Introduction
Nowadays, manufacturers in various industries discover liquid silicone rubber advantages for injection molding. Liquid silicone injection molding is an effective fabrication technology of molded parts. One of the primary advantage is that LSR molding is more efficiently and economically than other molding processes.
Thermoplastics are also a popular option for injection molding, because of their strong, lightweight and easy modification. However, thermoplastics cannot withstand extreme high temperature.
Liquid Silicone Rubber Properties
Silicone rubber is consisting of sand or silicone dioxide in basic form. These sand is processed into silicon metal and reacted with methyl chloride, then apply different additional fabrication methods to create different types and grades of silicone, like fluids, polymers, compounds and rubber bases.
Liquid silicone rubber (LSR) is a high-purity platinum cure silicone. Different from high consistency silicone (HCR), gum-based silicone, LSR materials has various types and grades, they provide different selection of texture from grainy to smooth for end-products. Furthermore, we also can modify different formulation and specific side groups to define required functions or specifications.
Although LSR materials have different specific properties, there are still same feature for all LSR materials. Liquid silicone rubber has processing attributes similar to thermoplastics. Meanwhile, LSR also some prominent characteristics as thermoset materials.
The common liquid silicone rubber properties including:
- Chemical inertness,
- Clarity,
- High elongation,
- Excellent tensile strength, tear strength,
- Flexibility in temperature from -60℃ to 250℃,
- Low compression,
- Odorless and tasteless,
- Hydrophobic,
- Hypoallergenic,
- FDA bio-compatibility,
- Superior dielectric strength.
Liquid Silicone Rubber Grades
We can modify different liquid silicone rubber formulation to create various LSR grade with different properties. Firstly, one option is different levels of hardness, this is measured with a durometer by final intended usage. Secondly, different sterilization methods is another option, such as: ethylene oxide (ETO), steam autoclaving, gamma radiation, e-beam, dry heat. Thirdly, we can formulate LSR materials by additives and other fillers, in order to withstand higher temperature, oil and other specific conditions. Finally, adding phenyl units to LSR materials to extend the low-temperature capabilities.
The common liquid silicone rubber grades include as following:
- Medical grade LSR,
- Food grade LSR,
- Electrically conductive LSR,
- Self-adhesive LSR,
- Self-lubricating LSR,
- FDA compliant LSR,
- Impermeability LSR,
- High optical LSR,
- Reinforced fiber LSR,
- Radio Opaque LSR,
- Flame Retardant LSR,
- Fast Cure LSR.
LSR is easily applied to create products with complex geometries and undercuts. This is difficult or impossible to produce by thermoplastics. Medical grade liquid silicone rubber is suitable for custom medical device components, ranging from analysis machines to surgical instruments. Comparing to plastics, LSR materials never stick on the patient’s skin, nor harmful to body tissues.
In final curing stage, LSR materials solidify through a method without hazardous substances. Therefore, there is no risk of hazardous chemical release.
LSR Value in Different Industries
In medical industry, medical grade LSR has high purity and distinctive properties. It is more appealing than normal silicone materials. Medical grade LSR materials have excellent bio-compatibility with human tissues, hypoallergenic features and high performance. Furthermore, it is also resistant to chemicals and bacteria.
Food grade LSR has high strength and stress resistance for kitchen products. Its applications range from gaskets and seal in industrial cooking appliances to dispensers and baking trays in residential kitchens.
For normal purpose applications, we can apply common liquid silicone rubber materials. These materials have mid-level properties of temperature resistance, tear strength, elongation and solvent resistance.
LSR Advantages in Injection Molding
In injection molding process, different materials will result to various problem. However, once apply liquid silicone rubber materials in injection molding, there is less or none challenges to fix with.
By-products
Liquid silicone rubber (LSR) never generate by-products in molding process, its unique molecular structure make it suitable for wide applications. However, the superior bonding strength in molecular structure also means it is unable to recycle. Furthermore, this silicon and oxygen atom bond strength prevent its degradation in high temperature. LSR is chemical inertness, and able to endure sterilizations process. Therefore, it is an optimal material choice for medical devices, food applications, and baby care products.
