Preform screens

The rovings in a plenum chamber are fed into a cutter and after being cut to the desired lengths, fall either into a plenum chamber or perforated screen where the air is exhausted from under the screen. A plastic binder of usually up to 5wt% is applied and is later cured. As the glass falls into the plenum chamber, the air flow pattern and baffles inside the screen control its distribution. Preform screen rotates and sometimes is tilted to ensure maximizing uniform deposits of the roving. 

With the directed fiber system strands are blown onto a rotating preform screen from a flexible hose. Roving is directed into a chopper where air flow moves it to a preform screen. Use can be made of a vertical or horizontal rotating turntable. This process requires a rather high degree of skill on the part of the operator however, automated robots are used to provide a controlled system producing quality preforms. 

The preform is usually heavy on the flat top and light on the edges and corners. Internal baffles may be added in the preform screen to control... 

Preform screens are usually made from 16-gauge perforated material with Vs in. holes on / 6 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. 

Roving is fed into a cutter on top of plenum chamber. Chopped strands are directed onto a spinning fiber distributor to separate chopped strands and distribute strands uniformly in plenum chamber. Falling strands are sucked onto preform screen. Resinous binder is sprayed on. Preform is positioned in a curing oven. New screen is indexed in plenum chamber for repeat cycle. 

Uniform depth of loading in dryers and furnaces handling particulate solids is essential to consistent operation, minimum heating cycles, or control of final moisture. After a tray has been loaded, the bed should be leveled to a uniform depth. Special preform devices, noodle extruders, pelletizers, etc., are employed occasionally for preparing pastes and filter cakes so that screen bottom trays can be used and the advantages of through circulation approached. 

Preforming method or feed slots Rolling extruder slots Filtered and perforated plate Fiber feed screen Pressed screen Extrusion Rolling extruder... 

The chemical adjustment of charge density is another potentially rewarding technique. It does, however, require either the synthesis of new polymers or the chemical modification of preformed polymers (e.g., supersulfation of hyaluronic acid [16]). This procedure has not been evaluated in our screening. 

The reinforcements amenable to RTM are similar to those used for pultrusion, except that they need not be continuous in nature. Thus, E-glass, S-glass, aramid, and carbon fibers are commonly used, as are discontinuous filaments such as wood fiber and polyesters. Even metal and ceramic fibers can be used in this technique. In one method, the preform is fabricated by spraying 12- to 75-mm-long chopped fiber rovings onto a preshaped screen. A binder sprayed with the fibers keeps them in place and holds the preform shape, which is then placed in the mold. 

Through circulation dryers employ perforated or open screen bottom tray construction and have baffles that force the air through the bed. Superficial velocities of 150 ft/min are usual, with pressure drops of 1 in. or so of water. If it is not naturally granular, the material may be preformed by extrusion, pelleting, or briquetting so that it can be dried in this way. Drying rates are greater than in cross flow. Rates of 0.2-2 lb/(hr)(sqft tray area) and thermal efficiencies of 50% are realized. Table 9.7(d) has performance data. 

Thus a new felting die assembly was required. The successful procedure involved a die formed from brass 0.0625 thick, perforated with 1/16 holes in a hexagonal pattern. This die was covered with a small "sock made of 44 x 44 mesh Saran screen, 0.010 thick. The sock has been sewn in the desired form with very fine cotton thread. The preform was felted onto this sock, which was then slipped from the die, collapsed withdrawn from the interior of the preform... 

In a further extension, they demonstrated screening of submicroliter volumes with fluorescein diphosphate (FDP) as substrate against multiple enzymes (lysozyme, RNase, catalase and alkaline phosphatase) using preformed arrays of nanoliter plugs in a three-phase (liquid-liquid-gas) plug flow (Scheme 4.99) [418],... 

Preform. A metal screen is made in the shape of the final product. Glass fiber is chopped 2 in. long and sprayed uniformly all over the shaped screen, using vacuum on the back side of the screen to assist the process. A small amount of binder, typically 5 percent of polymer in latex form, is sprayed onto the fiber to hold its shape. It is then removed from the screen, placed in the mold, saturated with an equal weight of liquid resin, and the mold is pressed at 1380 kPa (200 psi) and heated until cured, typically 3-15 min. This early process has been largely replaced by SMC. 

---