Molding process, hand processing

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. 

Basically, the hand lay-up molding process was used, with 98wt% of the structure via... 

The other difference [21] is fhaf fhermosef composife shows higher balanced sfrength and toughness compared to thermoplastic composite, with a little brittleness. Thus, thermoset composite plates can be fabricated relatively thinner and have more applications. On the other hand, as mentioned before, a well developed injection molding process has been applied as the fabrication process of thermoplastic-based composite plates to assure high quality and low cost. 

Thermosets, on the other hand, are polymers formed by the mixing and chemical reaction of fluid precursors into a mold once the precursors react, a crosslinked network that cannot flow anymore under heating is created therefore, reaction and molding into the final shape usually take place at the same time (by the RIM or reaction injection molding process). Examples of common thermosets are some polyesters, phenol-formaldehyde resins, epoxy resins, and polyurethanes, among others. Chapter 28 of this handbook elaborates on this topic. 

RTM is compatible with a variety of thermosetting polymers including polyester, vinyl ester, epoxy, phenolic, modified acrylic, and hybrid polyester-urethane. A convenient typical viscosity of thermoset precursors is in the range of 0.2-0.6 Pa s. The RTM process has the following advantages (i) emissions are lower than in open-mold processes such as spray-up or hand lay-up (ii) it can produce parts faster, as much as 5-20 times faster than open-mold techniques (iii) the mold surface can produce a high quality finish (iv) complex mold shapes can be achieved and... 

Zero s use is not restricted to high cost aerospace applications as A P designed the febric to provide affordable but superior performance. It can be used in a wide range of RP molding processes including resin transfer molding (RTM), vacuum assisted RTM (VARTM), resin film infusion, and hand lay-up (Chapter 5). 

With the more mechanized molding processes, adequate space is obviously necessary, and special attention must be paid to safety precautions and guarding. The apparent simplicity of hand lay-up or spray-up should not lead one into thinking that generous space here is not equally essential. The ideal for all work is a large, airy, solid fectory building, free from draughts, and with a controlled temperature of 20-25C. 

Several different processes can be used to produce a laminate. In what is called the hand application process, for example, glass-fiber mats are impregnated with unsaturated polyester resins. The impregnated mats are then removed from the mold by hand and pressed between rollers. The final molding is effected by cold pressing. The method is suitable for small numbers of objects with large surface areas (e.g., boat hulls). 

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. 

Mat n. 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 lay-up, or contact-pressure molding. The mat is usually impregnated with resin just before or during the molding process. 

Vacuum molding n. This type of molding is used to mold fiberglass-reinforced plastics. The method allows molding without high temperatures and pressures. In this technique, layers of reinforced media are applied to the mold by hand. Resin is either sprayed or brushed on after each layer is positioned. The flexible sheet (usually cellophane or polyvinyl acetate) is placed over the lay up. Joints and seals are sealed and a vacuum causes the b to collapse over the face of the product not contacting the mold. The resultant pressure tends to eliminate voids and forces out any excess resin or entrapped air. Madox DM (1998) Handbook of physical vapor deposition (PVD) processing. Noyes Data Corporation, New York. 

The resin transfer molding and liquid transfer molding processes are used principally for manufacturing fiber-reinforced composite parts in moderate to high volumes. The process combines the techniques of hand lay-up, liquid resin casting, and transfer molding, as discussed earlier in this chapter. 

Autoclave molding is also similar to vacuum bag and hand-layup molding, except that the pressure plate is removed and the entire mold and layup are placed in an autoclave. The chief advantages of autoclave molding processes are faster cycle times and improved physical properties. 

Although this process is inexpensive and easy to perform, the molded parts are not of the highest quality and may not completely have the properties expected. The laminates produced by this method contain more resin than those produced by other methods, and more voids may be present. Little control can be exercised over the uniformity of the wall thickness. Typical cure time for polyesters is about 10 h at room temperature. Epoxies are used in special applications. They can be cured in as little as 3 h at 82°C. Hand layup is very labor intensive, but it is an excellent molding process when large sizes and a few units are needed. 

---