Rubber Rollers
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Introduction
This article offers industry insights about rubber rollers. Read further to learn more about:
- What is a Rubber Roller?
- Advantages of Rubber Rollers
- Construction of Rubber Rollers
- Rubber Roller Manufacturing Process
- And much more&#;
Chapter 1: What is a Rubber Roller?
A rubber roller is a machine component consisting of an inner round shaft or tube covered by an outer layer of elastomer compounds. The inner shaft is typically made from steel, aluminum alloys, or other strong, rigid materials. The outer layer is usually composed of polymers such as polyurethane, silicone, EPDM, neoprene, or natural rubber. Rubber rollers are utilized in various manufacturing processes for operations including:
- Printing
- Pushing and Pulling
- Film Processing
- Material Conveying
- Squeezing and Wringing
- Straightening
- Cooling and Uncooling
- Pressing
- Laminating
- Driving
- Deflecting
- Feeding
- Spreading
- Coating
- Grains Milling
- Pushing
- Pulling
- Wringing
- Straightening
- Coiling/Uncoiling
- Driving
- Deflecting
- Metering
- Embossing
- Tensioning
- Converting
- Sanding (belt)
- Cutting (bandsaw)
- Sorting
Rubber rollers leverage the advantageous properties of elastomers, including impact strength, shock absorption, compression and deflection, abrasion and chemical resistance, a high coefficient of friction, and adjustable hardness. These qualities make rubber rollers ideal for handling manufactured goods without causing damage, unlike metal rollers. Additionally, the rubber covering can be reworked or repaired, often with less time and cost compared to metal core repairs. Rubber rollers are preferred in applications that require high surface durability combined with low to medium hardness. With appropriate design and engineering of the rubber compound, these rollers can endure mechanical and thermal stresses effectively.
Chapter 2: What Are the Advantages of Rubber Rollers?
Rubber rollers are favored for their unique elastic properties, which metals cannot provide. Metals can corrode, scratch, dent, and crack easily and frequently, while their texture and hardness can damage the materials they contact. Although fiber-reinforced composites offer superior quality, they are typically more expensive and less readily available. Rubber rollers, on the other hand, are a cost-effective solution that offers extended longevity and distinctive mechanical properties, including:
- Surface with a high coefficient of friction: The coefficient of friction between steel surfaces under clean and dry conditions is about 0.5 to 0.8. Aluminum to steel also yields a comparable value of about 0.45. Rubber, on the other hand, has a coefficient of friction with a range of 0.6 to 1.2 for various materials. This makes rubber a suitable lining material for conveying equipment such as rollers. A high coefficient of friction prevent the items from sliding, especially when transferring items in a non-level plane.
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No burrs from scratches and tears: Metals can easily be scratched by harder materials. These scratches can develop burrs on the surface of the roller, which damage products during operation. The covering of rubber on rollers protects the metal core from damage. Any damage to the surface of the rubber is not as detrimental to operation, in contrast with the sharp burrs from a scratched metal.
- Resists deformation from impacts: Because of their elasticity, rubbers are known to have good impact strength. They can easily absorb energy and dissipate it to a larger area while returning to their original shape. This prevents surface indentations and cracks which can cause the roller to prematurely fail.
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Better chemical resistance: Specific rubber types can withstand different chemical exposure. Covering the roller core can prevent corrosion, which can cause permanent damage to the roller. The most popular option for a metal roller that resists chemical attacks is stainless steel, which is far more expensive than rubber linings.
- Replaceable lining: Since the rubber lining takes the most damage during operation, the rigid roller core is preserved, with no structural damage. The roller core can easily be serviced by removing and replacing the used rubber lining. This extends the life of the roller core and the whole equipment. It also prevents expensive repairs like replacing the whole roller or cylinder.
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Chapter 3: How Are Rubber Rollers Constructed?
The two main components of a rubber roller are the roller core and the rubber cover. The roller core serves as the primary structural element and is connected to the main drive unit. The rubber cover is the part that makes contact with the load. Each component is detailed further below.
Roller Core
The roller core is the rigid structural component that supports the load. It is typically constructed from high-strength materials such as carbon steel, stainless steel, alloy tool steel, or aluminum alloys. Roller cores are designed based on their specific applications and can be divided into several parts for further specification.
