“Thermoplastic” and “thermoset” sound related but are not interchangeable. LSRs are thermosets and TPE materials are thermoplastics. Each has different material properties and they behave inversely during the molding process.
Thermoplastic elastomers and liquid silicone rubber have similar properties and are beneficial alternatives to synthetic or natural rubber. Both types of materials are ideal for many types of applications.
However, there are significant differences between thermoplastic (thermoplastic elastomer) characteristics and those of a thermoset (silicone elastomer).
What Is LSR?
Silicones are made from quartz sand, a raw material available in practically unlimited quantities. Liquid silicone rubber is a synthetic resin where polymers join together by a chemical bond. Heating the mixture causes polymer cross-linking which results in a chemical bond giving the substance permanent strength and shape after the curing process.
When you apply heat to LSR it hardens, which is the opposite of TPE. When LSR is heated, initially during the molding process and secondarily during the post-cure process, it vulcanizes and locks-in the physical properties (i.e. tensile strength, flexural strength, and heat distortion temperature). One of the primary purposes for performing post curing (heating the molded parts in an oven) is to accelerate vulcanization, maximizing some of the material’s physical properties, and in some cases is used to drive out volatiles.
Platinum-cured LSR is a 2-part formula, mixed prior to their reaching the injection mold. One element acts as the catalyst that initiates the cross-linking progression. Rubber undergoes chemical cross-linking during the molding process, which is called curing or vulcanization. The vulcanization process takes time to formulate, anywhere from seconds to several hours.
Silicone’s structure has alternating oxygen and silicon atoms and can come in many forms, including LSR, heat-cured rubber, and room temperature vulcanized rubber.
What Are Thermoplastic Elastomers?
Also known as thermoplastic rubber, a thermoplastic elastomer is a blend or compound of polymers that melt and form into plastic when heated. It hardens when cooled, but its chemistry doesn’t change from one form to the next.
A thermoplastic is a plastic polymer material and is commonly fossil-based. Bio-based TPE solutions derived from sources such as corn, sugar cane, beet, soya beans, cellulose, or vegetable oil or plants or algae are being introduced.
Manufacturers shape and fabricate TPE when temperatures reach their melting point. Although it has a similar elasticity to that of cross-linked materials, like LSR, its elastomeric behavior is not due to the cross-linking properties.
Chemical bonding does not take place within thermoplastic elastomers. By applying heat to the plastic pellets, they liquefy. They are then pressure-molded into various components that strengthen and retain their shape during the curing process. The seven types of TPE solutions include:
- Styrenic block copolymer
- Melt processable rubber
- Thermoplastic olefinic elastomer
- Thermoplastic polyester elastomer
- Thermoplastic vulcanisate
- Thermoplastic amide elastomer
- Thermoplastic polyurethane elastomer
In short, TPE is a class of copolymers, often made with a mix of plastic and rubber.
Choosing the Best Material for your Application
Both LSRs and TPEs have advantages and choosing the best material will depend on your product, your budget, and most importantly the performance required.
When is LSR the best choice?
When you’re choosing silicones or thermoplastic elastomers, temperature, compression set, and heat resistance are critical considerations. Other rubbers such as natural rubber, nitrile rubber, ethylene polypropylene diene rubber (EPDM), and polyacrylic elastomer tend to have modest working temperature ranges. These include ranges of -50 to less than 100 degrees C for natural rubber, for example, and -25 to 150 degrees C for nitrile rubber. When comparing thermoplastic elastomers vs. silicone elastomers, LSR offers unwavering performance in a wide range of temperatures and exceeds TPE’s on both ends of the scale – high and low.
LSR can operate in 350 to 400 degrees F (175 – 205 degrees C) without altering its shape. Over a range of extreme temperatures — whether hot or cold — the material and its properties remain stable.
For example, it maintains its flexibility and elastomeric characteristics (compression set) even down to temperatures as low as -100 degrees F. Compression set means it returns to its original shape when stretched. These attributes are desirable in the automobile industry, especially for parts used on automobile exteriors – from sensors to connectors, seals and gaskets that are used under-the-hood. LSR is also found in parts used inside the automobile, including vibration dampening cushions in HVAC systems, and gaskets and keypads used in key FOBs. Unlike LSR’s, thermoplastic elastomers are not suitable and do not perform well in these ranges of temperatures.
