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ASTM D 4918
Coefficient of Static Friction of Uncoated Writing and Printing Paper by Use of the Inclined Plane Method
The ASTM D 4918 Paper COF Slide Angle test describes a procedure for the determination of the coefficient of static friction (COF) of paper measured when sliding against itself. The inclined plane procedure is used.
Although this test method is basic in concept, the precision statement has been developed on uncoated writing and printing papers. While the use of this test method is recommended for those grades only, it may be used with other types of paper giving specific attention to special paper characteristics and with the understanding that the precision may not be the same.
Static COF relates to the force required to initiate movement between two surfaces while kinetic COF relates to the force required to cause continuation of the movement at uniform speed. Kinetic COF cannot be measured by this test method. Both static and kinetic COF can be measured by Test Method D 4917.
One specimen of the paper sample is clamped to an inclined plane, the other to a rubber-faced sled. The sled is placed on the inclined plane and the plane raised until the sled begins sliding. The coefficient of static friction is equal to the tangent of the angle at which sliding begins.
The coefficient of friction of printing and writing papers indicates the ease with which the top or bottom sheet of a stack of paper will slide across the succeeding sheet, such as occurs on the infeed of a sheet-fed printing press or the sheet transport into a copier machine. A minimum coefficient of friction is required to prevent the double-feeding of any sheets.
Since each sheet is removed from the stack only once, a single slide of each pair of specimens is performed and the value recorded.
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