Liquid Silicone Rubber (LSR) Injection Molding, also known as Liquid Injection Moulding (LIM), is a thermoset method for producing flexible silicone components. Two compounds are combined, heated within the mold, and a platinum catalyst is used to speed up the curing process.
What is liquid silicone rubber?
Liquid silicone rubber (LSR) is a thick liquid polymer that is produced using a clean liquid injection molding technique to create components.
LSR mouldings are dependable and excellent for difficult applications because of their low compression set and resistance to severe temperatures. LSR elastomer provides great optical clarity, toughness, and design flexibility while maintaining good mechanical qualities across a wide range of temperatures (-50oC to 250oC).
The most stable LSRs are two-part, platinum-cured polymers used in liquid injection molding. LSR that has been platinum-cured is received in drums A and B. The vulcanization or curing process starts when the A and B material components are pumped into the injection barrel of the molding machine. A third liquid containing the color pigment is also pumped into the barrel if colored parts are desired. When it is injected into the heated mold cavities, where it hardens and becomes the precise shape of the item, the curing process is hastened even further.
Benefits of LSR Injection Molding
The advantages of injection molding include:
- High-volume manufacturing
Complex liquid silicone injection molding parts may now be produced in great volumes and at a low cost thanks to advanced LSR materials.
- Low potential for contamination
In contrast to the labor-intensive compression molding of silicone rubber gum, the liquid injection molding technique uses automated molding equipment that minimize human contact with the material. With liquid injection molders that feature completely automated, hands-free manufacturing processes, the possibility of component contamination in LIM is significantly diminished. ISO-certified clean rooms offer an environmentally controlled manufacturing environment with stringent sanitary guidelines for goods with hygienic production needs.
- a short cycle time
The geometry of the component, the general vulcanization properties, the temperature of the mold, the material when it reaches the mold, and the chemistry of the curing substance all affect the cycle time for LSR components. Faster curing is possible with two-part, platinum-cured LSRs by preheating the injection barrel and cold runner to 40–80 degrees Celsius.
- reduces waste and flash
Material scrap is nearly eliminated since the molds work flashlessly and have few material runners.
- Enhanced safety
Operator entry into the molding area is not essential thanks to the automation possibilities. Robots, chutes, or conveyor belts are used to extract and transfer parts from the injection molding process, eliminating handling and the possibility of burns or other safety issues.
Disadvantages of Liquid silicone rubber
- LSR cannot be recycled since once it cures, it cannot be moulded again. LSR
- Long and expensive production times
- If left untreated, their sticky texture will cause them to gather dust and other debris.
- When exposed to certain substances like sulfur and latex, LSR will prevent their ability to cure, hence they are not suited for submerged applications in alcohol or gasoline.
- Limited options for materials used in the LIM process Silicone comes only a small range of grades.
Types of LSR Injection Molding
There are many types of LSR materials. Some include:
- medical levels
The ability to be extracted for medical applications, which have extremely delicate criteria to prevent interfering with tissues, will be removed from an LSR in subsequent phases. The strength of the material is also increased by removing the extractables. Restrictive (implant for fewer than 30 days) and unrestricted are the usual classifications for these grades (implant greater than 30 days).
- Self-lubricating:
It is possible to combine incompatible fluids with the parts. This substance will bloom to the surface to provide a slick surface when the silicone has dried and hardened. Then, depending on the pace at which the consumer wants the fluid to bloom, 1.5 to 3 percent of a phenyl fluid is added to the silicone.
- Conductive:
To the LSR components is put conductive-grade carbon black. In comparison to silicones packed with metal particles, these grades have very high mechanical qualities and may achieve volume resistivity as low as 9 ohm-cm.
- Self-bonding:
The addition of adhesion promoters. It provides primer-free adherence to a range of surfaces. The substance adheres nicely to polysulfones, stainless steel, aluminum, and nylons with glass filler. For composite seals, it provides the ideal means of reducing assembly components or achieving even greater sealing between two mating components.
- Radio Obscure
In order to make the silicone component visible under X-Ray, barium sulfate is applied. This works nicely for medical uses like catheters and wound drains.
- Flame Resistant
Without any additions, the majority of LSR rubber will satisfy a UL 94 HB grade. Higher grades, such V1 or VO, need for chemicals that break down and release water.
- Quick Cure:
A maker of LSRs can regulate the quantity of functional groups added as cure sites in the material, just like with ethylene propylene, where a terpolymer with double bonds is added, to adjust the degree of crosslinking of the elastomer. Higher reactivity is the outcome, which leads to extremely short cycle times for the mass manufacture of silicone components.
The LSR manufacturing process
Some materials require an additional step of adding cures (which were left out to increase the shelf stability of the material). All those steps are eliminated by an LSR. Here is an overview of the LSR production process:
1. A 55-gallon drum or a 5-gallon pail are the typical containers for LSR A + B components. The buckets are compressed by a process-controlled pumping kit, which feeds the material into the static mixer and injection unit.
2. After that, pigment streams can be added and manipulated. Press material will be combined with a screw and pushed forward during the injection process to create a "shot" of material, or the quantity required to fill the mold.
3. The material will remain clamped together after being injected into the mold to cure the LSR. The cured product is then removed from the mold by a human or a machine, and the cycle is then repeated. It is a very quick and effective process that usually takes 30 seconds or less.
4. In contrast, making a part typically takes six minutes or longer when using organic materials or high consistency silicone. Even more time is required to cure materials using peroxide, typically nine minutes. Compared to a silicone material with a high consistency, LSR can result in significant labor cost savings.
5. Add-on procedures like post bake can help improve compression set and lower volatilities. Deflashing also eliminates unnecessary flashing. After being thoroughly cleaned of any foreign matter, it is then packaged for shipping.
Why Is Liquid Silicone Rubber Ideal For Injection Molding Applications?
Easy-to-mold (and demold) low or high viscosity rubber silicones are needed by manufacturers in a variety of industries, including automotive, healthcare, general industry, infant care, consumer products, and electronics, to make components quickly and affordably.
The material of choice for injection molding applications is liquid silicone rubber (LSR). The repeating Si-O chains that make up the silicone backbone are very flexible and provide this chemical structure special qualities such a wide working temperature range, weatherability, hydrophobicity, bio-inertness, electrical insulation, and a long lifespan.