In the 1960s, the production of synthetic varieties of rubber surpassed natural rubber output. Today, synthetic rubbers comprise about two-thirds of the world’s rubber supply. Among the different categories of manufacturing materials, odorless and tasteless silicone rubber may be one of the most versatile engineering products available. Silicone rubber surpassed its image long ago as a material primarily used to make containers, waterproofing materials, elastic bands, and flexible tubing.
The unique characteristics of silicone rubber elastomers perform very differently from metals and plastics, especially the manner in which they deform and recover under load. Many components and products used in commercial and industrial applications require a high degree of elasticity. This ability of the material to regain its original shape and size after episodes of stretching, bending, twisting, and compressing, under an array of conditions makes it extremely attractive to manufacturers, regardless of the industry.
Silicone rubber has excellent resiliency. An attribute of inertness, resiliency is the measure of an item’s ability to snap back into place quickly. This characteristic of being non-reactive and safe for highly sensitive applications appeals to not only the foodstuff and medical industries, but to the electrical, aerospace, automotive, machinery, paper, furniture, and construction sectors.
Nearly every product imaginable seems to require some degree of sealing, shock absorption, vibration damping, electrical resistance, thermal insulation, and other properties commonly associated with silicone rubbers. Common applications include the following:
- Food Preparation and Dispensing
- Infant and Baby Care
- Personal Protection and Masks
- Headlamps and Tail lamps
- Data Entry Keypads
- Safety Cables
- Fuel Connectors
- Engine Mounts
- General Wire and Cable
- Intercooler and Radiator Seals
- Connector Seals and Gaskets
- High Voltage Composite Insulators
Medical applications may require a special medical grade rubber.
SEALING AND NON-SEALING APPLICATIONS
The types of products made from elastomers fit under two general categories: sealing and non-sealing applications.
Sealing applications refer to precisely formed, molded, or machined shapes that seal fluids and gases, such as the cylinders and pistons used in the automobile industry or the keg seals and bottle lifters prevalent in the food sector.
Non-sealing applications include face masks, nasal units, ankle and wrist seals, transmissions, flexible hoses, and drive systems.
HIGH CONSISTENCY RUBBER VS LIQUID SILICONE RUBBER
The silicone rubber manufacturing process uses a variety of forms, compounds, and fabricating methods. When choosing elastomers, the material options are typically narrowed down to High-Consistency Rubber (HCR) and Liquid Silicone Rubber (LSR).
For companies looking to manufacture a product, or enter a new market, the due diligence process for High Consistency Rubber vs Liquid Silicone Rubber can present a challenge. Designers, engineers, and managers will need to gather and evaluate data in order to make informed decisions about the type of silicone rubber they will use to produce their part. This decision will determine equipment and floor space requirements, as well as labor costs for the device manufacturing process.
Decision-makers will need to obtain as much data as possible about silicone rubber elastomers, their qualities, fabrication technologies, and benefits.
This guide will help your evaluation process as it relates to material options and fabrication methodologies.
WHAT IS HIGH CONSISTENCY RUBBER?
Heat-cured rubber, high temp vulcanizing (HTV), or high consistency rubber contains polymers with a high molecular weight and long polymer chains. The properties of HCR epitomize those types of rubbers, such as resiliency and recovery after elongation or compression.
HCR has excellent resistance to heat, cold, and other extreme weather conditions, and also has excellent electrical attributes. Fabricators routinely formulate HCR to unique specifications requirements by milling the base with additives, which modify the material’s physical properties. High consistency rubber has high viscosity and the consistency of putty.
High consistency rubber silicone that goes through the correct catalyzation process provides the raw materials that can be used to produce a broad range of applications that require finishing by molding and calendaring (sheets of varying thicknesses). HCR is the main material used for extruded tubing.
THE HCR FABRICATION PROCESS
Rubber manufacturers make solid silicone rubber in large batches, with the components mixed into a homogenous blend at high temperatures. The addition of peroxide catalyst facilitates the curing process. Fabricators allow the cross-linking of the molecules to begin but roll out the sheets of rubber materials for shipping or storage prior to the completion of the vulcanization process.
