Tech Talk
What is involved in the manufacturing of coated abrasives?
The key step in coated abrasive manufacture is the application of the abrasive. This may be done by pouring the abrasive in a controlled stream onto the adhesive impregnated backing, or more commonly running the impregnated backing through a tray of abrasive and allowing it to pick up the grain. Electrostatic attraction and orientation is also used to cause the grain to imbed itself firmly into the adhesive, oriented “sharpest point uppermost”, resulting in a very aggressive and fast cutting tool.
The pattern and amount of abrasive being placed onto the backing can be very closely controlled to provide two types of abrasive systems: an open coating (covering 50 to 70 percent of the surface), which has spaces between the abrasive grain for chip clearance, or a closed coating, where there are virtually no spaces between the grains. Open coating is best suited for more flexible applications and closed coated for very arduous conditions. Once the grain has been deposited, the belt is coated with the final “sizing” coat to anchor the abrasive onto the backing, and then rolled into a giant roll for storage.
Before the coated product is used, it has to be processed into a marketable form. The adhesive will cause the belt to become very stiff and has to be “broken” in a controlled manner, usually perpendicular to the edge of the belt. The break lines are determined by the grain size of the abrasive – closer for fine grain and wider for coarse grain. Other break line patterns are used for a variety of applications. Too much flexing and breaking of the bonding reduces the life of the belt and, therefore, is kept to a minimum.
Papers and sheets are cut from the large production rolls of coated abrasives and packed. Belts have to be cut and joined. The joint is a lap joint, generally 45 degrees to the edge of the belt. However, for narrow belts the angle is usually more acute, and for wider belts more obtuse. To form the lap joint without a significant lump in the belt, one end of the belt has the abrasive removed from the backing and the other end has a very small amount of the backing removed, so that when the joint is made, it is virtually invisible and will not upset the surface finish of the workpiece being machined. Coarse grain belts, which are not expected to produce high quality surface finishes, are usually lap jointed by removing the abrasive from only one end of the belt. The lump in the belt will not effect the resultant, rough surface finish.
Unlike the system for grinding wheels, there is no standard nomenclature for coated abrasives. However, there is a proposed unofficial systems with similar information.
