Fabrication hand layup

Some plastics, such as the TSs and in particular the TS-RPs (RTSs), can produce products with exceptionally tight tolerances that practically meet zero tolerances. In injection or compression molding of relatively thin to thick and complex shapes, tolerances can be held to less than 0.001 in. or to even zero, as can also be done using hand layup RP fabricating techniques. 

Apart from the above three types there are custom built rubber products such as expansion joints, flexible cell covers and large size rubber foils for the caustic soda industry, and many inflatables, fabric reinforced products and thick moulded sheets for specialty applications in certain process plants. These are all hand formed in aluminium or cast iron moulds or forms by laying up process and then cured in autoclave. Here the flow of the un-vulcanized rubber during cure is not very important as the shape is already formed rather the green strength and the stiffness of rubber stock with a low scorch time are the important requisites. A rubber expansion joint made by a hand layup method and cured in autoclave is shown in the following figure 14.1. 

The sprayup process is ideal for low to medium volume applications and is particularly well suited for efficient fabrication of large shapes. The decision to choose sprayup over hand layup is based on both the mechanical properties required and the processing constraints. This process has been superseded in some applications by other advanced fabrication techniques such as RTM which are discussed in later sections. 

There are reinforcement preform constructions. A preform is a method of making chopped fiber mats of complex shapes that are to be used as reinforcements in different RP molding fabricating processes (hand layup, injection, etc.). Oriented fiber patterns can be incorporated in the preforms (Chapter 5). 

Hand Layup (Contact Molding or Open Mold Process). The oldest and simplest technique is primarily manual. An open mold is waxed to lubricate it. The surface of the proposed product is applied to the mold, usually as an unreinforced gel coat or sometimes as a thermoformed plastic sheet. A layer of fabric is hand cut and hand laid into the mold. Catalyzed resin is poured over the fabric and worked into it with brush, roUer, and squeegee to eliminate air bubbles. Another layer of fabric is laid over this, often oriented in a specific direction to optimize properties. Again, catalyzed resin is poured over it and worked into it. The process is repeated as many times as required to build up the desired thickness and optimum orientation of the layers. If the cure reaction is inhibited by air, a wax may be dissolved in the resin it exudes to the surface and forms a barrier to exclude air and permit complete cure. The resin may be allowed to cure at room temperature, or the assembly may be heated to complete or hasten the cure reaction. 

Hand Lay-uplSpray up Spray up and open contact molding (hand lay-up) in one-sided molds is one of the cheapest and most common process for making fiber composite products. Typical products are boat hulls and decks, truck cabs and fenders. In a typical open mold application, the mold is first waxed and sprayed with gel coat and cured in a heated oven at about 49°C. In the spray up process, after the gel coat is cured, catalyzed resin (usually polyester or vinyl ester at 500-1,000 cP viscosity) is sprayed into the mold, along with chopped fiber. A secondary spray up layer imbeds the core between the laminates (sandwich construction). Then it is cured, cooled, and removed from the reusable mold. In hand layup processing, continuous fiber strand mat and other fabrics such as woven roving are manually placed in the mold. Each ply is sprayed with catalyzed resin (1,000-1,500 cP) and the resin is worked into the fiber with brush rollers to wet-out and compact the laminate. 

The major types of layup configuration include hand layup, automatic tape laydown, filament winding, resin transfer molding, and pultrusion. The configuration will depend on the type of reinforcement that is used (i.e., tape, tow, fabric, preform, chopped fiber, etc.) and whether or not it is impregnated with resin at the time of laying in the mold. 

Polyester can be used to fabricate a variety of products by many techniques such as open-mold casting, hand layup, spray-up, vacuum-bag molding, matched metal die molding, filament winding, pultrusion, encapsulation, centrifugal casting, and injection molding. 

Mat A fabric or felt of glass or other reinforcing fibrous material cut to the contour of a mold, for use in reinforced-plastics processes such as matched-die molding, hand layup, or contact-pressure molding. The mat is usually impregnated with resin just before or during the molding process. 

O Open-Mold Process n Any technique for fabricating reinforced plastics in which a one-sided male or female mold is used, with no or low pressure being required. See Hand Layup, Sprayup, and Bag Molding. 

Figure 6.1 Lab-scale fabrication of multi-ply, unidirectional kenaf, and fiber-reinforced ortho-UP composites by hand layup [12].

Many such objects are fabricated by a process known as hand layup. The mold surface is often first sprayed with a pigmented but nonreinforced gel coat of the liquid resin to provide a smooth surface finish. The gel coat is followed up by successive layers of fiberglass, either in the form of woven cloth or random matting, impregnated with the liquid resin, which is then cured (crosslinked) to give the finished product. 

For both small boats and racing yachts, the adhesive bond line thickness is rather variable it is not possible to maintain tight composite tolerances with this type of hand layup or oven manufacture, so adhesive thickness can reach several millimeters. This is much thicker than for aerospace structures, produced using film adhesives with carrier fabric in autoclaves, for which a typical bond line thickness is less than 0.2 mm. The consequences of this larger bond line thickness may be significant, resulting in ... 

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