Silicone foam molding is a good way to make silicone foam.Silicone foam permeability, sound absorption, buffer, excellent sealing and other functions are the reasons for its application in many fields. This article describes the production process, process advantages, comparison with other processes, and the application of silicone foam from several aspects. If you want to customize silicone foam, this article can provide you considerable assistance.
What is silicone foam molding？
Silicone foam molding is a process that uses molds to simultaneously complete foaming expansion and vulcanization crosslinking of a silicone rubber mixture containing a foaming agent. The main raw materials of this process are liquid silicone rubber(LSR) and high-consistency silicone rubber(HCR).
By adding foaming agents, tiny bubbles will form inside the compound and expand at a specific temperature, eventually resulting in a lightweight and flexible material. The final product is widely applied in multiple fields such as medical care, environmental protection, and building materials.
Equipment for Silicone Foam Molding
Compression Molding Machine
This is the traditional and most widely used equipment.And the key equipment is the platen vulcanizing press. This type of equipment is suitable for medium and small batch production or components that are relatively thick in size or not too complex in structure.
Directly place the pre-weighed compound into the heated mold cavity. After the mold is closed, the hydraulic cylinder will apply pressure to the mold and keep it. The compound will complete foaming and vulcanization under the set temperature, pressure and time.
The platen vulcanizing press can provide precisely controlled temperature and molding pressure for foaming vulcanization. This type of equipment requires relatively low investment, has a relatively simple mold structure, and is more adaptable.
Injection Molding Machine
The commonly used equipment in the silicone molding production process is the silicone injection molding machine. The equipment is suitable for large-scale, structurally complex, precise or thin-walled silicone foam products, with a high degree of automation.
The silicone injection molding machine is composed of an injection unit, a clamping unit and a temperature control system. Some will have vacuum systems. The injection unit includes the barrel, screw, etc. It is mainly responsible for the transportation, metering and injection of the compound. The clamping unit provides a huge clamping force to prevent the mold from being stretched apart by the injection pressure. Although a vacuum system is not necessary, it can greatly reduce defects such as bubbles and material shortages. It is indispensable for high-quality foam products.
Silicone injection molding machines can quickly inject compounds and precisely control the injection volume of the compound, which is conducive to uniform foaming, reduces material waste and guarantees the consistency of product weight. The equipment can achieve a fully automatic production process, with high production efficiency and high dimensional accuracy of the products.
Mold
Molds are the core of process equipment. Molds are usually made of mold steel and aluminum alloy materials. The former is usually more wear-resistant and has a longer service life. The latter has a faster heat conduction speed, a shorter production cycle and a lower production cost. The quality of molds can directly determine the quality of products and production efficiency. The surface treatment method will also directly determine the surface effect of the product.
Silicone Foam Molding Process
Mold Design and Manufacturing
Firstly, before production, a mold that matches the shape and size of your customized product needs to be made. You can choose the appropriate mold material according to the characteristics of the products you produce.
After the mold is made, it needs to be closed and heated to the initial temperature set by the product production process. Subsequently, evenly spray a small amount of water-based or solvent-based mold release agent on the surface of the preheated mold cavity. The mold release agent will form a release film on the surface of the cavity.
Compound Mixing and Weight
According to the design formula of the products you produce, uniformly mix the base polymer, reinforcing fillers, foaming agents, vulcanizing agents and various additives. Ultimately, a compound with uniform composition and good processing performance will be formed. Well-mixed compounds are the material basis that determines the final performance of the product.
The mixed compound needs to be precisely weighed according to the volume of the cavity and the density of the customized product. The compound needs to be performed into a simple shape similar to the mold cavity. This is conducive to the compound being placed into the mold cavity and promotes the flow and exhaust of the compound.
Material Loading
Place the preformed compound into the lower mold cavity. It should be noted that the compound needs to fully fill the cavity but avoid overfilling to prevent possible defects in the product or problems such as silicone overflow.
Vulcanization and Foaming
After the compound feeding is completed, start the equipment to quickly close the mold. After the mold is closed, the equipment will apply initial pressure, making the compound to soften and start flowing during the heating process, filling the bottom of the mold cavity.
The temperature of the mold will make the compound gradually reach the decomposition temperature of the foaming agent.When the temperature reaches, the compound will start to produce gas inside. When the temperature continues to rise to the activation temperature of the vulcanizing agent, the silicone will gradually change from a viscous flow state to a solid elastomer.
