In-mold cure

This section discusses general principles of resin cure in LCM. For a further discussion on the cure and kinetics of thermosets, see Chapter 2 [32]. 

The cure of thermoset resins involves the transformation of a liquid resin, first with an increase in viscosity to a gel state (rubber consistency), and finally to a hard solid. In chemical terms, the liquid is a mixture of molecules that reacts and successively forms a solid network polymer. In practice the resin is catalyzed and mixed before it is injected into the mold thus, the curing process will be initialized at this point. The resin cure must therefore proceed in such a way that the curing reaction is slow or inhibited in a time period that is dictated by the mold fill time plus a safety factor otherwise, the increase in viscosity will reduce the resin flow rate and prevent a successful mold fill. On completion of the mold filling the rate of cure should ideally accelerate and reach a complete cure in a short time period. There are limitations, however, on how fast the curing can proceed set by the resin itself, and by heat transfer rates to and from the composite part. 

An ideal resin processed in optimized conditions should have  

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Regardless of the variant of LCM the process can be subdivided into a number of steps. The first step is preforming, which means that dry reinforcement is tailored to the shape of the mold. This can be done in many different ways depending on the complexity of the geometry and the requirements on mechanical performance of the part. The preform is then placed in a mold cavity that is subsequently closed. The preform is usually compressed slightly by the mold. The next step is resin injection into the mold cavity until the preform is fully impregnated. The final step is in-mold cure (i.e., curing inside the mold until the part is sufficiently stiff to be demolded). A controlled postcure is sometimes performed to ensure that optimum properties are obtained. 

We have chosen to divide the rest of this chapter into three sections on the important topics of preforming, mold filling, and in-mold cure, followed by a section with a brief review of mold design and related topics. 

Reaction-Injection Moiding System Liquid compositions, mostly polyurethane-based, of thermosetting resins, prepolymers, monomers, or their mixtures. Has good processibility, dimensional stability, and flexibility. Processed by foam molding with in-mold curing at high temperatures. Used in auto parts and office furniture. Also called RIM. 

Early phenoHc resins consisted of self-curing, resole-type products made with excess formaldehyde, and novolaks, which are thermoplastic in nature and require a hardener. The early products produced by General BakeHte were used in molded parts, insulating varnishes, laminated sheets, and industrial coatings. These areas stiH remain important appHcations, but have been joined by numerous others such as wood bonding, fiber bonding, and plywood adhesives. The number of producers in the 1990s is approximately 20 in the United States and over 60 worldwide. 

Films from prepolymer solutions can be cured by heating at 150°C. Heating the prepolymer in molds gives clear, insoluble moldings (38). The bulk polymerisation of DAP at 80°C has been studied (35). In conversions to ca 25% soluble prepolymer, rates were nearly linear with time and concentrations of bensoyl peroxide. A higher initiator concentration is required than in typical vinyl-type polymerisations. 

Melamine resins were introduced about ten years after the Beetle molding compound. They were very similar to those based on urea but had superior quaHties. Henkel in Germany was issued a patent for a melamine resin in 1936 (7). Melamine resins rapidly supplanted urea resins and were soon used in molding, laminating, and bonding formulations, as well as for textile and paper treatments. The remarkable stabiHty of the symmetrical triazine ring made these products resistant to chemical change once the resin had been cured to the insoluble, cross-linked state. 

Cured phenolics are universally brittle in nature. This is true of both resoles and novolacs and does not depend much on the source of methylene used to promote cure. Consequently, the fillers used in molded articles are highly important to the design of the manufactured product. With resoles, the fiber or filler are usually the primary component of the final composite, with the resole acting as a binder or impregnating agent. With novolacs the resin may be the major component in the molded part. Poly-silanes and other organic polymers are also added in some applications to promote impact resistance and toughness [192]. 

Mulden-blei, n, pig lead, -heizung,/. (Rubber) cure in molds. 

Insulation formed by slurry casting or heat curing under pressure in molds in a number of insulation types. Most common moldings are preformed bends, valve boxes and flange covers. 

Coating with Bulk Polybutadiene. E-glass fabric was embedded In Firestone s Diene 35 NFA using procedures very similar to those used to prepare peel test specimens. Rubber, which had been mill-mixed with 0.05% dlcumyl peroxide, was premolded between Mylar sheets to the desired thlckness(0.308, 0.151, or 0.100 cm) and size ( 30.5 X 18 cm) by molding for 1 hour at 60 C. and 40,000 lbs/5" ram. Fabric was cut so that the final size was at least one Inch smaller than the rubber sheets In all directions. A sandwich was made from the fabric and two premolded rubber sheets of the same thickness and about half the total thickness of the final sandwich. The sandwich was cured In a press for 2 hours at 150°C and 5000 lbs/5" ram. In the cured specimen the fabric was embedded In the center of the molded specimen (0.15 -0.40 In thick). Samples were Immensed In alkali before cutting to size for tensile tests. 

C-stage Final reaction stage of various thermosetting resins. In this stage material is insoluble and infusible. Resin in fully cured thermosetting molding is in this stage and is referred to as resite. 

Copper naphthenate added to the resin at levels between 100—200 ppm effectively extends gel and cure characteristics, resulting in a reduction in exothermic heat (Fig. 7). Copper additives are used widely in commercial laminating resins to modify process exothermic effects. OC-Methylstyrene [98-83-9] substituted for styrene at levels of 5—8% has also been used effectively in resins cured at above ambient temperatures. The inhibitor 2,5-di-/-butylhydroquinone exerts significant exotherm suppression at levels of 200—400 ppm and is useful in high temperature molding processes. 

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