| CEM (COMPOSITE EPOXY MOLDING) |
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CEM(Composite Epoxy Molding) are usually used to replicate parts for prototyping purposes. CEM(Composite Epoxy Molding) by RIM(Reaction injection molding) are suitable for the project to manufacture huge prototypes such as automotive bumpers and fenders, with acceptable defects, and design without complex structure, such as undercut. CEM tools bridges the gap between silicone rubber soft molds and aluminum "soft tools". It has advantages in between the two RT methods: * Composite tooling can use production materials (thermoset plastics). * Less expensive than an aluminum tool. * Lower lead time than aluminum tool. * From one mold, 50-500 pieces can be produced. * Part can be of moderate to high complexity. Since the consistency of the mold is hard, it cannot be distorted to ensure part release. Therefore, elimination of undercuts and the implementation of draft is necessary, as shown in the figure below. As can be seen in the figure, much detail is lost because of the need to make a non-distorting tool that releases the part.  Composite Epoxy Molding Process Description (a) A master male pattern is made. The master can be made from any material that can tolerate epoxy, such as foam or wood. However, typically the master pattern is a stereolithography part. (b) An RTV mold is made of the master pattern and a polyurethane male pattern is made from the mold. The reason for this is that in the epoxy mold fabrication process, the male pattern is typically destroyed. A parting line surface is added, as shown below.  (c) Epoxy and glass cloth are layered around the male pattern to form the mold and are cured successively in layers. The figure below shows the beginning of the "lay up" process for the top half of the mold.  The following figure shows the completed lay up assembly for the top half of the mold.  (d) Curing of the mold epoxy can take from 0.5 to 40 hours. Aging the mold at room temperature increases the longevity of the mold. (e) To structurally strengthen the mold and for fixturing, the epoxy lay up can be potted in a block of epoxy. (f) After construction of the epoxy mold, steel inserts can be added for complex slides and coring and machining of the epoxy may need to be undertaken for example, in areas where sprues or gates could not be molded in. RIM(Reaction injection molding) - replication process for CEM Reaction Injection Molding (RIM) is similar to injection molding except thermosetting polymers are used, which requires a curing reaction to occur within the mold. Common items made via RIM include automotive bumpers, air spoilers, and fenders. Process First, the two parts of the polymer are mixed together. The mixture is then injected into the mold under high pressure using an impinging mixer. The mixture is allowed to sit in the mold long enough for it to expand and cure. If reinforcing agents are added to the mixture then the process is known as reinforced reaction injection molding (RRIM). Common reinforcing agents include glass fibers and mica. This process is usually used to produce rigid foam automotive panels. A subset of RRIM is structural reaction injection molding (SRIM), which uses fiber meshes for the reinforcing agent. The fiber mesh is first arranged in the mold and then the polymer mixture is injection molded over it. The most common RIM processable material is polyurethane, but others include polyureas, polyisocyanurates, polyesters, polyepoxides, and nylon 6. For polyurethane one component of the mixture is polyisocyanate and the other component is a blend of polyol, surfactant, catalyst, and blowing agent. Advantages and disadvantages Reaction injection molding can produce strong, flexible, lightweight parts which can easily be painted. It also has the advantage of quick cycle times compared to typical vacuum casting materials. The bi-component mixture injected into the mold has a much lower viscosity than molten thermoplastic polymers, therefore large, light-weight, and thin-walled items can be successfully RIM processed. This thinner mixture also requires less clamping forces, which leads to smaller equipment and ultimately lower capital expenditures. Another advantages of RIM processed foam is that a high-density skin is formed with a low-density core. The disadvantages are slow cycle times, compared to injection molding, and expensive raw materials. Tooling Machined steel or aluminum; cast aluminum; silicone rubber; epoxy resin. The machines can be large or small depending on the size of part required. |
