Rubber Extrusion

Chapter 1: What is Extruded Rubber?

Extruded rubber refers to products created by softening and pressurizing an elastomeric compound and then forcing it through a die a hard tool with a specific shape. This process produces a continuous piece of material with a uniform cross-section along its length. The cross-sectional shape is determined by the die profile, and thousands of different profiles are employed across various industries, including automotive, aerospace, electronics, building construction, consumer goods, and more.

Chapter 2: Benefits of Extruded Rubber

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.

Benefits of Extruded Rubber

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.

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Chapter 3: Common Extruded Rubber Products

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.

  

  
  
• Rubber Weatherstrips: Extruded rubber weatherstrips are similar to rubber gaskets but are more exclusively used in automotive and building construction. They are made to effectively isolate the inside room from external elements such as outdoor air and pollution, extreme heat and cold, and water. They are also designed to have great elasticity to allow for frequent opening and closing of doors and windows. When used in automotive vehicles, they must also resist vibrations while maintaining a tight seal.

• 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 low production cost, versatility, and strength of medical tubing make it a perfect product for helping medical professionals assist patients. Since it is unaffected by high temperatures, it can be sterilized multiple times without degrading. In addition, its transparency makes it ideal for monitoring the flow of medications or drainage.

Chapter 4: Rubber Extrusion Materials

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.

• Butadiene Rubber (BR): Butadiene rubber or BR is an elastomer from the polymerization of the monomer, butadiene. It accounts for about 25% of the world’s production of synthetic rubbers. Most of the BR produced are used to make tires, while some are blended with other rubbers to improve the final product's properties. Generally, BR has good cracking, abrasion, and rolling resistance but is prone to ozone degradation.

• 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.

  

  
  
• Acrylonitrile Butadiene (Nitrile) Rubber (NBR): Nitrile rubber, more popularly known as NBR or Buna-N, is made from the copolymerization of acrylonitrile and butadiene. It is the most common rubber-type when it comes to automotive applications because of its resistance to petroleum-based chemicals such as oils, fuels, and grease. Thus, they are used as excellent gaskets and sealants. However, they have low tensile strength and poor low-temperature performance, which is mitigated by compounding reinforcing fillers.

• 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).

  

  
  
• Butyl Rubber (IIR): Butyl rubber or IIR is a copolymer of isobutylene and isoprene. Its elastomer chain is mostly unsaturated because of its isoprene content which is only around 3% of the copolymer. The low unsaturation of IIR enables it to repel most chemicals (gas and liquids) and is highly resistant to aging when vulcanized properly. Because of this property, IIR is used to produce O-rings, gaskets, window and door trims, and hoses.
• Halogenated Butyl Rubber (CIIR, BIIR): This type of rubber is produced from the modification of butyl by reacting allylic chlorine and bromine. This improves the ozone and chemical resistance of the material but decreases its insulation capabilities and water resistance when compared to IIR. Regardless, its permeability to moisture and gas is still low compared to other rubber types. Its areas of applications are similar to IIR.
• Chloroprene (Neoprene) Rubber (CR): Chloroprene, known commercially as Neoprene, is one of the first synthetic rubbers ever made. Developed by DuPont in 1930, it is made to have better aging resistance and durability than natural rubber. Natural rubber shortage during World War 2 only made chloroprene more prevalent. Today, it is widely used in the production of weatherstripping seals, gaskets, and hoses because of its inherent chemical inertness. However, because of its many compounding additives, it is much more expensive than natural 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.

  

  
  
• Natural Rubber (NR): The discovery of natural rubber paved the way for the development of vulcanization, a key process in the production of more advanced rubber materials. Its main chemical compound, latex (polyisoprene), is harvested from barks of plants, particularly the Hevea tree. Natural rubber is preferred because of its excellent heat buildup and fatigue resistance compared with other rubbers.
• Isoprene Rubber (IR): Isoprene or IR is similar to natural rubber in terms of polymer chain structure. IR are widely-used, general-purpose rubber made from the polymerization of isoprene monomers. Since they are made artificially, they are purer than natural rubber, with more controlled and superior properties. Like natural rubber, they have good heat buildup and fatigue resistance, on top of their inherent impact absorbing capability. Thus, they are well-suited in producing shock absorbers and rubber bushings.

• 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.

