Processing, thermosets preforms

Molding or laminating at pressures below 200 psi. Process of molding plastic articles from powdered, granular or preformed plastics by fusing the material in a chamber with heat and forcing the mass into a hot chamber for solidification. Used primarily on thermosetting plastics. 

The use of preformed nonmiscible TP powders enables the TP load to be increased in the final material while keeping a continuous thermoset matrix. The final morphology is more easily controlled than in the in-situ phase-separation process. 

Wet layup consists of placing a layer of dry reinforcement inside a mold and then applying an uncured, low-viscosity thermoset resin as shown in Fig. 2. Woven glass fibers are the prevalent reinforcing preform utilized in layup processes, although carbon and aramid fibers are also used to a lesser extent. Typical fiber volume fraction of composites manufactured via wet layup range between 30% and 50%. The resin can be poured, sprayed, or brushed on top of the preform layer either by hand or by machine. The fiber preform layer is rolled on or pressed after the application of resin to evenly distribute the resin and remove air pockets. Resin is applied on top after each layer of fiber mats is properly placed. This process is repeated until the desired thickness is reached. To provide a smooth surface finish on the mold side, a thin layer of mold release is often applied prior to starting the layup. Thereafter, pressure and heat are applied to allow the composite to cure. Pressure can be applied... 

When using a thermosetting resin, it is usual to take the carbon fiber reinforcement and prepare a prepreg by impregnating with the chosen resin. Alternatively, a dry preform of opf can be used for conversion to carbon fiber and subsequent initial treatment by the CVI process to provide a preform with some integrity [38]. Polymerization shrinkage will occur, which can be minimized by controlling the rate of temperature rise. 

Dielectric heating can be used to preheat powder, pellets, or preforms to remove water before processing the material. Dielectric heating can also be used to generate the heat necessary for curing polar, thermosetting adhesives, and it can be used to quickly evaporate water from a water-based adhesive formulation. 

In the compression molding process, a thermoplastic or a partially polymerized thermosetting polymer is placed in a heated cavity. The material is nsnally preheated and preshaped with a form roughly similar to that of the cavity. The mold is closed and pressure is applied in order to force the material to fill the mold cavity. In the process, the polymer undergoes complete polymerization or crosslinking. At the end, the mold is opened, the part is ejected and the cycle can start again. This process wastes very little material. However, it is difficult to produce parts with close tolerances because the final size of compression molded products depends on the exact amount of the preform. Moreover, it is not possible to obtain parts with a complex shape, for example with deep undercuts (Osswald and Hemandez-Ortiz, 2006, Tadmor and Gogos, 2006). 

New glass fibre-reinforcement products for u.se in fibre-directed preform processes are another focus of development, designed specifically for liquid composite moulding applications such as resin transfer moulding with thermoset polyesters. 

The common feature of all LCM processes is the injection of a liquid thermoset resin into a bed of stationary (or almost stationary) fibrous preform (see Fig. 9.1). Although this chapter is devoted to the RTM process only, we will point out advantages/disadvantages of RTM compared to the other processes also addressed in this book. First, we will describe the RTM process and briefiy introduce other LCM processes in reference to RTM before highlighting the advantages and shortcomings of RTM as compared to the latter mutations of this process. 

The RTM process begins with a dry fiber preform. The preform is placed into a matched metal mold and the mold is closed resulting in the compaction of the preform to the specified fiber volume fraction. A liquid thermosetting resin is then injected into the mold (typically at high pressure, such as 5-7 bar). The mold and resin can be preheated before injection, or the mold can be heated after injection to cure the resin. Due to the high injection pressures and often high temperatures involved, RTM tools are bulky and costly to manufacture and to process. To aid filling of the mold, vacuum may also be applied to remove trapped air. In addition, vacuum can be used to optimize the resin infusion/injection. This leads to VARTM. More information on RTM can be found in Chapters 9-11. 

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