This article takes an in-depth look at extruded rubber.
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Extruded rubber refers to items manufactured by heating and pressurizing an elastomer compound and pushing it through a die, a precise tool with a specific shape. This process results in a continuous material with a consistent cross-section throughout its length. The shape of the cross-section depends on the die design, and a multitude of profiles are utilized across various sectors, such as automotive, aerospace, electronics, construction, consumer products, and more.
Rubber extrusion is a popular manufacturing process that efficiently produces uniform shapes and profiles with minimal waste and at a cost-effective price. Products made through rubber extrusion are known for their durability, longevity, and resistance to chemicals, heat, cold, impact, and abrasion.
Extrusion is a widely used process that began in the 1930s for producing continuous-profile products like rods, pipes, and tubes. This straightforward method has been adapted to manufacture a variety of materials that can be softened and extruded. In addition to rubber, extrusion is also applicable to metals and plastics.
Rubber is an elastic material that can be modified with various compounding ingredients. Similar to plastic, it is lightweight, durable, and easily molded using a profile die. Despite requiring less energy for processing and shaping, the final products retain all of rubber's advantageous properties. A key benefit of rubber is its low production cost, regardless of the type of rubber being used.
Plastic and rubber can be combined through a process known as co-extrusion, which produces products that are both rigid and soft. This technique leverages the beneficial properties of both materials, resulting in a more economical product that is high-quality, versatile, and readily available.
The extrusion of rubber offers numerous benefits, as it enables the production of a wide variety of products. Customers can select from different shapes, configurations, designs, and lengths, all manufactured efficiently with minimal waste.
Quality – A key advantage of extruded rubber products is their high quality, characterized by excellent resistance to heat, cold, aging, friction, abrasion, and everyday wear and tear.
Resistance – Extruded rubber is designed to endure the typical wear and tear that products face during use.
Cost Savings – Extruded rubber is popular largely because of its cost-effectiveness, which stems from the minimal waste generated during the extrusion process.
Diversity – Rubber extrusion can create a vast array of products, including special, unique, and unconventional designs. It accommodates countless varieties, types, and shapes, regardless of their complexity or uniqueness.
The rubber industry is predominantly driven by tire production, which constitutes two-thirds of global demand. The remaining demand is divided among non-tire automotive applications, industrial uses, footwear, electrical components, and other categories. Extruded rubber is widely utilized in most non-tire automotive and industrial applications. Examples of extruded rubber products are listed below.
Rubber Bushings: Extruded rubber bushings act as shock absorbers within equipment, isolating moving parts and minimizing the transfer of vibration and noise. They are frequently used in automotive systems, especially in suspension components, to enhance performance and comfort.
Rubber bushings are typically shaped as hollow cylinders, a form that is easily produced through extrusion. They are cut to length based on the dimensions of the part or shaft they are intended to fit. While some rubber bushings are used as bare rubber, others are enclosed and bonded within metal casings or sleeves for added durability and functionality.
Rubber Trims: Rubber trims are extruded products designed to safeguard the edges or surfaces of rigid objects from sudden impacts. They are commonly used on panels, windows, doors, removable covers, and hatches. In addition to impact protection, rubber trims also offer air-tight or water-tight seals between mating parts.
There are thousands of rubber trim profiles available, many of which are custom-made for specific applications. One of the most common shapes is the U-channel, where the edge of the rigid part is inserted into the channel and secured with fasteners or adhesives.
Rubber Tubing, Hoses, and Piping: Tubes, hoses, and pipes, regardless of the material, are produced through extrusion. These components are commonly used in hydraulic and pneumatic systems, with silicone tubing being suitable for manufacturing food and drug-related products due to its high safety and quality standards.
Rubber’s exceptional flexibility allows it to be bent into various shapes without damage. This characteristic makes rubber tubing, hoses, and piping ideal for connecting different parts of equipment and transferring fluids like compressed air, oil, water, and chemicals.
Rubber Bumpers: Rubber bumpers, like rubber trims and bushings, utilize the material’s shock-absorbing properties. Additionally, the compounding ingredients used in rubber bumpers provide abrasion resistance, enhancing their durability. They are commonly found in trailer truck docks, loading bays, jetty and boat fenders, and road safety devices. Typically, rubber bumpers feature a D-shaped profile, where one side is mounted on a wall while the other side absorbs impact. The rounded shape of the impact-absorbing side helps prevent damage to colliding vehicles.
Rubber Gaskets and Seals: Although most gaskets and seals are produced through die-cutting and injection molding, extrusion is also a viable method for manufacturing these components. Extruded gaskets and seals are commonly used for weatherproofing cabins, enclosures, vehicles, equipment housings, and electrical panels. The rubbers selected for these applications are chosen for their chemical resistance and durability against external factors like heat, sunlight, and oxidation. Rubber gaskets and seals come in both hollow and solid profiles, and it is crucial that the material can be easily compressed between mating surfaces to create an effective seal.
Rubber Cord Stock or O-Rings: O-rings are a specialized type of gasket with a uniform cross-section that forms a continuous ring when the ends are joined. Although O-rings are typically circular in cross-section, other shapes can also be used. While O-rings made through injection or compression molding are commonly used in small equipment like valves and quick-connect joints, larger equipment such as vessels and tanks often utilize O-rings made from extruded rubber cord stock. This cord stock is cut to length based on the circumference or perimeter of the mating parts. To create the O-ring, the ends of the cord stock are bonded together using a strong adhesive or through vulcanization.