  

  
  
• Polyurethane Rubber: Polyurethane rubber is one of the most versatile types since it can be formulated in various ways to have specific properties. It combines the design flexibility of both plastic and rubbers. Different degrees of hardness, compression set, tensile strength, abrasion resistance, and chemical resistance can be obtained by using different types and amounts of compounding ingredients. They are used in shock absorbers, dampers, protective sleeves, trims, and bushings.

Chapter 5: The Rubber Extrusion Process

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:

  

  
  
  • Fillers: Fillers are used to increase the strength of the elastomer structure. They act as reinforcing materials that enhance the tensile strength, impact strength, tear resistance, and abrasion resistance of the finished rubber compound. Typical fillers are carbon black, clays, silica, and calcium carbonate.
  • Stabilizers: Stabilizers are compounded with rubber to enhance their resistance to weathering and degradation. They act as antioxidants and antiozonants. Stabilizer molecules become free radical scavengers that bind to the elastomer’s active sites that are prone to oxidation. Oxidation of these sites makes the rubber stiff and prone to tearing and fatigue. Common antioxidants are phosphites, phenols, and hydroquinones. Antiozonants, on the other hand, are typically PPDs of paraphenylene diamines.
  • Vulcanizing and Curing Agents: Vulcanizing is an important part of extruder rubber production, which fixes the final properties of the rubber material. To achieve a perfect vulcanizing process, certain chemicals are added to the rubber mix. Examples of these are sulfur, peroxides, accelerators, activators, and retarders. Depending on the rubber compound and manufacturing processes involved (primary and secondary), the composition of the vulcanizing agents may vary.
  • Special Ingredients: These are chemicals added to the rubber compound which impart minor property modifications. Some of these include colorants and pigments for adding color, process oils for improving processability, and resins and fibers for improving tensile strength and chipping resistance.
  • Heating and Pressurizing: This process involves further masticating the rubber compound. The typical equipment for performing this process is a screw extruder which can either be a single screw or twin screw. A single screw is commonly used since its control is much simpler and its cost is cheaper. The screw is enclosed in a large, pipe-like chamber called the extruder barrel. Depending on the complexity of the machine, the rubber compound is fed into the machine using a hopper, a set of counter-rotating drums, or an injection port.

  

  
  

  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.

• Secondary Processes: This stage of the production process involves several operations such as cutting, splicing, spooling, drilling, notching, coating, and so on. Such operations are performed according to the general features of the product. The most common secondary processes are cutting and splicing, where the extruded rubber is cut into shape and the ends connected to form a continuous loop of rubber. These are used to manufacture extruded rubber gaskets, seals, and trims.

Chapter 6: The Co-Extrusion Process

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.

Salient Factors in Regard to Co-Extrusion

• Temperature – As with the production of so many products, temperature is a vital consideration in the co-extrusion process. Every type of plastic and rubber reacts differently to a specific environment and temperature. During the design phase of the process, engineers select materials that can endure the conditions for which the product is being created.
• Adhesives – Adhesives are a necessary part of combining plastic and rubber. In most cases, rubber adhesives are used since plastic adhesives are more expensive. A factor that determines the type of adhesive is the overall needs of the application and the component being produced.
• Recyclability – The choice of plastic is strongly influenced by the environmental concerns of the manufacturer. Adherence to environmental standards determines the type of plastic that will be used, which, in this case, would be one that is recyclable.
• Color – Plastic combined with rubber offers several varieties of color in the final product. While rubber does not offer many choices in this area, plastic can be colorized in multiple ways using different colorants.
• Cost – The main cost in the co-extrusion process is the cost of the die that has to be produced from a CAD rendering. In most cases, rubber co-extrusions are less expensive than plastic co-extrusions.

Conclusion

• Extruded rubber is a group of products characterized by having the same cross-section throughout its length. It is made by softening and pressurizing an elastomeric compound and forcing it to flow through a hard tool called a die.
• Common extruded rubber products are rubber bushings, trims, tubes, hoses, pipes, gaskets, seals, O-rings, and weatherstrips.
• The type of rubber material is determined by the kind of elastomer chain used in the production process. Examples are NBR, SBR, BR, EPDM, FKM, etc.
• Rubber extrusion involves compounding, heating, kneading, pressurizing, extrusion, and vulcanizing. These processes are done in an extruder and a vulcanizing oven or autoclave.

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