Roller Core Shaft
The shaft is the component that connects the entire roller to the motor, sprocket, or other drive units. It is solidly constructed to provide high strength and uniform hardness, designed to withstand both bending and torsional stresses. Bending stresses arise from radial forces exerted on the roller, while torsional stresses result from the torque generated as the roller rotates to move loads tangentially. The shaft can be coupled to the drive unit using either a key and keyseat or set screws.
Roller Core Cylinder
The cylinder is a hollow component, usually shaped like a pipe or tube, onto which the rubber lining is wrapped and bonded. It is designed with sufficient thickness to resist deflection under load. While steel is the most common material used for the cylinder, other rigid yet lightweight materials, such as aluminum and reinforced plastics, can also be employed.
Roller Core Flange
The flange or end plate connects the cylinder to the shaft. The shaft, cylinder, and flange are typically secured together by welds. In some cases, particularly in smaller roller constructions, flanges are pressed into place and held by interference fit.
Roller Core Bearings
Bearings are employed to minimize friction between static and rotating parts. The configuration, mounting, and type of bearing can vary based on the roller's design. In the previously described configuration, the shaft is installed alongside the roller cylinder. In other designs, the bearing may be mounted on the roller while the shaft remains static on the main equipment.
Rubber Cover
The rubber lining is the outer cover that interacts with the load or process material. It endures the most wear and tear, protecting both the roller core and the load surface. The choice of rubber material and grade depends on the specific roller application. Below are the types of rubber recommended for various properties:
- Hardness: SBR and FKM for high hardness (Shore A 60 to 95); NBR and PUR for a wider range (Shore A 10 to 95)
- Abrasion Resistance: SBR, PUR, XNBR, HNBR, CSM
- Tear Strength: SBR, PUR, XNBR, HNBR, CSM
- Compression Set: NBR, CR, Silicone, PUR
- Thermal Resistance: SBR, EPDM, Silicone, FKM, CSM
- Low-Temperature Toughness: NBR, CR, EPDM, Silicone
- Aging Resistance: Butyl, CR, EPDM, Silicone
- Acid and Alkali Resistance: Halogenated Butyl, EPDM, CSM
- Water Resistance: Halogenated Butyl, Silicone, EPDM, CSM
- Oil Resistance: NBR, CR, FKM
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Solvent Resistance: NBR for petroleum-based solvents; CR, EPDM, Silicone, and Butyl for alcohol-based solvents; CR, EPDM, CSM, and Butyl for ketone and ester-based solvents.
Chapter 4: What Is the Rubber Roller Manufacturing Process?
The manufacturing of a rubber roller involves a series of straightforward steps, including fabricating the roller core, compounding the rubber, bonding, covering, vulcanizing, grinding, and balancing. The roller core may be sourced from external fabrication shops or produced in-house by the rubber roller manufacturer.
Roller Core Fabrication and Preparation
The cylinder or hollow tube is formed through sheet rolling and welding. This can be done by the rubber roller manufacturer or by a separate plant that supplies steel tubes. The ends of this tube can be machined to receive bearings. If required, flanges or support discs are cut that are sized to fit inside the cylinder. A shaft is fabricated by turning a metal stock in a lathe machine producing a cylindrical core. This shaft can either be welded to the flanges as stated above, or be slid into the bearings on each end of the tube. All dimensions must be accurate to attain the roller's required diameter, roundness, and balance. The flanges are then welded to the ends of the cylinder with the shaft. After fabrication, the roller core is subjected to secondary processes such as blasting and cleaning to remove any traces of corrosion and contaminants on its surface.
Rubber Compounding
Rubber compounding is the formulation process in which specific chemicals are added to raw rubber to alter its final mechanical and chemical properties, reduce its cost, and improve its processability and vulcanization. This involves heating and masticating the rubber, which breaks down its polymer chains, making it more receptive to the compounding ingredients. The process is carried out using roll mills, Banbury mixers, or screw kneaders (extruders). Common ingredients in rubber compounding include filler systems (such as carbon black, silica, and calcium carbonate), plasticizers (for softening and processing), stabilizer systems (such as antioxidants and antiozonants), and vulcanizing agents (such as sulfur and peroxide).
Bonding and Building
Bonding is the process that involves adhering the rubber cover to the surface of the roller core using a chemical bonding agent or ebonite base layer. Once the bonding components are applied, the rubber building process can begin. Rubber building is the process of covering or lining the rigid roller core with the rubber compound. Some of the general methods for rubber building are explained below.
