Spray-up process

In this process, chopped glass fiber roving and resin are simultaneously applied on a mold surface. The mold release agent, and the gel-coat are first applied on the surface. 

The mix material on the surface is rolled by hand to lay down fibers, remove air and to smooth the surface. The thickness is controlled by the operator and sufficient thickness can be built up in sections likely to be highly stressed. The success highly depends on the skill of the operator. 

The main advantage of this method is that the equipment is portable, therefore on- site fabrication can easily be accomplished. In addition, low cost molds can be used and complex shapes with reverse curves can be formed more easily than the hand lay-up process. 

The spray-up process is used in making boats, display signs, truck roofs and other roofings, and in lining tanks. Large objects with connected parts can also be produced by this technique. 

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The hand lay-up or spray-up process, used universally for the production of laminar composites incorporating glass fiber reinforcement, is most efficient for the manufacture of large parts, such as boats, bathtubs, tanks, architectural shapes, and recreational accessories. Resins intended for spray-up processes are usually modified with thixotropic additives, such as fumed siHca (1%), to reduce the risk of drainage when appHed over large vertical mold surfaces. Molds are also made from ERP for short-mn products usually surfaced with a tooling gel coat to provide consistent surface quaHty and appearance. 

In the spray-up process a reinforcement, usuaHy glass fiber, is substituted for the mat and a special spray gun simultaneously chops the glass fiber and appHes it with catalyzed resin to the mold surface. Hand rolling techniques then consoHdate the fiber and resin to conform to the mold surface contours. The shorter chopped fibers aHow for more intricate design detaHs than do mats. Both processes rely heavily on the operators skiHs for product quahty. These two processes require the least capital investment and have the largest product size capabHity of aH the processes. A single-surface mold produces a part with one controHed (usuaHy the visible) surface. 

Although most boats are produced via the open mold spray-up process, environmental pressures are mounting due to the high styrene monomer emissions that are inherent in open molding techniques. Closed molded techniques are beginning to be commercially practiced, and it is anticipated that these processes will replace the open mold process. 

The use of FRP is desired to replace older materials of construction such as concrete, stucco, etc. They have been used in many historical preservation projects. The reason they are desired is because of light weight, durability ease of installation, and low maintenance (no dry rot, attack by insects, mold, etc.). These are typically nonstructural parts made by spray-up process or continuous panel process. An example are shown in Figure 23.11. This market is expected to continue to grow. The main resin used for exterior architectural applications are halogenated polyester resins. Since there is not a smoke requirement, resins that meet the ASTM E-84 flame spread requirement of <25 can be used for this application. The other common material used for these applications is polyester resin or modified acrylic resins that are filled with ATH. The ATH filled resins can also be used in interior applications since they will typically also have low smoke values. 

In the spray-up process, a mould is made, just as in the hand-lay-up process. Continuous roving is fed into a chopper gun, which as its name suggests chops the fibres and sprays them onto the mould resin is also sprayed at the same time. The fibre and resin streams meet each other on the mould surface. The laminate is then rolled to remove as much air as possible. The fibre content achieved with this process is typically between 25-35 wt%. This method is rarely used by itself for corrosion-resistant vessels but is used in combination with filament winding for larger vessels. 

The assembled composite may be cured at room temperature or at elevated temperatures for faster cycles. This procedure, which was originally called contact molding, may be upgraded by the application of a vacuum or pressure bag placed over a Cellophane film on the final layer to reduce void formation in the composite. The laminate may also be built up by a spray-up process in which a mixture of chopped glass strands and catalysed resin is sprayed on the gel coat instead of resin-impregnated mat. In any case, the inner surface will be less smooth than the first layer formed by the gel coat. Tanks, boats and pipe may be fabricated by this technique. 

It is possible to automate the spray-up process with, for instance, a robot, but the hand operated method is most common. 

The spray-up process employs a continuous tow of carbon fiber, fed to a dual applicator gun where it is both chopped and admixed with cement slurry [10] and finally discharged onto a mold surface. 

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. 

Cold-press molds are plastic, which makes them comparatively inexpensive. Relatively long cycle times and postmolding machining result in parts which are more expensive than compression-molded parts, but less costly than parts produced by the lay-up or spray-up processes. 

A special case of hand layup is the spray-up process. This is accomplished with a special type of spray gun. Molds similar to those used in hand layup are used. The gun chops glass fiber into predetermined lengths, mixes them with resin, and deposits them onto the mold surface. The mold surface is often pretreated with a mineral-filled resin that has been partially cured. This surface is called a gel coat When enough glass has been deposited, an impregnated glass mat has been formed. From this point on, the process is identical to the hand layup process. The process is attractive because it makes use of glass fiber in its least expensive form. Parts with complicated shapes can be made, and size is not critical. Skilled operators are needed if control over wall thickness is required. 

Several forms of roving are available. Spun roving is among the least expensive, consisting of a single continuous strand that is looped and wound into rovinglike form. Roving products for use in gun spray-up processes can be easily cut. 

Bag moulding processes use pressure applied to uncured resins on the mould in order to compact the laminates and to drive out volatiles. It can be in two different forms vacuum bag or pressure bag moulding. Either of these can be used to supplement curing in the lay-up or spray-up processes. 

Both hand lay-up and spray-up processes essentially involve placing reinforcement and liquid resin onto the surface of an open mold. As the two names suggest, hand lay-up involves applying the resin and reinforcement (for example, glass fiber chopped-strand mats) by hand, while spray-up uses spray equipment to deposit resin and reinforcement (for example, chopped glass fibers) onto the mold. These relatively cheap techmques are often used to produce large, complicated, strong composite parts such as boat hulls. 

Both lay-up and spray-up processes use only one-sided molds. Female molds are preferred for ease of demolding, since the resin shrinkage on curing tend to lock the part onto a male mold. 

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