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The words “thermoplastic” and “thermoset” sound similar but are not synonymous. LSRs are thermosets, while TPEs are thermoplastics. Each has unique material properties and behaves inversely during the process of injection molding service.

Thermoplastic elastomers and liquid silicone rubber have properties identical to synthetic or natural rubber and are advantageous alternatives. Both materials are suitable for a wide range of applications in injection molding companies.

However, there are major variations between thermoplastic (thermoplastic elastomer) and thermoset properties (silicone elastomer).

WHAT IS LSR?

Silicones are manufactured from quartz sand, a practically infinite supply of raw material. Liquid silicone rubber is a synthetic resin formed by the chemical bonding of polymers. Heat induces polymer cross-linking, which results in a chemical bond that gives the material permanent strength and shape after curing.

As heat is applied to LSR, it hardens, which is the inverse of TPE. As LSR is heated, it vulcanizes and locks in the physical properties, first during the molding process and then during the post-cure process (i.e., tensile strength, flexural strength, and heat distortion temperature).

One of the main reasons for post-curing (heating the molded parts in an oven) is to accelerate vulcanization, which maximizes some of the material’s physical properties and pushes away volatile situations.

Platinum-cured LSR is a two-part formula that must be blended before entering the injection mold. One factor serves as a trigger, causing the cross-linking process to begin. During molding service, rubber undergoes chemical cross-linking, which is known as curing or vulcanization. The vulcanization process can take anywhere from seconds to several hours to complete.

Silicone has alternating oxygen and silicon atoms in its composition and can be used in various ways, including LSR, heat-cured rubber, and room temperature vulcanized rubber.

A thermoplastic elastomer, also known as thermoplastic rubber, is a polymer mix or compound that melts and forms into plastic when heated. It hardens as it cools, but the chemistry remains constant from one form to the next.

A thermoplastic is a synthetic polymer substance that is typically derived from fossils. Bio-based TPE solutions extracted from corn, sugar cane, beet, soya beans, cellulose, or vegetable oil, as well as plants or algae, are being introduced.

As temperatures exceed their melting point, manufacturers shape and fabricate TPE. Since it has elasticity comparable to cross-linked materials such as LSR, its elastomeric behavior is not due to the cross-linking properties.

Chemical bonding does not occur in thermoplastic elastomers. The heat causes the plastic pellets to liquefy. They are then pressure-molded into different components that reinforce and maintain their shape until curing. TPE solutions are classified into seven types:

• Styrenic block copolymer
• Melt processable rubber
• Thermoplastic olefinic elastomer
• Thermoplastic polyester elastomer
• Thermoplastic vulcanizate
• Thermoplastic amide elastomer
• Thermoplastic polyurethane elastomer

In short, TPE is a class of copolymers, often made with a mix of plastic and rubber.

Both LSRs and TPEs have advantages, and the right material to choose by the injection molding companies will rely on the product, budget, and, most importantly, the necessary efficiency.

WHEN IS LSR THE BEST POSSIBLE OPTION?

Temperature, compression set, and heat resistance are all important factors to remember when selecting silicones or thermoplastic elastomers by the injection molding companies. Natural rubber, nitrile rubber, ethylene polypropylene diene rubber (EPDM), and polyacrylic elastomer have lower working temperature levels. 

Natural rubber, for example, has a temperature range of -50 to less than 100 degrees Celsius, whereas nitrile rubber has a temperature range of -25 to 150 degrees Celsius.

As thermoplastic elastomers are compared to silicone elastomers, LSR provides consistent performance over a broad temperature spectrum and outperforms TPEs on both ends of the temperature scale – high and low in the injection molding service.

LSR can run in temperatures ranging from 350 to 400 degrees Fahrenheit (175 to 205 degrees Celsius) without losing shape. The material and its properties remain stable over a wide spectrum of extreme temperatures, whether hot or cold.

It, for example, retains its stability and elastomeric properties (compression set) even at temperatures as low as -100 degrees Fahrenheit. When compressed, it returns to its original shape when stretched.

