The Scratch, Adhesion and Mar Tester (S.A.M. Tester), is a pocket size universal instrument loading handle for measuring, referred to as the paint chemist's "Educated Knife".
Original Hoffman Tester Concept - The Hoffman Tester is one of the “old line” developments in the paint industry and comes as close as any instrument to date of being “The Paint Chemist’s Educated Knife”. The “blade” of the original Hoffman instrument is made of hardened tool steel, is cylindrical in shape, with the axis of the cylinder held at 45 degrees by a beam carrying a sliding weight. The beam is attached through a hinge arrangement to a small four wheel trolley. Sufficient downward force on the trolley lifts the weight which imposes a force on the cutter against the test specimen in proportion to the positioning of the weight on the beam. The trolley is then pulled across the test sample and the effect, if any, of the “blade” is observed.
The Hoffman Tester when used in the lower range of up to 250 grams loading has been used for determining scratch resistance of a surface and is so listed in STP 500, 18.104.22.168, Paint Testing Manual, published by the ASTM. In this lower range it has also been used for evaluating mar resistance as also listed in STP 500, 22.214.171.124.4. It has had its greatest application, however, in the high range, up to 2500 grams, for cutting completely through the coating to the support surface for measuring properties such as degree of cure and adhesion. Information on its use in measuring adhesion is contained in STP 500, 126.96.36.199.
A major advantage of the cylindrical “blade” used in the original Hoffman instrument is that by a slight rotation, a fresh cutting edge is exposed for use. A major disadvantage is that the “blade” loading is by gravity which dictates that the instrument may be used in the horizontal position only and this adds to the disadvantage of having to accommodate a relatively long weight beam under steady conditions while moving the trolley across the test surface.
Scrape Adhesion and Mar Testing Concept - The most widely used instrument for evaluating coatings for scratch adhesion and mar resistance is the Balanced Beam Tester. Stylus of the exact contour, both loop and needle, developed for the Balanced Beam Tester are now used in the S.A.M. Tester. The advantages are greater sensitivity in the lower loading range on the stylus and portability with ease of use in any position and under most any conditions. (Note: load on the stylus is limited to 2,500 grams.) Load on the laboratory model instrument may be increased to 10,500 grams.
Crosscut Adhesion Testing Concept - Three of the largest variables in measuring adhesion with a multi-tooth cutter as prescribed in ASTM standard D3359, Method B, are now brought under control by the use of the S.A.M. Gardco Instrument Loading Handle.
In addition, an extended life of the cutters will be experienced under most conditions as excessive load on the cutter is largely eliminated. For consistent results, the downward force on the adhesion cutter must be just enough to cut through the test material. If complete penetration is not obtained throughout the cross cut pattern, the individual squares within the pattern are not isolated and erroneous results will be obtained. If too much force is applied, the tips of the cutter teeth penetrate the supporting base resulting in a decrease in area of the squares in the cross-hatch pattern. For example, with a 1mm cutter tooth spacing, the individual squares have an area of 1 sq mm or 0.00155 sq. in. Each of the teeth in the cutter have an included angle of roughly thirty degrees. Therefore, for each mil that the cutter teeth penetrate the base, the area of each square is reduced by about three percent.
Balance Beam Use
CrossCut Adhesion Use
Important Instructions for Changing Range of Tester