LSR vs TPE are the typical thermoset and thermoplastic materials. Thermoset and thermoplastic seam related but never interchangeable. LSR is thermoset, but TPE materials is thermoplastic. Each material has different material properties and inverse behavior in molding production process. Thermoplastic elastomer (TPE) and liquid silicone rubber (LSR) have similar properties, both of them are beneficial alternatives to natural or synthetic rubber. However, there are significant differences between thermoplastic elastomer (TPE) and thermoset elastomer (LSR) characteristics.
What is LSR (Liquid Silicone Rubber)?
Liquid Silicone Rubber (LSR) is a common thermoset material, it is a synthetic resin with chemical bond polymers. Heating LSR mixture will result in polymer cross-linking in chemical bonds, and create permanent strength and shape substances after curing process.
Opposite to TPE, heat to LSR will give rise to its hardens. Once LSR is heated, initially in the molding process and secondarily in the post-cure process, it will vulcanize and lock-in the final physical properties, such as tensile strength, flexural strength, and heat deformation temperature. Post curing is a process of heating molded LSR parts in ovens, the primary purpose is to accelerate rubber vulcanization and maximize material physical properties. In some specific case, post curing can also eliminate volatiles.
Platinum-cured LSR is a 2-compound formula, these two parts are mixed in static mixers before injection process. One part(A or B)of LSR contains catalyst will accelerate the cross-linking progression. Silicone structure is oxygen and silicon atoms alternation, it has different forms such as liquid silicone rubber(LSR), heat consistency rubber(HCR), and room temperature vulcanized rubber (RTV). The chemical cross-linking of rubber in solidification process is curing or vulcanization.
What is TPE (Thermoplastic Elastomer)?
Thermoplastic Elastomer (TPE) is also known as thermoplastic rubber, it is a compound or blend of polymers. It is melted and formed into plastic once heated, and hardened once cooled. Its chemistry will never change in form alternation. TPE is a class of co-polymers with mixture of plastic and rubber.
TPE is shaped and fabricated once reach melting point temperatures. Although it has similar elasticity as cross-linked materials of LSR, its elastomeric behavior is not determined by the cross-linking properties. Thermoplastic Elastomer has no chemical bonding in molten and solidify process. The plastic pellets will liquefy once heated, then pressure-mold into various components, and retain shapes in the curing process. The normal types of TPE solutions including:
- Olefinic thermoplastic elastomer
- Polyester thermoplastic elastomer
- Amide thermoplastic elastomer
- Polyurethane thermoplastic elastomer
- Thermoplastic vulcanization
- Styrenic block copolymer
- Molten processable rubber
Select The Best Material
Both Liquid Silicone Rubber (LSR) and Thermoplastic Elastomer (TPE) have unique advantages, so select the best material deto dependpend on the product, budget, and required performance.
When is LSR the Best Choice?
Once you are choosing silicone or thermoplastic elastomers, there are several critical considerations, such as temperature, compression set, and heat resistance. Normal rubbers like natural rubber, nitrile rubber, ethylene polypropylene diene rubber (EPDM) and poly-acrylic elastomer, have the modest working temperature range. For example, natural rubber has a temperature range of -50℃ to 100℃, while nitrile rubber is -25℃ to 150℃. Once compare thermoplastic elastomer and silicone elastomer, LSR provides consistent performance in wider temperature range than TPE, both in high and low of scale.
Wide Operation Temperature
LSR is able to operate in 175℃ to 205℃ temperature without shape alternation, it has stable properties in extreme temperature whether hot or cold. Such as, LSR can maintain flexibility and compression set in temperature as low as -40℃. These properties are desirable in automotive industry, especially for exterior part on automobile from sensors, connector, seals to gasket. In addition, we also can apply LSR for inside automobile parts, including gaskets and keypads in intelligent keys, vibration dampening cushions in Air Conditioner systems. However, thermoplastic elastomer is not able to perform as well as LSR in these ranges of temperatures.
High Flexibility
LSR has low viscosity, ease of process, and different durometer availability. Therefore, this offers designers and engineers the added design flexibility to achieve complex features’ requirement: tight dimensions, thick and thin details, consistency from soft to firm. In reason of its thin filling ability and wide temperature stability, LSR is the perfect material for pressure-sensitive applications. For example, gasket and membranes in gas appliance applications, pups and surgical deviance in ophthalmology surgeries, thin membranes and gasket in medical fluid management devices. LSR is available in different durometers from soft to firm, its most common stiffness is between 20 and 80 Shore A.
Stability
Silicone elastomer is UV-stable, it is impervious to UV rays and resist weathering. This make it an excellent choice for products with long-term exposure to UV rays. These products can withstand harsh outdoor conditions for decades without deterioration. LSR is also ozone-stable, which endow its superior humidity and moisture resistance.
Bio-compatibility
LSR also has excellent chemical resistance to several solvents and cleaning agents. It is ideal for medical and healthcare applications, especially for those are exposure to other ambient contaminants, and require regular cleaning with harsh cleaning agents. LSR is the material choice for products with the body contact, its bio-compatibility and hypo-allergy make it the most suitable material in healthcare industry. Normally, it is applied in respiratory devices and wearable medical devices without skin irritation.
