General Information in the recommendation of coated abrasive products
The manufacture of coated abrasives is performed by a combination of three basic raw materials:
- a mineral
- a backing on which to put the mineral
- an adhesive bond used to hold the mineral to the backing.
A great variety of coated abrasive products may result from the many variations of combining these three raw materials. In order to select the abrasive product which will give you the best results at the least expense, the following background information on the three basic raw materials will be very helpful.
COATED ABRASIVES - MINERALS
There are six basic minerals used in the manufacture of coated abrasives. These six fall into two general classifications:
- Aluminum Oxide
- Silicon Carbide
Aluminum Oxide - is brown in color. This synthetic material is extremely tough and is characterized by enormous penetrating strength, even at high speeds. Aluminum oxide has the greatest variety of uses among the coated abrasive minerals. It is the toughest of the grains and its toughness gives long life while its sharpness delivers a fast cut. It is recommended for most of the metal finishing applications.
Silicon Carbide - is shiny black in color. It is very sharp, practically as hard as diamond, and brittle. It fractures into new cutting points during use. It is best suited for very hard or very soft materials. It is preferred for titanium, rubber, leather and similar materials and for finishing and rubbing .lacquers and enamels. It is also well suited for hard plastics.
Flint - is actually quartz. It is white in color, and used commonly by the home craftsman.
Emery - is dull black in color. It is a very hard grit and round in shape. Used widely for polishing. This is a natural form of Aluminum Oxide.
Garnet - is a reddish brown abrasive. Due to its medium hardness and relative sharpness, it is widely used in the woodworking industries.
Crocus - is a deep brown or maroon abrasive. It is very soft, and is used primarily in polishing soft metals.
Mineral Grading Grits - There are a total of twenty-two different grain particle sizes (grits) ranging from No. 12 (the coarsest) to No. 600 (the finest). The grit comparison chart will show the breakdown of these grits.
COATED ABRASIVES - BACKINGS
The backings used in the manufacturing of coated abrasive products are classified into four basic groups: 1) Cloth, 2) Paper, 3) Fiber and 4) Combination. These four backings are further subdivided as follows:
- "J" Weight (Jeans)- light weight and flexible - used mostly for finishing and fine polishing.
- "X" Weight (Drills) - heavier, more durable and stronger than "J" weight - recommended for medium and heavy-duty work.
- "A" Weight - lightest and most flexible of the papers - used primarily for finishing operations.
- "C" and "D" Weights - intermediate weights which are stronger but less flexible - used for hand sanding and for oscillating machine sanding.
- "E" Weight - strongest, most durable and least flexible. Has the most resistance to tear and is used primarily for mechanical sanding.
- Fiber - made from impregnated paper which has been condensed and hardened. Fiber is tough, strong, hard and able to withstand heat used for discs and drum sanding.
- Paper and Cloth laminated together and used on high speed drum sanders and for floor sanding.
- Fiber and Cloth laminated together and used for discs.
COATED ABRASIVES - ADHESIVE BONDS
The third basic raw material used in the manufacture of coated abrasives is the bond. There are two layers of adhesives that anchor and lock the mineral to the backing. The first layer of adhesive is applied to the cloth then the abrasive is applied to that layer and locked on with a top layer of adhesive. There are basically five types of bonds used.
- Glue - used on both layers (glue over glue and usually results in the smoothest or finest finishes.
- Glue and Filler - this is a fine filler added to animal hide glue, producing a bond both durable and strong.
- Resin over Glue - Glue is used for the first (bond) coat and a synthetic resin for the second (size) coat. This bond has better heat resistance than glue.
- Resin over Resin - The synthetic resin is used for both coats and thus produces a bond which is the toughest, strongest and has the greatest resistance to heat.
- Waterproof - This bond consists of a synthetic resin used for both coats on a waterproof backing. This permits the use of water or other liquid lubricants with the coated abrasive.
COATED ABRASIVES - COATINGS
There are two basic types of coatings: closed coat and open coat.
- Closed Coat - refers to coating in which the entire surface of the backing is covered by abrasive grain, with no voids between -pieces. This is by far the most general in use and permits the greatest degree of stock removal.
- Open Coat - refers to coating in which 50% to 70% of the backing is covered by abrasive grain. There are voids between pieces of grain. This is used to prevent the small chips removed by the abrasive from building up on the abrasive and thus "choking'! the effective use of the coated material. Use of an open coat material, if recommended, is generally in the coarse grit applications.
FLEX - COATED ABRASIVES
Coated abrasives are stiff, when manufactured, due to the hardness of the bond. The stiffest combination is Resin over Resin on X weight backing. The most flexible, as manufactured, is Glue over Glue on J weight backing.
Most uses require a controlled flexing of the coated abrasive, base on the contour of the object to be polished and the diameter and size of the contact wheel. The polishing of a flat object with a hard, large diameter contact wheel requires little or no flex. A very irregularly contoured object polished on a small or soft contact wheel will require a very flexible coated abrasive to permit the abrasive to follow the contour. Flexing of the coated abrasive decreases its polishing life; therefore, no more flex should be requested than is required for its specific use.