This article offers industry insights about rubber rollers. Read further to learn more about:
- What is a rubber roller?
- Advantages of rubber rollers
- Manufacturing process
- And much more…
Chapter 1: What is a Rubber Roller?
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. The inner shaft is made of steel, aluminum alloys, or other strong and rigid material composites. The outer layer, on the other hand, is typically fabricated from polyurethane, silicone, EPDM, neoprene, and natural rubber. Rubber rollers are used in different manufacturing processes for performing operations such as:
- Film Processing
- Material Conveying
- Grains Milling
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. These properties make them suitable for handling manufactured goods without causing damage to the item or to itself, in comparison with metal rollers. They are the preferred machine parts for applications requiring high surface durability with low to medium hardness. Rubber rollers, with proper design and engineering of the rubber compound, can withstand degrading forces brought by mechanical and thermal factors.
Chapter 2: Advantages of Rubber Rollers
Rubber rollers are used because of the elastic properties of rubber which cannot be offered by any metal. Metals can be corroded, scratched, dented, and cracked which can happen easily and frequently. Other materials such as fiber-reinforced composites can provide better quality but are not readily available and are more expensive. Rubber rollers are the most economical method while offering extended roller longevity and unique mechanical properties.
- 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 prevents the items from sliding especially when transferring items in an inclined direction.
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 can damage products during operation. The rubber lining of rubber 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.
Better chemical resistance: Specific rubber types can withstand different chemical attacks. By completely covering the roller core, it can prevent corrosion which can cause permanent damage to the roller. The most popular option for a metal roller to resist chemical attacks is by using stainless steel which is far more expensive than rubber linings.
- Replaceable: Since the rubber lining takes the most damage during operation, the rigid roller core is preserved. The roller core can easily be serviced by removing and replacing the damaged rubber lining. This extends the life of the roller and the whole equipment. It also prevents expensive repairs such as replacing the whole roller or cylinder.
Chapter 3: Rubber Roller Construction
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. The rubber cover, on the other hand, is the component that is pressed against the load. These parts are further elaborated below.
Roller Core: The roller core is the rigid structural member which supports the load. It is typically made of high-strength materials such as carbon steel, stainless steel, alloy tool steel, and aluminum alloys. Roller cores are designed according to their applications. They can be further broken down into several parts.
Shaft: The shaft is the machine part that connects the whole roller to the motor, sprocket, or other drive units. It is solid in construction with high strength and uniform hardness. The shaft is designed to endure bending and torsional stresses. Bending stresses are caused by the radial forces against the roller; while torsional stresses are from the torque generated by rotating the roller to tangentially move loads. The shaft can be coupled to the drive unit by means of a key and keyseat, or by set screws.
- Cylinder: The cylinder is a hollow part typically in the form of a pipe or tube. This is where the rubber lining is wrapped and bonded. It has sufficient thickness to resist deflection upon application of load. The cylinder is usually made from steel, but other rigid but light materials can be used such as aluminum and reinforced plastics.
- Flange: The flange connects the cylinder onto the shaft. The shaft, cylinder, and flange are held together by welds.
Bearings: Bearings are used to reduce friction against the static and rotating parts. The configuration, mounting, and type of bearing can vary depending on the design of the roller. The configuration described previously has the shaft installed together with the roller cylinder. In other designs, the bearing can be installed on the roller while the shaft is static on the main equipment.
- Shaft: The shaft is the machine part that connects the whole roller to the motor, sprocket, or other drive units. It is solid in construction with high strength and uniform hardness. The shaft is designed to endure bending and torsional stresses. Bending stresses are caused by the radial forces against the roller; while torsional stresses are from the torque generated by rotating the roller to tangentially move loads. The shaft can be coupled to the drive unit by means of a key and keyseat, or by set screws.
Rubber Cover: The rubber lining is the outer cover that comes into contact with the load or process material. This part takes the most wear and tear with the intention of protecting the roller core and the surface of the load. The type of rubber material and grade are based on the roller application. Summarized below are the types of rubber that are recommended for providing a specific property.
- 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
- Tear Strength: SBR, PUR
- Compression Set: NBR, CR, Silicone, PUR
- Thermal Resistance: SBR, EPDM, Silicone
- Low-Temperature Toughness: NBR, CR, EPDM, Silicone
- Aging Resistance: Butyl, CR, EPDM, Silicone
- Acid and Alkali Resistance: Halogenated Butyl, EPDM
- Water Resistance: Halogenated Butyl, Silicone
- Oil Resistance: NBR, CR, FKM
Solvent Resistance: NBR for petroleum-based solvents; CR, EPDM, Silicone, and Butyl for alcohol-based solvents; CR, EPDM, and Butyl for ketone and ester-based solvents.