These characteristics are ideal in the injection molding companies of the automotive sector, especially for parts used on the exterior of automobiles – from sensors to connectors, seals, and gaskets used under the hood. 

LSR is also used in interior car parts, such as vibration dampening cushions in HVAC systems and gaskets and keypads in key FOBs. Unlike LSRs, thermoplastic elastomers are not ideal and perform poorly in these temperature ranges.

Because of its viscosity, ease of handling, and availability in various durometers, LSR allows engineers of injection molding companies to achieve complicated component features such as tight dimensions, thick and thin part details, and consistency from soft to firm.

LSR is the superior material for pressure-sensitive applications in the injection molding companies such as thin membranes and gaskets used in gas appliance applications such as water heaters or gas heaters, as well as medical equipment used for fluid handlings, such as pumps and surgical devices used in ophthalmology operations, due to its ability to fill thin walls and flexibility in a broad range of temperatures. 

LSR is available in various durometers (a scale of stiffness) ranging from very soft to very rigid. The most commonly used durometers are usually ranging from 20 to 80 Shore A.

The chemical properties of silicone rubbers are also advantageous. LSR has strong chemical resistance to a variety of cleaning agents and other solvents, making it suitable for medical and life science applications that require regular cleaning with harsh cleaning agents and exposure to other atmospheric pollutants.

Since silicone elastomers are UV-stable, they are resistant to weathering. Since silicone is impervious to UV radiation, it is ideal for products exposed to the elements for an extended time. 

They can survive harsh outdoor temperatures for decades without deterioration or the use of special additives. They are even more resistant to humidity and moisture because they are ozone-stable.

LSR is the preferred material by injection molding companies for products that come into contact with the body. Its biocompatibility and hypoallergenic properties make it a perfect commodity for the health industry. Injection molding companies can use it for respiratory devices and portable medical devices (glucose monitoring, for example) without inducing skin irritation.

• Clarity
• 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 in injection molding companies are not limited to only the industries mentioned above 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
• Bellows
• Septa
• 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:

• Appliances
• Consumer Electronics
• Sanitary
• Mother & Child Care
• Building Technology
• Food & Beverage

SEE ALSO :

• Dual-shot vs Rubber Injection Molding vs Thermoforming – How to Choose?

• Types of Injection Molding Processes & Equipment Maintenance

WHEN SHOULD TPEs BE USED?

From a logistics standpoint, thermoplastic elastomers provide cost advantages to processors and are less expensive than silicone elastomers, appealing to their consumers. Since the method prevents cross-linking, TPEs use less energy during processing. 

TPE melts at higher temperatures and hence has recyclability properties, allowing for reprocessing if not contaminated.

TPEs have some of the same advantages as silicone elastomers, such as vibration tolerance, hypoallergenicity, and compression set, but they are limited in some temperature ranges.

As a result, they are not well-suited for certain automotive under-the-hood applications or exterior components with these operating temperatures.

TPE has benefits over LSRs and other silicone elastomers in some applications. TPEs provide good protection against oxidation, moisture, and pressure, and vacuum retention in seals and other permeability-related applications. 

The material can also be designed to be non-tacky, which aids in the repellence of dirt and other contaminants, making it an excellent alternative for consumer goods and other products where aesthetics and surface finish are significant.

Additional advantages include:

• Abrasion resistance
• Excellent colorability
• Good electrical properties
• Heat-sealable
• 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 major disadvantage of TPEs is their reduced dimensional stability, which can be caused by inadequate cross-linking. Furthermore, additives such as plasticizers, antioxidants, and processing promoters can result in higher levels of leachable and extractable volatiles. 

These thresholds are normally carefully watched whether the components may be used in medical equipment or life science applications.

TPE may be used in several industries for a variety of purposes. TPEs may be used for a variety of purposes, depending on the needs and the environment.

• Baby bottles
• Bottle caps
• Closure liners
• Impact-resistant devices and component housing
• Seal rings
• Seals

A thermoplastic elastomer is a better choice than latex, PVC, or rubber, especially in medical and health applications such as gloves.

