Silicone rubber is a thermoset with a backbone of alternating silicone and oxygen atoms with methyl or vinyl side groups. It has earned a reputation as the best-in-class elastomer, especially for extreme environments. In addition, it is one of the most permeable elastomers.
There are two types of silicone rubber — solid and liquid. Their fundamental structure is similar, but curing and processing are radically different. Here, we present a detailed review of two silicone rubbers, highlighting their similarities, differences, applications, and how to select the best material for your unique needs.
Applications — Liquid Silicone Rubber
LSR is excellent for making products that require a high level of precision and quality. These products include:
- Seals
- Gaskets
- Diaphragms and delicate membranes
- Cushioning pads in portable communications
- Data acquisition instruments
- Electronic devices used in rugged environments
- Infant bottle nipples and pacifiers
- Electrical connectors
- Wearables for health monitoring and drug delivery
- Medical applications, such as septum, actuators, and dosing valves
In addition, LSR has become a common material used to over-mold onto other parts manufactured from various plastics.
Applications — High-Consistency Rubber
High-consistency rubber undergoes catalyzation processes to the raw material that result in a broad range of silicone rubber products. The most common types of HCR require finishing by molding and calendaring. It’s also the material most commonly used for compression molding and extruded tubing. Medical device manufacturers use HCR to make implantable shunts, pacemaker lead coverings, pump diaphragms, and catheters.
LSR vs. HCR
The noteworthy difference between LSR and HCR is the “flow” or “liquid” nature of these silicone rubber materials — in other words, their viscosity. LSRs have a lower viscosity, meaning they flow more readily and are easy to process via injection molding. The automated injection molding process makes LSR preferable for high production volumes.
HCR’s high viscosity makes it tricky to handle. It requires labor-intensive processing methods, such as transfer and compression molding, for limited production volumes. Nonetheless for some applications where durability and toughness are important, HCR offers advantages and offers versatility over LSR rubber.
The costs involved in the liquid injection molding process add up quickly due to the investment in tooling and equipment, especially during the design and development process. When comparing HCR and LSR injection molding methodologies, note that though the liquid injection molding process involves fewer steps, all aspects of this fabrication method require expertise beyond typical plastic manufacturing.
The secret to a successful project involves identifying and collaborating with a manufacturing supplier that possesses the required means and capabilities. Such a manufacturing partner will guide you from the design and material selection process to full production.
What Is HCR?
Heat-cured rubber, high-temperature vulcanizing, or high-consistency rubber contain polymers with a high molecular weight and long polymer chains. HCR’s properties epitomize those types of rubber, such as resiliency and recovery after elongation or compression. Manufacturers produce it in large batches with the material components mixed at high temperatures using peroxide catalysts.
Characteristics of HCR
HCR is a solid, gum-like silicone elastomer with elastic mechanical vulcanization. It has tenacious resistance to heat, cold, and other extreme weather conditions, with excellent electrical attributes. Fabricators routinely formulate HCR to meet unique specifications and requirements by milling the base with additives, which modify the material’s physical properties.
HCR Fabrication Process
Manufacturers produce high-consistency rubber by mixing the components at high temperatures with a peroxide group that facilitates curing. During the reaction, the vinyl double bond is reactive, and the peroxide group produces an oxygen-free radical. The vinyl group forms a reactive-free radical, and the free radical forms a bridge by attaching to another polymer.
Fabricators allow the molecules to begin cross-linking but roll out the rubber sheets for storage or shipping before the vulcanization process completes. Then, workers mix the sheets with rollers and process them by transfer molding. The post-vulcanization occurs in an oven.
The actual rubber manufacturing process entails six steps.
- Mill softening and catalyzation: This first step, required for extrusion and molding applications, reverses any “crepe hardening” that results from the material’s storage at the fabricator or supplier. In addition, some processes use milling to add peroxide catalysts to a free radical elastomer or mix the two components of an additional cure elastomer before processing. This labor-intensive process requires a two-roll mill, which the elastomer must cycle through multiple times to blend the compound material into a homogeneous product. The sheet may need additional processing after its removal from the mold.
- Preparing a preform: The next step is also laborious, particularly in high-volume production environments that operate 24/7. The transfer molding process requires a die cutter to cut a plug from the preform that fits in the transfer press. The compression molding application is more complex because personnel must cut the preform into the same shape as the final component. For the extrusion and injection molding applications, workers prepare the preform by cutting the elastomer sheet into strips and feeding them into the molding equipment.
