Rubber extrusion is the process by which natural and synthetic rubber materials are strengthened and shaped into useful products. In other words, extrusions are parts that are prepared by forcing the material through a die of the desired shape, cut, or cross section, using a high-pressure extrusion machine.
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The History of Rubber Extrusions
Rubber is an elastic hydrocarbon polymer (or elastomer) and is found naturally in the Pará Rubber Tree. The origin of human cultivation of rubber trees predates history; the Mayans are believed to have sapped rubber trees of their natural latex and boiled it to make crude rubber around 1600 B.C.E. It wasn't until the 19th century that more advanced methods of processing were adopted on a substantial scale. At that time, tire manufacturers started relying on texture rubber profile design.
In 1909, a team employed by the Bayer lab in Elberfeld, Germany, polymerized Isoprene, the first synthetic rubber. The next year, in Russia, a scientist named Sergei Vasiljevich Lebedev created the first synthetic rubber made from butadiene. His formula went on to provide rubber material for Russia during WWI, when natural rubber was in shortage.
In 1931, at DuPont, a group of scientists first successfully synthesized neoprene. This highly resistant polymer allowed companies to create both standard and custom fuel hoses for cars. After this, scientists, primarily in the U.S., Russia/the Soviet Union, and Germany, competed with one another to develop cheaper and better polymer products. Once WWII hit, though, the USA began manufacturing rubber in earnest, as most of the world’s rubber was controlled by the Axis Powers.
By the 1960s, use of synthetic rubbers exceeded use of natural rubber. Today, natural and synthetic rubber products alike are employed in countless industrial, commercial, and consumer contexts. Although different rubber processing methods exist, extruded rubber remains a popular choice for many manufacturers because of its strength, uniformity and customizability.
Benefits of Rubber Extrusion
There are a ton of benefits to choosing rubber extrusion as your rubber manufacture method.
First, rubber extruders can form extruded shapes of any shape or size; they’re perfect for custom rubber applications. In addition to rubber extruders, rubber itself is incredibly versatile; the wide range of qualities offered by its many variations ensures that it can be made into virtually anything. From profiles to seals, rubber material offers endless uses.
Speaking of which, another one of the benefits of rubber is the fact that, because it is so strong but flexible and soft, it makes incredibly effective seals. While rubber and plastic extrusions often compete with one another for dominance of the market, rubber extrusions almost always come out on top.
Designing Rubber Extrusions
When designing a rubber extrusion, manufacturers consider a number of different application-related factors. First, they think about the material; the forming of shapes depends on the rubber compound nature and hardness. They also think about the shape of the die. For a more generic application, they may be able to use their standard dies already in stock. However, for a custom rubber extrusion, they’ll likely need to design an entirely new die. For a high volume application, they will likely make a profile die, which will allow them to manufacture extrusions with the same cross section profile from run to run.
Rubber is a great candidate for a custom extrusion application. This is because it can be cut and shaped in any desired shape without applying much labor and effort. In addition, because there is such a wide variety of rubber types, manufacturers can custom manufacture just about any product, from rubber sponge to shock resistant seals.
Process of Rubber Extrusion
During the rubber extrusion process, raw rubber material (or stock) is processed through a rubber extruder or a screw extruding machine.
Step 1: The process starts with selecting a suitable rubber material. (An external supplier provides the rubber in a form that complements the extrusion system.)
Step 2: Next is feeding the unvulcanized rubber into the extruder machine. The stock is deposited in a hopper, which is a container suspended above the conveyor.
Step 3: Gravity sends the stock through the bottom of the hopper onto the conveyor. (Note: pre-heating of the stock is optional, depending on the precision of the die and the desired qualities of strength. Extrusion with pre-heated stock is known as hot extrusion, while extrusion with solid rubber is known as cold extrusion.)
Step 4: From the conveyor, the rubber is taken through the die with the help of a flute/base that is connected with a rotating screw. In this stage, the temperature and pressure on the material is increased, so that the material can contact well with the die.
Step 5: The pressure, heating and shearing softens the rubber, allowing rubber to pass through the opening of the die. Inside the die, it swells and takes its shape.
Step 6: The rubber emerges from the extruder in a rubber profile that matches the shape of the die shape.
