Plastic & Rubber – An Overview
Plastics and rubbers, collectively called polymers, are synthetic materials that have a number of very useful properties. Polymers can be synthesized cheaply and can easily be formed into almost any shape without the use of cutting tools or high heat. Far more than any other material, polymers can be tailor-made to have many combinations of material properties. They can be hard or soft, light or heavy, clear or colored, flexible or rigid, conductive or insulating, sticky or slippery. The ability to adapt polymer materials to almost any application has made them useful in every field from furniture to aerospace.
Plastics are divided into two categories, thermoplastics and thermosets. Thermoplastics can be heated to a temperature that makes them the consistency of bread dough, forced into a mold, and allowed to cool. Thermosets are manufactured by mixing two chemicals which react to form a solid material that can withstand high temperatures without softening.
Thermoplastics can also be formed by injection molding, where heated plastic is forced into a closed mold and allowed to cool. Injection molding is probably the most common process for forming plastic parts, but it is mainly used when large numbers of a part have to be produced. Since the steel mold for injection molding is so costly, it only makes economic sense to invest in it if the cost is going to be spread out over a large number of parts. Many products are made in large enough quantities to justify injection molding. Plastic products from novelty items to power tools are made by injection molding.
Extrusion is a process where a thermoplastic is squeezed through a shaped hole in a metal plate to produce long pieces with an unchanging cross-section. The tooling for extrusion is very inexpensive, so if your part has a constant cross-section, extruded plastic may be the best way to make it. The weather strip under a door is one product made by extrusion.
Blow molding forms hollow parts by dropping a tube of hot plastic into a mold and inflating it with air so that it assumes the form of the mold. All kinds of plastic bottles are made with blow molding. Since the mold does not have to withstand high pressures, it can be made from inexpensive materials.
In the vacuum forming process, a sheet of hot plastic is draped over a form, and the air is sucked out from between the form and the plastic, causing it to take the shape of the form, then the sheet is cooled and removed. Vacuum forming molds are inexpensive and can even be made from a type of plastic themselves, but the required cooling time means that it is fairly slow to produce parts. However, if large thin parts are needed in small to medium quantities, vacuum forming is an ideal process. Golf cart roofs and toolbox trays are two examples of vacuum formed parts.
In the rotational molding process, a hot mold is filled with a small amount of plastic powder and rotated in every direction. The plastic melts onto the inside of the mold and forms a hollow part. Rotationally molded parts are very strong and can be made in large sizes with relatively inexpensive tools. However, the process is slow and is mostly used when a small or medium number of parts are needed. Almost all plastic playground equipment is rotationally molded.
Thermosets are usually formed by Reaction Injection Molding (RIM), in which the two-component chemicals are mixed and then injected into a mold much like in injection molded. High-quality plastic cookware is made from thermosets processed this way. Thermoset products can be extremely strong and can withstand exposure to high heat and many solvents, making them ideal for use in the kitchen and harsh environments.
Plastic and Rubber Processes Supported
Film or Sheet
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