermoplastics and thermosets can be formed by compression moulding (Fig. 24.5). The polymer, or mixture of resin and hardener, is heated and compressed between dies. The method is well suited to the forming of thermosets (casings for appliances, for instance) and of composites with a thermosetting matrix (car bumpers, for example). Since a thermoset can be removed while it is still hot, the cycle time is as short as 10 seconds for small components, 10 minutes for large tliick-walled mouldings. Pressures are lower than for injection mouldings, so the capital cost of the equipment is much less. 

E. C. Bahardt, Computer-aided Engineering for Injection Moulding, Hanser, 1983. 

A. Whelan, Injection Moulding Materials, Applied Science Publishers, 1982. 

BOBBIN PBT(30% FILL) INJECTION MOULDED WITH ITEM 5... 

Following the tolerance stack through the end assembly, the bobbin dimension of 22 mm from the outside face to the back face of the magnetic pole is analysed next. This characteristic dimension does not include the tolerance on the impact extruded pole. The pole is to be moulded into the bobbin and the pole face is considered to be part of a mould related dimension. The bobbin is injection moulded using 30% filled polybutylene terephthalate (PBT). The tolerance assigned to the bobbin dimension is 0.035 mm. 

Injection moulding - Elastomers Compression moulding - Composites Hand/spray lay-up - Composites... 

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. 

Through the development of such techniques as injection moulding it is possible to make highly complex parts in one operation without the need for assembly work or the generation of more than a notional amount of scrap material. 

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. 

Some typical data for this mouldability index are given in Figure 8.8. One limitation of these data is that they do not explicitly show whether or not a mould will fill in an injection moulding operation. This will clearly depend on the thickness of the moulding, the flow distances required and operational parameters such as melt and mould temperatures. One very crude estimate that is widely used is the flow path ratio, the ratio of flow distance to section thickness. The assumption is that if this is greater than the ratio (distance from gate to furthest point from gate)/section thickness, then the mould will fill. Whilst... 

Table 8.2 Some collected values for the flow path ratio of injection moulding materials...

An example of the effect on production rates is provided by injection moulding. The longer it takes after injection for solidification of the polymer to occur, the longer will be the overall cycle. (Provided the moulding is not distorted on ejection it will only be necessary to form a rigid skin to the moulding.)... 

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