Silicone Rubber
Odorless ans tasteless silicone rubber is the most versatile engineering materials, it is widely applied to produce containers, elastic band, waterproofing materials and flexible tubing. Silicone rubber elastomer has unique characteristic of deforming and recovering under load, it can regain its original shape and size after bending, twisting, compressing and stretching. Therefore, its high elasticity is suitable for many component in commercial and industrial applications. In addition, silicone rubber is inert, this is non-reactive and safe for high sensitive applications in various industry.
We apply silicone rubber for any products with requirement of sealing, vibration damping, electrical resistance, thermal insulation. The common applications as following:
- Food & beverage preparation,
- Infant & baby care,
- Diaphragms,
- Personal protection,
- Keypads,
- Safety cable,
- Inter-cooler and radiator seals,
- Connector seals and gaskets,
- High voltage composite insulators,
- Medical devices.
HCR vs LSR
Silicone rubber manufacturing technology apply different forms, compounds and fabrication methods. Once in elastomer material selection, the typical material options are High Consistency Rubber (HCR) and Liquid Silicone Rubber (LSR).
Normally, elastomer products are belonged to two general categories: sealing and non-sealing.
- Sealing application: precisely formed, molded shapes to seal fluids and gases. Like keg seals in food sector.
- Non-sealing application: face mask, flexible hoses, ankle and wrist seals.
In silicone rubber production, we need to obtain more data of silicone materials quality, fabrication technologies, benefits. In order to make the best decision in final manufacturing process.
What is High Consistency Rubber?
HCR epitomizes a series of rubbers, it includes high consistency rubber, heat-cured rubber and high temp vulcanizing (HTV) rubber. These materials contain polymer with high molecular weight and long polymer chains. Therefore, This make them have excellent resiliency and recovery after elongation and compression.
The HCR materials are resistant to heat, cold, or other weather conditions. In addition, they also have electrical attributes, high viscosity and putty consistency. Manufacturers can formulate HCR with additives to modify unique physical properties for specification requirements.
High consistency rubber with correct catalyst process provides raw materials for wide applications’ production. Meanwhile, Its manufacturing methods are molding and calendaring. It is the main materials for extruded tubing.
HCR Manufacturing Process
Solid silicone rubber is produced in large batches, then mix it into homogeneous blend at high temperature. Peroxide catalyst is also added to facilitate the curing process. Therefore, HCR sheets is easy for shipping or storage before the completion of vulcanization process.
Normal HCR manufacturing process includes six steps:
Mill Softening and Catalyst Addition
This is the first step for both extrusion and molding, it reverses any crepe hardening from materials storage and apply milling process to add peroxide catalyst to HCR materials. Normally, we use a two-roll mill to mix HCR elastomer through the mill multiple time, and blend the compound material into homogeneous.
Preform Preparation
Preform preparation is the next step. In the transfer molding process, a die-cutter will cut a plug from the preform to fit the transfer reservoir. In compression molding, personnel must cut the preform into similar shape as final components. In extrusion and injection molding process, the preform is cut into strips and fed into the extruder.
Extrusion
The preformed HCR strips will be feed into the extruder by a roller feed wheel, then HCR materials will be extruded through a die and mandrel assembly. The extruder outfits with cross-head assembly to the desired form. Meanwhile, another support extrusion process requires a cross-head assembly fitted on the extruder, the HCR will pass the supporting geometry through the cross-head and form a layer of silicone rubber.
Molding
Different high-consistency rubber require different molding equipment. In transfer and injection molding process, HCR materials are loaded into equipment by manual, and then removed finished components from molds. In compression molding process, workers need to place performs in each cavity of molds. High consistency rubber has a long cure cycle, so the molds can contain a large quantity of cavities.
Vulcanization
Normally, we apply hot-air vulcanizing ovens (HAVs) to vulcanize the extruded products. In vulcanization process, cross-linking reaction in molecules will increase HCR strength and durability. In addition, steam autoclave and radiant heat are the other two vulcnization methods in some HCR vulcanization processes.
Finishing
The final finishing extent depends on application requirements. For extrusion profiles, we require visual inspection and specified length cutting. For extrusion components with peroxide curing, a post-coring process is required to eliminate peroxide byproducts. Otherwise, trimming or de-flashing is needed for injection molding HCR parts.
HCR manufacturing process has a high labor cost and material waste. It needs additional tools and equipment. However, its equipment cost is cheaper than that of LSR silicone.
What is Liquid Silicone Rubber?
The primary difference between liquid silicone rubber and high consistency rubber is the liquid nature of LSR material. LSR material has very low viscosity, it is easy to cast or injection mold. LSR material consists of Part A and Part B, these two compounds start curing process once mix together. Heat treatment will accelerate the curing process.
