Processing, thermosets casting

Plastic, rubber, and non-iron metal processing (extrusion aid and release additive for plastics and rubber e.g., PUR, thermosets, casting resins, high melting point mold release agents and additives)... 

Shell molding n. In metal foundries, a process of casting metal objects in thin molds made from sand or a ceramic powder mixed with a thermosetting-resin binder. Some authors have misused the term by equating it to plastics processes such as dipping and slush casting. 

For prototyping or for limited production runs, liquid resin casting offers simplicity of process, relatively low investment in equipment, and fast results. Thermoset casting resins may be epoxies, polyesters, phenolics, etc. The resins harden by a polymerization or cross-linking reaction. Such resins are often poured into open molds or cavities. Because pouring is done at atmospheric pressure, molds are simple, often made of soft metals (Fig. 6.2). 

Casting is employed in producing unique shapes, sheets, films, tubes, and rods from both thermoplastic and thermoset materials. The crucial difference between most molding processes and casting is that there is no additional pressure required in casting. The... 

The first five of these techniques involve deformation and this has to be followed by some setting operation which stabilises the new shape. In the case of polymer melt deformation this can be affected by cooling of thermoplastics and cross-linking of thermosetting plastics and similtir comments can apply to deformation in the rubbery state. Solution-cast film and fibre requires solvent evaporation (with also perhaps some chemical coagulation process). Latex suspensions can simply be dried as with emulsion paints or subjected to some... 

This process, commonly used with thermosets, is sometimes used for specific thermoplastics such as monomers or pre-polymers of PMMA or polyamide (Nyrim). For the latter, a mix is made just before casting with ... 

In some cases the raw material is already in a fluid state with thermosets as a resin for casting or impregnating, with thermoplasts, as an exception, sometimes as a monomer which is able to polymerise in the mould. In these cases the forming processes are relatively simple, since they can be carried out on low-viscous fluids which do not require high pressures to be transported. 

Casting is the simplest processing technique the mould is filled with a liquid, which, at ambient temperature, has such a low viscosity that it is able to fill the mould under its own weight. After filling, a chemical reaction should take place to form the material. With thermosets this is a reaction between the components which, prior to filling, have been mixed together, and which react into a three-dimensional network. Sometimes a catalyst is added to initiate or accelerate this reaction. Dependent on the combination chosen, curing takes place at room temperature or at an elevated temperature. 

Thermoset polyurethanes are cross-linked polymers, which are produced by casting or reaction injection molding (RIM). For cast elastomers, TDI in combination with 3,3,-dichloro-4,4,-diphen5lmethanediamine (MOCA) are often used. In the RIM technology, aromatic diamine chain extenders, such as diethyltoluenediamine (DETDA), are used to produce poly(urethane ureas) (47), and replacement of the polyether polyols with amine-terminated polyols produces polyureas (48). The aromatic diamines are soluble in the polyol and provide fast reaction rates. In 1985, internal mold release agents based on zinc stearate compatibilized with primary amines were introduced to the RIM process to minimize mold preparation and scrap from parts tom at demold. Some physical properties of RIM systems are listed in Table 7. 

Spin casting can use plastic molds, such as silicone, to produce close tolerance, highly cost effective, limited production in a variety of materials. The process uses easily adjustable centrifugal force to inject liquid thermoset plastics into a circular disc-shaped elastomeric mold under pressure, completely and rapidly filling the mold cavities. 

Different foundry casting techniques are used. Included are plastic-based binders mixed with sand. Various types of molds and cores are produced that include no-bake or cold-box, hot-box, shell, and oven-cured. Usual binders are phenolic, furan, and thermoset polyester. There is the foundry shell casting, also called dry-mix casting. It is a type of process used in the foundry industry, in which a mixture of sand and plastic (phenolic, thermoset polyester, etc.) is placed on to a preheated metal pattern (producing half a mold) causing the plastic to flow and build a thin shell over the pattern. Liquid plastic pre-coated sand is also used. After a short cure time at high temperature, the mold is stripped from its pattern and combined with a similar half produced by the same technique. Finished mold is then ready to receive the molten metal. Blowing a liquid plastic/sand mix in a core-box also produces shell molds. 

The trickle impregnation process is a related process to thermoset plastic casting, potting, and encapsulation where it also uses a low viscosity liquid reactive plastic to provide the trickle impregnation. As an example, the catalyzed plastic drips on to an electrical transformer coil. Capillary action draws the liquid into its openings at a rate slow enough to enable air to escape as it is displaced by the liquid. When fully impregnated, the part is exposed to heat to cure the plastic. 

Many studies have used these methods for processing of both thermosetting and thermoplastic polymers. Y. Liao (53) dissolved epoxy in a well-dispersed, ultra-sonicated CNT suspension. The solvent was evaporated, and the epoxy was subsequently cured to form a nanocomposite in which the good CNT dispersion was achieved. Jin et al. (54) produced various types of polymer-coated and polymer-grafted MWNT solutions, in some cases evaporating the solvent and subsequently melt-mixing with another polymer. Yudasaka et al. (55) used a mixture of SWNTs and PMMA in monochlorobenzene (MCB) for dispersion, purification and subsequent spin-casting of the material. 

Casting is a relatively simple process (Figure 6.1) involving the pouring of a thermosetting liquid into a mould, where the liquid hardens into a solid, dimensionally-stable shape. 

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