Different Types of Silicone Rubber Parts
Silicone rubber parts provide durable protection from impact damage for handheld electronics and ruggedized sensors. They are also effective for sealing housings and protecting wire harnesses, and as dampeners or cushions in various applications.
This thermoset elastomer has short molecular chains and Silicone rubber parts can be molded through several processes. These include liquid injection molding, compression molding and vacuum casting.
Liquid Silicone Rubber (LSR)
Liquid silicone rubber (LSR) is a high-purity, platinum-cured material that’s injected into a mold to produce different parts with precision. Because it’s easy to mix in additives and fillers, LSR is a popular choice for producing medical-grade components like gaskets, valves, seals, and insulators. It’s also used to create different consumer products, including infant and kitchen goods, because it has a soft feel, is tasteless and odorless, and can meet strict FDA standards.
In contrast to HCR, LSR is very fluid and can flow into the most intricate part geometries with ease. It’s also able to withstand harsh environments, making it ideal for production of medical-grade silicone parts that require tight tolerances, heightened cleanliness, and tiny part geometries.
Like other elastomeric materials, LSR needs to undergo a polymerization reaction to form molecular chains. Vinyl and methyl siloxane groups are polymerized to form polydimethylsiloxane chains, which provide strength and flexibility to the finished product.
LSR comes in a variety of different grades and types for specific uses. For example, the ALPA-LSR product series crosslinks without byproducts and has a low compression set, making it an excellent option for creating complex-shaped molds. Similarly, the fluorinated LSR grades are self-lubricating and offer stable behavior for automotive connector seals, while electrical-grade LSRs are highly durable and can be used for air- and fluid-sealing applications.
Compression Molding
Compression molding is a manufacturing process that utilizes heat and pressure to shape solid thermoplastic materials like plastics or rubber. This type of mold production is ideal for medium-volume production. Injection molding is better suited for high-volume production, since it involves injection of liquid plastic into the mold cavity at a rapid rate.
The raw material is loaded into the mold and then compressed using a hot or cold compression process. The compressed material will take on the shape of the mold cavity. This allows for greater accuracy in the charge and reduces dimensional tolerances. It also eliminates short shots and flow lines. However, the process can produce excess plastic in the form of flash (excess material that is forced out of the mold at the parting line between the two halves of the mold).
A number of different types of mold are available to make this type of product. Some of the most common materials used in this technique are UP, vinyl ester and phenolic for thermosetting polymers, and PTFE or polytetrafluoroethylene for thermoplastics. Carbon fibers are often added to these materials to create composites that offer both the advantages of the individual constituents and provide unique properties like strength, ductility and electrical conductivity.
Regardless of the specific material, it is important to establish proper process parameters. This includes the appropriate temperature and pressure, as well as the mold design, which can impact how easily or difficult it is to push the material into the desired shape. The tooling can be designed with shear edges to help control the amount of flash that is produced, and it should also include water lines for heating, curing and cooling.
Injection Molding
The injection molding process is ideal for large scale production of polymer-based products with complex shapes. It is a repeatable process that produces a high quality of finished parts. Injection molding is used in the healthcare industry to produce a wide range of products from plastic syringes to instruments for medical procedures.
During this process, the raw material in the form of pellets is heated under pressure to a molten state and then injected into a mould in a very quick manner to fill the entire space. The build-up of pressure packs and holds the molten material together, solidifying into the shape of the product part. Once the molded part has cooled down, it is ejected from the mould by a clamping unit.
Achieving the desired appearance, strength, and performance of an injection molded product requires a precise blend of raw materials that can include virgin pellets, pelletized post-consumer waste, scrap regrind, colorants, performance or property additives, and other fillers. The exact formulation of these ingredients must be carefully controlled to maintain a consistent quality for every manufactured product. Injection molding is an environmentally friendly process that generates less waste than many other manufacturing methods. However, upfront costs can be quite high for this type of process due to the high initial investment in machine and mould tooling.
Vacuum Casting
Vacuum casting is a manufacturing process that draws liquid casting material into a mold through vacuum, unlike injection molding which pushes the material into the mold. This makes it ideal for making parts with undercuts and fine details. It also allows coloring pigments to be added for a wide range of colors. The molds for this type of casting are usually made from silicone, and they have a lifespan of around 20 casts before needing to be replaced. The molds must be cleaned regularly to prevent clogging by resins and other contaminants. They are also sprayed with special mold release agent and sometimes even put in a vacuum chamber for degassing and to ensure air bubbles are removed, before being placed in an oven to cure.
Before starting the casting process, a flawless master pattern must be made. This can be done through 3D printing or CNC machining, Silicone rubber parts depending on the design requirements. The finished master pattern must accurately embody the CAD model, and have ultra-smooth surfaces, hardness and chemical and temperature resistance.
A high-quality tin-cure or platinum-cure silicone is used for the vacuum casting. This type of silicone offers a low shrink rate and enables the part to retain its shape after casting. It is highly recommended to choose a 40-50 Shore A hardness for a durable and long-lasting mold.
