Rubber Injection and Transfer Molding ProcessRubber Injection MoldingBased off of a process intended for the molding of plastics, injection molding of rubber began in the mid 1960s. Rubber injection molding successfully alters the plastics process by heating the rubber and placing it under significantly more pressure per square inch of cavity surface in molding. This is different from the plastic injection molding process where the materials are cooled under less pressure. Through various innovations, injection molding has become one of the most efficient ways to create molded rubber products in many cases. The process of injection and injection-transfer molding starts with efficient material preparation. To begin, the material is mixed in bulk and then stripped immediately into continuous approx. 1.25" wide & .375" strips. These strips are then fed into a screw which, in turn, fills a barrel with the appropriate predetermined amount of rubber material. Advantages of injection molding:
Compression MoldingCompression molding is a process that involves taking a rubber compound or mixed raw material and creating "pre-forms" in the basic shape of the end product. The pre-forms provide a surplus of material to be placed in the cavity, thus ensuring a total cavity fill. Once in place, the mold is then closed, applying both heat and pressure to the pre-form and allowing it to fill the cavity. When the cavity is filled, excess pre-form material spills out into overflow grooves. Following this step the rubber is then demolded, usually by hand, leaving us with the molded rubber product. Compression molding is often chosen for medium hardness compounds in low volume production or in applications requiring particularly expensive materials. This process helps to minimize the amount of overflow, or flash created during the rubber molding process. In creating compression molded rubber products, the pre-forms can be difficult to insert into more complex mold designs. Furthermore, the compression molding process does not lend itself well to the material flow requirement of harder rubber compounds. Benefits of Compression Molding
Compression molding can be a cost effective solution in situations where:
Applications of compression molding range from simple o-ring drive belts to complex brake diaphragms with diameters of more than 10 inches. Timco can also offer a variety of other molded rubber products through compression molding. Transfer MoldingAs with compression molding, transfer molding requires secondary raw material preparation into pre-forms. It differs, however, in the placement of these pre-forms into a "pot" located between the top plate and a plunger. When the pre-forms are placed into the pot, the mold is then closed and the material is compressed by the plunger and transferred through sprues into the cavity below. Some possible advantages of transfer molding over compression molding can include:
The primary disadvantage of transfer molding is found in the increase of waste or excess flash. This can be seen in the "flash pad" or the rubber left in the pot after the transfer is cured which can either be recycled or discarded. DeflashingRemoval of the waste edge, or flash, from a molded rubber product can be accomplished in a number of ways. Depending on the material, part size, tolerance and quantity deflashing methods commonly include:
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