Molding Processes For Silicone Rubber Parts
Silicone rubber is a very versatile material that can be molded into many different shapes and sizes. It is used in a variety of applications, including voltage line insulators; automotive applications; cooking, baking and food storage products; apparel; electronics; and home repair and hardware.
Silicone rubber is a liquid in its uncured form and must be cured, or vulcanized, to turn into a solid. Silicone manufacturers use a number of processing techniques to make silicone parts and gaskets.
Liquid Injection Molding (LIM) or Liquid Silicone Rubber (LSR) Molding
This process molds high viscosity, 2 part liquid silicone rubber into a hot mold and cures in a Silicone rubber parts few seconds. This allows for short molding cycles that are cost-effective in medium to high volume production runs. The LIM/LSR molded parts also exhibit good physical properties such as excellent tolerance to extreme temperature, chemical resistance, and non-marking, which is particularly important for medical and automotive applications.
The closed system of the LIM/LSR molding process ensures that the uncured raw materials never come into contact with the human body. This feature makes it a good choice for sensitive industries such as medical, food processing, dairy, and semiconductor manufacturing.
During the process, the sealed raw materials are metered into a machine at a 1 to 1 ratio with pigments and then injected into the hot molds. The machines can also inject multiple colors simultaneously, reducing cycle times and labor intensity.
LIM molded components don’t shrink inside the mold as substantially as thermoplastics, but they will still shrink up to 3% after they are ejected and left to finish curing. This can cause drafts to form in the mold cavities, which can lead to flawed components. Gating and venting are critical considerations in the mold design to prevent air entrapment and avoid hot tearing of the silicone when releasing it from the mold.
Sheet Molding
As electronics continue to shrink and become more complex, thermal management is critical for their functionality. Silicone sheet is ideal for this application due to its high-temperature and flame-retardant properties as well as excellent work-ability, flow-ability, dimensional stability, low gas pressure loss and abrasion resistance.
SMI utilizes a specialized silicone compression molding process that allows us to create a wide range of high-quality silicone rubber parts and components including medical, LED lighting and industrial applications. This technique is a cost-effective fabrication method for prototypes, short runs and flat sheets, as tooling costs are much lower than injection molding.
This manufacturing process uses HCR compounds with adapted catalyst systems to preform the raw material into strips or loafs which are then pressed into a heated mold cavity via a funnel-shaped runner system known as a transfer pot. The mold is then closed and forced with a plunger until the material takes the shape of the cavity and is left to cure.
The calendering process is a mechanical process that involves pressing a raw material between pairs of hard steel rolls or multiple roller sets that are adjusted for pressure to produce the desired thickness, surface finish and texture for the finished product. This is done for silicone rubber products and can produce finishes such as matte, glossy, smooth or adherent.
Compression Molding
The compression molding process is used to create rubber parts by heating and pressurizing a mold. The mold is filled with raw rubber material, and then a plug is secured at the top to ensure the desired shape is formed. The rubber is then heated and under pressure until it is cured.
This process is suitable for both thermoset and thermoplastic polymers. The resulting rubber is strong, durable, and has good thermal and electrical properties. It is also highly resistant to moisture and oil. It’s used to make seals and o-rings, among other things.
Unlike injection molding, compression molding can produce complex shapes. It is also ideal for producing large, thick-walled components. It is also less prone to the formation of knit lines, which are visible cracks in the molded material. Additionally, it generates less waste, which makes it a cost-effective choice for manufacturing high-volume components.
The Xometry team recommends using this method when prototyping products that require the use of soft materials such as silicone, urethane, and other rubbers. In addition to its broad design flexibility, it’s also a good option for making small, durable parts like rubber gaskets. The two-part silicone putty Castaldo Quick-Sil is an example of a product developed through this process. It’s an essential component for OXO’s cocktail shaker, creating a watertight seal between the glass and lid of the container.
Extrusion
Extrusion is a manufacturing molding process that forces base material through a die to create objects with a fixed cross section. This method is ideal for making long, continuous parts such as pipe and tubing, door insulation seals, plastic bags, and window glazing. It can also be used to make textured vegetable protein (meat analogs), breakfast cereals, snack foods, and chewing gums.
There are two types of extrusion: hot and cold. In hot feed extrusion, the stock material is pre-heated to a temperature above its recrystallization point. This helps to reduce degradation of the material while also providing good flowability and a high degree of Silicone Rubber Parts – Supplier mechanical properties. Cold feed extrusion, on the other hand, is performed below the recrystallization point and uses low pressure. This provides good flexibility, but requires a higher maintenance level.
The key to a successful extrusion is the die, which gives the finished product its shape and profile. The feed-throat, breaker plate and barrel all help to channel the plastic through the die and maintain pressure. The breaker plate also filters out contaminants and prevents the plastic from rotating while entering the die. The die is also heated to melt the plastic and provide shear heat, which will help the strands of extruded rubber to take on their final form. Once the strands have taken on their desired shape, they are pulled and cooled to form solid products like elbow macaroni or hollow pasta.