Chapter 4: Manufacturing Process
The manufacturing of a rubber roller is a straightforward process that involves the fabrication of the roller core, rubber compounding, bonding, covering, vulcanizing, grinding, and balancing. The roller core can be supplied by other fabrication shops or can be made 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. Flanges or support discs are cut that are sized to fit inside the cylinder. Meanwhile, a shaft is fabricated by turning a metal stock in a lathe machine producing a cylindrical core. All dimensions must be accurate to attain the required diameter, roundness, and balance of the roller. The flanges are then welded to the ends of the cylinder together 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 where certain chemicals are added to the raw rubber to modify its final mechanical and chemical properties, lower its cost, and aid its processability and vulcanization. This process involves heating and masticating which breaks down the rubber’s polymer chain making it receptive to the compounding ingredients. It is done through roll mills, mixers, or screw kneaders (extruders). Common compounding ingredients are filler systems (carbon black, silica, calcium carbonate), stabilizer systems (antioxidants, antiozonants), and vulcanizing agents (sulfur, peroxide).
Bonding and Building: Bonding is the process that involves adhering the rubber cover to the surface of the roller core. It is done by using a chemical bonding agent or an 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: Plying is the most common method which involves rotating the roller core while feeding calendered rubber sheets or strips. The rubber sheets wind or wrap around the roller until the required diameter is acquired. The roller core can be pressed against two or three rollers to provide some pressure to tightly wrap the rubber cover.
- Extrusion: In this process, instead of using calendered rubber strips, rubber from an extrusion machine is bonded to the surface of a rotating roller core. This process is more suitable 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 creating crosslinks between the elastomer chains or the rubber compound. This makes the rubber more stable which enables it to resist the effects of heat, cold, and solvents. Vulcanization is done by applying heat into the system which initiates the bonding of the curative agents such as sulfur and peroxide. After heating, the rubber is allowed to cure for several minutes or hours. After which, the rubber roller is cooled for the next stages.
Crowning: Crowning is an optional process that is done by shaping 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.
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.
Grinding: This process smoothens the surface of the rubber cover by removing and protruding parts and leveling overlapping strips. Grinding is done by rolling the rubber roller against an abrasive wheel.
- Roller Core Balancing: A roller core can become imbalanced in two ways: static and dynamic imbalance. Static imbalance is described as the roller rolling to its heavy side when made to rotate freely. On the other hand, a dynamic imbalance is the generation of a rocking motion or vibration when the roller is rotated to its operating speed. Rubber rollers are typically inspected and corrected for dynamic imbalance. Dynamic balancing is done by testing the roller into a computer-controlled dynamic balancing system. It then determines the location and amount of counterweight needed.
Chapter 5: Characteristics of Rubbers for Roller Applications
The desirable properties of rubber compounds are brought by their molecular structure. Rubbers are polymers that have a highly elastic nature created by the crosslinking of long polymer chains into amorphous structures. This structure allows them to be deformed and absorb energy upon application of load without suffering permanent damage. Some of the important properties of rubbers are summarized below.
- Hardness: Hardness is the ability of a material to resist localized surface deformation. The harder the surface of the rubber roller, the more difficult it is to be penetrated, distorted, and compressed. 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 characterized by the Shore hardness number as measured by a durometer.
Abrasion Resistance. Abrasion resistance is the ability of the rubber surface to withstand a 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 causing erosion.
By intuition, it can 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, there are other factors that affect abrasion resistance. Some of these factors are compound composition, cure strength, temperature, and presence of degrading external elements such as moisture, oxygen, ozone, and ultraviolet light.
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 deteriorated.
- Compression Set: When subjected to compression, some rubber compounds tend to 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.
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 losses 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, at this brittle state, the material can easily develop tears or fractures which can easily propagate as the rubber contract. Low-temperature resistance depends on the type of rubber compound and can be increased by using additives such as plasticizers and softeners.
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 and can be further improved by 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: Types of Rubbers Used for Rubber Rollers
Different rubber compounds offer different mechanical properties and chemical resistance. The most common rubber compounds used for rubber rollers are listed below.
Polyurethane Rollers (PUR): Polyurethane or urethane rollers are often regarded as the most extensively used rubber rollers. Polyurethane is known for its wide range of physical properties. It is possible to make a variety of blends from different types and proportions of its compounding ingredients. It can virtually have almost any property to suit a particular use. It can be formulated for hard, tough, high-performance parts such as wheels and rollers; or, for soft, shock-absorbing applications such as impact absorbing pads and cushions. Different formulations are available in the market. Some are proprietary blends by large chemical producers.
