Liquid silicone rubber molding
April 18, 2024Silicone Rubber Injection Molding
Silicone rubber molding is a technique for shaping and creating silicone rubber components and products with the aid of a heated mold. This process entails compressing or injecting silicone rubber into a mold, allowing it to adopt the shape and structure of the mold cavity.
The methods employed to manufacture silicone rubber molded parts include injection molding, compression molding, and transfer molding, all of which utilize molds made from steel or cast iron.
Liquid Silicone Rubber Injection (LSR)
Injection molding represents the predominant method employed for the fabrication of silicone rubber components, which can differ significantly in terms of size, complexity, and intended application.
This technique utilizes a screw-type plunger akin to that found in an extruder. In a manner similar to the extruder process, the raw silicone material is introduced into a hopper that is connected to the barrel and screw mechanism.As the material progresses through the barrel, it is subjected to heating, ultimately reaching a precisely sealed die that is configured to match the desired shape of the final product.
1. Process
The distinction between injection molding and liquid injection molding lies in the mechanical amalgamation of the silicone rubber material prior to its injection into the mold, a process facilitated by two plungers.
One contains the base material, which is enhanced with additives and fibers, while the second plunger holds the catalyst compound that initiates the mixing process.
The benefits of liquid injection molding include the capability to manufacture intricate and complex designs with exceptionally precise tolerances, high-volume production efficiency, the potential for automated process design, and the application of thermal imaging technology to mitigate molding errors.
Generally, the complete injection molding procedure requires a duration of two to twenty minutes. This cycle is comprised of four distinct phases: clamping, injection, cooling, and ejection.
Silicone rubber is different from organic rubbers because it is a synthetic elastomer made from a crosslinked polymer, which is reinforced with silica.
This molding process is often used to produce seals, gaskets, buttons, medical devices, kitchenware, and other products that require high-temperature resistance, flexibility, and non-reactivity.
The characteristics and applications of silicone rubbers are diverse, including thermal conductivity, fire resistance, chemical stability, flame retardancy, as well as resistance to heat and cold.
These can be modified by altering the base polymer and incorporating reinforcing fillers and additives aimed to enhance the properties of silicone rubber to fulfill specific application requirements.
The categorization of silicone rubbers is determined by their distinct properties:
- General Purpose Silicone Rubber
- High Hardness Silicone Rubber
- Low Hardness Silicone Rubber
- High Strength Silicone Rubber
- Transparent Silicone Rubber
- Flame Retardant Silicone Rubber
- Thermal Conductive Silicone Rubber
- Electrically Conductive Silicone Rubber
- Heat Resistant Silicone Rubber
In this method, a pre-measured amount of silicone rubber compound is placed directly into the mold cavity. The mold is then closed, and pressure is applied, forcing the material to fill the mold cavity. The mold is typically heated, which causes the silicone to cure and take the shape of the cavity. After curing, the mold is opened, and the part is removed.
Similar to compression molding, transfer molding involves loading the silicone material into a chamber outside the mold cavity. Once the mold is closed, pressure is applied, and the material is transferred through a sprue into the mold cavity where it cures.
Liquid silicone rubber (LSR) is often used for injection molding. LSR is a two-component material, where the A and B components are kept separate until just before entering the injection barrel. These components are mixed, heated, and then injected into the mold cavity. Heat within the mold causes the silicone to cure rapidly, and the part can then be ejected. This method is especially suitable for high-volume production of precision parts.
This involves molding silicone rubber over a different material, typically a plastic or metal substrate. This process can create parts with combined properties, such as a rigid component with a soft-touch exterior.
During the pressure maintenance phase of the injection molding process, the silicone undergoes cooling, which results in expedited solidification, an increase in melt viscosity, and considerable resistance within the cavity.
As the pressure continues, the material density progressively increases, and silicone products are now also manufactured through injection molding techniques.
An advanced cooling system can substantially decrease the molding duration, improve injection output, and reduce costs, given that the cooling phase constitutes approximately 70% to 80% of the total molding cycle.
Ejector pins, commonly utilized in conjunction with thermoplastic molds, are not required in the process of liquid silicone rubber molding. Upon the completion of the molding cycle and the subsequent opening of the mold, the entire product remains attached to one half of the mold.
2. Advantages
It is possible to adopt multiple materials and colour and employ the Insert Mold method to craft a plastic part.
The selected color is “over-molded” onto the transparent silicone inserts, creating a seamless connection between the insert and the over-mold.
This process eliminates the necessity for the customer to design specialized elements to improve the usability of a product.
For example, the glass inserts positioned behind the silicone keypad can serve as display windows to showcase components or digital panels. A leak-proof effect or component can be achieved due to the cross-link bond established through the over-molding of silicone inserts.
Silicone rubber can withstand high temperatures without degrading, making it suitable for applications requiring heat resistance. This contributes significantly to its widespread application in industrial equipment as gaskets, seals, and plugs.
