Liquid injection molding is a process used to produce high quality, pliable and durable parts in high volumes. The key ingredient is liquid silicone rubber, which is a high purity platinum cured silicone. The benefits of injection molding include low viscosity nature, which is conducive to producing tight tolerances; closed process, to avoid contamination; and short cycle times to reduce production costs. Pressure plays a key role in the process. Knowing the pressures involved and regulating them ultimately improve the efficiently and successful production of high quality silicone rubber products.
Pressure is first used in hydraulic piston pumps (FAU) to transfer the Silicone rubber from its original container to a static mixer. Regulators that measure the pressure (150-220 bars) and volume flow of the silicone, control the flow between the metering device and the mixing chamber. Once in the mixer, color can be added and pressure is then used to inject the pigment into the static mixer at pressures of 15-40 psi and from 0.5-6%, depending on the desired finished color needed. The pressure of the silicone delivered through the pumps is reduced to between 30-70 bars before it reaches the injection unit.
Liquid silicone rubber has a low viscosity, which in turn requires low pressures to be injected into the mold. The exact pressure needed is determined by the cross-section of the feed channel. The best results can be achieved when injection speed and pressure is chosen so that the mold fills within 3 seconds. This is important to ensure even dimensions and curing of the silicone part. The expansion of the material in the hot mold increases the cavity pressure significantly. This makes holding pressure important to prevent silicone from leaking out of the gate. Adjusting the holding pressure on the mold, depending on the material and geometry of the gate, will hinder any discharge of material from the gate.
The vulcanization of the component being produced varies depending on many factors. The temperature of the mold is the key factor in cure time. The thickness of the part also depicts the cure time, the thicker the part the longer the cure time. The flow, viscosity, speed and pressure of injection will depict the exact amount of time needed for full vulcanization. The expected percentage of shrinkage can be slightly affected by the pressures used to inject and hold the part in the mold.
The balance and fine-tuning of these pressures will help to eliminate problems such as voids or air trapped in the part. Diagnosing pressure-related issues demands experience and expertise in the process of liquid silicone injection molding. With the right technology and equipment, silicone rubber molding can be a highly efficient, accurate solution for all your silicone needs. At SiTech, we employ the latest technology and innovations in LIM machines to provide our customers with the highest quality parts available for the life of your product. For more information on liquid silicone injection molding capabilities and products or to place an order for silicone rubber products, contact us at SiTECH.