- Extrusion: During the HCR extrusion press, the operator feeds preformed strips into a single-screw extruder via a roller feed wheel. The elastomer material extrudes through a die-and-mandrel assembly, which shapes the desired profile by outfitting the extruder with a crosshead assembly. Another processing option, the “support extrude,” necessitates fitting a crosshead assembly onto the extruder. The crosshead passes the supporting geometry through it and extrudes a layer of silicone rubber over it.
- Molding: Different high-consistency silicone applications require the use of various molding equipment. Transfer and injection molding fabrication need people to load the elastomer into the equipment and de-mold the finished components. In the compression molding process, workers must place preforms in each mold cavity. Because high-consistency rubber has slow cure cycles, the molds may have multiple cavities.
- Vulcanization: Fabricators use hot-air vulcanizing ovens to vulcanize extruded products. This process increases the material’s durability by cross-linking the molecules. The vertical oven has a variable-speed drum at the top that pulls the extruded profile up through the oven. Some vulcanization processes use a steam autoclave oven or radiant heat.
- Finishing: The extent of this step depends on the application. For extrusion profiles, the finishing process involves a visual inspection and cutting the tubes into specified lengths. For extrusion components cured with peroxide, the profile must go through a post-cured process to remove peroxide byproducts. The injection molding process also requires trimming or de-flashing of molded parts. The worker uses a die-cutting tool to cut the parts from the large, molded sheet. In some cases, the component may require post-cure.
Unfortunately, high-consistency rubber fabrication wastes material and results in high labor costs. It also requires manual labor and extra tools and equipment. However, the equipment is less expensive than the molds used to manufacture injection molded LSR products.
What Is LSR?
LSR consists of low-viscosity liquid silicone rubber processed through injection molding or cast into sheets. The raw uncured LSR material comprises two separate liquid components. Component A contains a platinum catalyst, and Component B contains methyl hydrogen siloxane as a cross-linker and an alcohol inhibitor.
When these components combine during fabrication, it initiates the curing process. Then, adding heat accelerates curing. During the processing, the mixing is precise, and no posturing is necessary. All curing occurs in the mold, and the parts obtained are repeatable. The elastomer’s long chains chemically cross-link during vulcanization.
Characteristics of Liquid Silicone Rubber
LSR, unlike many thermoplastic elastomers such as TPEs and TPRs (thermoplastic rubber), has impressive performance characteristics and fast cure cycles that make it the preferred material for small, intricate molded components. The material maintains its elasticity and flexibility down to temperatures of minus 60 degrees Celsius and retains these qualities up to 250 degrees Celsius.
LSR Processing
LSR processing typically involves combining the two components. The reaction starts with the reactive vinyl double bond — the platinum center has one free coordination site. The interaction with the platinum center activates the double bond, and the vinyl group cross-links by transforming the double bond. This reaction creates a single bond to a polymer chain — in this case, to a cross-linked molecule containing Si-H groups. The catalyst becomes free and is again available for further cross-linking.
LSR processing requires two steps.
- Meter mixing: Using hydraulic or pneumatic-driven reciprocating pumps, the meter-mixing delivers the two-part mixture at a 1-to-1 ratio into a static mixer with an accuracy of ±1%. The precision of the material allocation mitigates the occurrence of off-tolerance products and reduces material waste. The closed meter-mix system keeps the product clean because the mixer seals out dust and moisture. The system also allows for the controlled addition of pigments and other additives.
- Molding: The second stage of the liquid injection process requires highly automated machines specifically designed for LSR molding. Once operable, the machines require very little labor. The most substantial capital expense incurred in the liquid silicone rubber injection molding process involves the cost of the mold design and fabrication.
For most applications, LSR processing does not require finishing. Properly tooled injection molds cause minimal flash, eliminating the need for trimming. The additional cure mechanism used in liquid injection molding makes a post-cure cycle unnecessary.
Silicone Cure Mechanisms
Silicone rubber products have two primary cure mechanics — free radical and addition.
Free Radical Cure
Adding heat to silicone rubber products initiates the deposition of peroxide catalysts into two free radicals. The free radicals react with either an alkyl or a vinyl species along the polymer backbone, which transfers the free radicals to the silicone polymer. The reaction of the free radical on the polymer chain with an alkyl species on another polymer chain terminates the cross-linking mechanism.
Addition Cure
Sufficient for LSR and HCR, hydrosilylation, or addition cure, involves adding a silicon hydride to an unsaturated carbon bond in the presence of a noble metal catalyst. Often, hydrosilylation catalysts use platinum, but some processes employ palladium and rhodium. The silicone polymers in the elastomers must include alkenyl or vinyl functionality for the material to cure properly.
