Manufacturers commonly use Liquid Injection Molding (LIM) to form Liquid Silicone Rubber (LSR). Unlike compression molding, the mold is first closed and then material is introduced into the cavities through a system of sprues, runners and sub-runners. Once the parts are cured, the mold is opened, parts are removed and the process repeats.
Let’s take a look at the step-by-step process.
The Liquid Silicone Rubber Process
1. The uncured silicone starts off in two barrels — one marked Compound A and the other Compound B. One barrel contains the cures, while the other holds an accelerant. These two substances are combined and kneaded by an external mixer in a one-to-one ratio, along with any color that is needed.
2. The injection molding machine is programed and customized to every job. The injection settings are programed to the proper shot size. The platens and pistons that line up and close the mold have settings for temperature, pressure and cycle time that are adjusted accordingly.
3. Once setup is complete, the mold is heated to the appropriate temperature (typically between 300 to 350º Fahrenheit) and the clamping force is applied. The mold is closed and the screw then plunges forward to force the gathered material into the mold and the succeeding cavities.
4. Heat and pressure are applied to the LSR material until it is cured into a solid state. Once the cycle time is complete, the mold opens and a worker (or an automated process) removes the rubber part along with any flash. Subsequently, the mold closes again to repeat the same process.
5. The parts are moved through the manufacturing operation to undergo post-molding processing, which can include deflashing, post-curing, automated inspection and packaging.
Benefits of LSR Molding
So why choose LSR for parts and seals?
Great resistance. Since LSR has high resistance to water, ozone and weather, LIM molding with this type of material gives excellent resistance properties.
Stability. LSR molds have stability with low compression set, the ability to resist extreme temperatures (ranging from -65 to 200º Celsius) and can also be any color on the spectrum.
While many silicone parts are unpigmented, most customers opt for pigmented material to allow for easier identification and inspection of parts during assembly. Colored silicone is also used to create a particular “look” on the overall finished product. Adding color pigment generally does not change properties of the LSR, given that the process is done properly.
LSR capabilities. Liquid Silicone Rubber is a consistent molding material, and, LIM has the ability to produce high-precision parts that meet the following quality standards:
- Rubber Manufacturers Association, RMA-A1
- RMA Class A O-Ring tolerances or better
- ISO 3601-1A O-Ring tolerances
- ISO 3601-3 Grade N and Grade S tolerances
Minimized contamination. The LIM closed-system process minimizes raw material contamination by eliminating exposure to the environment. The process results in less scrap and material waste while still ensuring maximized efficiency.
Automation. All injection equipment, tooling and ejection equipment can be automated, which allows for minimal labor and reduced labor costs. These cycle times are typically measure in seconds instead of minutes.
Drawbacks of LSR Molding
Limited material. The material used for molding is limited to LSR options. Although characteristics can still vary among LSR, manufacturers are confined to this material.
Failure to meet aerospace specifications. When it comes to meeting aerospace specifications, LSR will not pass. Aerospace specifications require fillers and additives that simply cannot be added to LSR (because it will result in higher viscosity, meaning poor flow) so compression-molded material is preferred.
To avoid this, manufacturers quote compression-molded grades when it comes to aerospace specifications. Typically, customers buying LSR parts will not need the heat or oil resistance required to meet that specification.
Cost. Tooling is usually somewhat complex, leading to higher tooling prices. The complexity of the mold and set-up can lead to longer lead times.
Despite any drawbacks, LSR technology provides consistency and cost-effectiveness for a wide variety of applications such as medical, electronics, automotive, appliances and more. For questions on our silicone parts and seals, visit our website or contact our expert engineers — and be sure to connect with us on Twitter @AppleRubber.