Moulding reaction injection

Reaction Injection Moulding.— By contrast with the epoxy resins, polyurethanes cure at much lower temperatures, 40 °C upwards. The reactants are usually polyether or polyester triols and diols (polyols) and isocyanates or isocyanate-tipped polyester piepolymers. The process problems are essentially those of mixing the reactants sufficiently rapidly to achieve composition uniformity and to control the temperature in the mould. Where foams are produced, control of bubble nucleation is an additional complicating factor e.g. Menges and Schwesig ). 

Poly(methyl methacrylate) is another example of a within-mould reaction carried out at moderate temperatures ( 95 °C). The effect of certain fillers is to increase significantly toughness, hardness, and stiffness. This result will undoubtedly extend the range of applications of such glassy polymers, and induce a concurrent study of improved processing techniques. 

The basic RIM process is illustrated in Fig. 4.47. A range of plastics lend themselves to the type of fast polymerisation reaction which is required in this process - polyesters, epoxies, nylons and vinyl monomers. However, by far the most commonly used material is polyurethane. The components A and B are an isocyanate and a polyol and these are kept circulating in their separate systems until an injection shot is required. At this point the two reactants are brought together in the mixing head and injected into the mould. 

Since the reactants have a low viscosity, the injection pressures are relatively low in the RIM process. Thus, comparing a conventional injection moulding machine with a RIM machine having the same clamp force, the RIM machine could produce a moulding with a much greater projected area (typically about 10 times greater). Therefore the RIM process is particularly suitable for large 

In Section 4.2.7 we considered the process of extrusion blow moulding which is used to produce hollow articles such as bottles. At that time it was mentioned that if molecular orientation can be introduced to the moulding then the properties are significantly improved. In recent years the process of injection blow moulding has been developed to achieve this objective. It is now very widely used for the manufacture of bottles for soft drinks. 

The steps in the process are illustrated in Fig. 4.48. Initially a preform is injection moulded. This is subsequently inflated in a blow mould in order to produce the bottle shape. In most cases the second stage inflation step occurs immediately after the injection moulding step but in some cases the preforms are removed from the injection moulding machine and subsequently re-heated for inflation. 

Here two liquid streams, typically streams of a polyol and isocyanate for polyurethane RIM, are mixed in metered (usually stoichiometric) proportions as they are injected into the mould cavity. There are well-controlled, separate delivery systems to ensure that both 

Quality-control tests and important process variables 

Typical systems are two-part polyurethane systems that may produce rigid, rubbery, foamed or filled products. Typical fillers include chopped fibres and mineral fillers. Composite systems may also be produced by having pre-placed reinforcements in the mould, in which case the technique is known as reinforced reactive injection moulding (RRIM). 

Haagh et al. (1996) modelled the filling stage of a RIM process. They used the following equations to describe the cure kinetics and chemorheology  

Macosko (1989) examined process modelling for RIM processes. Interestingly, the kinetic models examined here are pseudo-mechanistic, like the following for polyurethane RIM processing  

Polymers are not generally end products in themselves so have to be formed into some artefact which serves a particular purpose, e.g. for domestic use or an 

RECIRCULATION-nihpiii forward 3. INJECTION - aapia mracted 

Reacting polymer expelled from mixhead into mould 

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

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. 

Reaction injection moulding techniques, developed primarily for polyurethanes (see Chapter 27), have also been adapted for nylon 6 in what must be considered as a variation of the polymerisation casting technique. 

Not only are these reactions of importance in the development of the cross-linked polyurethane networks which are involved in the manufacture of most polyurethane products but many are now also being used to produce modified isocycuiates. For example, modified TDI types containing allophanate, urethane and urea groups are now being used in flexible foam manufacture. For flexible integral foams and for reaction injection moulding, modified MDIs and carbodi-imide MDI modifications cU"e employed. 

The reinforced reaction injection moulding (RRIM) process is a development of RIM in which reinforcing fillers such as glass fibres are incorporated into the polymer. One advantage of such a system is to reduce the coefficient of thermal expansion, and with a 40-50% glass fibre content the coefficient is brought into line with those of metals. 

FRISCH, K. c.. Recent Developments in Urethane Elastomers and Reaction Injection Moulded (RIM) Elastomers, Rubber Chem. Technol., 53, 126 (1980)... 

The beneficial effects are demonstrated of heterogeneous secondary pyrolysis reactions on the liquid products of PU pyrolysis. Pyrolysis volatiles are passed through a packed bed of carbonaceous solids that promote the secondary reactions. Activated carbon and reaction injection moulded PU (RIM) char were found to be suitable bed materials. The long-term object was to develop marketable solid products by pyrolysis of wastes, so obtaining high char yields. In addition to affecting the liquid products, RIM char also increased the total char... 

The mixing can be done by hand, or in low pressure mixer/dispensers, and in reaction injection moulding (RIM) machines. In the latter operation the de-gassing operation is not required. 

Cast nylon is used for prototypes and small production runs processed by casting, roto-moulding and reaction injection moulding (RIM), possibly with structural reinforcements (SRIM) such as glass mat or fabric. 

Reaction injection moulding (RIM), commonly used with thermosets, is sometimes used for specific thermoplastics such as Nyrim. Nyrim is supplied in two parts ... 

RRIM - reinforced reaction injection moulding - the resin is reinforced with short fibres... 

SRIM or SRRIM - structural (reinforced) reaction injection moulding - mats or fabrics are placed in the mould before injection of the resin, possibly reinforced with short glass fibres. 

Note Reaction injection moulding commonly uses two-component precursors that produce polymer networks after mixing. 

Reaction injection moulding within which glass fibres are included to increase the strength of the moulding. 

Note 3 Reaction injection moulding and reinforced reaction injection moulding are types of reactive polymer processing. 

Kresta JE (ed.), Reaction Injection Moulding and Fast Polymerization Reactions, Plenum, New York, 1982. 

Macosko CW, RIM Fundamentals of Reaction Injection Moulding, Hanser, Munich, 1989. 

The metathesis polymerisation of dicyclopentadiene, an inexpensive monomer (commercially available cyclopentadiene dimer produced by a Diels-Alder addition reaction containing ca 95 % endo and ca 5 % exo form), leads to a polymer that may be transformed into a technically useful elastomer [144-146, 179] and thermosetting resin [180,181]. The polymerisation has characteristics that make it readily adaptable to the reaction injection moulding ( rim ) process [182], The main feature of this process comes from the fact that the polymerisation is carried out directly in the mould of the desired final product. The active metathesis catalyst is formed when two separate reactants, a precatalyst (tungsten-based) component and an activator (aluminium-based) component, are combined. Monomer streams containing one respective component are mixed directly just before entering the mould, and the polymerisation into a partly crosslinked material takes place directly in this mould (Figure 6.5) [147,168,183-186],... 

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