Resin Transfer Molding Processes

For the composite material like carbon fiber composite, resin transfer molding process can be used quite effectively for the production of composite product. Resign transfer molding process consist of following steps

1.      Placing the carbon fiber preform in mold
2.      Closed mold is heated under low pressure
3.      Resin is transfer into mold under pressure
4.      Mold is left for curing
5.      Finished product is ejected from the mold

From all the types of RTM processes available Shell ruminate RTM process is used because it is the process which have lowest tooling cost and it best for projects having beach production. Design of tooling for the joint brace will be a sensitive and long. Mold making of joint brace will required making of basic pattern of joint brace and then laminating it with different laminating materials. The lower mold of the joint brace will like regular open mold, but the upper mold of the joint brace will be a thin, flexible laminate. An inexpensive rubber extrusions form seal elements that facilitate vacuum clamping.

Pattern of fiber arrangement in the mold is very important because product has stresses in tangential and radial direction and composite fibers have highest mechanical properties along the length and less in width. So to overcome the stresses each fiber layer is place cross the other. Any misalignment of the fiber during manufacturing can change the mechanical properties of the product. Fatigue resistance of the product depends on the resin used. Epoxy base resins have much fatigue resistance than the vinylester and polyester.

Aluminum, nickel, steel and invar can be used for the tooling in manufacturing of composites product. Selection of material depends on the geometry of product, surface finish require, method of cure, molding method and on matrix. There are certain limits for a mold design like minimum inside radius of an object should be 6 mm, maximum section should be of 90 mm, smallest area that can be manufactured is 3 mm^2 and maximum dimension should be of 450 mm. if there is any thickness variation in mold it should be less than 2:1 and the preferred draft angle is about 2 to 3 degree.

Economic Considerations

Some of the factors like small lead time, fast curing speed, high production rate but exact production rate depends on the skills of labor and machine limitations. High degree of automation make this process suitable where quick production is required. Resin transfer molding process is an economical process because it utilizes most of the material, only 3 percent is scrap but that material cannot be reused directly and have moderate tooling, equipment and direct labor cost.

Quality Considerations

Due to mold flow characteristic, differential stress distribution can occurs which results into minor distortion. During the mold filling the temperature should be kept higher than the resin melting temperature. Resin pools and dry spot may occur at finish product surface because of the improper fiber preform placement. If mold have sharp corners then the resin concentration in these corners is difficult to control. This process cannot be used for the product which has to be subjected to high loading. Dimensional tolerances of 0.05 to 25 mm are achievable.


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