What is RTV Silicone Rubber?
RTV Silicone is a liquid silicone that cures at room temperature to form a flexible rubber. The term RTV means Room Temperature Vulcanising.
RTV silicone is available in two forms: as a one-component product or as a two-component (a base and curative). Manufacturers offer it in a range of hardness options, typically from very soft to medium, with a hardness measurement of 15 to 40 Shore A. To initiate the curing process of RTV silicone, manufacturers provide a catalyst that can consist of either platinum or a tin compound like dibutyltin dilaurate.
The applications of RTV silicones are diverse. They are used for low-temperature over-molding, enabling the creation of protective and insulating layers. RTV silicones are also utilized in the production of molds for reproducing objects, and certain optically clear grades find application in lens manufacturing. Moreover, they are widely employed in the automotive industry as an adhesive and sealant
Chemistry of RTV Silicone
RTV silicones are created by combining silicone polymers, fillers, and organoreactive silane catalysts. These silicones are composed of a silicon-oxygen (Si-O) bond but can have a wide variety of side chains. The production of silicone polymers often involves the reaction between dimethyl dichlorosilane and water.
The addition of acetic acid as a filler can accelerate the curing process, leading to a faster cure time. On the other hand, oxides and nitrides are utilized as fillers to enhance the thermal conductivity of the silicone. The tack-free time refers to the time required for the silicone to become non-sticky, typically ranging from a few minutes. The complete curing process usually takes several hours.
There are two kinds of RTV: one-component formulations (RTV-1), and two-component formulations (RTV-2).
One-Component Formulations (RTV-1)
One-part silicones make use of moisture in the atmosphere to cure from the outside toward the center. The time required for complete curing is influenced by factors such as temperature, humidity, and the surface area-to-volume ratio.
RTV-1 is supplied and ready to use. Various types of cross-linking agents are employed in RTV-1 silicones. Some of these agents release small amounts of acetic acid, amines, or alcohol during the setting process, while others are odorless.
RTV-1 silicones are highly regarded for their ease of use, good adhesion, and stability at high temperatures They use as sealants, bonding glues, and coatings in various industries and settings.
Two-component formulations (RTV-2).
Two-part silicones employ moisture from the second component, along with a cross-linker such as active alkoxy, to initiate the curing process. RTV-2 comes in two parts that have to be mixed just before they are used. It cures faster than RTV-1 once mixed.
RTV-2 silicone offers a wider array of product options, boasting exceptional mechanical properties over a broad temperature range.
Condensation-cured RTV-2 silicone tends to be more cost-effective than the platinum-cured variant. However, platinum-cured silicone holds the advantage of producing no byproducts during the curing process, which is particularly important for certain applications.
Features of RTV Silicone Rubber
Easy Operation: RTV silicone rubber offers excellent characteristics that make it easy to handle and work with. Its user-friendly nature simplifies the application process, ensuring convenience and efficiency.
Light Viscosity and Good Flow-ability: RTV silicone rubber exhibits a light viscosity, allowing it to flow smoothly and evenly into molds or other desired shapes. It possesses good flowability, ensuring uniform distribution and filling of intricate details.
Low Shrinkage: This means that the material maintains its original dimensions, resulting in the accurate and faithful reproduction of molds or objects.
Favorable Tension: RTV silicone rubber possesses favorable tension properties, enabling it to withstand stretching or pulling forces without losing its shape or structural integrity. This characteristic is particularly beneficial when durability and resilience are required.
No Deformation: RTV silicone rubber resists deformation, retaining its shape and structure even under external pressures. It maintains its desired form, preventing any unwanted distortions or alterations.
Favorable Hardness: RTV silicone rubber exhibits a favorable hardness level, striking a balance between flexibility and rigidity. This characteristic ensures optimal durability and stability for various applications.
High-Temperature Resistance, Acid and Alkali Resistance, and Ageing Resistance: RTV silicone rubber demonstrates exceptional resistance to high temperatures, making it suitable for applications involving heat exposure. It also exhibits excellent resistance to acids and alkalis, ensuring its durability and reliability in corrosive environments.
Molding Processes For RTV Silicone Rubber
The choice of molding processing method depends on the type of part, grade, and type of curing.