Plying Process
Plying is a widely used method where the roller core is rotated while feeding calendered rubber sheets or strips onto it. The rubber sheets wind or wrap around the core until the desired diameter is achieved. The core can be pressed against two or three rollers to apply pressure and ensure a tight and secure rubber cover.
Extrusion Process
In this process, rubber is extruded from a machine and directly bonded to the surface of a rotating roller core, rather than using calendered rubber strips. This method is particularly well-suited for large rollers, such as those used in big paper mills.
Casting or Molding
This process involves placing the roller core into a mold or die where rubber resin is transferred or injected. The resin covers the roller core and is introduced to high heat to cure the rubber.
Vulcanization and Cooling
Vulcanization, or curing, is the process of forming crosslinks between the elastomer chains or rubber compound, enhancing the rubber's stability and resistance to heat, cold, and solvents. This is achieved by applying heat, which activates curative agents such as sulfur and peroxide to bond with the rubber. After heating, the rubber is allowed to cure for several minutes to hours, after which it is cooled before proceeding to the next stages.
Roller Shaping and Crowning
Crowning is an optional process that shapes the roller to have varying diameters along its length. This creates a tapered, convex, or concave shape which allows a slight deflection when pressed against a load.
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Groove Cutting
Groove cutting is the creation of specially designed depressed and elevated regions on the surface of the roller to increase the surface area of the roller, to prevent slippage, to improve heat dissipation, and to apply embossings and print patterns.
Rubber Roller Grinding
This process smoothens the surface of the rubber cover by removing protruding parts and leveling overlapping strips. Grinding is done by rolling the rubber roller against an abrasive wheel, typically in some kind of turning lathe.
Roller Core Balancing
A roller core can experience imbalance in two ways: static and dynamic. Static imbalance occurs when the roller rolls to its heavy side when rotating freely. Dynamic imbalance, on the other hand, results in a rocking motion or vibration as the roller reaches its operating speed. Rubber rollers are usually inspected and corrected for dynamic imbalance. This is done by testing the roller in a computer-controlled dynamic balancing system at its normal operating speed, which then determines the necessary counterweight's location and amount.
Chapter 5: What Are the Characteristics of Rubbers for Roller Applications?
The desirable properties of rubber compounds stem from their molecular structure. Rubbers are polymers with a highly elastic nature, achieved through the crosslinking of long polymer chains into amorphous structures. This structure enables them to deform and absorb energy under load without permanent damage. Key properties of rubbers include:
- Hardness: Hardness is the ability of a material to resist localized surface deformation. The harder the rubber roller surface, the more difficult it is to penetrate, distort, or compress. Harder does not mean better since the rubber roller must be able to absorb some of the force or energy so that the material being handled will not be damaged. For rubbers, hardness is most commonly characterized by the Shore A hardness number (from 0-100) as measured by a durometer gauge or instrument.
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Abrasion Resistance: Abrasion resistance is the ability of the rubber surface to withstand the progressive removal of material through mechanical action. It can be classified into two types: sliding and impingement abrasion. Sliding occurs when a soft and hard material slides or rubs into each other, with or without contaminants between the surfaces. Impingement abrasion, in contrast, happens when particles impact the surface and cause erosion.
By intuition, it may be assumed that rubbers with high hardness values have better abrasion resistance. This correlation is true for homogenous material with a uniform or near-perfect microstructure, such as crystals and metals. However, this is not entirely true for rubbers since they have a different microstructure&#;chained and crosslinked polymer chain. Moreover, other factors affect abrasion resistance like compound composition, cure strength, temperature, and the presence of degrading external elements such as moisture, oxygen, ozone, and ultraviolet light.
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Impact Resistance: Impact resistance, often referred to as impact strength or impact toughness, is defined as the property of a material to resist sudden forces or loads. Rubber is one of the best materials that exhibit this property due to its inherent ability to take elastic deformation. They can deform to absorb the shock and return to their shape while dissipating the energy throughout the body of the material. Many materials can feature shock absorption properties as long as they have some degree of ductility or pliability. Rubber absorbs impact energy well without becoming damaged or deteriorating.