When the foaming process is basically completed, the silicone has initially taken on a cell structure but has not yet fully stabilized. At this point, the equipment needs to apply full pressure to solidify the foam structure, prevent foam from merging or collapsing, and guarantee the compactness and clear contour of the product. Additionally, the mold also needs to be kept at the set temperature and time for a period of time to guarantee full vulcanization.
Demolding
After the foamed silicone is completely cured, there needs to be sufficient time to cool the mold. After the cooling process is completed, open the mold and remove the product from it.
Surface Treatment
At this stage, it is necessary to adopt appropriate trimming methods to remove the excess material or flash on the product. If you have cleaning needs, the manufacturer will clean the product surface as required. If you have any special requirements for product texture processing, this step is also necessary.
Quality Control
The final process at the end of production requires a comprehensive quality inspection of the products. First, quality inspection personnel will conduct visual inspections and use professional quality inspection equipment to examine the size, appearance, hardness and density of the products.
In response to the usage requirements of the product, quality inspectors will conduct targeted tests on your product, such as tensile strength, elongation, compression set rate, resilience and flame retardancy, etc.
Packaging and Delivery
Products that pass the quality inspection and are finished in appearance should be packaged according to your packing requirements for convenient transportation and distribution. Appropriate packaging materials can prevent products from being damaged during transportation.
Benefits of Silicone Foam Molding
Foam-like Structure
Silicone foam molding produces silicone products with unique honeycomb structure bubbles. This type of product has good air permeability and sound absorption, and also features excellent buffering and compressibility. These characteristics make foamed silicone have a wider range of applications.
Excellent Insulation and Sealing Properties
The irregular surface of silicone foam can prevent the infiltration of air and water, thus having excellent sealing performance and avoiding contamination of the protected object.
Various Durometers Options
Silicone foam molding can provide you with silicone foam of all the hardness you need, whether soft, strong or flexible, all of these can be achieved. Customized production of foamed silicone can meet your unique application requirements and offer you flexible choices with different cushioning forces and comfort levels.
Consistency and Uniformity
Modern molding equipment and stable formulas can guarantee a high degree of consistency in performance and size of products during large-scale production. Moreover, the controllability of foam structure and density is also a reason for the high consistency of the product.
Aging Resistance
Anti-aging is the fundamental reason why silicone foam distinguishes itself from the vast majority of extruded foams.
Silicone can keep its elasticity and physical properties for a long time within an extreme temperature range, and can even withstand temperatures above 300℃ in the short term.And it at low temperature does not brittle or crack, at high temperature it does not soften or decompose. This is also the reason why it can be applied in the aerospace field.
Secondly, silicone has extremely strong resistance to ozone, ultraviolet rays, radiation and other substances. This is undoubtedly the reason why silicone foam products have a long service life.What’s more, silicone foam products have good resistance to solvents, oils, etc. This also makes it perform well in chemical and medical disinfection environments.
Safety and Environmental Friendliness
Silicone foam products with flame retardants added can easily meet the highest flame retardant standards. Even the white smoke produced during combustion has a very low density and releases very little toxic gas.
During the regeneration and use of foamed silicone, no harmful substances are released, which meet the increasingly strict environmental protection production standards nowadays. Even when applied in the high-standard medical industry, medical foamed silicone itself is non-toxic, odorless and does not cause allergies. Besides, it can withstand high-pressure and high-temperature sterilization. Therefore, foamed silicone products are also ideal materials for manufacturing high-quality medical gaskets and breathing mask seals.
Factors Affecting Silicone Foam Molding
Temperature
Temperature directly affects the two core chemical reaction processes of foaming and vulcanization in the silicone foaming molding process.
The decomposition temperature of the foaming agent is divided into two stages, namely the initial decomposition temperature and the peak decomposition temperature. Different vulcanizing agents also have their specific activation temperatures. Precise temperature control can make sure that the foaming agent decomposes vigorously earlier than the vulcanization reaction, thus allowing the production of products that have sufficient foaming, appropriate density, and meet the softness standards.
Besides, uneven mold temperature can also lead to differences in the foaming process and product density. Uneven vulcanization or deformation of the product is also a possible phenomenon. Therefore, the uniform heat distribution of the platen vulcanizing press and the injection machine is very crucial.
Time
The vulcanization time is related to whether the foaming reaction is sufficient. However, the foaming reaction also requires a comprehensive consideration of both time and temperature.
The actual vulcanization time required for silicone foam is longer than solid compounds of the same thickness.
