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Manufacturing Technology Insights | Monday, January 02, 2023
The bulk molding compound is heated and then injected into a heated mold, where it is held under pressure until it dries. In this process, relatively small components can be produced in a short time. Tooling costs are high, so the process is suitable only for medium to high-volume production.
Fremont, CA: Several layers of reinforcement and wet resin are applied to a mold and distributed by a roller until the desired thickness is reached. The resin is then cured, using heat, if necessary, to produce the finished product.
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Polyester resins reinforced with glass fibers are widely used in the marine industry. The process is flexible, and materials are relatively inexpensive. While labor-intensive, styrene emissions are high, and the process is labor-intensive.
Spray-up
To build up the desired thickness of the material, chopped fiber reinforcement, and wet resin are applied using a spray gun. A cure is then performed on the resin. Although the process is faster and cheaper than wet lay-up, its mechanical properties are lower.
In most cases, the process is used for large, relatively simple structures such as bathtubs, boat hulls, and storage tanks.
Compression molding
In order to achieve high-volume production, a high investment in heated tooling is required.
Injection molding
The bulk molding compound is heated and then injected into a heated mold, where it is held under pressure until it dries. In this process, relatively small components can be produced in a short time. Tooling costs are high, so the process is suitable only for medium to high-volume production.
Resin transfer molding (RTM)
In a heated mold, fiber preforms or fabrics are placed. A reactive resin is mixed and injected under pressure into a mold. In order to remove the part, the pressure is maintained until the resin has cured. Many industries use the process for complex, heavily loaded parts. VARTM (Vacuum Assisted Resin Transfer Moulding) is a variant of the process in which a vacuum is applied to the closed mold, allowing the resin to be injected under low pressure.
Vacuum infusion
Vacuum is used to impregnate fibrous preforms with low-viscosity resin. Preforms are typically impregnated with resin by causing it to flow over the surface and then impregnate through the thickness of the preform, minimizing the distance the resin must travel. In addition to boat hulls and wind turbine blades, the process is well-suited to large components. During the process, the tooling does not have to carry substantial loads.
Filament winding
Fibre tows are passed through a resin bath and applied to convex mandrels under tension. The fiber release is moved as the mandrel rotates to lay down fibers in the desired geometry up to the required thickness. After curing, the composite is allowed to cool at elevated temperatures if necessary.
Prepreg
A prepreg (pre-impregnated fiber) is composed of fibers, fabrics, or mats impregnated with resin. A thermosetting prepreg contains hardeners in the resin, so it has a limited shelf life and usually needs to be refrigerated. Plies are cut to the required shape and orientation before the prepreg is brought to room temperature.
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