Patent ID: 6423394
Filing Date: 2002-07-23
Classification: B29C,B29D,Y10S,Y10T

Abstract:
A method of producing a biaxially-stretched plastics material geogrid for use in a geotechnical construction, having a greater strength in a primary direction (PD) than in a secondary direction (SD) substantially at right angles to the PD, comprising:providing a substantially uniplanar plastics starting material which has a thickness of at least about 2 mm and has a pattern of through-holes on a substantially square or rectangular grid whose axes are substantially parallel to the PD and to the SD respectively, the sides of at least some PD end portions of said holes being defined by crotch-forming zones having protuberances; applying PD stretch to form oriented PD strands and to apply some orientation to the junction-forming zones so that orientation extends into and through the junction- forming zones; and applying SD stretch with an overall stretch ratio of at least about 1.5:1 as measured from the mid-point of one junction-forming zone to the mid-point of the adjacent junction-forming zone in the SD to form oriented SD strands; in the geogrid so produced, the mid-point of the junction zone being substantially thicker than the mid-point of any oriented strand entering the junction zone, and the edge of the crotch interconnecting adjacent sides of adjacent oriented PD and oriented SD strands being oriented in the direction running around the crotch with the orientation ratio decreasing significantly as one passes around the crotch edge either from the oriented PD strand or from the oriented SD strand, the crotch edge either a) having an unoriented part, or b) the thickness of the least oriented part of the crotch edge being reduced, or the length of the least oriented part of the crotch edge being increased, by no more than about 20% by the action of stretching, and the action of stretching not reducing the thickness of any point along generally diagonally-extending lines of maximum thickness on the biaxially-stretched mesh structure from the mid-point of the junction zone to said crotch edges to such an extent that the ratio of finished thickness to starting thickness at that point is less than about 80% of the ratio of finished thickness to starting thickness of the junction zone mid-point.