Silicone Calendering

Silicone Calendering

Silicone calendered products have been applied in various fields, like textile,automobile,industry,aerospace and more. As a silicone molding process, it can produce various products with composite functions. This article explains silicone calendering from definition, production process, factors influencing production, advantages and applications.If this process will serve your custom product, this article will certainly serve you well.

What is silicone calendering?

Silicone calendering is an indispensable step in the production and manufacturing process of silicone. It mainly continuously compresses the silicone raw materials into uniform silicone sheets through the steel rollers inside the calender. The rollers inside the calender determine the final thickness and mechanical properties of the product.

Different calendering speeds and pressures will produce products of different thicknesses and textures. The pressed products can also be embossed on different substrates, such as plastic, peel, fabric, etc. The final surface treatment effect and texture of the product can be transformed in different styles.

The Basic for Silicone Calendering Process

Composition of Silicone

The performance of silicone mainly relies on a complex production formula system. The following are the main components of silicone:

Base Polymer

Base Polymer is mainly polysiloxane, which is a basic polymer. The most commonly used type is methyl vinyl silicone rubber, in which the vinyl content can be adjusted over a wide range to balance the overall processability and physical properties of the material.

Reinforcing Filler

The most mainstream reinforcing filler is fumed silica. It can improve the tensile strength, tear strength and other properties of the silicone you customize. However, it can cause the compound to harden during storage, a phenomenon also known as the structuring effect, which usually requires structuring control agents to suppress.

In addition to the former, precipitated silica is also a type of reinforcing filler. Its reinforcing effect is not as good as the former, but the cost is lower. It is usually applicable to products with low performance requirements.

Processing Aids

Mold release agent: The addition of a small amount of internal mold release agent can prevent the material from sticking to the roller or mold during calendering or vulcanization.

Colorant: This is usually added as high-temperature resistant inorganic pigments or special organic pigments according to the requirements of your customized products.

Heat-resistant additive: It can significantly strengthen the long-term heat aging resistance of silicone.

Thermal/conductive fillers: As special functional fillers, this is usually added based on the performance of your product. Fillers mainly meet the application requirements of functionalized products.

Vulcanization System

The vulcanization systems mainly include peroxide vulcanization and platinum addition vulcanization.

The former is the most traditional vulcanization system, mainly through high-temperature decomposition to trigger cross-linking. The latter does not produce by-products during vulcanization, has no odor and is highly safe. This is also the key reason why it has become the preferred vulcanization choice for medical and food-grade products. But its cost will also be higher.

Types of Silicone

High Temperature Vulcanized Silicone Rubber(HTV): This type of silicone rubber is mainly a solid silicone rubber made by mixing base polymer with the above-mentioned fillers. It is also the main raw material for the calendering process.

Liquid Silicone Rubber(LSR): Traditional calendering processes usually do not directly use LSR. However, in recent years, there have also been dedicated LSR calendering machines for the production of high-precision and ultra-thin films.

Special silicone rubber: This type of silicone rubber is typically used in industrial scenarios and can usually satisfy the application requirements of oil resistance, radiation resistance, and solvent resistance.

Processing Characteristics of Silicone

Hot Tear Resistance

The tear resistance of semi-finished silicone at high temperatures is low. This means that if the compound is subjected to uneven tension or shear force during the calendering process, it is likely to break. Therefore, the calendering operation requires smooth and precise control of tension.

Shrinkage Rate

The coefficient of thermal expansion of silicone is relatively high. In other words, the high-temperature calendered silicone sheet shows obvious shrinkage after cooling and vulcanization. Therefore, when manufacturing your product, manufacturers usually accurately predict and compensate for the shrinkage rate in advance based on the roller size setting and your formula design. This can guarantee the final stability of the product’s size.

Temperature Sensitive

Temperature can greatly affect the viscosity of silicone. High temperatures can lead the compound to become soft, prone to sticking to the rollers and difficult to keep its size. Low temperatures will make the compound hard. This requires the calender to use greater pressure. However, the fluidity of hardened silicone decreases and the surface smoothness also declines. Therefore, the rollers must achieve high-precision temperature control to achieve a stable and good silicone sheet.

Silicone Calendering Process

Raw Material Pretreatment

Before using silicone, it is necessary to check whether the material is contaminated, damp or hardened. The pre-used fillers should be dried; otherwise, the damp fillers may cause bubbles in the vulcanized products and also reduce the material strength.

Mixing

Mix the base polymer uniformly with the structured control agent, silica and other additives in sequence. Uniform mixing leads to the formation of a compound with stable performance and appropriate plasticity.

