Step-by-step Liquid Silicone Rubber Injection Molding Process

  • Step-by-step Liquid Silicone Rubber Injection Molding Process

Liquid Silicone Rubber Injection Molding

Through liquid silicone rubber injection molding, you can make many rubber parts.  Ideally, liquid silicone rubber has two parts –

  • Parts A – Platinum catalyst
  • Part B – methylhydrogensiloxane; it functions as an alcohol inhibitor and cross-linker

More importantly, LSR is a thermoset. Let’s dive right in:

Step 1: Choose Liquid Silicone Rubber Injection Molding Machine

Liquid Silicone Rubber Injection Molding Process
Illustrating LSR injection molding

High quality machine is key during silicone rubber injection molding process. It directs LSR into the mold and facilitate curing.

Usually, the mold design resembles the final parts you wish to make. Let’s explore the key components and their main functions:

Machine Part Function
  • Pressurizes LSR material
  • Using pumping mechanism, it forces LSR into the machine system ready for the molding process
  • Allows you to precisely control pressure depending on the molded part requirements
  • Injects LSR in the mold
  • It LSR flow to prevent overfilling the mold and possible leakages
Metering system
  • Ensures the recommended material ratio flows into the machine – that is, liquid silicone rubber parts A and B
Plungers or supply drum
  • Helps in LSR blending
  • In most designs, you can connect it to the pumping system
  • It helps to blend liquid silicone rubber materials
Mold clamp
  • It fastens the mold during LSR injection molding


More Resources:

Step 2: Choosing Liquid Silicone Rubber Materials

There are many liquid silicone rubber grades available today. For example, you can choose the:

  • Heat stabilized LSR
  • Weather resistant LSR
  • Steam resistant LSR

They type of material will depend on your specific needs and requirements.

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 3: 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 4: 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 5: Mixing of the 2 Liquid Silicone Rubber Components

food grade liquid silicone rubber injection molding
LSR injection molding process

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 6: Actual Liquid Silicone Rubber Injection Molding Parts

Once you follow the required silicone injection molding design guide to make you tooling system. Definitely, you will get high quality LSR parts.

The process involves:

  • Transferring LSR material into the mold cavity
  • It is important to preheat the mold cavity
  • All material move to the mold cavity through the cooled runner system
  • It is in this mold where vulcanization happens to form the silicone parts

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 7: 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, you should pay attention to certain liquid silicone rubber injection molding temperature. That is, 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 8: Post Curing LSR Parts

Figure 1 Medical grade liquid silicone rubber
Medical grade LSR parts

Injection molded LSR parts leave the mold after achieving about 75 to 95% curing. Typically, this will prevent possible damage due to say tearing or 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.

Production Capabilities Injection Molding of Liquid Silicone Rubber

While evaluating the capability, you must consider these key options:

  • 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. For all your LSR injection molding, LegenDay is here to help.



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