Unlike TPE and TPR, LSR has excellent performance characteristics and fast curing cycle. It is a popular material option for small molded components. LSR can maintain its flexibility and elasticity in temperature range from -60℃ to 250℃. LSR is suitable for products with high precision level, including:
- Gaskets
- Seals
- Cushioning pads
- Infant products
- Kitchen goods
- Electrical connectors
- Medial device
In addition, LSR is a common material for over-molding onto other plastics parts.
LSR Molding Process
Meter-mixing
Hydraulic or pneumatic reciprocating pumps can meter-mix two LSR component A/B into a static mixer at 1:1 ratio with accuracy of ±1%. This meter-mixing can guarantee precision of material allocation and reduce material waste. This close meter-mix system can prevent from dust and moisture and keep product clean. In addition, this system also can control color pigment and other additives.
Molding
Normally, Miwo apply high automated and modified LSR injection molding machines in this molding process. Thess machines require little labor in operation. Therefore, the greatest expense in LSR injection molding is mold design and fabrication.
Finishing
LSR processing doesn’t require finishing in most applications, properly injection molds cause minimal flash and eliminate trimming. In reason of addition cure mechanism in LSR injection molding, there is no need of post-curing cycle. In addition, specific fillers can be added in LSR mixing stage, in order to increase final LSR parts properties. Such as, reinforcing fillers can strengthen finished LSR products, it consists of 5 to 20 manometer amorphous-fumed and precipitated silica. Barium sulfate fillers provide a specific performance attribute for medical devices.
Different from other elastomer, liquid silicone rubber only requires few additives. Stabilizers can optimize heat and media resistance. Furthermore, its transparency appearance can be colored as requirement. Manufacturers need to operate carefully once to add nonessential ingredients, in order to avoid compromise of mechanical properties.
Curing
Liquid silicone rubber (LSR) has two major curing mechanic: free radical curing, and addition curing.
Free Radical Curing
Heat initiates peroxide catalysts into two free radicals, these free radicals react with alkyl or vinyl groups on polymer backbone, and transfer free radicals to silicone polymer. Then these free radicals react with alkyl species on another polymer chains. Finally, create the cross-links in different polymer chains.
Addition Curing
Addition curing, or hydrosilylation is a reaction to add a silicon hydride to an unsaturated carbon-bond by a noble metal catalyst. Most metal catalyst apply platinum, some process use rhodium and palladium. Therefore, this silicone polymers must involve vinyl or alkenyl functionality for proper curing.
The materials in addition curing normally including two-part kit, one part contains catalysts, another contains cross-linker and inhibitor. These materials should never expose to peroxide, sulfur, amine, phosphorous, tin complexes. Otherwise, addition curing will be inhibited. This curing technology provides a cure reaction without by-product generation, there is no necessary for post curing. Nonetheless, some specific applications require post cure cycles to stabilize and enhance finished parts properties.
Material Selection Considerations
Once, evaluate the efficacy of High Consistency Rubber vs Liquid Silicone Rubber materials and fabrication. Industry-wide criteria is the base for their applications, such as hardness, tear resistance, thermal resistance, and compression set.
Hardness
The Shore A Hardness Scale is the common measurement of silicone material hardness, softness and flexibility. Silicone rubber hardness range from very soft to hard, this property is important once remove molded parts from molds. Liquid Silicone Rubber (LSR) has hardness from 10 to 80 durometer Shore A. LSR with 10 durometer is soft, gum-like quality. LSR of 80 durometer has a hardness exceeds shoe heel. Most applications hardness range from 20 to 60 durometer Shore A.
Tear Resistance
Tear resistance test is measurement of silicone rubber responds to tearing. The typical tear strength of solid silicone rubber is approximately 9.8 kN/m. We also can improve strength by polymer modification and addition of filler and cross-linker.
Compression Set Resistance
Silicone rubber material has ability to recover from compression deformation, it has consistent compression set in a wide temperature from -60℃ to 250℃.
Elongation
ElonAn elongationgation test is applied to measure the strain point of rupture, this is suitable for products with substantial stretching or sealing across large gaps.
Depending on different applications, there are other tests for silicone properties in applications, such as modulus, thermal resistance, abrasion resistance, toxicity and flame resistance, permeability, electrical testing. Finally, the specification and performance criteria for applications determine the minimal features for materials and manufacturing process.
Select Silicone Rubber for Your Project
Silicone rubber materials is widely applied in manufacturing and commercial, particularly in automotive, electronica, medical industry. Furthermore, advances in material technologies and manufacturing process expend its food and water applications for life-enhancing and critical-care applications.
The cost of LSR injection molding will increase dramatically in tooling and equipment, especially in design and development process. In comparing HCR vs LSR molding methodologies, we should notice that liquid injection molding requires more expertise than typical plastic fabrication.
In conclusion, the material choice and fabrication process for High Consistency Rubber vs Liquid Silicone Rubber depend on customers’ preference and products requirements. If you want to find out more about LSR and the injection molding process, contact us for your next project.