Medical Silicone Rubber Tubing: Among the various types of extruded rubber products, medical silicone rubber tubing is distinguished by its high quality and meticulous design, driven by the stringent standards of the medical industry. Manufacturers must comply with rigorous requirements to ensure that medical silicone tubing meets the highest levels of safety and performance.
The properties of extruded rubber products are primarily determined by the chemical and physical characteristics of their base elastomer chain. This elastomer chain consists of a carbon-based polymer backbone with various active sites or functional groups. The specific chemical composition of the elastomer chain dictates the type and properties of the rubber material.
In addition to general properties like elasticity, thermal and electrical insulation, and shock absorption, the choice of rubber material depends on specific attributes such as chemical resistance, aging and degradation resistance, mechanical properties, and cost. Below are the various types of rubber materials used in the production of extruded rubber products.
Styrene-Butadiene Rubber (SBR): Styrene-butadiene rubber (SBR) is a copolymer of styrene and butadiene. It is a crucial type of rubber used not only for extruded rubber parts but also in the production of tires, gaskets, and footwear. SBR is a general-purpose rubber that rivals natural rubber in market share, favored for its superior abrasion, tear, and thermal resistance compared to natural rubber.
Ethylene Propylene Rubber (EPM, EPDM): EPDM (Ethylene Propylene Diene Monomer) is produced by copolymerizing ethylene and propylene with an added diene to facilitate curing. Without the diene, the resulting material is EPM (Ethylene Propylene Monomer), which can only be cured with peroxide. Both EPDM and EPM offer excellent weathering resistance, superior insulating and dielectric properties, and outstanding mechanical performance at both high and low temperatures. They also exhibit strong chemical resistance. In automotive sealing applications, they are produced in volumes comparable to NBR (Nitrile Butadiene Rubber).
Fluorocarbon (Viton) Rubber (FKM): Fluorocarbon rubbers, commonly known as Viton (a trademark of DuPont), are fluoro-elastomers made from the copolymerization of vinylidene fluoride (VDF) with other chemicals like hexafluoropropylene (HFP) and tetrafluoroethylene (TFE). FKM is renowned for its exceptional resistance to nearly all chemicals, alongside its robust mechanical properties. This makes it ideal for producing cord stocks for O-rings and various sealing products.
Silicone Rubber: Unlike other rubber materials with a carbon-based polymer backbone, silicone rubbers feature a silicon-oxygen chain bonded with methyl, vinyl, and phenyl functional groups. These materials are known for their excellent resistance to oxygen, ozone, heat, light, and moisture. Despite their higher cost and relatively poorer mechanical properties compared to organic rubber, silicone rubbers are widely used in various industrial applications, with a prominent role in food and drug manufacturing.
Rubber extrusion is a downstream process that includes several stages: compounding, heating, kneading, pressurizing, extrusion, and vulcanizing. This process utilizes rubber and chemical feedstocks derived from various primary operations in the chemical and petrochemical industries.
Rubber extrusion, while fundamentally straightforward, involves several detailed steps. The basic process includes feeding rubber material into the extruder, collecting the extrudate, vulcanizing it, and then cutting and splicing it to meet the client’s specifications. However, successful extrusion depends on managing numerous parameters, such as the formulation of the rubber compound, extruding conditions (including temperature and throughput rate), and vulcanizing time. Precise control of these factors is essential to produce a consistent and reliable product.
Here is a step-by-step overview of the rubber extrusion process:
Rubber Compounding: In the compounding stage, the rubber formulation is prepared by mixing the primary elastomer, such as NBR, SBR, BR, FKM, or others, with various additive ingredients. These additives enhance and modify the intrinsic properties of the base elastomer to achieve the desired characteristics for the final product. Some common compounding ingredients used in rubber extrusion include:
As the rubber progresses through the extruder barrel, the rotating screw masticates and mixes the material. The screw consists of engineered, precisely machined elements that either heat, knead, or pressurize the rubber. Heating the rubber increases its flowability, making it ready for the subsequent stages of the production process.
Extrusion: The extrusion operation occurs at the end of the extruder barrel, where a die is installed. This die features an opening that shapes the cross-section of the finished material. The die is designed to account for any dimensional changes that may occur after extrusion, as the extrudate typically swells upon exiting the die and during the vulcanization process.
As the rubber is pressurized by the screw, it is pushed through the die to form the extrudate, which takes on the physical characteristics of the final product. Once ejected from the machine, the extrudate is directed by a series of rollers and wheels to proceed to the next stage of the process.
Vulcanizing or Curing: Vulcanization, or curing, is the stage where the physical and chemical properties of the rubber are permanently set. During this process, cross-links are formed between the elastomer chains in the rubber. Vulcanizing agents, such as sulfur or peroxide, facilitate the bonding of sites within the rubber’s long molecular chains, creating an interconnected network. This cross-linked molecular structure results in a stretchable material that returns to its original shape when relaxed.
To start the vulcanizing process, the extrudate is heated or maintained at elevated temperatures of approximately 140 to 160°C. The duration of this process can vary depending on the rubber compound and vulcanizing agents used, typically taking several minutes. Vulcanizing time can be reduced by incorporating vulcanizing modifiers such as activators and accelerators.
In the co-extrusion process, different plastics and rubbers can be combined to create composite components. The choice of materials depends on the design requirements of the final product. It is essential to carefully monitor this aspect of the process to ensure the quality and performance of the finished product.
Co-extrusion involves the simultaneous extrusion of multiple layers of materials, requiring multiple extruders to melt and prepare both the rubber and plastic before they are combined. This process leverages the beneficial properties of each material to produce a stronger and more resilient final product.
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