Silicone rubber is a unique synthetic elastomer that is made from a cross-linked polymer that is reinforced with silica. Some of the characteristics include stability at both high and low temperatures, no taste or smell, translucent and hence easy to color, wide hardness range, chemically resistance, weather-resistant, high level of sealing performance, electrical properties, and resistance to compression. There are four main molding methods: liquid injection molding, compression molding, transfer molding, and extruding/extrusion/extrusion molding (Dow Corning, 2007).
Liquid Injection Molding
This process typically involves injection molding a thermoplastic material such as PBT or nylon, followed by co-molding or over-molding with an LSR. LSR is vulcanized under heat using three main parts: an injection molding machine and a metering/mixing system and a specially designed mold to handle the material, which is cured to about 160°C to 200°C. Most injection molding machines for LSR use a reciprocating screw injection unit. The two-component LSR material is pumped to the injection unit using a meter/mix device with a static mixer mounted at the feed throat to aid in mixing and/or dispersion of additives. The LSR is injected into a mold, which is typically heated by four to six electric mold heating zones for each mold half. Adequate clamp force must be maintained throughout the molding cycle, since during the 10 to 100 seconds of curing, LSR will expand in volume by 1% to 2%, which is sufficient to cause flashing (Dow Corning, 2007). Pressure must also be maintained so that material is not permitted to migrate back through the bushing into the nozzle. Many LSR injection molding machines have water-cooled or insulated platens to ensure that the high temperature of the mold is not transferred to other areas of the injection molding machine.
Compression Molding
In compression molding, a preform is placed on one half of a heated mold. When the mold is closed and put under pressure in a press, the rubber is forced into all parts of the mold cavity; and excess rubber flows into a flash groove around the mold cavity.
Single cavity molds are loaded by hand. With some multiple cavity molds, loading boards may provide faster mold loading, which helps prevent scorching of the preforms. Enough pressure should be applied to obtain a sufficiently rapid flow of rubber in the mold. When molding thick sections or fabric-reinforced parts, it may be necessary to cool the mold before releasing the pressure -to help prevent backrinding, porosity, and delamination. The following factors must be taken into consideration during compression molding with various vulcanizing agents: molding time, vulcanizing agents, molding temperature (KDL, 2007).
Transfer Molding
In transfer molding, the unvulcanized rubber is placed in a chamber (called a pot), usually at the top of the mold, and the assembly is placed in a press. The press applies pressure to a piston-like plug in the open end of the pot, clamping the halves of the mold together and forcing the rubber to flow through one or more sprues into the heated mold.
Transfer molding is particularly useful in producing parts that require precise positioning. The time and temperature for transfer molding are the same as for compression molding. Transfer molding combines the advantages of injection molding with the ease of compression molding. This is the ideal process for forming parts that require exact positioning, bonding rubber to fragile metal parts – such as wire, mold designs that contain multiple cavities and can trap air, and intricate parts with lower volume requirements (Qualiform, 2007).
Extrusion Molding
Silicone rubber is extruded to make tubing, rods, gaskets, seals, wire insulation, and preforms used in compression molding. With this process, the rubber is continuously forced through a die that forms it to the desired cross-sectional size and shape. Screw extruders used with silicone rubber should have the following equipment: a screw designed for silicone rubber; a feeding roller attached to the screw by a gear or separately driven in the intake zone; an extended barrel, suited to the length of the screw; a breaker plate with recess to hold screens that will produce enough pressure in the rubber to ensure removal of trapped air; a spider flange or crosshead for holding purposes. In this process, silicone rubber moldings are made by forcing it through a shaped orifice by means of pressure. The rubber is continuously forced through a die that forms it to the desired cross-sectional size and shape before curing. Silastic silicone rubber should be extruded at room temperature. In fact, it should not be allowed to reach a temperature above 54°C during extrusion, since higher temperatures may produce scorching and loss of vulcanizing agent (Dow Corning, 2007).
In order to provide a silicone rubber composition that does not foam or have an unpleasant odor during its cure, it has been found that the silicone rubber composition must include a polydorganosiloxane gum, microparticulate silica, bis(ortho-methylbenzoyl)peroxide, and bis(para-methylbenzoyl)peroxide (Patent Storm, 1999). This has a high cure rate and provides silicone rubber extrusion moldings that post-cure and are either free of bubbles or contain bubbles to a limited extent. The silicone rubber composition includes 100 weight parts of a silicone rubber base compound and 0.1 to 10 weight parts of organoperoxides (Patent Storm, 1999).