The actual rubber manufacturing process entails six steps:
Mill Softening and Catalyzation
This first step, required for both extrusion and molding applications, reverses any “crepe hardening” that results from the storage of the material at the supplier or fabricator. In addition, some processes use milling to add peroxide catalysts to a free radical elastomer, when needed, or to mix the two components of an addition cure elastomer prior to processing. This labor-intensive process requires the use of a two-roll mill, as the elastomer must cycle through the mill multiple times to blend the compound material into a homogeneous product. After the removal of the sheet from the mold, it may require additional processing.
Preparation of a Preform
The next step involves preparation of a preform, which requires a lot of labor, 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 reservoir of the transfer press. The compression molding application entails a more complex process because personnel must cut the preform into the same shape as the final component. For the extrusion and injection molding applications, the preform is prepared by cutting the elastomer sheet into strips and feeding them into the extruder.
The extrusion press for HCR involves the use of a single-screw extruder. The operator feeds preformed strips into to an 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, called the “support extrude,” requires the use of a crosshead assembly fitted onto the extruder, which passes the supporting geometry through the crosshead and extrudes a layer of silicone rubber over it.
Different high-consistency silicone applications require different molding equipment. Both transfer and injection molding fabrication requires personnel to load the elastomer into the equipment and to de-mold the finished components. In the compression molding process, workers must place preforms in each cavity of each mold. High-consistency rubber has slow cure cycles, which means the molds may have a large number of cavities.
Fabricators use hot-air vulcanizing ovens (HAVs) to achieve vulcanization of extruded products. This process increases the strength and durability of the material by cross-linking the molecules. Available in both vertical and horizontal models, 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.
The extent of finishing 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, which eliminates peroxide by-products. 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.
The high-consistency rubber fabrication wastes material and has a high labor cost. It also requires additional tools and equipment. However, the cost of the equipment is less expensive than the molds required in the manufacturing of LSR silicone products.
What Is Liquid Silicone Rubber?
The primary differentiator between liquid rubber and high consistency rubber is the “flow” or “liquid” nature of LSR materials. While HCR has a peroxide cure, LSR has addition/cure/platinum cure. LSR consists of liquid silicone material that has a very low viscosity, which allows the material to be cast into rubber sheets or injection molded. Raw uncured material for LSR consists of Part A and Part B, liquid components that the fabricator mixes, which starts the curing process. The application of heat accelerates the curing process.
Unlike many thermoplastic elastomers such as TPEs and TPRs, LSR has performance characteristics and fast cure cycles, which makes it a popular material for small molded components. The material maintains its flexibility and elasticity down to temperatures of -60°C and retains these qualities up to 250°C. LSR often finds application in products that require a high-precision level, including the following:
- Cushioning pads in portable communications
- Data acquisition instruments
- Electronic devices used in rugged environments
- Infant products (bottle nipples)
- Kitchen goods (spatulas)
- Electrical connectors
- Medical applications (hoses)
In addition, Liquid Silicone Rubber has become a common material used to over-mold onto other parts manufactured from a variety of plastics.
Utilizing hydraulic or pneumatic-driven reciprocating pumps, the meter-mixing delivers the two-part mixture at a 1:1 ratio, into a static mixer, within an accuracy of ±1%. The precision of the material allocation prevents off tolerance products and reduces material waste. The closed meter-mix system also keeps the product clean because the mixer seals the mix from dust and moisture. The system also allows for the controlled addition of pigments and other additives.
The second stage of the liquid injection process requires the use of highly automated and modified LSR injection molding machines. Once operable, the machines require very little labor. The greatest 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, which eliminates the need for trimming. The addition cure mechanism, used in liquid injection molding, makes a post-cure cycle unnecessary.
Some manufacturers add fillers during the mixing stage, which add additional qualities to the material. For example, reinforcing fillers strengthen the finished product. Fillers with particle sizes of 5 to 20 nanometers in diameter consist primarily of amorphous-fumed silicas and some precipitated silicas. Extending fillers provide a specific performance attribute, such as barium sulfate filler, which is used to create certain products for the medical industry.
Unlike other elastomers, silicone rubber requires few additives. Some fabricators add stabilizers, which optimize attributes, such as heat and media resistance. Silicone rubber has a transparent appearance, but fabricators can color the material as required. While some compounds deliver the highest value in terms of performance, manufacturers need to operate carefully when adding nonessential ingredients in order to avoid compromise of mechanical attributes and other properties.