- Compression Set: When subjected to compression, some rubber compounds remain compressed or deformed upon the removal of load. This phenomenon is known as compression set and is seen as the decrease in thickness of a rubber lining. Compression set can also be described as the loss of resiliency after prolonged elastic deformation. Rubbers with a low compression set are desired for roller linings, especially in applications that require dimensional stability over dynamic application of loads. Compression set is affected by various factors such as the duration of load application, operating temperature, and rubber compound composition.
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Tear Strength: This is the ability of the rubber lining to withstand the application of tensile forces that tends to rip the material apart and propagate the tear throughout the body of the material. Tear propagation can vary depending on how the force is applied and the microscopic structure of the material. Tear strength can sometimes be correlated to abrasion resistance. Materials with good abrasion resistance are likely to have good tear strength.
- Low-Temperature Toughness: When cooled, rubber tends to change its mechanical properties. Rubber loses a significant amount of elasticity, making it stiff and slightly to moderately brittle. This process is physical rather than chemical, making it possible to be reversed. However, in this brittle state, the material can easily develop tears or fractures, which can easily propagate as the rubber contracts. Low-temperature resistance depends on the type of rubber compound and can be increased by using additives such as plasticizers and softeners.
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Aging Resistance: Aging is the degradation of rubber characterized by the loss of strength and elasticity. Rubber undergoes accelerated aging through high temperatures with the presence of oxygen. Aging is an irreversible process that changes the structure and composition of the rubber compound. Aging resistance varies on the type of rubber compound. Aging resistance varies depending on ther rubber compound and can be further improved using stabilizers and antioxidants.
- Chemical and Water Resistance: Aside from the composition and structure of the main polymer chain, certain functional groups are present along the molecule, which binds with other functional groups within the chain. This creates the amorphous molecular structure of the rubber. Acids, alkalis, organic solvents, and water can degrade the rubber compound by reacting with the functional groups on the elastomer chain. Different types of rubbers exhibit varying chemical affinities for specific groups of chemicals. An example is an application involving ketone solvents. EPDM and Butyl can easily resist chemical attacks while NBR cannot.
Chapter 6: What Types of Rubbers Are Used for Rubber Rollers?
Different rubber compounds provide varying mechanical properties and chemical resistance. The most common rubber compounds used for rubber rollers are listed below:
Polyurethane, or urethane, rollers are among the most widely used types of rubber rollers due to their versatile physical properties. Polyurethane can be blended from various types and proportions of compounding ingredients, allowing for nearly any desired property to suit specific applications. It can be formulated to create hard, durable components for high-performance uses like wheels and rollers, or softer, shock-absorbing parts for applications such as impact-absorbing pads and cushions. Numerous formulations are available on the market, including proprietary blends from major chemical producers.
Polyurethane rollers are favored for their toughness, high impingement resistance, shock absorption, and fatigue resistance. These properties result from the reaction of various chemicals in its polymer system, which consists of four main components: polyol, diisocyanate, curatives, and additives.
Polyol is the first component used to create the main polymer chain in polyurethane. This polymer chain can be either polyether-based or polyester-based. The second component, diisocyanate, reacts with the polyol to form longer molecular chains through polymerization. Together, the polyols and diisocyanates form the resin or prepolymer blend used in polyurethane production.
The third component is the curative, which facilitates crosslinking between functional groups along the polymer chains, giving polyurethane rubber its elastic properties. The final component, additives, enhances the polyurethane by providing additional properties such as anti-aging and low-temperature toughness.
Polyurethane rollers are highly versatile and suitable for nearly all rubber roller applications. Common uses include printing, milling, packaging, material handling, military and marine applications, aerospace, the food industry, and automotive maintenance and repair.
Silicone rollers are made from polymers with a silicon-oxygen backbone, rather than a carbon chain, and include methyl, vinyl, and phenyl groups. They offer excellent resistance to oxygen, ozone, heat, light, and moisture, and provide superior release properties. However, silicone rollers tend to be more expensive and have limited mechanical properties compared to other materials.
Chloroprene (Neoprene) Rubber Rollers (CR)
Neoprene is a polymer of chloroprene created through emulsion polymerization. The chlorine in the polymer chain enhances its resistance to oxidation, ozone, and oil. While chloroprene is a versatile polymer, it does not excel in any particular area. It is used in the roller industry for its tackiness and straightforward construction capabilities, though it is generally less common than NBR due to its higher cost.
