The term coating / coating type or coating density in the area of coated abrasives describes the density with which the abrasive grain is applied to the backing. There are 3 different types of coating:
Close coating: the backing is almost completely coated with abrasive grains. The large quantity of grain points used during this type of coating provides for high stock removal and a long service life of the abrasive. Close coating is typically applied on abrasives used for metal processing.
Semi-open coating: approx. 70 - 80% of the backing surface are covered with abrasive grains. The space between the individual grain points allows for the discharge of swarf. A semi-open coating is typically used for applications that involve increased stock removal, e.g. the grinding of varnishes, plastics and NF metals.
Open coating: approx. 50 - 70% of the backing surface are covered with abrasive grains. The greater chip clearance between the abrasive grains found in this coating reduces clogging and allows for the discharge of grinding chips from the grinding area. An open coating is typically used for abrasives used on wood and other long chipping materials that would otherwise quickly clog the abrasive.
The coating density can, furthermore, be used to influence the aggressiveness of an abrasive and the fineness of the ground surface.
Pressure = force/area (p = F/A)
the exerted force is spread across a smaller number of grain points in an open coating as opposed to a close coating. This allows the grain to penetrate deeper and provides for greater stock removal. On the other hand, the force exerted on each individual grain is, of course, lower in a close coating where a lot of grain points are in use; the grain does not penetrate as deeply, resulting in a more finely ground surface.
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