LSR possesses the ability to endure both elevated and reduced temperature fluctuations without alterations to its inherent properties.
For extended durations, silicone rubber retains its structural integrity at 200°C (392°F) and can withstand temperatures exceeding 350°C (662°F) for brief intervals.
Upon combustion, it generates minimal black smoke and does not emit harmful gases.
A material characterized as chemically or physiologically inert does not engage in reactions when in contact with other substances, which pertains to the configuration of electrons in the outermost shell of the material.
This distinctive characteristic of certain materials is designated as group 0 due to their lack of reactivity. Silicone rubber is categorized within this group owing to its inertness in relation to other materials.
Silicone rubber remains unchanged in its properties upon exposure to oils, solvents, and various other chemicals. When immersed in water, it preserves its mechanical strength and electrical characteristics.
Certain medical-grade silicones are biocompatible, making them ideal for medical devices and implants.
The primary advantage of silicone rubber lies in its versatility. By modifying the constituents utilized in its production or altering the curing agent, silicone rubber can exhibit a reduced compression set and enhanced resistance to flames, water, steam, and fluctuations in temperature.
It is available in both translucent and colored forms, and it is a dependable, reliable, and durable material that maintains its properties for decades.
Silicone parts are flexible, resistant to compression set, and have excellent longevity.
Silicone provides the capability to achieve tighter tolerances in part geometries than those attainable with thermoplastic elastomers.
Liquid silicone rubber (LSR) enables the creation of precise features, varying wall thicknesses, a diminished risk of sink marks, as well as the incorporation of small undercuts and negative drafts.
Certain silicone formulations can be clear, which can be useful for applications like lenses or medical devices.
Silicones can be excellent insulators, making them valuable in electronics and electrical applications.
3. Challenges
Silicone rubber can be more expensive than some other materials.
Some silicone parts may require a post-curing process to achieve their final properties or to ensure removal of any uncured components, especially in medical applications.
Silicone’s inherent stickiness can sometimes make mold release challenging. Special mold treatments or release agents may be necessary.
The manufacturing process for liquid silicone rubber necessitates the use of a specialized silicone metering and dosing machine, as opposed to a conventional material auto-feeder.
The silicone metering and dosing apparatus is characterized by its precision and is significantly more expensive than a plastic feeder.
The molding process for liquid silicone rubber requires that the raw material be maintained at a cooled temperature prior to its injection into the mold.
The injection molding process for liquid silicone rubber also necessitates the use of a specialized auto shut-off nozzle to prevent any leakage of the raw material.
4. Applications
A wide variety of LSR materials for various applications and industries is available.
Popular options include standard silicones along with medical-grade and fuel-resistant grade materials.
LSR molding is a great for gaskets, cushioning pads in portable communications, ruggedized electronic devices, and many more applications.
Silicone exhibits remarkable resilience to environmental influences and maintains its aesthetic appeal throughout the lifespan of the device.
It does not diminish in transparency over time or due to exposure to ultraviolet light, moisture, or elevated temperatures.
Additionally, it demonstrates a strong resistance to scratches, fractures, and breakage.
- Wide range of applications from -50 to +250 °C (specialized variants from -110 to +300 °C)
- Elevated flash point and favorable fire performance
- Excellent electrical insulating properties
- Outstanding resistance to weathering and aging
- High dimensional stability even under prolonged stress (sealing applications)
- Superior biocompatibility, along with neutrality in odor and taste
Due to the characteristics of LSR as a thermosetting polymer, its molded configuration is irreversible. In contrast to thermoplastics, it cannot be remelted.
Silicone rubber is frequently employed as an insulator for wires and cables, as well as for connector seals and switch boots.
Its exceptional resistance to high temperatures and its capacity to endure extreme environmental conditions render it an ideal material for insulation purposes.
Silicone rubber insulation retains its flexibility, waterproof characteristics, and resistance to arcing and cracking, even after prolonged usage over several years. The robustness and durability of silicone rubber insulation are instrumental in safeguarding the most sensitive electronic components.
Hoses, gaskets, seals, and various other components of a vehicle are vital for ensuring the safe and efficient operation of the vehicle.
These components must withstand the rigorous conditions they encounter during the functioning of the engine. Consequently, silicone rubber is the material of choice for manufacturing the most critical engine components.
It possesses the requisite strength, toughness, and impact resistance, making it a long-lasting solution for automotive applications.
Silicone rubber is utilized in the manufacturing of food-related items and cookware owing to its non-toxic and food-safe properties.
As a culinary material, it meets the criteria established by the Food and Drug Administration (FDA) and can be produced in visually appealing hues. Silicone rubber is extensively employed in bakeware, as it does not leach substances that could alter the flavor or aroma of the food being prepared.
Summary
In summary, silicone rubber molding is a versatile process that caters to the unique properties of silicone rubber.
It allows for the production of parts that can withstand harsh conditions, remain flexible over time, and be safe for various applications, including those in the medical field.
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