Comparison of the three methods: In all these methods, rubber is placed in a mold and subjected to pressure and heat in order to shape and vulcanize the product. The methods differ from each other in a number of ways such as mold loading method, time and temperature of the molding cycle, etc. The source material is in the form of a mixture of compounds in the case of the injection method and extrusion method. In the compression method and transfer method, unvulcanized rubber is used. The final product of the extrusion method is odor-free and does not foam. Similar molders are used in all the processes. Preforms are rough pieces of unvulcanized rubber that are placed in the compression mold, the cylinder of the injection molding machine, or in the pot of the transfer mold. Strip preforms are especially suited to injection molding. For injection and transfer molding, preforms can be of any shape that is convenient for feeding the injection cylinder or transfer pot.
Transfer molding offers several advantages over other methods by providing: shorter production cycle maintains closer dimensional tolerances than compression molding, provides uniformity and fast mold setup.
Properties
Silicone rubber has some unique properties such as excellent electrical properties, high chemical resistance, and can be used in temperatures ranging from –50°C to +200°C. Moreover, it is also translucent and can readily be colored. They are resistant to temperature extremes, sunlight, and aging. They are increasingly being used to substitute for organic rubbers, because of their advantageous properties, such as high and low-temperature stability, inertness (no smell or taste), low toxicity, colorability, and transparency, combined with good electrical properties. The hardness range is wide, from 10-80 Shore A (WP, 2006).
Products
The development of silicone rubber with improved physical strength and excellent dielectric and moisture resistant properties has resulted in the application of these materials as primary insulation for transformers, large industrial motors, and traction equipment (Naegele et al, 2006). These applications have taken advantage of the natural properties of silicone rubber that are superior to other insulating systems. Silicone rubber is used widely in the following products: automotive applications, electrical/electronic applications, components/seals for medical devices in health care, in cooking, baking, and food storage products, in the textile industry, in the footwear industry, and for making gaskets, domestic appliances, fabric coatings (e.g. airbags), baby bottle teats, and medical devices. Silicone rubber is compression-molded to form gaskets, seals, 0-rings, flat sheets, fabric reinforced laminates, and many other types of industrial rubber goods, of almost any size desired. Silicone rubbers are frequently used for the extrusion molding of a tube, tape, sheet, wire and cable coatings, and the like because these rubbers offer excellent heat resistance, cold resistance, weathering resistance, and electrical characteristics. Silicones have been in use in medical applications for over 30 years because of their long-term stability and biocompatibility. High gas permeability is a positive property in many medical devices; silicones have up to 400 times the permeability of butyl rubber at room temperature. A particular grade of liquid injection molded silicone rubber is ideal for applications where contamination is a problem or where high volumes are required. The Medical Industry prefers this grade because it is a closed system of molding, meaning, the pail or drum kits of uncured liquid silicone are loaded into the pumping system, injected, and cured without human contact (KDL, 2007). Liquid Injection Silicone Molding is also ideal because it is less expensive compared to other products. Silicone rubber is also used in cosmetic applications, where its colorability and sensory properties are important (a soft, skin-like touch and appearance can be achieved) (WP, 2006).
Medical Grade Silicone Rubber
Bio Medical Grade Liquid Silicone Rubber is made up of the following: two-part (1:1 by weight), platinum-catalyzed liquid silicone rubbers; Typical Applications include injection molding of precision and intricate parts of medical devices (O-rings, stoppers, and closures) and mesh coating (DCH, 2007). There are different grades of silicone rubber used for other medical applications.
Food Grade Silicone Rubber
Food grade silicone contains silicone, meets FDA Regulation for Incidental Food Contact, provides excellent lubrication, and is nonstaining, noncorrosive, and fast evaporating. A multi-purpose silicone spray is used for food processing and handling applications (LPS, 2007). It is recommended for use in food processing plants on conveyors, guides, roller chains, bakery equipment, racks, and cookers and in bottling and packaging applications (Jet-Lube, 2007).
Other Grades
There are also many special grades and forms of silicone rubber, including Steam resistant, metal detectable, glow-in-the-dark, electrically conductive, chemical/oil/acid/gas resistant, low smoke emitting, and flame-retardant (2007). General Purpose grades are 20 – 90 shore hard. FDA177.2600 and WRC approved. They are suitable for food and pharmaceutical use. They also have a temperature range of – 80°C to +200°C. Another grade: BS6853 Cat 1 & 2 is used for fire-sensitive applications that require low smoke and low toxicity levels. It has high-temperature grades stable to 315°C.
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