Polyurethane rollers are a popular choice due to their toughness, high impingement resistance, shock absorption, and fatigue resistance. These properties are brought by the reaction of different chemicals. Its polymer system is composed of four components: polyol, diisocyanate, curatives, and additives.
The polyol is the first component used to make the main polymer chain. A polyurethane polymer chain can be either polyether or polyester-based. The second component, the diisocyanate, is used to bind and polymerize with the polyol compounds to make longer molecule chains. The polyols and the diisocyanates form the resin or prepolymer blend.
The third component is the curative. It helps form the crosslinking between functional groups along the polymer chains that give the polyurethane rubber its elastic property. The last component, the additives, are used to give additional properties such as anti-aging and low-temperature toughness.
Polyurethane rollers are versatile and can be used in almost all rubber roller applications. Common uses of polyurethane rubber rollers are printing, milling, packaging, material handling, military, and marine, aerospace, and automotive maintenance and repair.
- Silicone Rollers: These polymers, instead of having a carbon backbone, have a silicon-oxygen chain with groups of methyl, vinyl, and phenyl. Silicone rollers have good oxygen, ozone, heat, light, and moisture resistance. However, silicone rollers are more expensive and have poorer mechanical properties than organic rubber.
- Chloroprene (Neoprene) Rubber Rollers (CR): Neoprene is a polymer of chloroprene produced from emulsion polymerization. The presence of chlorine in the polymer chain improves resistance to oxidation, ozone, and oil. However, they are more expensive than natural rubber and have poor low-temperature performance.
Styrene-Butadiene Rubber Rollers (SBR): Styrene-butadiene is a type of copolymer of butadiene with styrene. They are usually copolymerized through emulsion (chain-growth) polymerization (E-SBR). SBR 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.
- Polybutadiene Rubber Rollers (BR): This is produced from the polymerization of butadiene monomer. Polybutadiene is available in three types which are due to the different isomers of butadiene. Generally, butadiene rubber has good cracking, abrasion, and rolling resistance but is prone to ozone degradation.
- Butyl Rubber Rollers (IIR): This is a copolymer of isobutylene and isoprene; hence, the abbreviation IIR. Isoprene only consists of around 3% of the copolymer which gives the required unsaturation for vulcanization. The low unsaturation of IIR enables it to repel most chemicals (gas and liquids) and is highly resistant to aging when vulcanized properly.
- Halogenated Butyl Rubber Rollers (CIIR, BIIR): This is a rubber compound produced from the modification of IIR. Halogenation is done by inserting allylic chlorine (CIIR) and bromine (BIIR) into the double bonds of the isoprene monomer creating new crosslinking chemistry. Like the IIR, halogenated IIR has superior air impermeability, and good resistance to moisture, chemicals, and ozone.
- Acrylonitrile Butadiene (Nitrile) Rubber Rollers (NBR): This rubber is a copolymer of acrylonitrile and butadiene. They are polymerized in an emulsion similar to SBR polymerization systems. NBR is widely used due to its resistance to oils and petroleum-based solvents. However, they have low tensile strength and poor low-temperature performance. Reinforcing fillers are added to solve these problems.
- Ethylene Propylene Rubber Rollers (EPM, EPDM): These rubbers are products of copolymerization of ethylene and propylene. Initially, only ethylene and propylene are copolymerized which results in a rubber compound that can only be cured by peroxide. The addition of diene enables the polymer to be curable with sulfur. The main desirable characteristics of EPM/EPDM are good weathering resistance, good insulating and dielectric properties, excellent mechanical properties both at high and low temperatures and chemical resistance.
- Fluorocarbon (Viton) Rubber Rollers (FKM): Fluorocarbon rubbers are a family of rubber mainly composed of vinylidene fluoride (VDF) copolymerized with other chemicals such as hexafluoropropylene (HFP), tetrafluoroethylene (TFE), and so on. Terpolymers and tetrapolymers are also possible. Generally, FKMs have good mechanical properties and excellent resistance to oils and greases.
- Natural Rubber Rollers (NR): Natural rubber comes from latex harvested from barks of plants, mainly from Hevea tree. It contains the polymer chain polyisoprene. Natural rubber is preferred because of its excellent heat buildup and fatigue resistance when compared with other types of rubbers.
- Polyisoprene Rubber Rollers (IR): Isoprene rubbers are general purpose rubbers created from the polymerization of isoprene monomers. The polymer chain is similar to that of natural rubber. Because IR is synthesized in a controlled environment, it is chemically purer than natural rubber with similar or more superior characteristics.
- 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.
- The manufacturing of a rubber roller is a straightforward process that involves the fabrication of the roller core, rubber compounding, bonding, covering, vulcanizing, grinding, and balancing.