After the compound is evenly mixed, the silicone needs to be pressed into thin sheets on an open mill to dissipate heat. After the temperature of the compound drops below 70℃, the vulcanizing agent should be added. It is essential to make sure that the vulcanizing agent is evenly dispersed. Finally, the compound is cut into sheets of uniform thickness.

Filtration and Reprocessing

The plasticity of the compound will decline after it has been parked. Therefore, before calendering, the compound still needs to be re-mixed on the open mill. The silicone after re-refining will become soft and have uniform fluidity. If your product application has high requirements for cleanliness, then the compound also needs to be filtered.

Primary Processing Steps

Choose a Suitable Calendering Machine for Your Product

The calender is the most core production equipment in silicone calendering. The calender mainly consists of metal rollers, a temperature control system and gap adjuster. Calenders mainly include three roller calendering machines and four roller calendering machines.

The former is the most commonly used machine, suitable for sheeting or one side topping. The compound needs to pass through two roller gaps in sequence. The latter can be used for double-sided topping. The compound needs to pass through three roller gaps simultaneously. This kind of equipment is more efficient, but it also requires higher equipment precision and control requirements.

Preheating Compound

After re-mixing, the compound will be cut into strips of uniform width. Then the compound strips will be continuously and evenly conveyed to the gap between the feeding rollers of the calender through the feeder. It should be noted that the temperature of the compound strip needs to be preheated. This can reduce the heat load of the calender and guarantee the stability of the feed.

Calendering Process

The compound usually accumulates before entering the first roller gap. By observing the shape, size and stability of the bank, it can be determined whether the calendering can be carried out successfully. If operated properly, the bank usually takes on a thick pencil shape and can rotate continuously and stably.

When the compound passes through an extremely narrow roller gap, it will be subjected to extremely high pressure and shear rate. This can help further plasticize the compound and assist in eliminating internal air bubbles.

The calendering roller needs to be adjusted to an appropriate temperature and speed to guarantee that the compound can pass through smoothly. Excessively high temperatures and overly fast speeds of the rollers may cause silicone to adhere. Therefore, precisely controlling the speed ratio between the two rollers can guide the smooth transfer of the compound and improve the surface finish.

If your product also needs to be compounded with the substrate, the substrate is usually closely bonded to the silicone at the last roller gap under the action of pressure and temperature.

Surface Treatment

If your product surface has specific pattern requirements, the material will also pass through a roller with engraved patterns. The roller leaves textures or patterns on its surface. This step can meet your product’s aesthetic requirements and specific functional needs.

Online Monitoring and Control

The online monitoring instrument will continuously scan the thickness of the monitoring film and then feed the data back to the control system. If adjustments are needed, the system will automatically fine-tune parameters such as the roller distance. Monitoring equipment will also detect whether the surface of the silicone contains bubbles, impurities, scratches, etc.

Post-processing

Cooling

The film or composite material coming out of the calender still needs to pass through the cooling roller for rapid cooling. Rapid cooling can prevent premature vulcanization caused by heat accumulation inside the product. In addition, it can also fix the size and prevent the product from shrinking.

Vulcanization

Calendered products also need to conduct vulcanization to obtain the final product performance. Silicone sheets need to be placed in a curing oven to endow the product with a series of core properties of silicone, such as chemical stability, heat resistance, and biocompatibility. For complex composite products, they may need to be cut and then undergo final shaping and vulcanization.The product after vulcanization still needs to be cooled to better keep its properties.

Cutting and Winding

At this stage, the manufacturer will cut the silicone sheet into the shape and length you need, and then roll up the material through a winding machine.

Quality Inspection

The product still needs to undergo a comprehensive quality inspection before being delivered to you. The inspection content includes the size, width, length and appearance of the product, such as surface finish, color, bubbles, impurities, etc. Additionally, the performance of the product also needs to be tested. The specific content of the test needs to be determined based on your usage requirements, such as the heat resistance, flame retardancy, tear strength of the product.

Package and Delivery

According to the product characteristics, the rolled products will be packed in dust-proof packaging and placed in packing boxes for delivery to you.

Factors Affecting Silicone Calendering

Roller Temperature Control

Roll temperature is the most primary and sensitive control variable in the calendering process. It will directly affect the viscosity, fluidity and viscoelasticity of the compound. The high coefficient of thermal expansion and thermal tear resistance of silicone determine the gradient temperature of the roller. The gradient temperature between the rollers can guide the compound to flow and transfer along the preset path.

The specific impact of roller temperature on the product is manifested in that if the temperature is too high, the compound will soften excessively. This will cause the compound to stick to the roller or the bank in the roller gap to be unstable. Besides, the thickness control of the compound will also deteriorate.

