Liquid silicone molding is a process where liquid raw materials are injected into a hot mold to create the finished part. It is a versatile production method with quick cycle times and the ability to incorporate molded features.
Supply drums, commonly referred to as plungers, serve as the primary mixing containers. These are connected to a static mixer and then paired with a nozzle to enhance injection.
Process
Liquid injection molding is a process that transforms liquid silicone rubber (LSR) into critical performance components for medical, power, consumer, automotive and aerospace applications. This method of fabrication allows manufacturers to design parts that cannot be molded using other methods and then fabricate those parts in high volumes for low costs.
The uncured LSR material comes in pail or drum kits and is injected into the molds in a closed-loop system, which reduces contamination. This also eliminates the need for human handling, which minimizes the possibility of air pockets and design errors.
The two raw materials, Compound A and Compound B, are combined in a machine that is designed for this type of molding. A metering unit releases the two materials concurrently at a steady ratio, and a static mixer blends the components. The resulting compound is pumped into the mold through a heated nozzle, where it forms around the product inserts. Once the molded part has cooled and hardened, it is ejected from the mold using an ejector pin.
Materials
The materials used in liquid silicone molding are biocompatible and can be molded into very complex shapes. They are also resistant to water, ozone and extreme temperatures from -65 to 200°F. This makes them ideal for a wide variety of sealing applications in many different industries.
The uncured liquid silicone starts in two primary containers – Barrel A holds the base-forming silicone, while barrel B holds the catalyst. Both of these connect to a pumping system, which releases the two substances at a consistent one-to-one ratio, as well as any color pigmentation additives for colored parts.
The mixed materials then go through a cold runner static mixer, where they are combined and pushed into a heated mold cavity through a nozzle. This nozzle often has an automatic shut-off valve to prevent leakage and overfilling. The resulting part is then cured to solidify. The injection molding machine has settings for the temperature, pressure, injection rate and cycle time that are adjusted in accordance with the customer’s part design.
Equipment
Liquid silicone molding uses a specialized injection machine. The injection unit, which includes a barrel, screw and nozzle, is responsible for melting and injecting the liquid silicone rubber into a mold cavity. The clamping unit, which secures the mold and platens is also incorporated into the machine.
Unlike compression molding of gum base silicone, which is a labor-intensive process, LIM requires precise tools designed to create high-quality parts. Because the molded items are injected rather than compressed, this process offers tighter control over dimensional tolerances in both the closure (cross section) dimensions and plan view (length and width) dimensions.
During the injection process, a combination of heat and pressure is applied to the molded item until it solidifies and cures in the mold cavities. The cured items can then be removed from the mold, de-flashed and cleaned for inspection. Stockwell Elastomerics recently invested in a cryogenic de-flashing system that utilizes liquid nitrogen and small tumbling beads to remove parting line flash, which allows the company to maintain its fast cycle times and keep costs low.
Automation
Unlike thermoplastic elastomers that melt and lose their shape when subjected to high temperatures, liquid silicone rubber is able to withstand those conditions. As such, manufacturers across a wide range of industries utilize it to create injection molded products.
Injection molding machines use a combination of heat, pressure and liquid silicone to mold the product into its final form. They consist of a fixed and moving plate with tools bolted to both sides. The fixed plate is heated through resistance or induction while the moving plate applies clamping force to the custom rubber mold.
The metering, mixing and dispensing systems of liquid silicone molding machines can be automated, which reduces labor costs. This helps maintain cycle times, eliminates errors and improves process controls. Networking via OPC UA and MES authentig ensures data tracking at the machine and mold level. The closed mold is then vented and vacuumed to prevent air pockets in the resulting molded product.