LSR Advantages
LSR also have other advantages, including:
- Ease and clean processing,
- High clarity and light transmittance,
- Odorless and tasteless,
- Radiation, UV light and bacteria resistance,
- Strong dielectric property and superior insulation,
- Minimal water uptake and water resistance,
- Extensive pigment colors.
Applications of LSR
LSR also has thousands of applications in various industries like consumer electronics, sanitary, maternal and infant health care, food&beverage, these LSR products including:
- Connector seal in all industries,
- Membranes, diaphragms and vales,
- Gasket and hardware,
- Bellows and septa,
- Electronic sealed housing,
- Pump housing,
- Hearing aid parts,
- Respiratory masks,
- Baby nipples and pacifiers.
When is TPE the Best Choice?
Thermoplastic elastomer provides cost saving to manufacturers in processing perspective. Firstly, its material price is less than silicone elastomer. Secondly, TPE also consumes less energy in production, due to avoid cross-linking in solidification process, . TPE materials will melt once heated in high temperature, and it has recyclability and reprocessing properties.
TPE has some similar benefits as silicone elastomer including hypoallergenic, vibration resistance and compression set. However, these properties are limited in certain temperature ranges. In reason of this, TPE is not well-suited for many automotive exterior parts with high operating temperature.
TPE provides advantages over LSR and other silicone elastomer in some application. For seals and permeability applications, TPE has strong oxidation and moisture resistance, pressure and vacuum retain. Specific TPE materials with non-tacky property also can repel dirt and contaminants. Therefore, this kind of material is a good choice for consumer products with critical aesthetic and surface finish requirement.
TPE Advantages
Additional TPE advantage including:
- Excellent colorability,
- Heat seal-ability,
- Abrasion resistance,
- Good electrical properties,
- High elasticity,
- High fatigue resistance and impact strength,
- Low density and compression set,
- Low specific gravity,
- Chemicals and weathering resilience,
- Strong and flexibility in room temperature.
The main drawback of TPE materials is compromised dimension stability, which result from incomplete cross-linking. Otherwise, additives as plasticizes, anti-oxidants may result in high content of leachable and extractable volatile. But we should monitor these levels closely in medical devices and healthcare applications.
Applications of TPE
Depending on different requirements and environments, We apply TPE materials in various industries, including:
- Bottle caps,
- Baby bottles,
- Impact-resistant component and devices housing,
- Seal rings,
- Closure liners,
Thermoplastic elastomer is alternative than latex, PVC or rubber, especially in medical and healthcare applications.
LSR vs TPE Comparison
LSR has ability of sterilization withstanding and capability with miniaturization, this encourages its rapid growth in medical and healthcare applications. Therefore, LSR is suitable for human contact products. LSR hardness range from 3 to 80 Shore A, the most common is 50 Shore A with the best tensile and tear strength. LSR hardness of 30 Shore A has the best sealing property. While low durometer silicone elastomer as gels can reach less than 10 Shore A.
TPE materials is reusable in many times within acceptable levels. Furthermore, there is no negative impact of integrity and performance. TPE can retain its compression set property after several times of reprocess and remold by re-heating, softening and re-hardening. The typical hardness of TPE is around 70 durometer A.
LSR vs TPE Molding Process
The primary difference between LSR vs TPE molding is that liquid silicone rubber undergoes a chemical cross-linking reaction. Liquid A/B material compounds are mixed in injection barrel and heated in the mold. While TPE material pellets are melted in injection barrel and cooled in the mold.
Over-molding and 2-shot molding apply both LSR and TPE materials in molding process. TPE can bond to a wider range of plastic materials, in reason of its low melting temperature. However, LSR has high processing temperatures, it only can bond with high performance engineering thermoplastics like polymides, polycarbonates, and glass reinforced PBTS. For LSR 2-shot molding, We mold both LSR and TPE materials in same mold for superior bonding and integration, these materials should have similar melting temperature.
LSR Injection Molding
In LSR injection molding process, we apply customized fabrication cells to fit customer specific requirements. Normally, We divide LSR into equal A and B components, in order to prevent chemical reaction. While component A contains the catalyst, and B contains the cross-link.
Then feed A&B components into barrels in 1:1 ratio by metering pumps, and add color pigment before it reaches static mixer. These LSR mixture is chilled and inserted into hot molds, LSR molds have low injection pressures, always below 69 Mpa.
In reason of ow viscosity of LSR, its molds require high precision to avoid flashing, and apply vacuum for venting. Liquid silicone rubber has high shrinkage than thermoplastic elastomer, therefore we can pull out silicone parts with undercuts easily from mold without complex mechanical slides.
TPE Injection Molding
TPE has less demanding in injection molding process, we add thermoplastic elastomer in pellets form to injection molding machines by the hopper. The pellets flow through hot barrels and inject into molds with temperature between 20℃ to 65℃.
Product Performance
In the comparison of LSR vs TPE, LSR provides higher quality with higher price. In addition, LSR has higher performance than any other rubber on the market.
LSR Solutions from Miwo
Miwo engineering team apply LSR unique material characteristics to create high quality LSR solutions. We have advanced technology and experience for consistent LSR molding process.
