What is a Coated Abrasive?
A coated abrasive is a product that consists of a thin layer of abrasive grain attached to a substrate such as paper, cloth, etc. Coated abrasives come in a variety of forms such as sheets, discs, rolls, specialties, or belts.
Components of Coated Abrasives
Coated abrasives are manufactured using abrasive grains; the most common are aluminum oxide, zirconium, ceramic, silicon carbide and garnet. The crude grains are crushed and separated into sizes, called grit sizes, using calibrated screens. Grits range from 12 (very coarse) to 1200 (very fine). Once the grains are separated into sizes, they are attached to a backing material using various bond techniques. Below are descriptions of the most common abrasive grains:
A high performance, man-made abrasive material. Very uniform, high density grain structure is extremely durable and self sharpening for longer life and cooler cut. Excels on tough to grind materials.
A tough, blocky shaped, man-made grain used for high speed grinding and finishing of metals, wood, and other high tensile strength materials without excessive fracturing or shedding. Wherever the ability to resist fracturing is the main consideration, aluminum oxide will outperform all other coated abrasive grains.
A very fine, dense, man-made crystalline grain which can be used for aggressive stock removal. Zirconium is a very dense material with a unique self-sharpening characteristic which gives it long life on heavy stock removal operations.
A very hard, very sharp, man-made abrasive suited for non-ferrous materials and non-metallic materials such as concrete, marble and glass. A very friable grain, silicon carbide cuts faster under light pressure than any other grain used in coated abrasives.
Garnet is made of natural aluminum oxide which is a relatively sharp, but very weak bonding structure. Very inconsistent when compared to synthetics. It is used primarily in woodworking as garnet dulls too quickly to be used in metalworking.
Backing Materials (Substrates)
Below are the four major types of backing materials:
Specialized technical papers are used as substrates for coated abrasives. They are identified by letters representing weight and flexibility:
"A" and "B" weights are light weight and highly flexible. "C", "D", "E", "F" weights are medium to heavy weight for more strength and less flexibility.
Cloth backings are more durable than paper backings. There are several types of cloth backings: cotton (ie: Egyptian), polyester and polyester-cotton blends. Cloth backings are identified by weight and flexibility:
"F" weight (J-Flex) is light and highly flexible. This lighter weight is suitable for cleaning, finishing, and polishing.
"J" weight (Jeans) is highly flexible and suitable for cleaning, finishing, and polishing of contour surfaces.
"X" weight (Drills) is medium to heavy weight for more strength and durability. It has low flexibility suitable for grinding, deburring and finishing.
"H" (Heavy Duty) is a heavier weight than "X" weight. It has low flexibility and is suitable for heavy duty grinding and deburring applications. Excellent edge stability.
Fiber backing is a tough vulcanized material made from rag stock. This backing is usually used for abrasive fiber discs.
Combination backing is laminated paper and cloth, and is very sturdy and shock resistant. Combination backings typically are used for a wide range of grits and mounting techniques.
Other Backing Materials
There are also a variety of other substrates such as nylon fiber or screens that can be coated for special applications. Non-woven nylon impregnated with abrasive grain is another substrate that can be used for cleaning, polishing, or blending.
The grains are locked to the backing material using a bond technique that involves a two layer process. There are three major typesof bond techniques used in coated abrasives:
Resin over Resin
A very strong bond resistant to heat and moisture. Resin over resin bonds are durable for heavy stock removal and is by far, the most popular bonding method.
Resin over Glue
A bond that is resistant to heat. It is more aggressive than glue over glue bonds and leaves a finer finish than resin over resin bonds.
Gue over Glue
A bond that is less durable than resin bonds, but glue over glue bonds produce a more uniform finish. Glue over glue bonds soften under heat, thereby reducing the life of the product.
All coated abrasives are stiff and rigid after the drying and curing of the bond. To achieve the flexibility required for the application, a mechanical flexing process is required. This process is actually a controlled cracking of the bond in one or more directions to achieve the desired flexibility.
Open Coat vs. Closed Coat
These terms refer to the spacing between the abrasive grains bonded to the backing.
Closed coat means the abrasive grains are adjacent to each other with no space between. The majority of applications will benefit from closed coat material because it allows for more material removal.
Open coat means the grains are set apart from each other, achieving a surface coverage of about 60% or more. In situations where loading is likely (soft, non-ferrous materials, painted surface, wood, etc.) open coat will resist loading and clogging and extend the useful life of the abrasive.