Materials that depend on the addition cure mechanism come in two-part kits. One part contains the catalyst species, and the other contains a silicon hydride functional cross-linker and an inhibitor. The inhibitor allows for working time after mixing the two ingredients. The additional cure offers the advantage of producing a cure reaction that generates no byproducts, eliminating the need for post-curing.
Nonetheless, some applications require a post-cure cycle to stabilize or enhance the properties of finished parts. The manufacturer must take exceptional care to avoid exposing the material to peroxide, sulfur, phosphorous, amines, tin complexes, peroxides, and peroxide byproducts, which can inhibit the addition cure regimen.
New, low-volatile silicone rubber specialized grades are available to eliminate the need for post-curing. These new grades may be appropriate for some applications.
Considerations for Material Selection
Silicone rubber elastomers are also available as sheets with a predetermined thickness or rolls of strip-formed material divided into various thicknesses and widths and ready for feeding into the injection molding machine and extrusions.
Decision-makers must consider the virtues of HCR vs. LSR materials and the fabrication processes in choosing the material best for their applications based on industry-wide criteria. Below, we discuss the most critical properties to consider when selecting silicone rubber material.
Hardness
The Shore A Hardness Scale measures the hardness of materials, as well as the flexibility and softness of moldable silicone rubbers. These silicone materials range from very soft and flexible to medium and somewhat flexible to hard with almost no flexibility at all.
The Shore A Hardness scale ranges from 10 to 100, with most applications falling within 20 to 80. An LSR with a 10 durometer has a gum-like quality and feels softer than some sponge products or extra-firm silicone foam. Contrastingly, an LSR material with a Shore A durometer of 80 has a hardness exceeding that of a shoe heel.
Tear Resistance
A tear resistance test measures how silicone rubber materials respond to tearing, calculated by dividing the breaking force in pounds by the cross-section of the outstretched specimen in square inches.
Solid silicone rubber typically has a tear resistance of around 9.8kN/m. Generally, large, molded products, reverse tapered forms, and intricately shaped objects use high-strength materials with a tear strength between 29.4 kN/m and 49.0 kN/m. Manufacturers improve strength through polymer modification and the liberal use of fillers and cross-linkers.
Compressions Set Resistance
The compression set resistance is a measurement that determines silicone rubber’s ability to recover from compression deformation. Silicone rubber has a consistent compression set resistance spanning a broad range of temperatures, from minus 60 degrees Celsius to 250 degrees Celsius.
Elongation
For products that require substantial stretching or seals to operate across large gaps, the fabricator will need to conduct elongation testing — a measurement of the strain at the point of rupture. Depending on the application, manufacturers may perform other tests, such as thermal resistance, toxicity, modulus, permeability, flame resistance, abrasion resistance, electrical testing, and environmental testing.
Ultimately, the specifications and performance criteria for the finished application determine the minimal features required for the materials and fabrication process.
For example, an electronic parts manufacturer may choose general-purpose silicone rubber because it has good mechanical properties, a hardness in the 30 to 80 durometer range, tear strength of 8, and compression set resistance between 12 to 26.
A specialty material such as low-compression set silicone rubber that also meets other application requirements may be more appropriate for O-rings and gaskets used in the automotive and aerospace industries. However, low-hardness silicone rubber is better for sensing membranes or baby care items.
Biocompatibility
Biocompatibility refers to a material’s ability to interact with living systems or tissue without toxicity or causing an immunological rejection. Silicone rubber materials are biocompatible because they’re nontoxic, stable, and insoluble in bodily fluids. LSR is more widely used for biomedical applications because it has a more regular network of cross-links, high gas permeability, and is easy to mold.
Choose Silicone Rubber for Your Commercial and Manufacturing Projects
The manufacturing and commercial sectors have used silicone rubber for many years, particularly the medical, automotive, industrial, and consumer electronics industries. Advances in material technologies and the fabrication process have extended its use for various life-enhancing and critical-care applications.
The material choices and manufacturing processes for high-consistency rubber versus liquid silicone rubber depend on the requirements of the product or device. If you have additional questions or want to learn more about the LSR injection molding process, contact an expert with multidisciplinary experience who can help you find the ideal solution for your project.
SIMTEC is a reliable silicone rubber manufacturer that has produced billions of LSR parts since the inception of our company in 2002. We produce parts for the leading manufacturers in nearly every industry, and we look forward to finding the best value-added custom solution that will enhance the quality and performance of your products, streamline your process, and meet your program objectives.