LSR injection molding is a thermoset procedure that involves mixing two compounds in a mold and then heat-curing them utilizing a platinum catalyst.
The process offers the most economical and efficient solution for the rapid manufacturing of quality silicone parts.
Therefore, keep reading to learn the step-by-step process of silicone injection molding.
How Liquid Silicone Rubber Injection Molding Works
LSR is the ideal material for the silicone injection molding process since it allows you to manufacture parts with complex geometries. The material permits combining of several functional elements into one part because of its tooling and design flexibility.
You will need an integrated LSR injection molding machine to ensure accuracy and efficiency during the molding process. For that reason, let’s look at the main parts of silicone injection molding machine.
- Injector is instrumental in pressuring the LSR to allow for its injection in the pumping segment of the system. You can adjust the injection rate and pressure as you desire.
- Metering Unit pumps the two main LSR parts (A and B), and any additives. The device ensures a constant ratio as it releases the two materials concurrently.
- Supply Drums are also known as plungers, which act as the main containers for blending silicone rubber materials. Both pigment container and supply drums connect to the primary pumping mechanism.
- Mixer can be either a dynamic or static mixer, which helps in blending materials after they leave the metering device.
- Nozzle facilitates the deposition of the silicone rubber material blend into the mold. Often, it has an automatic shut-off valve that is instrumental in preventing leakage and/or overfilling of the mold.
- Mold Clamp fastens the mold during silicone injection molding, and unlocks the mold after completion.
That said, let’s now discuss the main stages in liquid silicone rubber injection molding process.
6 Steps For Liquid Silicon Injection Molding
The process entails feeding LSR into comparatively cool barrel of liquid injection machine and then forcing it inside a heated mold for curing. Molds utilize either cold or hot runner systems.
With hot runner system, you remove the whole runner cures with the silicone part. However, a cold runner system maintains the LSR material in chilled conditioned to avoiding runner curing.
Step 1: Feeding Liquid Silicone Rubber Material into the Injection Molding Machine
Utilizing the supply drums, add the base-forming silicone material and platinum catalyst components of LSR compounds (A and B) into the pumping unit.
The supply drums are the main containers for LSR parts A and B. There is a separate container that carries the color pigments and other additives.
Step 2: Injecting LSR into the Metering Unit
The injector system then pressurizes the liquid silicone material to help feeding into the metering pumps. Fill pressure and injection rate limit this phase of liquid silicone molding process.
Minimize the injection pressure to avoid over-pressurization and flashing within the cavity and gate. Also, you can optimize the injection phase by profiling it into several steps when you need to manufacture silicone parts with complex geometries.
Step 3: Mixing of the 2 Liquid Silicone Rubber Components
The metering pump then blends the two LSR parts, making sure that they maintain uniform ratio of 1:1 while being supplied simultaneously.
You can vary the blending ratio for the LSR parts up to 5 percent off the 1:1 ratio. This variation will not have a substantial effect on the process or end product properties.
Step 4: Molding the Silicone Part
From the metering and mixer unit, the liquid injection molding machine pushes the mixed compound via cooled runner system into heated mold cavity. It is in this mold where vulcanization happens to form the silicone parts.
A nozzle assists in the mixture deposition into the mold. Most nozzles feature an automatic shut-off valve to prevent mold overfilling and leakage.
You need to ensure adequate clamp force to stop the mold from unlocking under cavity pressure and reduce flash. A mold camp helps in securing the mold cavity tightly throughout the liquid silicone injection molding process.
Step 5: Silicone Rubber Curing
LSR material in contact with the hot mold starts to undergo silicone curing process. The cure rate considerably rises with increasing temperature.
Typically, the preset curing temperature within the mold falls between 120 – 160 degrees Celsius. But, you can set the mold temperature beyond this range depending on distinct material or additional process parameters.
Curing causes development of the silicone polymer network, leading to enhanced limitations to chain motion. This lowering of mobility leads to viscosity increase till the LSR transforms into an elastic solid.
Step 6: Post Curing of Silicone Rubber
Often, the formed silicone parts get ejected from the mold after attaining 75-95 percent curing to avoid tearing and deformation. You can achieve full conversion through a post-bake or post-cure process.
However, some silicone rubber formula and applications might not need a post-cure. But most liquid silicone rubber suppliers suggest heating parts following ejection for an extended duration.
This heating makes sure that the silicone parts realize full conversion, thereby attaining final properties. Typical silicone rubber post curing conditions are 4 hours at 200 degrees Celsius within an oven.
- Injection Molding Liquid Silicone Rubber
- Liquid Silicone Rubber Injection
- Liquid Silicone Rubber Overmolding
Production Capabilities In Liquid Silicone Injection Molding
There are 5 primary types of liquid injection molding that you can perform when working with LSR material:
- Single-shot Molding: Here you use only a single liquid silicone materials for the injection molding process.
- Multi-shot molding: This process entails combining the LSR with a substrate and polymer, or two or beyond polymers for more complex silicone rubber parts.
- Overmolding: Here, you mold the liquid silicone rubber material around a finished part or substrate.
- Micromolding: This liquid silicone rubber injection molding process manufactures tiny, intricate parts, normally with weight below 1 gram.
- Hygienic Molding: Here, you produce the LSR parts in a clean environment that is highly controlled for several sanitary silicone applications. They can be for use in white rooms or more strict ISO cleanrooms of different classifications.
Contact us today to know more about our liquid silicone injection molding abilities. We can also assist you with silicone injection molding design guide specific to your product requirements.