CAPITULO 5 Métodos y prácticas de moldeo.

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For advanced composite part that require an elevated temperature cure, an oven or an autoclave (a cylindrical pressure vessel with heating and cooling capabilities) may be prescribed. The parts are generally laid-up, vacuum bagged, and placed in the oven or autoclave for processing.

Composite pultrusion is analogous to aluminum extrusion, but as the name suggest, fiber reinforcements are pulled through a heated die to form a cured composite profile. Pultrusion can created almost any shape including a wide variety of bars, beams stringers, stiffeners, pipes and angles. These shapes ca be solid or hollow, as long as the cross sectional are is contact. The heated die forms the shape without the use of conventional molds.

The pultrusion process stars with racks or creels holding roll of fiber, mat or fabric reinforcement. As the reinforcement is pulled off the racks/creels, it is guided through a resin bath or resin impregnation system. Resin can also be injected directly into the die in some pultrusion system. In the non-wet-out and are then guided through a series of custom tooling called a pre-former, which helps arrange and organize the reinforcement into the final shape while excess resin is squeezed out.

Thermoforming is one of the oldest and simplest plastic forming process. Baby rattles and teething rings were thermoformed in the 1980s, using cellulose-based plastics. The process saw major growth in the 1930s with the development of the first roll-fed thermoforming machines in Europe.

In the thermoforming process, heat and pressure are used to transform flat sheet thermoplastic into a desired three-dimensional shape. The sheet is preheated using one of three methods:

1.- conduction via contact heating panels or rods;

2.- convection heating using ovens which circulate hot air;

3.- radiant heating achieved with infrared heaters.

The preheated sheet is then transferred to a temperature-controlled, pre-heated mold and conformed to its surface until cooled. The final part is trimmed from the sheet, and the trim can be reground, mixed with virgin material and retroceded. This ability to recycle trim waste is typically most viable for unreinforced and some chopped glass-reinforced thermoplastics.

Compression molding used a heated matched mold similar to matched die forming, and has traditionally used thermoset sheet molding compound or bulk molding compound as molding materials. A hydraulic cylinder is used to bring the mold halves together, providing simultaneous heat and pressure. This caused the thermoset matrix to liquefy and flow into the voids in the mold where it chemically reacts and cures into the final shapes. This molding process is capable of producing high volume, complex composite shapes while holding a very high tolerance. The material properties are less than that of conventional composite parts because of the discontinuous fibers in the molding material. Molds can be made from steel, cast, iron or aluminum and are built with integral heating and cooling for rapid turnaround.

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