This article offers industry insights about rubber rollers. Read further to learn more about:
A rubber roller is a machine part 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. On the other hand, the outer layer is typically fabricated from a polymer such as polyurethane, silicone, EPDM, neoprene, and natural rubber. Rubber rollers are used in different manufacturing processes for performing operations such as:
Rubber rollers take advantage of the desirable properties of elastomers, such as impact strength, shock absorption, compression and deflection, abrasion and chemical 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 itself, compared to metal rollers. Plus, the rubber covering can be regrouped or repaired to continue its service life; in most cases, in less time and investment than a metal core repair. They are the preferred machine parts for applications requiring high surface durability with low to medium hardness. With proper design and engineering of the rubber compound, rubber rollers can withstand degrading forces brought by mechanical and thermal factors.
Rubber rollers are used because of the elastic properties of rubber that cannot be offered by any metal. Metals can be corroded, scratched, dented, and cracked which can happen easily and frequently. In addition, the texture and hardness of metals scratch and damage whatever medium they touch. Other materials like fiber-reinforced composites can provide better quality but are more expensive and not readily available. They are not readily available and are typically more expensive. Rubber rollers are the most economical method while offering extended roller longevity and physical and mental unique mechanical properties such as:
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 on below.
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.
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; 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.
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, such as aluminum and reinforced plastics, can be used.
The flange or end plate connects the cylinder to the shaft. The shaft, cylinder, and flange are typically held together by welds. In some cases, flanges are pressed into place and held by interference fit, which is normally seen on smaller roller constructions.
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.
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 to protect 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.
Manufacturing a rubber roller is a straightforward process involving 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 made in-house by the rubber roller manufacturer.
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 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, banbury mixers, or screw kneaders (extruders). Common compounding ingredients are filler systems (carbon black, silica, calcium carbonate), plasticizers (for softening and processing), stabilizer systems (antioxidants, antiozonants), and vulcanizing agents (sulfur, peroxide).
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 is the most common method, involving 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 pressure to tightly wrap the rubber cover.
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.
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 or curing is the process of creating crosslinks between the elastomer chains or the rubber compound. This makes the rubber more stable, enabling it to resist the effects of heat, cold, and solvents. Vulcanization is done by applying heat to the system, which initiates the bonding of 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.
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.
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.
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 at its normal operating speed. It then determines the location and amount of counterweight needed.
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.
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.
Different rubber compounds offer different mechanical properties and chemical resistance. The most common rubber compounds used for rubber rollers are listed below.
Polyurethane or urethane rollers are often considered 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 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 popular 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.
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 provide 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, the military, marine applications, aerospace, the food industry, and automotive maintenance and repair.
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 and excellent release capabilities. However, silicone rollers are more expensive and have limited mechanical properties.
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. Chloroprene is a good all-around polymer but not exceptional in any specific area. It is used in the roller industry due to its tack and simple construction capabilities. However, it typically is not as prevalent as NBR due to its cost.
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 and is preferred due to its better abrasion, tear, and thermal resistance than natural rubber.
This is produced from the polymerization of a butadiene monomer. Polybutadiene is available in three types due to the different isomers of butadiene. Generally, butadiene rubber has good cracking, abrasion, and rolling resistance but is prone to ozone degradation.
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.
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.
This rubber is a copolymer of acrylonitrile and butadiene that are polymerized in an emulsion similar to SBR polymerization systems. NBR is widely used in the roller industry due to its resistance to oils and petroleum-based solvents, as well as its abrasion resistance and high hardness capability.
However, NBRs have low tensile strength and poor low-temperature performance. Reinforcing fillers are added to solve these problems. Carboxylated Nitrile (XNBR) and Hydrogenated Nitrile (HNBR) are used to significantly improve many of the overall physical properties of NBR, which can compete with PUR in physical characteristics, including superior heat resistance.
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, insulating and dielectric properties, excellent mechanical properties at high and low temperatures, and chemical resistance.
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 comes from latex harvested from barks of plants, mainly from the Hevea tree. It contains the polymer chain polyisoprene. Natural rubber is preferred because of its excellent heat buildup and fatigue resistance compared to other types of rubbers.
Isoprene rubbers are general-purpose rubbers created from the polymerization of isoprene monomers. The polymer chain is similar to that of natural rubber. IR is synthesized in a controlled environment, making it chemically purer than natural rubber with similar or superior characteristics.
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