| 摘要 |
PROBLEM TO BE SOLVED: To prevent crack of a flute bottom while improving draining performance and improve durability. SOLUTION: A groove wall 5 of a flute inclines in the direction that the groove width extends and at fifteen degrees or less. An intersection part 6 of this groove wall 5 and a groove base 4 are formed with circular arc 7 of which radius R1 is 2mm or less, and the groove base 4 extends substantially parallel to the flute and has the plural projections 10 rising slightly from the deepest part 9 of the groove base 4. Moreover, a rising height H of this projection 10 from the deepest part 9 is set in a range of 0.5 to 1.0mm, and the width W of the projection 10 is set in a range of 1.0 to 1.5mm. |
