Liquid Silicone Injection Molding: How It Works
Injection molding of liquid silicone rubber (LSR) is a process to produce consistent, durable parts at an efficient rate. There are many mechanical devices involved in the make-up of an LSR injection system. The silicone delivery system includes a two-part silicone base, generally referred to as the “A” component and “B” component. Silicone rubber is clear in color and has the consistency of thick molasses. The “A” component contains a catalyst, which is a platinum agent which causes cross-linking as temperature is introduced. It also contains an inhibitor that helps to keep the mixed silicone from curing within a certain temperature. The “B” component contains a cross-linking element. These can vary between manufacturers.
The silicone is transferred into the barrel section of the molding machine using a fluid automation unit (FAU). At this point, pigment can be added for color of the silicone. Then, either a static or dynamic mixer is used to blend the materials, generally a ratio of 1:1 is used. The resulting mixture is housed in what is known as the “barrel.” The barrel is water cooled to keep the material from curing before being injected into a mold. At the end of the barrel is a nozzle which is also water cooled. The nozzle is the most likely place that the silicone would cure, because of its consistent contact with a hot mold.
In liquid silicone injection molding machines, the mold cavity consists of a fixed plate and a moving plate. Both plates are heated through either resistance or induction. Tools are bolted to both sides of the mold. One side of the tool corresponds to the fixed-plate side of the mold. This side will also be the injection side for the silicone. The moving side of the custom rubber molding tool is matched up visually to the tool already in place to avoid damage before being bolted in place. Tools vary greatly in size, shape, material, finish and geometry of the part being molded. Molds, depending on the design of the tool and the part being molded, are generally under a vacuum system connected to the mold. The importance of this is to quickly evacuate air from inside the tool so that it doesn’t get trapped inside the finished custom rubber molding part.
Liquid silicone injection molding machines have become more intricate and digital over the years. Newer machines have advanced features to control all aspects of the shot. They are also very accurate on dispensing silicone, which cuts down tremendously on waste. New machines can be operated effectively with knowledge of the process and the know-how in programming and troubleshooting for the specific job. The cycle of a part can be broken down to the individual aspects of the process.
First, the mold closes and seals, and at this point the air is vacuumed out of the cavity. The vacuum step is important to pull air out of the tool, so that there are no air pockets in the final product.
Second, the tip of the barrel moves up to mold in direct contact with the tool. Next, the metered (measured) amount of silicone rubber is injected into the mold. In the next step, the silicone is held under pressure and the shut-off valve in the barrel tip is closed to avoid the backflow of material back into the barrel. During the cure time, the next shot is metered and mixed. The last step is for the mold to open and the finished molded silicone rubber part to be retrieved. At this point, any flash should be removed from the tool to avoid flash getting in the next part. At the end of this cycle, the barrel should be programmed to back off the mold, even if just a little, to avoid liquid silicone material from curing while retrieving the finished part.
Liquid silicone injection molding is a highly productive way to produce quality custom rubber molding parts. It is a technologically-demanding method, but once variables are fine-tuned, a far more accurate part is produced. It’s a superior method of silicone molding for most applications.