ENTRY SHEET FOR DRILLING

摘要

THE OBJECT IS TO PROVIDE AN ENTRY SHEET FOR DRILLING HAVING A LAYER OF CRYSTALLIZABLE WATER-SOLUBLE RESIN COMPOSITION WITH A THICKNESS OF 0.02 TO 0.3 MM FORMED ON AT LEAST ONE SURFACE OF A SUPPORTING METAL FOIL, BEING EXCELLENT IN HOLE LOCATION ACCURACY, HAVING NO RESIN ATTACHED TO A DRILL BIT, AND PREVENTING BREAKAGE OF A DRILL BIT. CRYSTAL GRAINS OF THE WATER-SOLUBLE RESIN COMPOSITION HAVE AN AVERAGE GRAIN SIZE WITHIN A RANGE OF 5 TO 70 Um AND A STANDARD DEVIATION OF NOT MORE THAN 25 UM, THE DRILL BIT ENTRY SURFACE OF THE LAYER OF WATER-SOLUBLE RESIN COMPOSITION HAS A SURFACE ROUGHNESS SM OF NOT MORE THAN 8 UM, AND THE LAYER IS FORMED BY, DIRECTLY ON THE SUPPORTING METAL FOIL, COATING A HOT MELT OF THE WATER-SOLUBLE RESIN COMPOSITION, OR COATING A SOLUTION CONTAINING THE WATER-SOLUBLE RESIN COMPOSITION AND DRYING, AND THEN COOLING FROM A TEMPERATURE OF 120°C TO 160°C TO A TEMPERATURE OF 25°C TO 40°C IN 60 SECONDS AT A COOLING RATE OF NOT LESS THAN 1.5°C/SEC.