If the vulcanized time is too long, it will lead to a decrease in product elasticity and an increase in product odor. Insufficient time can lead to substandard physical and mechanical properties of the product, unstable foam structure and other phenomena.
Therefore, precise time-temperature control is a necessary condition for obtaining high-performance products.
Pressure
It should be noted that the pressure is not constant during the silicone foam molding process. Pressure needs to be adjusted over time, which is also a key condition for shaping the ideal foam structure.
At the initial stage of mold closing, the pressure is usually kept at a relatively low level, which provides sufficient space for gas expansion and cell growth. When foaming is nearly completed but vulcanization is not yet complete, the equipment needs to apply rapid pressure to stabilize the cell structure. The products produced in this way have a dense and smooth surface, without bubbles or roughness. And the size of the product will also be more precise.
Unstable cell structure, excessively high open cell rate, loose product surface and excessive flash may all be caused by insufficient pressure. On the contrary, excessive pressure will instead inhibit foaming and increase the density of the product. The products produced in this way are extremely inconsistent with your application requirements.
Mold Status
Molds usually transfer the pressure and temperature of the equipment to the material. Therefore, the state of the mold will also directly affect the characteristics of the product.
The surface finish of the mold directly affects demolding and the appearance of the product. Molds with high surface finish are conducive to demolding, and the surface finish of the product is high. Molds with specific textures will also replicate the textures on the product surface. If there is dirt or damage inside the mold, it may cause difficulty in demolding. In severe cases, it can scratch the surface of the product and contaminate it.
The material of the mold will also have a certain impact on the molding of silicone foam. Aluminum molds usually conduct heat faster and have a shorter heating cycle. Steel molds are stronger, more wear-resistant and have a longer service life. The former is more suitable for small-batch production or product sampling, while the latter is more suitable for large-batch production.
A well-designed mold exhaust system can expel air, moisture and by-product gases generated during foaming from the cavity. Poor exhaust may cause local bubbles in the product, coking or thick flash.
Applications of Silicone Foam Molding
Automotive Parts
The application of silicone foam in this field mainly meets the requirements of high-temperature resistance, oil resistance, shock absorption and sound insulation. For instance, engine hood gaskets are typically used for heat insulation, noise reduction and fire prevention. It can also be used as a buffer pad between battery modules. Or the sealing strips for car doors and windows.
Electrical Insulation
The core value of silicone foam in this field lies in flame retardancy, safety and thermal management. For example, insulating sheaths or spacer pads for high-voltage lines. It can provide excellent electrical insulation and flame retardancy. At the same time, it can buffer mechanical stress. Silicone foam can also be used as a thermal interface material or an electromagnetic shielding pad.
Comfort Products
The biocompatibility, soft adhesion and disinfecting property of silicone foam can meet the requirements of comfort products and core applications. For instance, some helmet linings, padding for key parts of sports protective gear, pressure-relieving layers at puncture points, and so on. Silicone foam can buffer impact energy and provide protection for users.
Building Seals and Gaskets
Silicone foam gaskets and silicone foam seals can be used as building sealing materials or sealing strips for fireproof doors and windows. Besides, industrial high-temperature ovens also need to use silicone foam gaskets, which can provide long-term high-temperature resistant sealing, preventing heat loss and energy waste.
Silicone Foam Molding vs Other Molding Process
Silicone Foam Molding vs Compression Molding
Material: Silicone compression molding mainly involves uniformly mixing base polymers, reinforcing fillers, vulcanizing agents, then achieving complete and dense cross-linking under heating. Foam molding is a chemical reaction in which a foaming agent is added to the compression molding formula, and the material will eventually generate gas and complete cross-linking.
Pressure Control: In the production process of compression molding, the compound is placed in a heated mold to soften it and allow it to flow and fill in the mold cavity, while simultaneously triggering a vulcanization reaction. Throughout the entire production process, the temperature and pressure of the equipment remain relatively stable. However, the pressure control in foaming molding is not always kept at a stable level but is perfectly matched with the vulcanization rate to manage the foaming process.
Properties: Silicone compression products are usually solid and have a high density. This type of product typically features high strength, high elongation, excellent permanent deformation resistance, and outstanding anti-permeation and pressure-resistant sealing capabilities. Foamed silicone products, such as foamed sponges, usually have open-cell and closed-cell structures inside and have a low density. Foamed sponge is light in weight, soft, highly compressible, highly resilient, and has excellent shock absorption, buffering, heat insulation and sound insulation properties.
