This article takes an in depth look at rubber extrusion.
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Rubber Extrusion is a group of products made by softening and pressurizing an elastomeric compound and forcing it to flow through a hard tool called a die. The resulting product is a continuous piece of material that has the same cross-section throughout its length. The shape of the cross-section depends on the die profile. Thousands of different die profiles are available in the market that caters to many industries such as automotive, aerospace, electronics, building construction, consumer goods, and much more.
Extrusion (the process) and rubber (the material) are two words that go well together from a manufacturing perspective. Extrusion is a popular process for producing products with a continuous profile such as rods, pipes, and tubes. The concept of this process is very simple and can be adapted to the manufacture of all sorts of materials that can be softened and extruded. Aside from rubbers, other materials that can be extruded are metals and plastics.
On a different note, rubbers are kinds of materials that have elastic properties. They are related to the "wonder material," plastics, which can be engineered by using different compounding ingredients. They are light, durable, and can be easily shaped according to the profile of the tool used. Manufacturing rubber products does not take as much energy as processing other types of materials. Thus, they can be made at a very low cost, on top of the very wide range of properties that can be obtained by varying its formulation.
Combining the two together yields a type of product that is cheap, readily available, high-quality, and versatile.
The rubber industry is currently dominated by tire production, which accounts for two-thirds of global demand. The balance is split between non-tire automotive applications, industrial, footwear, electrical, and others. Extruded rubber is used in the majority of non-tire automotive and industrial applications. Examples of extruded rubber products are detailed below.
Extruded rubber bushings are used as shock absorbers in a working piece of equipment. They isolate moving parts from each other and prevent vibration and noise from being transferred throughout the equipment. They are commonly seen in automotive systems, particularly in suspension components.
Most rubber bushings are shaped as hollow cylinders, which can be easily produced from an extrusion process. It is cut to length according to the dimensions of the part or shaft where it goes into. Some rubber bushings are used as bare rubber only, while some are enclosed and bonded in metal casings or sleeves.
Rubber trims are extruded products that are used to protect the edges or surfaces of rigid objects from sudden impacts. Examples of such objects are panels, windows, doors, removable covers, and hatches. Aside from impact protection, they also provide an air-tight or water-tight seal between the mating parts.
There are thousands of rubber trims profiles available, and often, they are custom-made to suit a special application. One of the most common rubber trim shapes is a U-channel. The edge of the rigid part is inserted on the channel and is fixed into place by fasteners and adhesives.
Tubes, hoses, and pipes, regardless of the material type, are commonly produced by an extrusion process. These are commonly used industrial equipment’s hydraulic and pneumatic control. The use of food-grade materials such as silicone allows its application to expand to food and drug manufacturing.
Rubber’s excellent flexibility allows it to be bent into shape without incurring damage. This property makes these types of products desirable for connecting different parts of equipment for transferring fluids such as compressed air, oil, water, and chemicals.
Like rubber trims and rubber bushings, rubber bumpers take advantage of the material’s shock-absorbing capabilities. Aside from this property, the compounding ingredients for making rubber bumpers add some amount of abrasion resistance to enhance their durability. They are commonly used in trailer truck docks and loading bays, jetty and boat fenders, and road safety devices. The typical cross-section of rubber bumpers is a D-shape profile. This allows one side of the bumper to be fixed on a wall while the other side absorbs the impact. The circular shape of the impact-absorbing side prevents damage to the colliding vehicle.
While most gaskets and seals are made from die-cutting and injection molding, they can also be made from extrusion. Extruded gaskets and seals are typically used in weatherproofing cabins, enclosures, cars, equipment housings, and electrical panels. The rubbers used in the production of these products are selected for their chemical resistance and resilience against external harm such as heat, sunlight, and oxidation. Rubber gaskets and seals are available in both hollow and solid profiles. Regardless of the profile, it is important that the material can easily be compressed between the mating surfaces to properly form a tight seal.
O-rings are special types of gaskets that have a uniform cross-section and have ends connected together forming a ring. While popularly known as having a circular cross-section, other shapes are sometimes used. Typical O-rings made from injection or compression molding processes are limited to small equipment such as valves and quick-connect joints. However, when it comes to large equipment such as vessels and tanks, they are made from a long cord stock. The cord stock is an extruded rubber cut to length according to the circumference or perimeter of the mating parts. To form the O-ring, the ends of the cord stock are bonded together by a strong adhesive or through vulcanizing.
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.