COMPARISON OF LSR AND THERMOPLASTIC ELASTOMER

Because of its capacity to survive sterilization, its micro-molding capability with miniaturization is on the rise. Because it is suitable for human contact products, replacing latex in certain cases, LSR sees exponential growth in medical and healthcare applications.

LSR has a hardness range of 3 to 80 durometer A, but the most typical is 50 durometer A, which has the highest combination of tensile and tear strength. Most manufacturers use liquid silicone rubber with a hardness of 30 durometer A to achieve the best sealing properties. Low durometer silicone elastomers, also known as gels, may have a Shore A of less than 10.

Since thermoplastic elastomers can be melted and molded, the resin can be reused several times until adversely affecting the material’s integrity and efficiency. TPE retains its compressive strength, including the need to reprocess and remold it several times by re-heating, softening, and re-hardening.

Elastomers are measured in terms of softness and hardness as they transition from a liquid to a solid-state. A Shore durometer scale can be used to determine the value. Soft gel TPE materials, for example, vary from 20 Shore OO to 90 Shore A. 

If a product’s hardness increases, so does its durometer scale. It begins to reach the Shore D mark as hardness values reach 85 Shore D, indicating that the solid is extremely hard.

The hardness of more elastic materials ranges from 20 to 95 durometer A. The average hardness of thermoplastic elastomer products is about 70 durometer A.

The primary distinction between silicone and TPE injection molding service is that liquid silicone rubber undergoes a chemical reaction involving cross-linking by combining the two A/B material components (usually liquid) in the injection barrel and heating the mold. 

The material pellets for thermoplastic elastomers are melted in the injection barrel and then cooled in the mold.

Both processes make use of overmolding and the technology of a 2-shot molding service. This is where a product comprises two elastomeric components or a mixture of an elastomer and a plastic.

TPEs bond to a wide variety of plastic materials, and because of their low melting temperature, they can be molded from lower-cost bulk plastics with lower melt temperatures. LSRs, on the other hand, are usually processed at a high temperature to ensure short cycle times. 

Because of the higher processing temperatures, the materials to which they can bind are more constrained. They go well with higher performance manufacturing thermoplastics such as polycarbonates, polyamides, and PBTs that are ideally glass reinforced. 

For two-shot LSR molding, where both components are molded in the same phase for superior bonding and incorporation, the melting temperatures should be identical.

Customized manufacturing cells are used to meet the unique needs when making LSR products through injection molding service. Silicone is delivered in drums containing equal parts component A and component B, combined to form the silicone elastomer. 

The chemical reaction is prevented from continuing due to the use of separate tanks. The catalyst is component A, and the cross-link is component B.

The A and B components are then fed into a barrel in a 1:1 ratio using metering pumps for precision and minimum pollution. LSR has the consistency of peanut butter or honey. Before pouring the A and B materials into the container, they combine, and color can be applied before reaching the temperatures.

The silicone is cooled during the transition process before being poured into the hot mold. LSR molds at low injection pressures, sometimes less than 10,000 psi.

Since silicone has a low viscosity and flows quickly, it is important to use high precision molds for liquid silicone rubber to prevent flickering and apply vacuum for venting. Since liquid silicone rubber parts shrink faster than TPEs, parts with undercuts can be taken out of the mold without using complicated, expensive mechanical slides.

TPE injection molding service is a less demanding operation. Pellets of thermoplastic elastomers are fed into the injection molding system through a hopper. The color can be mixed at the machine or compounded into pellets before the operation by the material supplier. 

The pellets pass through a heated barrel before being injected into a mold of temperatures ranging from 70 to 120 degrees Fahrenheit.

When it comes to silicones vs. TPEs for injection molding service, silicones appear to have better efficiency at a higher price. After being more expensive per pound, LSR provides superior product performance over any other rubber on the market.

Copolyamide elastomers (COPAs) and copolyester elastomers (COPEs) rank second and third in quality, respectively, with thermoplastic products ranking in the center of the pack in both measures.