Thursday, July 29, 2010

Pencil Hardness Test - Wolff-Wilborn Pencil Tester

Website: http://www.sheeninstruments.com/products/hardness/wolff-wilborn.htm
Test procedure and datasheet:
http://www.sheeninstruments.com/datasheets/wol.pdf


PHOTO: Wolff-Wilborn Pencil Tester Sheen 720N
http://www.elektrophysikusa.com/images/content/pencil_hardness.jpg

The purpose of scratch hardness tests is to determine the resistance of coating materials or lacquers to scratch effects on the surface.



PHOTO: http://www.byk.com/instruments/products/images/products/146/HR9500a.tif.jpg



PHOTO: Twenty pencils in the range of grades from 9B to 9H ( ISO 15184 / BS 3900 - E19 )
http://pad2.whstatic.com/images/3/30/Pencil_hardness_452.jpg


In the Wolff-Wilborn Test, twenty pencils in the range of grades from 9B to 9H ( ISO 15184 / BS 3900 - E19 ) are used, each being moved over the surface under a fixed pressure and angle to the surface. The degree of hardness of the pencil which damages the surface is taken as a measurement of scratch hardness e.g. '3H' hardness.


PHOTO: http://4.bp.blogspot.com/_3cvpfoj7mb8/SwrJC-XEQcI/AAAAAAAAA9g/z34ubSjeAwo/s1600/PencilGradingChart.png


Complies with ASTM D3363, ECCA-T4-1, BS 3900-E19, ISO 15184.



INSTRUCTIONS FOR USE
The test methods describe how the relative hardness of a coating is to be determined. This is achieved using a set of pencils of known hardness.

The test surface needs to be correctly prepared and smooth. (Please refer to the relevant test standard.) The test should be performed at 23°C ± 2°C with a relative humidity of 50 ± 5% (ECCA T4) or 25°C ± 2°C (ASTM D3363).

1. Select a mid range pencil eg. 2H.

2. For wood pencils, remove approximately 3/16 to 1/4 in. (5 to 6 mm) of wood from the point of each pencil using a draftsman-type mechanical sharpener, being careful to leave an undisturbed, unmarked, smooth cylinder of lead. Holding the pencil holder ( when using drawing leads) at an angle of 90° to the abrasive paper, rub the lead against the paper maintaining an exact angle of 90° to the abrasive paper until a flat, smooth and circular cross section is obtained, free of chips or nicks in the edge of the cross section.



PHOTO: Step 3
http://www.gardco.com/images/hardness/wolffwilbornGARDCO/calibraation_red.jpg

3. Stand the tester onto its end face and insert the pencil until its point touches the flat surface. Tighten the pencil clamping screw.

4. Place the 720N on the test surface and push it forward by approximately 1/4 -1/2" (6-12mm). Be sure to hold the gauge by the indentations on the wheels.

5. Rotate the pencil through 90° and move the gauge 1/2" (12mm} to one side of the first test. Repeat step 4.

6. Repeat step 5.

7. Examine the coating for indentation and scratching. If there is none, repeat the test using a harder pencil eg. 3H. If the surface is scratched or indented repeat with softer pencil eg. H.

8. Repeat step 7 until a pair of pencils is found, one of which scratches / indents the coating, the other does not.


PLEASE NOTE: This method is only applicable to smooth surfaces and coatings.




Reference