CAPITULO 5 Métodos y prácticas de moldeo.

CAPITULO 5
Métodos y prácticas de moldeo

UNIVERSIDAD AUTÓNOMA DE BAJA CALIFORNIA
Escuela de Ingeniería y Tecnología
Asignatura: Mecánica estructural de materiales compuestos  
Alumno: Navarro Arredondo Mario Ernesto.
Profesor: Montoya Patiño Iván

There are many different molding method and techniques employed in the manufacture of advanced composite parts. Some method use a single mold surface with a vacuum bag on the backside of the layup, and some use a cavity mold or die-set. The material selection complexity of the shape, and production rate requirements often drive the choice of molding methods for a specific composite structure.

 There are two basic categories of composites processing.

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In molding and assembly of composite parts, high performance release agents are used for releasing parts from sticking to the mold or fixture. Mush like a Teflon coated pan, semi-permanent mold release system are in fact cured polymers that are bonded to the tool surface. (They are not wax) typically, a semi-permanent system is made up of two different polymers:

1. Tool sealer coat to fill the micro-porous sites on the tool surface (reducing mechanical attachment)
2.  Low-energy release agent to prevent chemical attachment

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The primary function of vacuum bag is to provide compaction pressure and consolidation of the plies within the laminate. This is done by creating a pressure differential between the inside and the outside of the bag using a vacuum pump, a hose, and a sealed film membrane (bag), and result in down-force on the bag equal to the ambient atmospheric pressure.

The secondary function of vacuum bag, used along with a bleeder/breather mechanism for processing outside of the autoclave, is to facilitate removal of excess resin and/or gases from a laminate during processing. The routes for extraction of gases and resins are via the bleeder/breather sequence. The compaction^[a] pressure provided by a vacuum bag is crucial for good quality laminates when they are cured within a vacuum bag only.

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Wet layup is the one of the simplest, least expensive, yet most labor intensive methods of composite manufacturing because it involves manually wetting out the reinforcement fibers with resin.

The fibers can be any of the normal reinforcement and forms available, however the resins are normally (but not always) cured at room temperature. These include polyester, vinyl eser and epoxy resin system.

Wet layup processing is often performed with fairly unskilled labor; however, variation in skill level and handling technique can result in significantly different laminate properties.

Vacuum bagging a wet layup can enhance the quality of the end product. A properly designed bleeder and breather schedule allows for good compaction, excess resin extraction, and removal of trapped air and gas in the laminate.

Vacuum infusion processing is a cleaner and more precise process than wet layup because all reinforcement are placed dry, vacuum bagged and then resin is introduced through using vacuum pressure and cured. (This is also called resin infusion) Thus, workers never have to roll the resin into the reinforcement. The dry fiber reinforcement or preform is positioned in the mold (either with or without a surface coat layer), then a series of resin supply tubes/channels and vacuum bag is installed over the entire assembly.

Vacuum is pulled on the bag and the preform is completed in place before resin is introduced into the laminate. The process work based on standard fluid dynamic, when the flow of the resin is dictated by the low-pressure differential created under the bag.

Prepreg hand layup is one of the most widely used methods of fabricating aerospace composite parts and structure. Prepreg material allow for very precise resin to fiber ratios and accurate control of fiber orientation in the final laminate.

Prpreg are almost always processed within a vacuum bag, and cured in either an over or an autoclave, depending on the required structural properties of the end structure. Although some flat or gently shaped prepreg parts are hand laid-up and press-molded without the aid of a vacuum bag.

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