The inherent characteristics of extruded rubber products depend on the chemical and physical properties of its base elastomer chain. The elastomer chain is composed of the carbon-based polymer backbone with several active sites or functional groups. The chemical composition of the elastomer chain determines the type of rubber material.
On top of the general properties such as elasticity, thermal and electrical insulation, and shock-absorbing properties, a specific kind of rubber material is chosen for its inherent chemical resistance, aging and degradation resistance, mechanical properties, and cost. Below are the different types of rubber materials used in the production of extruded rubber.
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 or SBR is a product of copolymerization between styrene and butadiene. SBR is one of the most important types of rubber which are used to produce not just extruded rubber parts but also to manufacture tires, gaskets, and shoes. It is a general-purpose rubber that competes with natural rubber in terms of market share. SBR is preferred due to its better abrasion, tear, and thermal resistance than natural rubber.
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.
EPDM is made from the copolymerization of ethylene and propylene with the addition of diene, which facilitates curing. When diene is not used, the result is referred to as EPM and is only curable by peroxide. Both EPDM and EPM have good weathering resistance, good insulating and dielectric properties, excellent mechanical properties both at high and low temperatures and chemical resistance. They compete with NBR in terms of production volume when used in automotive sealing applications.
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.
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, 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.
FKM rubbers are known as fluoro-elastomer materials. They are made from the copolymerization of vinylidene fluoride (VDF) with other chemicals such as hexafluoropropylene (HFP), tetrafluoroethylene (TFE), and so on. It is known commercially as Viton, which is a trademark of DuPont. FKM is generally known to be highly resistant to almost all chemicals, aside from having good mechanical properties. They are perfect to produce cord stocks for making O-rings and other sealing products.
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 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.
Unlike other rubber materials, which have a carbon-based polymer backbone, silicone rubbers have a silicon-oxygen chain bonded with methyl, vinyl, and phenyl functional groups. These materials are characterized to have good oxygen, ozone, heat, light, and moisture resistance. However, they are more expensive and have poorer mechanical properties than organic rubber. They are used in many industrial applications, but they are most popularly used in food and drug manufacturing.
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.
Rubber extrusion is a downstream process that involves compounding, heating, kneading, pressurizing, extrusion, and vulcanizing. It uses rubber and chemical feedstock from a number of primary plant processes in the chemical and petrochemical industries.
By itself, rubber extrusion is a fairly straightforward process. In the simplest sense, it only involves feeding the rubber material into the extruding machine, gathering the extrudate, vulcanizing it, and then cutting and splicing according to the client’s specifications. However, there are many parameters at play. One is the formulation of the rubber compound. The second is the extruding parameters such as temperature, throughput rate, and vulcanizing time. The process must be carefully controlled to create a reliable product.
Below is the step-by-step rubber extrusion process.
The compounding process is where the formulation of the rubber material takes place. The main elastomer compound, which can either be NBR, SBR, BR, FKM, and so on, is thoroughly mixed with additive ingredients to obtain the desired properties of the product. The additive ingredients improve and complement the intrinsic set of properties of the main elastomer. Below are some of the compounding ingredients manufacturers use in rubber extrusion.
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 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 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.
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.
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 goes through the extruder barrel, the constant rotation of the screw masticates and mixes the material. The screw is composed of a set of engineered and carefully machined screw elements that have a profile that either heats, kneads, or pressurizes the rubber. As the rubber is heated, its flowability increases, which prepares it for the next stage of the production process.
The extrusion operation takes place at the end of the extruder barrel. At this point, a tool called a die is installed, which has an opening that contains the cross-section of the finished material. This opening is designed to compensate for any dimension changes that occur after extrusion. Typically, the extrudate swells after leaving the die and during the vulcanization process.
As the rubber is pressurized by the screw, it is forced to flow through the die to become an extrudate. This extrudate now resembles the physical features of the final product. The extrudate is ejected from the machine and guided by a set of rollers and wheels to bring it to the next step.
The vulcanizing stage, as mentioned earlier, is done to fix the physical and chemical properties of the finished product. In a more detailed perspective, this process is where the cross-links between elastomer chains are created. Sites within the rubber’s long molecular chain bind with each other using vulcanizing agents such as sulfur and peroxide. This yields an interconnected molecular structure that acts like a stretchable, meshed material that returns to its original shape in its rested state.
To initiate the vulcanizing process, the extrudate is further heated or held at elevated temperatures of around 140 to 160°C. This could take several minutes depending on the rubber compound and vulcanizing agents used. Vulcanizing time is shortened by vulcanizing modifiers such as activators and accelerators.
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.
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