Moulding processes flexible mould

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 RIM process was originally developed for the car industry for the production of bumpers, front ends, rear ends, fascia panels and instrument housings. At least one mass-produced American car has RIM body panels. For many of these products, however, a number of injection moulding products are competitive, including such diverse materials as polycarbonate/PBT blends and polypropylene/EPDM blends. In the shoe industry the RIM process has been used to make soling materials from semi-flexible polyurethane foams. 

Vulcanised rubbers possess a range of very desirable properties such as resilience, resistance to oils, greases and ozone, flexibility at low temperatures and resistance to many acids and bases. However, they require careful (slow) processing and they consume considerable amounts of energy to facilitate moulding and vulcanisation. These disadvantages led to the development of thermoplastic rubbers (elastomers). These are materials which exhibit the desirable physical characteristics of rubber but with the ease of processing of thermoplastics. 

Variations on this basic process are (i) vacuum bag moulding and (ii) pressure bag moulding. In the former process a flexible bag (frequently rubber) is clamped over the lay-up in the mould and a vacuum is applied between the moulding and the bag. This sucks the bag on to the moulding to consolidate the layers of reinforcement and resin. It also squeezes out trapped air and excess resin. The latter process is similar in principle except that pressure is applied above the bag instead of a vacuum below it. The techniques are illustrated in Fig. 4.67(b) and (c). 

A copolymer of ethylene and vinyl acetate which processes like a plastic and performs like a rubber. The copolymer, of which there are various grades, is similar to rubber in softness and flexibility but can be injection moulded without vulcanisation. 

This process uses the plasticising and heat advantages of the injection unit to impart good flow properties to the rubber mix. It also offers the advantages of the flexibility of the transfer layout without the sprue and runners of the balanced runner system required by injection moulding. The space used by runners in other systems can be profitably used by more mould cavities. 

Poly(butylene terephthalate) (PBT) is a semicrystalline, thermoplastic polyester which is completely analogous to PET except that it has a longer, more flexible butylene chain linkage which imparts a rapid crystallization rate, thus making PBT well suited to injection moulding processes. This polyester is used widely for electrical and electronic components due to its high temperature resistance and good electrical properties (Chapter 8). 

Apart from the above three types there are custom built rubber products such as expansion joints, flexible cell covers and large size rubber foils for the caustic soda industry, and many inflatables, fabric reinforced products and thick moulded sheets for specialty applications in certain process plants. These are all hand formed in aluminium or cast iron moulds or forms by laying up process and then cured in autoclave. Here the flow of the un-vulcanized rubber during cure is not very important as the shape is already formed rather the green strength and the stiffness of rubber stock with a low scorch time are the important requisites. A rubber expansion joint made by a hand layup method and cured in autoclave is shown in the following figure 14.1. 

The more flexible comonomer is introduced to reduce the melting point, so that the polymer can be processed in the molten condition, e.g. in injection moulding and extrusion. The end products show superior mechanical properties, but are, as a matter of fact, highly anisotropic. 

Thermo set as well as thermoplastic and elastomer binders are used in compression, injection and calendaring process respectively. Extrusion moulding uses elastomers as well as thermoplastic materials such as PVC. Compression and injection moulding processes make only rigid magnets, calendaring makes flexible magnets and extrusion can lead to both states. 

Low clamping pressures only are needed, and this in turn means that quite large products can be made on inexpensive plant. By varying the raw materials it is possible to produce either rigid or flexible articles, microcellular or otherwise, and (if required) to include fillers or reinforcing sections. Thermosets other than polyurethane can be used—like epoxides and polyesters (with the latter, the process is known also as resin transfer moulding ). 

Details are given of the design of an autoclavable medical device using flexible PVC. The device was injection moulded and exposed to steam autoclaving. Surface delamination was investigated. Experiments on residual stresses after moulding and statistical analysis of the moulding process were conducted. 8 refs. 

Coextrusion, e.g. a process incorporating two or more plies of a plastic, will undoubtedly find more use in flexible pharmaceutical packs. Coextrudates can also be moulded into rigid containers, bottles, tubes, tubs, etc. subject to the quantities justifying the costs. 

The polyether triols are the most important class of polyether polyols and they are used in flexible PU foam fabrication. The majority of polyether triols used in flexible foams are copolymers of PO-EO. Random copolymers are used in continuous slabstock flexible foams and block copolymers (PO-EO), with terminal poly[EO] block, are used in moulded foams (hot moulding and cold cure moulding processes). 

PHD polyols are successfully used for high resilience flexible PU foams (made by the cold cure process), for continuous slabstock flexible PU foams and for elastomers obtained especially by reaction injection moulding (RIM) technology. 

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