• A Beginner's Guide to Rubber: Press Vulcanization
    In case you missed it, see part one. Rubber molding is one of three ways to form rubber; once the rubber compound is created it needs to be formed into its final product. This is accomplished by adding pressure and heat necessary to vulcanize the rubber. This guide will cover the various press vulcanization methods commonly used to mold rubber. Press vulcanization takes place in presses that supply pressure and heat. A very simple mold consists of two metal plates with cavities which conform to make the outside shape of the desired finished part. The presses hydraulically pushes plates heated by electricity steam or hot oil together vulcanizing the rubber. Molding processes can be classified under three broad headings: compression molding, transfer molding and injection molding. Compression molding involves placing properly shaped unvulcanized stock in each cavity of the mold; a cavity is the void where rubber will fill to form the shape of the finished product. The mold is then closed and maintained at a certain temperature for a preset amount of time. Under the pressure and heat the stock will flow and fill the cavity, the small amounts of excess flows out special grooves. This extra is known as mold flash. At the end of the cure time the press is opened and the rubber product is removed from the cavity. The challenge with compression molding is that it is necessary to accurately cut or weigh just enough stock for each cavity. Too little stock or incorrect placement can result in defects such as flow lines, blisters or unfilled parts. Excessive amounts of rubber can cause heavy flash that is difficult to remove. Additional challenges are longer curing cycles and more labor required to load and unload the mold. Transfer molding differs from compression molding mainly in that the stock is transferred through a hole into the mold cavity. In its rudimentary form, a transfer mold consists of a plunger, a cylinder (pot), and the mold cavity. A piece of unvulcanized stock in placed into the pot and is covered by the piston. The piston then presses the rubber with added heat forcing the stock through the hole into the mold cavity. After the mold has been cured the mold is separated and the cavities are unloaded. All transfer molding leaves a flash pad between the piston and the pot, which is thrown away, this additional cost may be offset by the shorter cure times and reduced stock preparation costs as several cavities can be filled with a single piece of stock. Injection molding is similar to transfer molding in that rubber stock is forced into a closed mold cavity through a nozzle. A strip of rubber stock fed into the press which is masticated and pushed through a large screw. Generally the screw fills a ram and then a piston forces rubber out of the ram into the mold cavities. This process significantly reduces cure times and can be cheaper than compression and transfer molding. In our next guide we will cover the other methods of bringing rubber compound to a finished product: extrusion, die-cut sheet rolling, open vulcanization and continuous vulcanization.
    Posted Thursday, November 14, 2013 by: Blog User
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