Because of its viscosity, ease of processing, and availability in a range of durometers, LSR offers engineers the added design flexibility to achieve complex part features including tight dimensions, thick and thin part details, and consistency from soft to firm. Because of its ability to fill thin walls and stability in a wide range of temperatures, LSR is the superior material for pressure-sensitive applications, such as thin membranes and gaskets used in gas appliance applications like water heaters or gas heaters, as well as in medical devices used for fluid management, including pumps and surgical devices used in ophthalmology surgeries. LSR is available in a range of durometers (a measurement of stiffness) from very soft to very firm with the most common durometers used are typically between 20 – 80 Shore A.
Silicone rubbers’ chemical properties are also an advantage. LSR has good chemical resistance to a range of cleaning agents, and other solvents, making it ideal for medical and life science applications subjected to regular cleaning with harsh cleaning agents and exposure to other ambient contaminants.
Silicone elastomers are UV-stable, so they resist weathering. Because silicone is impervious to UV rays, it is an excellent choice for products with long-term exposure to the elements. They can withstand harsh outdoor conditions for decades without deteriorating and without requiring the use of special additives. And because they are also ozone-stable they have superior resistance to humidity and moisture.
LSR is the material of choice for products that come in contact with the body. Being biocompatible and hypoallergenic make it the ideal material for the health industry. It can be used for things like respiratory devices and wearable medical devices (glucose monitoring, etc) without causing irritation to the skin.
Other advantages of silicone include:
- High light transmittance
- Ease and clean processing
- Odor-free and tasteless
- Resistant to radiation, UV light and bacteria
- Resistant to other light energy, including VIS, IR, and microwave radiation
- Strong dielectric properties that provide superior insulation
- Water resistance and minimal water uptake
- Minimal flammability
- Pigmentable in an extensive range of colors
The uses of LSR are not limited to only the above mentioned industries or items. Liquid silicone rubber has thousands of applications such as:
- Connector seals across all industries
- Duckbill valves, membranes and diaphragms
- Gaskets and hardware in appliances
- Sealed housings for sensors and other electronics
- Plunger plugs/tips
- Pump housings
- Hearing aid parts/ear tips-buds
- Respiratory masks
- Baby bottle nipples / Pacifiers
Other industries LSR products can serve to include:
- Consumer Electronics
- Mother & Child Care
- Building Technology
- Food & Beverage
When are TPEs the best choice?
From a processing perspective, thermoplastic elastomers offer cost savings to processors and is priced less than silicone elastomers which is also attractive to their customers. TPEs consume less energy during production because the process avoids cross-linking. TPE melts when exposed to higher temperatures, thus has recyclability properties, permitting the reprocessing, if not contaminated.
TPEs offer some of the same benefits as silicone elastomers including vibration resistance, hypoallergenic, and compression set, however with limitations in certain temperature ranges. For this reason, they are not as well-suited for many automotive under-the-hood applications or exterior parts with these operating temperatures.
For some applications, TPE can offer advantages over LSRs and other silicone elastomers. For seals and other permeability-related applications, TPEs provide a strong barrier against oxidation, moisture, and retain pressure and vacuum. The material can also be formulated to be non-tacky, which helps repel dirt and other contaminants making them a good choice for consumer products and other products where aesthetics and surface finish are important.
Additional advantages include:
- Abrasion resistance
- Excellent colorability
- Good electrical properties
- High elasticity
- High fatigue resistance
- High impact strength
- Low compression set
- Low density
- Low specific gravity
- Resilient to chemicals and weathering
- Strong and flexible at room temperature
The main drawback of TPEs are its compromised dimensional stability that can result from incomplete crosslinking. In addition, the use of additives such as plasticizers, anti-oxidants and processing promoters may result in higher contents of leachable and extractable volatiles. If parts will be used in medical devices and life science applications, these levels are usually closely monitored.
Uses of Thermoplastic Elastomer
You can implement TPE in a variety of industries for different applications. Depending on your requirements and environment, uses of TPEs include items such as:
- Baby bottles
- Bottle caps
- Closure liners
- Impact-resistant devices and component housing
- Seal rings
A thermoplastic elastomer is a better alternative than using latex, PVC or rubber — especially in medical and health applications like gloves.
LSR COMPARISON VS. THERMOPLASTIC ELASTOMER
The use of LSR is experiencing rapid growth in medical and healthcare applications because of its ability to withstand sterilization, its micro molding capabilities with miniaturization on the rise, and because it is ideal for human contact products, replacing latex in some instances.
The hardness of LSR ranges from 3 to 80 durometer A, but the most common is 50 durometer A, where it has the best blend of tensile and tear strength. To receive the best sealing properties, many manufacturers utilize liquid silicone rubber with a hardness of 30 durometer A. Low durometer silicone elastomers, known as gels, can reach less than 10 durometer Shore A.