The post-processing step called curing, also known as vulcanization, is what gives rubber its physical properties, which makes it capable of withstanding challenging environments. As the shaped rubber exits the die, this process starts almost immediately. After the curing step, further post-processing steps involve splicing, taping end joining, coiling, drilling, and cutting.
Curing is commonly done using one of three methods:
- Using a Mold
- In this method, manufacturers use a metal mold tool to match the finished product shape and size. They inject a raw feed of rubber into the mold. There, with the help of a press, the rubber is heated and pressurized for a predetermined period of time. After this, they let the mold cool down, and then their rubber product is removed from the die. This process provides accurate results. It is often used to cure tires.
- Using an Autoclave
- This curing method starts when an extruded rubber product, sitting on a metal rack with other products, is put inside an autoclave, which is a type of pressure chamber. Inside the autoclave, extruded products are exposed to elevated pressure, up to 300 PSI, and elevated temperature. This removes the porousness and adds structural strength. However, the method is slow, and at times, can be inaccurate. Nevertheless, the process is inexpensive and can cure a number of rubber products at once.
- Continuous Curing
- A high-speed manufacturing process that works in conjunction with the extruding machine. It involves the use of microwaves, vulcanizing hot air, salt baths, oil, and/or glass beads. Extruded rubber products that are cured via continuous curing method have high stress tolerance. Continuous curing cuts cost in terms of labor, energy, and space. However it has higher set up and raw material costs. Moreover, there are certain sizes and shapes that cannot be cured by this method.
Rubber Extrusion Types
- A variation of the basic extrusion process, where two separate extruders connect to a single die and two or more separate batches of rubber material are mixed by separate screws, and then fed into the die at the same time. This allows for the convergence of rubber materials that contain different properties like color and consistency.
- Cold Feed Extrusion
- Involves performing the extrusion process without preheating. This is best suited to produce profiles, hoses, cable and sheaths.
- Extruded Rubber Seals
- Products that are used to seal or fill gaps, insulate surfaces or spaces from water or atmospheric conditions and protect surfaces from corrosion.
- Extruded Rubber
- Rubber that has been melted and forced into a die.
- Hot Feed Extrusion
- Involves pre-heating the rubber material before performing the extrusion process.
- Pin Barrel Extruders
- Have a special design with many pins protruding from the cylinder wall toward the screw's center that enhance the mixing and dispersing of the rubber as it is kneaded between the screws and the cylinder. This method can be applied to many rubber compound formulations for diverse applications.
- Rubber Bumper
- Serve as shock absorbers in consumer and commercial contexts. They also absorb vibration and noise caused by moving parts in industrial machinery.
- Rubber Bushings
- Placed between moving parts to absorb vibration; they have uses in vehicle suspension systems, manufacturing machinery and more.
- Rubber Connectors
- Extruded rubber products that join the ends of tubes or pipes together.
- Rubber Extruders
- Machines that mold raw rubber material into usable products with heat, pressure and a die.
- Rubber Grommet
- Rubber rings inserted into an opening in sheet metal to protect cords or electrical wires from abrasion.
- Rubber Profiles
- Extruded rubber products in any shape or form.
- Rubber Sheets
- Used for a variety of purposes including matting, lining, floor covering and stair treads.
- Rubber Trim
- A long, thin piece of extruded rubber that can be used in many applications.
- Silicone Extrusions
- Extruded rubber products that are processed from raw silica stock.
- Vented Extruders
- Have screws of an original and unique design. This type is easy to maintain and occupies very little floor space.
- Used to insulate a building to make heating and cooling more efficient.
Materials Used in the Extrusion Process
The qualities of the finished product will depend on the extrusion process and on the qualities of the raw rubber stock. Rubber exists in many forms, each with its own set of unique properties, allowing for custom creations. Properties like tensile strength, hardness, aging, flexibility, permeability and speed of recovery from deflection will vary in different materials.
Natural rubber, also called "gum rubber," has excellent abrasion and acid resistance.
Recently, silicone extrusion products have become popular after getting certification from FDA for being non-reactive to a variety of things. Silicone rubber can endure both high and low temperatures, without undergoing any change. Even biological fluids do not react with silicone, making the silicone product a darling of the beauty industry. In addition, because of silicone’s heat resistance and chemical inertness, silicone extrusions are particularly attractive to customers in the healthcare and food service industry.
