This article gives detailed insights into polyurethane rollers. Read further to learn more about:
Polyurethane rollers are cylindrical rollers covered by a layer of elastomer material called polyurethane. Depending on the application, the inner roller core is prone to scratches, dents, corrosion, and other types of damage. The layer of polyurethane has intrinistic properties the inner roller core, such as abrasion resistance and impact strength. Polyurethane rollers are used in many manufacturing processes for performing operations such as:
Polyurethane is the most extensively used elastomer material for rollers. It is possible to make a variety of blends from different types and proportions of its compounding ingredients. Through blending, varying mechanical properties are obtained to suit a particular use. The most desirable properties of polyurethane are its toughness, high impingement resistance, shock absorption, and fatigue resistance.
Formulations of polyurethane can produce rollers that are hard and firm or soft and pliable. Polyurethane rollers are processed in hard to soft durometers depending on the requirements of an application. Polyurethane's durability and shock resistance make it one of the most popular types of roller compositions. In addition, polyurethane is an elastomer that can be thermoformed into any possible or conceivable shape.
Polyurethane and rubbers, such as nitrile and neoprene, belong to a family of materials called elastomers. Their elastic properties make them an ideal choice as roller-covering material. Polyurethane rollers have several advantages over ordinary rubber rollers. Their impressive mechanical properties make them more viable than other types. The many benefits of polyurethane rollers include:
Processing polyurethane does not require expensive and complicated equipment. A simple blending operation may only require a batch mixer (no heating equipment needed). This is in contrast with other rubber types, which employ equipment for mastication and heating.
Rubber roller linings typically have carbon black added as filler and reinforcing components. When rolled against a surface with sufficient force, they can leave marks that are unpleasant in applications such as finished goods handling and printing. Polyurethane rollers do not have this problem since they do not use carbon black.
Depending on the type of polymer system used, polyurethane can withstand water, oils, and other petroleum-based solvents. Water resistance is a required property for rollers under wet service or in environments with frequent washdown. Oil resistance is required for handling hydrocarbon-based solvents or chemicals such as inks.
Most of the engineering work for polyurethane rollers happens during the production of the elastomer lining. Four components make up the material: the polyol compound, the diisocyanate compound, the chain extender or curatives, and the additives.
The mixture of the polyol and the diisocyanate compounds forms the prepolymer resin. They combine to form a simple polymer chain from the reaction of a polyol component (a carbon-chained molecule with alcohol on both ends) with a diisocyanate component (a molecule with isocyanate on both ends). This results in a molecule with reactive alcohol on one end and a reactive isocyanate on the other. The alcohol end further links to another isocyanate end or terminal, while the isocyanate end of the same chain further reacts with chain extender compounds or curatives such as hydroxyl and amines. This process continues, making a long, chained polyurethane molecule.
The mechanical properties depend on the formulation of the prepolymer resin and the curatives. Additives are used to further improve the properties of the polyurethane, such as resin curing time, machinability, color, UV protection, and etc. Careful proportioning of additives relative to the amount of resin in the mixture is required since it can weaken the properties of the product.
The previous chapter briefly described how polyurethane is made and discussed the roles of the four different components. In this chapter, the focus is on the different chemicals used to make polyurethane and their implications on the final property of the product.
Like the polyols, diisocyanate compounds form the resin side of the polyurethane system. There are two main types of diisocyanate: aliphatic and aromatic.
Curative - Curatives are mixed with the polyol and diisocyanate prepolymer to form a solid or semi-solid elastomer. There are two basic types of curatives: hydroxyls and amines.
Polyurethane is regarded as an engineering material because of its excellent properties. Most of which are attributed to its highly elastic nature. Below are some of the properties of polyurethane significant to roller applications.
Materials are graded according to their hardness relative to one another. For elastomers, hardness is characterized by the Shore Hardness Number. This is measured by a durometer. There are 12 different Shore Hardness Scales; each scale has its indenter configuration, profile, and force applied. The Shore scales used for polyurethanes are Shore A and D. Shore A scale measures the hardness of soft, semi-rigid polyurethanes, while Shore D measures hard rubbers and rigid polyurethanes. However, keep in mind that high hardness does not correspond with high rigidity or strength.
With impingement abrasion, the paths of the particles impacting the surface of a material can be perpendicular or at an angle. The attacks of the particles strike specific areas, causing high strain and breaking off small bits of the surface.
Regardless of the type of abrasion, polyurethane is highly capable of resisting both sliding and impingement. Since it has a low friction coefficient and high strength, it is unaffected by sliding abrasion. With impingement abrasion, it absorbs the energy from the particulate attacks, distributes the stress, and immediately recovers without any signs of damage.
