Sanding Belt Material Explained
The Construction of Coated Abrasives
Coated abrasives are products manufactured from three basic raw materials; a mineral, a backing and an abrasive bond. Minerals do the job of grinding, finishing and polishing. There are seven abrasive minerals;
Natural; Emery, Garnet
Synthetic; Aluminum Oxide, Silicon Carbide, Alumina-Zirconia, Ceramic Aluminum Oxide, Diamonds
Emery is dull black in color. It is hard, round and blocky in structure.
Garnet is a reddish brown in color. It is of medium hardness with good cutting edges and has the tendency to break or refracture when in use, thus creating new cutting edges.
Aluminum Oxide is a brown colored mineral that is tough and durable and the most widely used abrasive grain. It is coated in the entire grade range and is most commonly used in the metal, wood, leather and shoe industries.
Silicon Carbide is a shiny black, sharp, very hard mineral that is coated in the entire grade range. It is used for a wide range of non-ferrous metals such as brass, titanium, the finishing of stainless steel, shoe trades, extensively for sanding primer, sealer, and top coats of paint and clear finishes. It is also the most effective grain for removing primary metal scale.
Alumina-Zirconia is an alloy mineral of aluminum and zirconium oxide that is very effective for coarse grade stock removal of metal and wood.
Diamonds are extremely hard and used to cut ceramics, stone, and very hard metal alloys such as tungsten carbide. Because of high cost, usage is limited to material that cannot be cut with other abrasives.
Ceramic Aluminum Oxide is a mineral that is neither sintered nor fused. Compared to conventional aluminum oxide, this white mineral is two to three times tougher.
Backings are the base for the mineral and are classified into five basic groups; cloth, paper, fibre, combination, film.
Paper - There are five weights or thicknesses of paper backings designated A,C,D,E and F weights.
"A" weight is a lightweight paper stock coated in the medium and fine grades for use by hand pad and disc sanders.
"C" and "D" weights are intermediate weight paper coated in the medium grades, sold in sheets, discs, and for some belt applications.
"E" weight is a strong and durable heavy paper stock coated in a complete range of grades and primarily used for mechanical sanding operations.
"F" weight is an extra heavy weight paper designed for heavy duty mechanical operations. Belts, drums, cones, and discs.
Cloth is identified by weight and flexibility. The cloth backings are treated with adhesive films prior to coating. These treatments can have a pronounced effect on the backing characteristics, most apparent in flexibility. The most widely used fiber is cotton. Rayon is used as a flexible and firm backing for high strength resistance to tearing and edge wear. It is identified by the addition of the letter "E" to the backing weight. JE is rayon jeans cloth. Polyester fibers are coated where high strength and water resistance are desirable. Polyester is identified by the addition of the letter "F". XF is polyester drills cloth.
"J" wt... (jeans) is the lightest and most flexible cloth.
"X" wt... (drills) is a heavier backing coated in the entire grade range for the widest range of applications.
"Y" wt... a heavy drill for the stress of heavy stock removal and/or high horsepower.
"T" wt...(twills) for selected use requiring a heavy weight cloth.
"M" wt...(duck) for extremely high horsepower abrasive belt use.
"YF" wt...heavy polyester drills
"MF" wt... polyester duck
"JE" wt... rayon jeans
"XE" wt...rayon drills
Fibre -vulcanized fibre- is a heavy weight extremely tough backing made from rag stock. It is used in a heavier weight as a backing for fibre discs and in a lighter weight as a cover for drum sanders.
Polyester film is the newest type of backing that serves as a base for all grades of mineral grains. Used in Imperial brand lapping film for precision finishing in the computer and electronic fields, in Imperial Microfinishing film in the medium grades for lapping and generating uniform finishes and in coarse grade Aluminum Oxide and in Silicon Carbide products for extended life and more uniform finishes.
Coated abrasives have two adhesive layers which anchor and lock the mineral to the backing. The first is called the make coat and the second is the size coat. There are five types of adhesive bonds used; glue, glue and filler, resin over glue, resin over resin, waterproof.
Glue is a straight animal hide glue that is used for both the make coat and size coat. This type is referred to as gluebond.
Glue and Filler is a hide glue to which a fine, heat resistant inert filler has been added producing a bond that is durable and strong.
Resin Over Glue is a combination of pure hide glue for the make coat and synthetic resin for the size coat making a bond that is exceptionally resistant to heat.
Resin Over Resin is a synthetic resin used for both the make and size coats. This type of bond is resistant to heat and moisture.
Waterproof is a synthetic resin used for both for make and size coat on a waterproof backing permitting the use of water or other water containing lubricants with the coated abrasives.
A basic and reasonable rule to keep in mind is to follow the practice of using an abrasive which is no more flexible than is required to conform to the shape being sanded. The less flexible the abrasive is, the more aggressive will be the cutting action and the longer the life. Single flex provide little or no crosswise flexibility, intermediate flexibility is derived from double flexing and maximum flexibility from full flexing or "L" flexing. Coated abrasives following curing or drying of the bond re stiff and rigid. As the weight and thickness of the coating increases with the coarseness of the grade, the flexibility decreases. As required, a mechanical flexing is applied to create ease of handling in normal usage. This flexing is actually a controlled cracking of the bond in one more directions. The designations for the specific flexes are below.
Noflex; No mechanical flex. This unflexed or stiff construction is furnished as standard in items such as sheets, in fine grade paper products for hand or machine sanding where added flexibility is not desirable or necessary.
Singleflex; A single mechanical flex which creates flex lines at 90 degrees to the edge or running direction of the material. This flexing provides a minimum handling flex and the necessary conformability to run over even small diameter pulleys and contact wheels.
Doubleflex; (2-45 degrees angle flexes.) Two mechanical flexing operations which make the material moderately flexible in all directions.
Fulflex; (2-45 degrees angles and a 90 degree angle flex.) Three mechanical flexes resulting in a product that is soft and flexible, regardless of the direction in which it must bend or conform.
"L" Flex; A mechanical flex applied to impart flexibility in the crosswise direction to enable the belt to conform to contours. The flex lines are predominantly in the lengthwise direction. Since the abrasive in belt form, for example, must have crosswise flexibility for handling purposes without harmful cracking, a minor 90 degrees or crosswise component is a portion of the total flex.
There are two types of abrasive coatings that are known as closed coat and open coat.
Closed Coat - In this type the abrasive grains are adjacent to one another without voids between so that the backing is completely covered with abrasive grain.
Open Coat - In this type the abrasive grains are set at a pre-determined distance from one another with a void between. The surface coverage by the abrasive grain is about 50-70%. Open coating is generally preferred when abrading soft materials that tend to load or fill the abrasives.
Gravity Coating Process
In this process, the abrasive grains are dropped from an overhead hopper onto the coated backing. It can be modified in a way to present the sharp cutting edge of the abrasive grain to the best advantage.
Electrostatic Coating Process
This is a process whereby the backing, coated with an adhesive bond, and the abrasive grains pass through an electrically charged field. As the abrasive grains and the coated backings pass simultaneously through the electronic static field, the abrasive grains are propelled upward and imbedded in the adhesive on the backing.