Preform process

With water slurry, chopped strands are in water (take-off used by the paper pulp industry for centuries). It produces intricate shaped preforms 

Preform screens are usually made from 16-gauge perforated material with in. holes on A6 in. centers. This produces about 40% open area. For some operations, a more open area is required. Perforation patterns are also used to develop specifically designed reinforcement directional properties. The screen is usually designed so that the outside contour is identical with the contour of the mating half of the mold. A screen, which is not of the correct size, will cause a great deal of difficulty in 

The preform is usually heavy on the flat top and light on the edges and comers. Internal baffles may be added in the preform screen to control the airflow, thus giving a uniform deposition of glass. The exact area of the baffle usually has to be worked out on a trial-and-error basis until experience is developed. Close cooperation with the preform-machine manufacturer is helpful. 

When molding a product with a variable wall thickness, it is possible to vary the thickness of the preform. This is usually accomplished by baffling. Another approach that can be used is to completely block off areas where no fiber is desired. This action saves material that would otherwise be trimmed off and probably discarded. It has also proven practical to combine two or more preforms into one molded part. This technique is very useful where the thickness of the molded part prohibits the collection of the preform in one piece. 

Also prepared is mat preforming. It is the usual flat fiber mat that is formed into a shape usually using a set of matched dies (often made of fiber reinforced epoxy). A mat is cut to the correct dimensions and 

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CaulTman. S., Brown, R.. and Bartos, A. (1994), Video monitoring of a large-scale commercial three-dimensional braider for advanced composite preform process control, Proc. of the Conf. on NDE Applied to Process Control of Composite Fabrication, Oct. 1994, pp. 127 142. 

Figure 5.6 Schematic of the direct preform process spraying two molds...

Two of the four known silica giass fiber processes are dry spinning processes. They are continuous processes, which afford either a continuous sliver or staple product, i.e., a cohesive array of discontinuous fibers, or a continuous multifilament yam, respectively. The other two processes are continuous processes yielding continuous fibers and multifilament yarns. Of these, the preform process (Chapters 4 and 6) relies on the use of silica preforms, and produces continuous, ultrapure siiica giass fibers, or quartz fibers. This process yields the strongest silica giass fibers on record. 

Najjar, W., et al., 2013. Analysis of the blank holder force effect on the preforming process using a simple discrete approach. Key Eng. Mater. 554—557,441-446. 

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. 

Reinforcement material is provided to the process in form of one or more stacked, continuous plane textile structures. Thus, the profile s geometry must be representable with this material by shaping. Alternatively, special preforms can be used so that the geometry is limited by the performance of the preforming process. 

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