Polyurethane is blended to produce a low coefficient of friction, high tear strength, and elasticity, properties that help it withstand abrasion. The composition of its resin produces its exceptional abrasion resistance. Among the polyol compounds for making polyurethane, polyesters have the best tear and abrasion resistance.
Polyurethane tear strength is measured in pounds per inch of thickness. The testing for tear strength is dependent on the application for which the polyurethane will be used. Tear strength tests have been established by the American Society for Testing Materials (ASTM). There are three ASTM tests used to determine the tear strength of polyurethane, which are:
Polyurethane exhibits good thermal aging resistance when formulated with certain compounds such as PPDI and CHDI. Typical polyurethanes have a maximum operating temperature of about 90 to 100°F (32 to 37.7°C). Special but more expensive formulations can reach 302° F (150°C).
The manufacture of a polyurethane roller is a straightforward process that involves the fabrication of the roller core, balancing, polyurethane blending, bonding, building, curing, machining, and quality testing. The fabrication process is similar to that of other rubber roller types. The main difference is cover building since the polyurethane resin is in liquid form.
Polyurethane rollers have a solid core or a keyway and space for bearings, which are designed for one or both ends of the roller. Bearings reduce friction against the static and rotating parts. The configuration, mounting, and type of bearing can vary depending on the design of the roller.
All dimensions of a polyurethane roller must be accurate to attain the required diameter, roundness, and balance of the roller. After fabrication, the roller core is subjected to secondary processes such as blasting, lapping, and cleaning to remove any traces of corrosion and contaminants.
The formulation can be done through different processes. These are known as the single shot, prepolymer, and quasi-prepolymer processes.
The single-shot process involves having all components in separate chambers. These will then be blended by a mixing head and poured or injected into the mold.
The second option is the prepolymer process. This process is carried out by mixing the polyols and diisocyanates before pouring them into the mold. This process helps dissipate the heat produced from the exothermic reaction of the compounds.
Last is the quasi-prepolymer process. Quasi-prepolymers consist of polyols partially reacted with the diisocyanate compounds. This simplifies the formulation process since the quasi-prepolymers are less viscous and require low processing temperature.
Other rubber compounds used for roller linings are in the form of calendered sheets and strips. They are joined to the roller core by plying and extrusion. Polyurethane formulations are available as liquid mixtures. Thus, the processes of building polyurethane are casting and injection molding. Both of these methods use liquid resins.
Casting involves placing the roller core into a mold where the polyurethane polymer is poured. The process of casting is an economical method for manufacturing polyurethane rollers and is much less expensive than injection molding.
Injection molding requires expensive dies that must be machined and configured to the shape of the desired polyurethane roller. It involves a great deal of time and the use of expensive machinery and equipment.
Other polyurethane systems employ an additional curing process known as post-curing. Post-curing further improves the mechanical properties of the cast, as well as its temperature aging resistance.
The choice of polyurethane rollers for an industrial application is mainly based on their durability and wide range of durometers. While tooling for manufacturing rubber rollers is expensive and time-consuming, polyurethane casting uses aluminum molds that are easily formed and shaped to the proper dimensions of the designed roller.
Although polyurethane rollers are easily produced, they have exceptional durability, are resistant to abrasion damage, and come in sizes to fit all types of roller applications.
The choice of polyurethane rollers as conveyor rollers is due to their noise abatement and abrasion and solvent resistance. In packaging and shipping, conveying systems are the foundation of efficient and smooth operations. Materials have to move quickly without being damaged or mishandled. Polyurethane rollers are an important part of the effortless conveyance of parts, tools, customer orders, and equipment. Their noise suppression properties assist in keeping the work environment stress free for workers.
An essential part of a conveyor roller system is idler rollers, which perform different functions to assist in the movement of materials along a conveying belt. The two types of idler rollers are carrying and return, with carrying rollers moving materials while return rollers support the conveying belt during its return.
On the return side, flat return idlers are used to support the return of the conveyor belt. They have a steel support rod and lifting brackets to keep the belt from stretching, deforming, and slacking.
Industrial polyurethane coated casters are a rugged, durable, tough, and strong type of caster capable of carrying up to five tons. They have a higher capacity than rubber wheels and are made of high density thick polyurethane. These polyurethane casters are chosen because they can carry exceptionally heavy loads without damaging floors or other surfaces.
As with polyurethane rollers used on conveying systems, industrial polyurethane casters operate noiselessly and have a larger footprint, which reduces stress on the floor. The longevity of polyurethane industrial casters is due to their elasticity, which provides ergonomic benefits.
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