Ethylene propylene diene terpolymer, or EPDM, is a synthetic rubber material with qualities of heat resistance, UV resistance, aging resistance, ozone resistance, and oxidation resistance. EPDM products, such as EPDM sponge rubber and EPDM foam rubber, are great for use in a wide variety of commercial and industrial settings. In addition, EPDM is the perfect material for the creation of door gaskets, weather stripping, window gaskets, roofing membrane, tarp straps, and much more.
Styrene-butadiene rubber, or SBR is a rubber product that offers particularly high abrasion resistance. It’s perfect for automobile products like belt covers and motor mounts.
Also known as synthetic natural rubber, polyisoprene is a flexible type of polymer with a mild resistance to abrasion.
Nitrile rubber, also known as nitrile butadiene rubber (NBR), Buna-N, Perbunan, or acrylonitrile butadiene rubber, is a synthetic rubber copolymer. NBR is highly resistant to oils. Therefore, it used to make rubber products that will be exposed to oil, such as hydraulic seals and hoses, and O-rings.
Neoprene is resistant to a number of things, including: heat, exposure to water, oil, and ultraviolet rays. Moreover, the polymer has high-tensile strength and fire resistance. These properties make neoprene a preferred choice for many applications.
Machinery Used in the Extrusion Process
The machinery used to extrude rubber, the rubber extruder, is made up of two main parts: a heated shearing screw conveyor and a die through which pressurized rubber is squeezed.
Shearing Screw Conveyor
A shearing screw conveyor is as its name suggests: a conveyor and a screw. The screw is positioned parallel to the conveyor. As the screw turns, it pushes the rubber material while it becomes heated in the conveyor channel.
Extrusion dies are purpose-built tools designed by cutting a shape, to reflect exactly an intended extruded rubber. In advance of the actual extrusion process, an engineer will design the die, either according to the specifications of his project or those of his customers, and install it in the extruder. Most of the time, extrusion dies are made from a hard metal, like steel, so as to have sharp edges. Once the mold or die has been created, the rubber is forced through it, applying high pressure in a high temperature zone.
Rubber Extrusion Applications
The rubber extrusion process is used for a wide variety of reasons. For example, rubber extruded profiles are known to make appliances waterproof. Likewise, they’re effective for applications related to climate control and moisture control. In addition, extruded rubber is an excellent material for connecting applications (connecting aerospace hoses, etc.) They’re also used to make equipment and working environments, especially floors, safer.
Rubber extrusions connect tubing, insulate electronics and erase mistakes on paper, among other applications. Extruded rubber profiles perform under most conceivable circumstances, including in space, under the sea, and on the road.
Some of the industries that rely on rubber extrusion products include: automotive, aerospace, appliance, electronics, sports and recreation, flooring, industrial cleaning, marine, and more.
Rubber Products Produced
Some of the most common extruded rubber products out there are extruded rubber seals, rubber sheeting, rubber bushings, gaskets, rubber profiles, rubber channels, rubber trim, rubber weatherstripping and rubber tubing.
Weatherstripping, rubber trim, and extruded rubber are essential for effective climate and moisture control. Rubber sheeting enjoys an almost universal presence in the entrance to many homes and businesses as textured floor matting. Rubber bumpers, bushings, grommets and connectors all contribute to effective, efficient and safe workplaces, vehicles and homes. For every other context in which a rubber product or rubber part is needed, rubber extruders can produce custom rubber profiles to meet unusual or unique needs.
In homes, rubber trim can be found lining baseboards, protecting paint and wallpaper from damage. Likewise, a rubber u-channel can be used as a door seal or trim seal. Rubber mats can be placed under rugs to protect hard floor surfaces and prevent injury from slipping.
Rubber grommets line the openings in sheet metal power strips, protecting the cords from wear and the consumer from the risk of shock. Rubber sealants keep bathroom fixtures watertight and kitchens water-efficient. Weather strip products have become increasingly important for commercial and residential builders with an interest in maintaining energy-efficiency.
In industrial contexts, rubber rods, rollers and sheets absorb vibration and noise from machinery with moving parts. Rubber bushings make vehicle suspension systems effective, and rubber bumpers protect freight trucks and loading docks. Rubber profile production is limited only by its designer's creativity.
Finding Rubber Extrusion Manufacturers
The quality of the rubber product you receive is correlated to the expertise of the supplier or manufacturer with whom you choose to work. So, the question is, how do you find the best extruded rubber supplier?