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 vs LSR injection molding methodologies, note that although the liquid injection molding process involves fewer steps, all aspects of this fabrication method require expertise that goes beyond typical plastic manufacturing.
The key to a successful project involves identifying and collaborating with a supplier that has the required means and capabilities, a supplier that will guide you from the design and material selection process to full production.
SILICONE CURE MECHANISMS
Silicone rubber products have two major cure mechanics: free radical cure and addition cure.
FREE RADICAL CURE
The addition of heat initiates the deposition of peroxide catalysts into two free radicals. These free radicals react with the either an alkyl or a vinyl species along the polymer backbone, which transfer 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.
Sufficient for both 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. Most hydrosilylation catalysts use platinum, but some processes employ rhodium and palladium. The silicone polymers in the elastomers must include vinyl or alkenyl 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 component contains a silicon hydridefuntional cross-linker, as well as an inhibitor. The inhibitor allows for working time after mixing the two ingredients.
Addition cure offers the advantage of producing a cure reaction that generates no by-products, which means that post curing is not necessary. Nonetheless, some applications require a post cure cycle to stabilize or enhance the properties of finished parts. The manufacturer must take great care to ensure that the material is not exposed to peroxide, sulfur, phosphorous, amines, tin complexes, peroxides, and peroxide by-products, which can inhibit the addition cure regiment.
CONSIDERATIONS FOR MATERIAL SELECTION
The elastomer material is available as a sheet that has 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 evaluate the efficacies of High Consistency Rubber vs Liquid Silicone Rubber materials and fabrication for their applications based on industry-wide criteria such as hardness, tear resistance, thermal resistance, and compression set.
The Shore A Hardness Scale measures the hardness of materials, as well as the softness and flexibility of moldable rubbers. Rubbers can range from a hardness of very soft and flexible, to medium and somewhat flexible, to hard with almost no flexibility at all. This attribute is important to consider when it comes to removing the component from the rubber mold.
LSR has a hardness range of 10 to 80 durometer Shore A. LSR with a 10 durometer has a soft, gum-like quality and feels softer than some sponge products or certain extra firm silicone foam. At the other end of the spectrum, LSR material with a durometer of 80 has a hardness that exceeds that of shoe heel. The hardness of most applications ranges from 20 to 60 durometer Shore A.
A tear resistance test provides a measure of how the silicone rubber responds to tearing. It is calculated by dividing the breaking force in pounds by the cross-section of the outstretched specimen in square inches. The typical tear strength of solid silicone rubber is around 9.8kN/m. For large molded products, reverse tapered forms, and complexly-shaped objects, use high-strength materials with a tear strength of 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.
Compression Set Resistance
This measurement determines the ability of silicone rubber material to recover from compression deformation. Silicone rubber has a consistent compression set over a broad range of temperatures, from -60C to +250 C.
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 modulus, thermal resistance, toxicity and flame resistance, permeability, abrasion resistance, electrical testing, and environmental testing.
Ultimately, the specifications and performance criteria for the finished application determines the minimal features required for the materials and fabrication process.
For example, a manufacturer of electronic parts 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 a compression set resistance between 12 to 26.
An O-rings and gaskets supplier to the automotive and aerospace industries may select a specialty material such as low compression set silicone rubber, which also meets other application requirements. However, the manufacturer of swimming caps or baby care items may choose a low hardness silicone rubber for its applications.
CHOOSE SILICONE RUBBER FOR YOUR PROJECT
Silicone rubber has been employed for many years in the manufacturing and commercial sectors, particularly for defense, electronics, and automotive. Advances in material technologies and the fabrication process have extended its use for food and water applications and for a variety of life-enhancing and critical-care applications.
The material choices and manufacturing processes for High Consistency Rubber vs Liquid Silicone Rubber depend on the fabricator’s preference and the requirements of the product or device. If you have additional questions or want to find out more about LSR and the injection molding process, contact an expert with multi-disciplinary experience who can help you find the right solution for your project.
Trust SIMTEC Silicone Parts
SIMTEC has produced millions of LSR parts since the inception of our company in 2002. We produce parts for clients in nearly every industry, and we look forward to helping you streamline your process by finding the right solution for your needs.