Restrictive Design
Liquid silicone rubber molding has similar design guidelines as thermoplastics, but it provides more freedom in certain areas. LSR provides designers and engineers more flexibility in designing process.
Planes
Plane features on mold opening direction require necessary drafts, in order to allow milling of molds. The mold design requires almost 1 degree draft in 25mm mold depth to prevent scraping. Because shear-thin of liquid silicone rubber, it can navigate easily into thin walls. This allows parts design with different thickness.
Sink
As liquid silicone rubber materials solidify as heated, most materials cure before cool down. Sink is not an issue in LSR molding process. Designers and engineers can make silicone parts with thicker features than thermoplastic materials.
Undercuts
Undercuts handling is the biggest difference between liquid silicone rubber and thermoplastics in injection molding process. As high flexibility of liquid silicone rubber materials, there is less limitation in undercut features. However, rigid thermoplastic materials requires complicated methods. Such as apply side-action to produce undercut feature, then withdraw the material prior before mold opening.
Compress-ability
The innate compress-ability of liquid silicone rubber create some unattainable capabilities with plastic in parts design. Furthermore, LSR is able to combine with different thermoplastics without priming preparation. Therefore, we can add LSR materials on part surface for functional purposes, and eliminate additional substrate to withstand heat in vulcanization process. This result to smaller parts with less weight in a more economical fabrication way.
LSR Injection Molding Advantages
Liquid silicone rubber materials provide various advantages in injection molding process:
- Shorter molding cycle time in seconds than thermoplastics in minutes.
- Close premixing system prevent contamination in environment.
- High viscosity properties make LSR ideal for small and complex silicone parts with high tolerance.
- Less materials waste in molding process than other methods.
- Ability to over-mold on thermoplastics, metal and other materials.
In Miwo, our experienced engineering team will assist you in the selection of appropriate liquid silicone rubber materials. In order to guarantee your final applications. The superior resistance of thermal, electrical and chemical make LSR an excellent choice for various application.
Liquid Injection Molding Advantages
Another important consideration to select liquid silicone rubber is the liquid injection molding (LIM) manufacturing process.
LIM technology start with liquid silicone formula with compound A & B, one contain catalyst, another contain cross-linker. Typically, these two compound are delivered into a static mixer by metered pumping system at constant ratio of 1:1. Then, the static mixer blend two compounds into a homogeneous concoction, and trigger the cure system. After that, mixed LSR materials flow into injection unit, and feed into mold cavity by the runner and gate system. Finally, The LSR is held in mold under high pressure and temperature at an established cycle time, in order to ensure optimal level of rubber vulcanization. Once the cycle finish, we remove cured parts from the cavities, and start a new cycle.
LIM technology is able to fabricate a wide range of precise and flash-less silicone parts with high-volume and tight tolerance. The main advantages of liquid injection molding (LIM) including:
- High automation level
- Excellent flash and waste control
- Quicker cycle time than thermoplastics
- Superior clarity and excellent pigment ability
- Closed molding for complex geometries and over-molding
- UV degradation resistance
Comparing to other silicone molding process, LIM has higher startup and shutdown cost. This make it suitable for high-volume production.
Why Choose LSR for Your Project?
Liquid silicone rubber have higher performance gains, processing stability and competitive pricing than thermoplastics. It is a better option in various industries. Such as medical, automotive, industrial, electronic.
We must evaluate a wide range of LSR materials properties and processing methods for critical silicone applications. In order to satisfy rigid performance and budget requirements. Therefore, it is important to define performance standard in certain properties, like elongation, durometer, and thermal resistance. Otherwise, selecting the wrong LSR materials or process will result in project failure.
Miwo is a top silicone molding company in China, we work exclusively with liquid silicone rubber(LSR). Contact our engineering team to learn more about LSR advantages in injection molding.
