It is important to consider what method of molding a silicone rubber manufacturer uses when deciding to choose one. There are a few different methods of molding silicone components, each with their own benefits and disadvantages. The main methods used are compression molding and liquid silicone injection molding (LIM).
Compression molding consists of a process that uses labor intensive silicone rubber. The silicone rubber used is pre-catalyzed by adding powder to the silicone. This enables the silicone to cure once introduced to heat. To add pigment to the silicone, the silicone must be mixed using a two-mill roll. This is a labor-intensive process that has the possibility for contamination of the silicone. Once the material is pre-catalyzed and pigment is added it is weighed and often shaped to the approximate size and configuration of the desired part. It is then placed directly into the mold cavity. The rubber is then compressed between the top and bottom plates. These plates are heated to melt the silicone and vulcanize (cure) the silicone.
Compression molding is the simplest form of silicone rubber molding, but it is also an open-air labor intensive process. This allows for the possibility of contamination to get into the silicone. Compression molding also has the use of fillers that are added to the silicone to lower the cost of material for manufacturing.
Liquid silicone injection molding or LIM, is a custom injection molding method of liquid silicone rubber (LSR). While normal injection molding typically uses a High Consistency Rubber (HCR) which is also known as gum stock, LIM uses a high viscosity liquid silicone rubber. LSR consists of two parts, an A and a B. Both parts are a liquid, but only one part contains the catalyst which is platinum. LSR is naturally translucent and typically comes in pail kits or drum kits which are 80lbs and 800lbs. LIM is a closed-loop system which means that the raw silicone is sealed, as well as any pigment that may be added. The Part A, and Part B are connected to a dispensing machine that seals the material and uses pressure to pump the material from the original containers into the machine. The pigment which colors the silicone is a silicone based pigment that is connected to the dispensing machine as well. The material and pigment are injected into a water-cooled barrel that keep the material from catalyzing. The material is then injected into a hot mold but the runner gate and sprue are typically smaller due to having a lower viscosity than gum stock. Due to the platinum cure system, Liquid Silicone Rubber cures at a much faster rate than typical gum stock. This allows for frequently lower piece prices than traditional compression molding. The rapid curing also makes LIM the desired choice for high volumes. This can cause an issue for low volume production that requires consistent material purging which can add to the final cost of the product.
Using a closed-loop system helps keep contamination to a minimum. Lowering the amount of handling greatly decreases the chance of contaminating the silicone components. LIM is the desired choice for the Medical Industry because the pails or drum kits of uncured LSR are loaded and sealed into the dispensing machine which can be injected or cured without human contact.
Here at SiTECH we use liquid silicone injection molding machines. Our facility in Newport News, Virginia provides you with a competitive domestic manufacturer of liquid silicone injection molding products. At SiTECH, we use the latest technology in silicone injection molding machinery to produce the highest quality silicone rubber products the industry can offer. If you are in need of assistance with your silicone component or keypad, or you simply would like a quote. You can contact us here.