When the temperature is too low, the compound is not plasticized sufficiently, which leads to excessive viscosity and poor fluidity. This will undoubtedly increase the load of the calendering machine, and the processed sheet will have a rough surface and uneven thickness.

Roller Speed

The ratio between the roller speed and the speeds of two adjacent rollers determines the shear rate, production efficiency and the transfer process of the compound. The calendering speed of silicone usually depends on the thickness, width of the sheet and the compound formula. The speed is directly related to production efficiency.

Excessive roller speed may lead to insufficient and uneven plasticization of the compound. Moreover, the accelerated heating of the compound will lead to roller sticking and coking. Slow roller speed not only reduces production efficiency but also causes the compound to coke on the high temperature roller.

An appropriate speed ratio not only helps to disperse fillers and expel air bubbles, but also improves the density of the compound. It can also help the compound achieve a smoother surface. An improper speed ratio may form bubbles or cause uneven thickness of the compound.

Roller Gap

Roll spacing usually has a direct impact on the thickness of calendered products. The actual thickness of the calendered product is also affected by the bank between the roll gaps. Under ideal conditions, the bank volume is stable and moderate.The bank can generate a certain pressure to force the compound to pass through the roller gap.

It should be noted that the compound will still have a certain degree of elastic recovery after being extruded through the roller gap.It will slightly increase the thickness of the product. Excessive bank will cause an increase in the thickness fluctuation of the compound. Too little bank will cause the edges of the sheet to be uneven. Therefore, when adjusting the thickness, it is also necessary to observe the size of the bank simultaneously.

Advantages

Dimensional Stability

Silicone calendering can produce products ranging from extremely thin to relatively thick through precise rollers and online thickness measurement systems. And it can carry out high-precision continuous production within this production range. Strict control of process parameters can reduce the shrinkage rate and deformation of products and improve the consistency of product batches.

Superior Surface Quality

The high-hardness rollers inside the calendering machine can directly produce sheets with high gloss, smoothness and completeness. Moreover, the calendered silicone sheet can be directly applied to scenarios with high appearance requirements, such as the outer layer of medical catheters, etc. It can also effectively avoid surface defect problems such as bubbles.

Wide Range of Applications

Compared with the silicone extrusion process, silicone calendering can handle silicone compounds with high viscosity and poor fluidity. Moreover, the applications of calendering and forming process products are versatile.

It can be compounded with fabrics such as nylon and polyester fibers. For filling applications, siliconized polyester fiber uses a silicone finish to improve smoothness, loft and processing performance. Besides fabrics, the compounding of various substrates such as films and metals is also not a problem at all. It can endow products with supporting or functional features and can be applied in various industries.

High Production Efficiency

Silicone calendering can be produced continuously on a large scale. Once the production equipment is set up stably, it can implement high-speed production for several kilometers. Moreover, the calendering process, the compounding of the base material, vulcanization and winding can all be integrated on a single production line. This production mode greatly simplifies the production process, reduces production costs and energy consumption.

Flexible Functional Design

Silicone calendering can design and produce multi-layer composite structures according to your product requirements. And it can achieve comprehensive functions that single-layer materials cannot.

Applications

Automobile and Transportation

The calendering process can produce large-area plates with uniform thickness and no impurities, and can guarantee insulation reliability. It can meet the application requirements of thermal insulation gaskets and buffer sealing gaskets in the automotive industry. And it can also produce products such as insulating protective sleeves and heat-conducting gaskets.

Medical Products

Medical products such as electrode sheets of defibrillators and conductive silicone calendered sheets can all be made by calendering process. It needs to combine the biocompatibility, softness and conductivity of silicone. The calendering process can verify the uniform thickness of the product. Additionally, the softness and consistent stickiness of silicone are also applicable to silicone gel sheets. It can also be combined with other materials to make silicone gel dressings.

Industry and Energy

Silicone can solve the material problems of long-term UV resistance and temperature resistance for photovoltaic equipment. The calendering process can also produce composite gaskets with insulation and tear resistance. It can solve the problem of seal failure.

Moreover, calendering is the only process that can economically and continuously produce insulating silicone rubber tape. It can guarantee uniform electric field distribution and excellent electrical insulation.

Textiles and Outdoor Products

Firesuits are usually made of fiberglass cloth and silicone by calendering process. Calendering guarantees the flexibility and durability of the material, allowing it to withstand bending and water washing, while also reflecting thermal radiation.

Outdoor products such as inflatable tents and kayaks are usually a composite of nylon and silicone. Calendering can form a truly impermeable and waterproof composite layer. Moreover, the product is extremely light in weight, making it highly suitable for outdoor use.