Styrene-Butadiene Rubber Rollers (SBR)
Styrene-butadiene rubber (SBR) is a copolymer made from butadiene and styrene, typically produced through emulsion (chain-growth) polymerization (E-SBR). SBR is a versatile, general-purpose rubber that competes with natural rubber in the market. It is favored for its superior abrasion, tear, and thermal resistance compared to natural rubber.
Polybutadiene Rubber Rollers (BR)
Polybutadiene is a polymer made from the polymerization of butadiene monomers. It comes in three different types, depending on the isomer of butadiene used. Butadiene rubber is known for its excellent resistance to cracking, abrasion, and rolling, but it is susceptible to ozone degradation.
Butyl Rubber Rollers (IIR)
This material is a copolymer of isobutylene and isoprene, abbreviated as IIR. Isoprene makes up only about 3% of the copolymer, providing the necessary unsaturation for vulcanization. The low level of unsaturation in IIR allows it to resist most chemicals, both gases and liquids, and it exhibits excellent aging resistance when properly vulcanized.
Halogenated Butyl Rubber Rollers (CIIR, BIIR)
This rubber compound is derived from modifying IIR through halogenation, which involves introducing allylic chlorine (CIIR) or bromine (BIIR) into the double bonds of the isoprene monomer. This modification creates new crosslinking chemistry. Like unmodified IIR, halogenated IIR retains excellent air impermeability and provides strong resistance to moisture, chemicals, and ozone.
Acrylonitrile Butadiene (Nitrile) Rubber Rollers (NBR)
This rubber is a copolymer of acrylonitrile and butadiene, polymerized in an emulsion process similar to that used for SBR. Known as NBR, it is widely utilized in the roller industry because of its excellent resistance to oils and petroleum-based solvents, along with its abrasion resistance and ability to achieve high hardness.
However, NBRs have limitations, including low tensile strength and poor performance at low temperatures. To address these issues, reinforcing fillers are added. Carboxylated Nitrile (XNBR) and Hydrogenated Nitrile (HNBR) are variants that significantly enhance many of NBR's physical properties, allowing them to compete with polyurethane (PUR) in terms of characteristics such as superior heat resistance.
Ethylene Propylene Rubber Rollers (EPM, EPDM)
These rubbers are produced by copolymerizing ethylene and propylene. When only ethylene and propylene are copolymerized, the resulting rubber can only be cured with peroxide. Introducing a diene into the mix allows the polymer to be cured with sulfur. EPM/EPDM rubbers are known for their excellent weathering resistance, insulating and dielectric properties, as well as their superior mechanical performance at both high and low temperatures and resistance to chemicals.
Fluorocarbon (Viton) Rubber Rollers (FKM)
Fluorocarbon rubbers are a group of elastomers primarily made from vinylidene fluoride (VDF) copolymerized with other substances like hexafluoropropylene (HFP) and tetrafluoroethylene (TFE), among others. These rubbers can also be formulated as terpolymers or tetrapolymers. Fluorocarbon rubbers, or FKMs, are known for their excellent mechanical properties and outstanding resistance to oils and greases.
Natural Rubber Rollers (NR)
Natural rubber is derived from latex harvested from the bark of the Hevea tree and is composed primarily of the polymer chain polyisoprene. It is highly valued for its superior heat buildup resistance and fatigue resistance compared to other types of rubber.
Polyisoprene Rubber Rollers (IR)
Isoprene rubbers, or IR, are general-purpose elastomers produced by polymerizing isoprene monomers. The polymer chain of IR closely resembles that of natural rubber. Synthesized in a controlled environment, isoprene rubbers are chemically purer than natural rubber while often exhibiting similar or enhanced properties.
Conclusion
- A rubber roller is a machine part that is composed of an inner round shaft or tube covered by an outer layer of elastomer compounds.
- Rubber rollers take advantage of the desirable properties of elastomers, such as impact strength, shock absorption, abrasion resistance, high coefficient of friction, and controllable degree of hardness.
- The two main parts of a rubber roller are the roller core and the rubber cover. The roller core is the main structural component connected to the main drive unit. On the other hand, the rubber cover is the component that is pressed against the load.
- Manufacturing of a rubber roller is a straightforward process involving the fabrication of the roller core, rubber compounding, bonding, covering, vulcanizing, grinding, and balancing.
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