Use a Business Directory
Finding a supplier via search engines is like shooting an arrow in the dark, as you don’t really know whether or not the supplier is reputable. To circumvent falling into the trap of a notorious or unknown supplier, consider taking help from a directory.
Many high-traffic online business directories, like IQS Directory, have lists of reliable and experienced companies, suppliers, and manufacturers that can provide you with high quality extruded rubber products. Use such a directory to locate a dependable supplier.
Ask the Right Questions
Once you’ve found three or four suppliers in which you are interested, reach out to them and inquire about their products and prices. Make sure to cover your product requirements (size, shape, stiffness, flexibility, color, quantity, etc.), as well as a date of delivery. Also, it’s incredibly important to note whether or not you feel that the supplier has good customer service skills. Sometimes, you will find a manufacturer that offers great prices, but subpar services. It’s better to bypass a manufacturer like this, because potential problems in their service may end up costing you more in the long run. Compare the services and offers from three to four suppliers and finalize your deal.
Variations and Similar Processes
The rubber extrusion process is similar to plastic extrusion, where a raw material, in the form of nuggets or kernels, is fed into a screwing column, which transforms the raw material into semi-fluid by a combined action of pressure and temperature, and then the material is passed through a die. The die gives a predefined shape to the raw material. The difference emerges after that, plastic extrusion has its own post-processing steps and rubber extrusion has its own. When it comes to rubber extrusion, the physical properties that a finished rubber product will have are determined at the curing step.
A common alternative to rubber extrusion is rubber injection molding. Manufacturers often mold rubber this way when working with thermosetting polymers, elastomers, or highly complex designs. This rubber molding process involves heating a material until warm or molten, and then injecting it into a mold cavity, where it shapes, cools, and hardens.
A variation on rubber extruding is feed extruding. A feed extruder is simply an extrusion machine designed to create animal feed on a commercial scale. It does so by taking the combination of ingredients it is fed and sending them through the extruder. Inside the machine, the feed is created in three steps: pre-conditioning, extrusion cooking, and texturing/shaping.
Rubber Extrusion Terms
- A substance that quickens the pace of vulcanization and lowers the operating temperature required during the process.
- Term for the bond between a rubber surface and a non-rubber surface.
- Air Curing
- Vulcanizing a rubber product in the air instead of in a press or steam vulcanizer.
- Atmospheric Cracking
- Crevices that form on rubber surfaces due to exposure to environmental conditions, such as temperature extremes and precipitation.
- Atmospheric Pressure
- The amount of force the atmosphere exerts upon the earth’s surface, measuring 14.7 psi at sea level.
- The development of a powdery residue on a rubber surface as a result of surface breakdown.
- Small cracks on the surface of rubber, usually from environmental damage.
- Compression Set
- A measurement of a rubber material’s return to its initial shape after deformation.
- Deformation on a rubber surface due to the application of stress.
- Die Swell
- The incidence of swelling that occurs after the extruded profile comes out of the die. The rubber product’s properties determine the amount of swelling that will occur.
- A change in a rubber material’s hardness over time.
- An apparatus used for measuring the hardness of rubber.
- Applying powder to rubber to prevent adhesion to something else.
- A characteristic of rubber, describing its tendency to return to its initial shape after warping.
- A material capable of returning to its initial length after being stretched at room temperature to twice its original length.
- An increase in length after the application of stress. This occurs during stretching.
- Flex Test
- A laboratory test that measures a rubber material’s resistance to bending deformation.
- The resistance to motion when different surfaces are in contact.
- Energy loss in the form of heat that results from the deformation of elastomeric material, caused by the application of stress.
- The softening of raw rubber by mechanical and atmospheric forces.
- The extent of a rubber material’s ability to resume its original shape after deformation.
- The amount of stress required to create a certain amount of elongation.
- Optimum Cure
- The point during vulcanization at which rubber material attains the intended properties.
- The degree to which rubber will retain deformation.
- Pounds per square inch. This measurement indicates pressure level.
- The comparison of the amount of energy needed to create an elastic deformation, and the amount of energy needed to recover from such a deformation.
- Tensile Strength
- The maximum amount of stress that may be applied to rubber before breaking occurs.
- The process of increasing the strength and elasticity of rubber materials through the application of heat and pressure.