Silicone Foam Molding vs Extrusion Molding
Material: Both can produce foamed silicone using formulas containing foaming agents. However, the foaming process of foaming molding is completed within the mold cavity. Precisely controlled pressure and temperature are used to shape the cell structure. Foaming and vulcanization can almost be completed simultaneously in the mold. The foaming process of extrusion molding usually occurs at the initial stage when the compound leaves the die and enters the vulcanization channel. Moreover, extrusion foaming has extremely high requirements for the decomposition temperature of the foaming agent, extrusion speed and vulcanization temperature.
Product Features: The products made from silicone foam are mostly discrete and three-dimensional complex parts. For example, silicone foam gasket, silicone foam sponge and gaskets with three-dimensional structure. The products produced by silicone extrusion are mostly continuous linear profiles with constant two-dimensional cross-sections. For example, hoses, sealing strips, O-ring cord, etc.
Production Efficiency: The silicone foam molding process is intermittent production. In other words, each product has a separate cycle period. The thickness of the product and the vulcanization time are the main factors affecting efficiency. Silicone extrusion is usually carried out continuously. Once the equipment is started, it can operate around the clock without interruption, achieving extremely high production efficiency.
Silicone Foam Molding vs Injection Molding
Material: The material used in injection molding is liquid silicone rubber. It usually adopts a platinum addition curing system. The production process does not foam, and the products are solid. The material used in silicone foam molding is high consistency silicone rubber.
Production Process: Injection molding typically involves high-speed, high-pressure, and precisely measured filling. The compound is injected into the heated mold at an extremely fast speed. The injection pressure is high and stable. The foaming reaction and the curing reaction of foam molding are controlled in a coordinated manner. While filling the cavity, the compound generates internal cells. Moreover, the pressure of the production equipment is also changing dynamically.
Product Features: Injection product size precision is particularly high. It can shape thin walls, fine texture and structure. The products usually have no flash and excellent physical and mechanical properties. The dimensional accuracy of silicone foam is relatively low due to the influence of foaming shrinkage. The fine details of a product are usually difficult to form. The products are lightweight, soft, highly elastic, compressible, and capable of absorbing energy, buffering, insulating and soundproofing.
FAQ
Is Silicone foam molding product resistant to permanent compression set?
Yes. The compressive deformation resistance of silicone mainly benefits from the flexibility and thermal stability of silicone. Its resistance to compression is superior to that of other organic foams, especially over a wide temperature range and after long-term aging.
Can the product meet food contact or medical standards?
Yes. However, silicone foam products must meet food contact and medical standards through dedicated formulas, strict production processes and authoritative certifications. The raw materials must be silicone and additives that comply with FDA or other certifications. Clean production processes, biocompatibility tests and appropriate vulcanization systems are also necessary.
How to control the shrinkage rate of silicone foam during the production process?
Precise formula design, pre-mold compensation, stable process control and adequate vulcanization can keep the shrinkage rate of silicone foam within a predictable range. Systematic materials engineering and process control, and precise compensation during the mold design stage, can ultimately achieve the dimensional accuracy of the product.
Can the surface texture of the product be customized in a personalized way?
Sure. By altering the surface treatment effect of the mold cavity, various personalized textures can be precisely replicated on the product surface. However, custom textures can affect the touch, gloss and demolding difficulty of the product.When you customize the product, you need to communicate with the manufacturer about the production details for confirmation.
High consistency silicone rubber foam or liquid silicone rubber foam, which one is better?
Which raw material to choose mainly depends on your core needs. HCR has a wider operating temperature range and lower material costs, making it suitable for manufacturing thicker or larger-sized foam components. Liquid silicone foam features excellent biocompatibility and higher dimensional accuracy and detail restoration capabilities. It features a faster production cycle and purer products. LSR is usually the first choice for medical equipment, baby care, and precision electronic components.
How to decide whether to choose an open cell foam or a closed cell foam for molding?
Open-cell foam has good air permeability, absorption capacity and resilience. Closed-cell foam has good waterproof sealing and heat insulation properties, but its compression set capacity is relatively low. Therefore, if you need breathable products, choose open-cell foam. If you need to seal the new product, choose closed-cell foam.
Final Thought
Silicone foam molding technique is versatile and it contains many advantages that other molding techniques do not have. As a professional silicone manufacturer, Legensilico has a significant advantage in the production of silicone foam. If you want to customize the application of silicone foam, please feel free to contact us to get your own silicone foam customization solution.