Because you can melt and mold thermoplastic elastomers, you can reuse the resin many times over within an acceptable level before negatively impacting the materials integrity and performance. With the freedom to reprocess and remold a number of times by re-heating, softening and re-hardening, TPE will still retain its compression set properties.
Changing from a liquid to a solid state, elastomers are measured in terms of softness and hardness. You can determine the value using a Shore durometer scale. For example, soft gel TPE materials range from 20 Shore OO to 90 Shore A. As the hardness of a product increases, so does the durometer scale. It starts to reach the Shore D level when hardness values reach up to 85 Shore D, meaning the solid is very hard.
Regarding more elastic materials, hardness ranges from 20 to 95 durometer A. A typical hardness is around 70 durometer A for thermoplastic elastomer products.
The primary difference between molding in silicones and TPEs is that liquid silicone rubber undergoes a chemical reaction involving cross-linking by mixing the two A/B material components (usually liquid), in the injection barrel and applying heat to the mold. For thermoplastic elastomers, the material pellets are melted in the injection barrel then cooled in the mold.
Overmolding and 2-shot molding is also a technology used by both processes. This is where one product is composed of two elastomeric elements or a combination of an elastomer and plastic. TPEs bond to a broad range of plastic materials, and because of its low melting temperature, it can be molded with lower-cost commodity plastics that also have lower melt temperatures. LSRs on the other hand usually use a high processing temperature to ensure fast cycle times. Because of the higher processing temperatures, they are more limited in the materials to which they can bond. They are well-matched with higher performance, engineering thermoplastics like polycarbonates, and polyamides and PBTs preferably glass reinforced. For two-shot LSR molding where both materials are molded in the same process for superior bonding and integration, the materials should have similar melting temperatures.
LSR Injection Molding Process
When producing LSR products using the injection molding process, customized manufacturing cells are used to fit your specific needs. Silicone arrives in drums with equal parts of component A and component B mixed to form the silicone elastomer. Separate containers prevent the chemical reaction from starting. Component A is the catalyst, and B is the cross-link.
The next step is to feed both the A & B components into a barrel in a 1:1 ratio, using metering pumps for accuracy and for minimal contamination. LSR has a consistency similar to peanut butter or honey. The A and B components mix before injecting into the mold, and color can be added before it reaches the static mixer.
During the transfer phase, the silicone is chilled and then inserted into the hot mold. LSR molds at low levels of injection pressures, often below 10,000 psi.
It’s imperative to use high precision molds for liquid silicone rubber to avoid flashing and to apply vacuum for venting as silicone has a low viscosity meaning it flows very fast. Liquid silicone rubber parts shrink more than TPEs, therefore parts with undercuts can be pulled out of the mold without complex, costly mechanical slides.
TPE Injection Molding Process
The injection molding process of TPE is less demanding. Thermoplastic elastomers come in the form of pellets that are added to the injection molding machine via a hopper. Either the material supplier compounds the color into the pellets prior to the process, or the color can be blended at the machine. The pellets flow through a hot barrel and inject into a mold that has temperatures between 70 and 120 degrees F.
In the battle of silicones vs. TPEs, silicones tend to offer higher quality at a higher price. Though it can be more expensive per pound, LSR offers higher product performance than any other rubber on the market. Products like copolyamide elastomers (COPAs) and copolyester elastomers (COPEs) come in a close second and third in quality, with thermoplastic materials coming in in the middle of the pack in both metrics.
Liquid Silicone Rubber Solutions from SIMTEC Silicone Parts
Liquid silicone rubber is our exclusive focus, and we have the experience, knowledge and advanced technology that has been perfected for a consistent stable process. SIMTEC produces custom manufactured LSR parts, LSR multi-shot, and LSR overmolded components. We create high-quality solutions using the unique material characteristics of LSR that make it ideal for a range of applications, environments and industries.
Our SIMTEC experts work with customers one-on-one to convert concepts into a reality. With our proficiency in LSR and LSR multi-shot injection molding, the possibilities are endless.
Our forte is high volume, long term production utilizing high-capacity molds. We have years of experience in processing LSRs, which sets us apart from other manufacturers and a reputation for low parts-per-million defect rates.
The benefits of choosing LSR for your high volume component requirements range from biocompatibility, thermal stability and low compression set to chemical and UV resistance, hypoallergenic and many more. We know every aspect of silicone and how it can benefit your company and your products’ performance.
To learn more about LSR solutions at SIMTEC Silicone Parts, please contact us for more information or to evaluate your current or upcoming project.