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Compression Molding

Tuesday, December 20, 2011


Compression molding is one of the oldest methods for creating rubber parts. It was developed very early in the history of the rubber manufacturing industry from techniques used in baking. It is a very efficient process for creating large, flat or slightly curved rubber or rubber parts, and it's cheaper and creates less waste than either injection or intrusion molding.

Compression molding is a simple process. The rubber is put inside a mold, and both heat and pressure are applied. As the rubber melts, it will take the shape of the mold. The pressure is maintained until the rubber has cooled into the shape of the mold.

The compounds most commonly used for compression molding are a bulk molding compound (BMC) or a sheet molding compound (SMC). The BMC most often comes in a pellet form, and must be spread evenly across the mold for the best results. SMC comes in a sheet and can be cut to the exact size of the material required to make the part, preventing waste. The sheet also sits evenly across the mold, reducing movement in the material during the compression stage, thereby improving the quality of the final product.

Compression molding presses come in a variety of sizes and consist of two parts. The first is a hydraulic press with a male mold on the end. The second is a female mold which is held still though the compression process. The molds are usually made of metal and have grooves to help remove the excess rubber.

Compression molding is typically used to create large flat or slightly curved high quality rubber products. It is often used in the automotive and airline industries. Compression molding is also used to create many household and everyday items.

On big advantage of compression molding is ease with which glass fibers can be introduced to the manufacturing process. The fibers can be laid in the mold either unidirectionally, randomly or in a weave. As the rubber is compressed it will merge with the fibers and create a strong and light composite. Other rubber manufacturing techniques don't allow as much flexibility in the orientation of fibers, and sometimes injection and extrusion molding damage the fibers before the product is finished.

Engineers decide several basic things when be successful when compression molding. They determine how much material is needed and whether they will use an SMC or BMC compound. They also decide how long the material needs to be heated before it reaches its melting point, how long it has to be held at the melting point and compressed. Thirdly, they decide how much pressure must be applied to the molten rubber to get the desired part thickness. Finally, they decide how they are going to cool the part and how long it will take to cool.

Compression molding has several advantages over other rubber manufacturing techniques. The set-up and running costs are relatively low when compared to other process, like extrusion and injection molding. In addition to the set up costs, the amount of material wasted is very low, cutting down on recycling and disposal costs for wasted material.

Quality-wise, the parts can be much larger than their extrusion molded counterparts. They also have a much better surface finish than either extrusion or injection molded parts. Finally, compression molding creates less degradation of any glass fibers used to reinforce the rubber.

Unfortunately, the quality of parts created with compression rubber molding can be inconsistent. Compression molding also cannot be used for parts with undercuts, overlaps or strong curves as the mold cannot be accurately shaped. Compression molding also takes a lot longer than either extrusion or injection molding.

Compression molding is a great technique for creating large rubber parts. It has several useful advantages over other rubber manufacturing techniques, including its cost, the amount of waste and the surface finish of the final product. However, it is slower than other techniques and sometimes creates parts of inconsistent quality.


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