Injection Molding
Description: Injection molding is suitable for complex products. The material is forced into the mold’s hot cavity using a ram or plunger. The part is formed when the two halves of the mold close and the material solidifies.
Material: Low viscosity and short cure time.
Variables: Pressure, temperature, viscosity, shot volume (amount of material injected into the cavity).
Advantages: High productivity, automated process, low cost per part, short cycle times, minimal flash (excess material), and tight dimensional tolerances.
Disadvantages: Material used in the runners (waste), and defects such as warpage, bubbles, and sinks can occur, making it more suitable for parts with thin walls.
Applications: Injection molding is commonly used for small parts and thin-walled components, such as automotive body panels.
Gravity Casting:
Description: Gravity casting utilizes the force of gravity to fill the mold cavity.
Material: Medium viscosity and long cure time.
Variables: Viscosity of the material.
Advantages: Gravity casting is a low-cost process, and the equipment required is also inexpensive compared to other methods.
Disadvantages: Gravity casting has long cycle times, and it is not suitable for producing thin-walled products.
Applications: Gravity casting is commonly used for scale models and dental plates.
Vacuum Casting:
Description: In vacuum casting, the mold includes a jacket that creates a vacuum. The air is pulled out from the mold and allowing the silicone rubber to fill even small and intricate spaces
Material: Vacuum casting typically uses low-viscosity materials with a medium cure time.
Variables: The viscosity of the material and the level of vacuum applied.
Advantages: Vacuum casting is particularly advantageous for creating parts with thin walls and small details. The vacuum assists in drawing the material into complex shapes and intricate features.
Disadvantages: One drawback of vacuum casting is the relatively high implementation costs compared to other molding methods. The equipment required to create and maintain a vacuum adds to the overall expenses.
Applications: Vacuum casting is commonly used for producing cases and housings
Thin Wall Reaction Injection Molding (RIM)
Description: Thin wall reaction injection molding (RIM) is a process similar to injection molding. In RIM, a vacuum is used to mix the silicone rubber before it is injected into the mold cavity.
Material: RIM typically uses low-viscosity materials with a short cure time.
Variables: The variables in RIM include viscosity, temperature, and pressure during the mixing and injection process.
Advantages: RIM offers close tolerances, fast production times, and is suitable for producing large parts.
Disadvantages: The equipment required for RIM can be expensive, making it economically viable primarily for large production runs.
Applications: Thin wall RIM is commonly used for manufacturing bumpers, cases
Advantages and disadvantages
RTV silicone rubber offers outstanding release properties in comparison to mold rubbers, making it highly advantageous for the production casting of resins such as polyurethane, polyester, and epoxy. One significant benefit is that it eliminates the need for a release agent, thus eliminating the requirement for post-production cleanup. Additionally, silicones demonstrate excellent chemical resistance and can withstand high temperatures, including temperatures of 205 °C (400 °F) and even higher. As a result, silicone molds are well-suited for casting low-melt metals and alloys like zinc, tin, pewter, and Wood’s metal.
The disadvantage of RTV silicone is its higher cost compared to other materials. Additionally, it has a tendency to attract dirt and residue. Due to the curing process that relies on air moisture, it requires rapid application, which can be challenging. Moreover, RTV silicone is thick and porous, necessitating degassing with a vacuum to eliminate air bubbles that may impact its application.
Applications
RTV silicone rubber is highly versatile in casting and mold-making processes, allowing for the reproduction of intricate details. It finds extensive applications in both industrial and artistic fields, including prototyping, furniture production, sculpture, and architectural elements
RTV silicone rubber is capable of casting various materials such as wax, gypsum, low-melting alloys/metals, urethane, epoxy, or polyester resins,
A recent advancement in this field is the ability to 3D print RTV silicones, opening up new possibilities for complex and customized designs
Industrial sectors such as aviation, aerospace, consumer electronics, and microelectronics also benefit from the use of RTV silicones. In aviation and aerospace, they are utilized for cockpit instruments, engine electronics potting, and engine gaskets. The exceptional mechanical and thermal stress resistance of RTV silicones makes them suitable for these demanding applications.
For more information on how using RTV silicone can help you and your business, contact A1 Silicones or give us a call and speak to one of our professional representatives at 0969.965.008