Moulding liquid thermoplastics

Discontinuous coating by dipping in a bath of liquid thermoplastic is very similar to moulding by dipping as used for PVC gloves (see Section 5.1.8 above). Wire articles, for example, can be coated with PVC to protect them and obtain a softer touch. 

In RIM, as in injection moulding, whether thermoplastic or thermoset, there is a transition zone between the nozzle (mix head) and the mould. In injection moulding, this transition zone includes the sprue/runner/gate system, and, in special cases, can also include a static mixer. The analogy to RIM is very apt. The transition zone for the RIM process also includes a sprue/runner/gate system, as well as an after mixer. One function of the RIM transition zone is to convert the mixing head liquid stream from a turbulent state to one which is in a laminar flow mode. If this does not occur, air traps (i.e. large subsurface voids in the finished part) can become a problem. 

Kel-F Trade name for a polymeric chlorotri-fluoroethene, often copolymerized. May be a liquid or solid. Inert to chemical attack and a thermoplastic (Teflon cannot be moulded). 

Creep of polymers is a major design problem. The glass temperature Tq, for a polymer, is a criterion of creep-resistance, in much the way that is for a metal or a ceramic. For most polymers, is close to room temperature. Well below Tq, the polymer is a glass (often containing crystalline regions - Chapter 5) and is a brittle, elastic solid -rubber, cooled in liquid nitrogen, is an example. Above Tq the Van der Waals bonds within the polymer melt, and it becomes a rubber (if the polymer chains are cross-linked) or a viscous liquid (if they are not). Thermoplastics, which can be moulded when hot, are a simple example well below Tq they are elastic well above, they are viscous liquids, and flow like treacle. 

At yet higher temperatures (>1.4T ) the secondary bonds melt completely and even the entanglement points slip. This is the regime in which thermoplastics are moulded linear polymers become viscous liquids. The viscosity is always defined (and usually measured) in shear if a shear stress o produces a rate of shear 7 then the viscosity (Chapter 19) is... 

Table 2.17 analyses some examples of the relative processing cost versus the annual production in units. The various technologies listed are not suitable for all materials or parts machining is used with numerous materials rotational moulding uses liquid resins vacuum forming uses thermoplastic sheets blow moulding uses special grades of thermoplastics... 

Reaction injection moulding is associated with polyurethanes and is different entirely from moulding techniques used commonly for thermoplastics in that the starting materials usually are liquids at room temperature. As the name implies, a chemical reaction takes place in the course of moulding and complex polyurethane items can be made in one step from mixtures comprising polyol, polyisocyanate, and a suitable promoter of the reaction. 

Advanced Composites, D.K. Thomas, RAE, Famborough. Liquid Crystal Polymers, M.K. Cox, ICI, Wilton. CAD/CAM in the Polymer Industry, N.W. Sandland and MJ. Sebborn, Cambridge Applied Technology. Engineering Thermoplastics, I.T. Barrie, Consultant. Reinforced Reaction Injection Moulding,... 

Several companies (Expancel, Kureha) have developed hollow thermoplastic microspheres (containing a volatile liquid, usually isopentane or isobutane) that can be expanded to make foams for use in wallcoverings and shoe parts. They can produce fine-cellular foams by rotational moulding. 

Prepolymers One-shot systems Thermoplastic Millable gums Castable liquids Reaction injection-moulded and cast systems Melt-processed by injection moulding or extrusion Conventional rubber processing... 

A trend of recent years has been to supply additives processed into safe forms, in pellets, capsules, liquids and masterbatch concentrates and so, for the compounder and processor, the hazards have largely been taken out, but it is still important that they should be aware of them. For most compounders and processors of thermoplastics, the hazard from additives can now be safely contained upstream, during manufacture of the additives. In moulding and fabricating wet polymer systems, such as polyesters and polyurethanes, however, it is essential to take precautions. 

Description Alcohol phosphates, mixtures of fatty alcohols, acids and derivatives with microcrystalline waxes, amides and organic acid derivatives and mixtures of synthetic resins liquid or powder, added to thermoplastics or thermoset compound to migrate to surface and prevent adhesion to a mould ... 

Polymer matrix composites (PMC) are used to increase the rather low stiffness or strength of polymers by adding stiffer or stronger fibres. If thermoplastics are used as matrix material, short fibres can be added to the granulate material that is subsequently softened by heating and processed, for example in injection moulding. If thermoset resins (duromers) are used, the fibres can be placed into the liquid resin before curing. 

Polymers are nsed in fnel cells. Those of particular interest are the polymer electrolyte membrane (PEM) and the phosphoric acid fuel cell (PAFC) designs. The latter design contains the liquid phosphoric acid in a Teflon bonded silicon carbide matrix. In March 2005 Ticona reported that it had bnilt the first fnel cell prototype made solely with engineering thermoplastics. They claimed that this approach rednced the cost of the fuel by at least 50% when compared with fuel cells fabricated from other materials. The 17-cell unit contains injection moulded bipolar plates of Vectra liquid crystal polymer and end plates of Fortron polyphenylene sulfide (PPS). These two materials remain dimensionally stable at temperatures up to 200 "C. The Vectra LCP bipolar plates contain 85% powdered carbon and are made in a cycle time of 30 seconds. 

Figure 11.6 shows the strong dependence of solidification time for the thermoplast and/ or curing time for the liquid silicone rubber from moulding temperature. It is not intended to be symmetric as it may differ for various pairs of materials. 

Shorter curing times in the silicone cavity lead to shorter contact times for the thermoplastic. The shorter the contact time, the higher the temperature of the silicone cavity that can be used without it being critical for the plastic substrate. In other words, the temperature of the silicone rubber mould strongly depends on the speed of curing of the liquid rubber used in the processing. 

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