Geogrid or mesh structure

A geogrid made by stretching and biaxially orienting a plastics starting material which was provided with an array of holes, the geogrid comprising:

• a first set of substantially straight oriented strands extending at an acute angle to a first direction;
• a second set of substantially straight oriented strands extending at an acute angle to the first direction and, as considered in a second direction at right angles to the first direction, alternate (angled) strands of the two sets being angled to the first direction by substantially equal and opposite angles;
• further substantially straight oriented strands extending in said second direction;
• and
• junctions each interconnecting four of the angled oriented strands and two of the further oriented strands, at substantially each junction the crotch between each pair of adjacent strands being oriented in the direction running around the crotch, whereby there is continuous orientation from the edge of one strand, around the crotch and to the edge of the adjacent strand.

A method of making a biaxially oriented plastics material geogrid, comprising:

• providing a plastics sheet starting material which has holes in an array of hexagons of substantially identical shape and size so that substantially each hole is at a corner of each of three hexagons, there being within the hexagon no holes of a size greater than or equal to the size of the first-mentioned holes;
• applying a stretch in a first direction to stretch out strand-forming zones between adjacent holes on sides of the hexagons and form oriented strands from such zones;
• and
• applying a stretch in a second direction substantially at right angles to said first direction to stretch out strand-forming zones between adjacent holes on the sides of the hexagons and form oriented strands from the latter zones, whereby centre portions of the hexagons form junctions interconnecting the oriented strands, the stretching being applied to such an extent that the orientation of the strands extends into substantially each junction so that at substantially each junction, the crotch between each pair of adjacent strands is oriented in the direction running around the crotch, whereby there is continuous orientation from the edge of one strand, around the crotch and to the edge of the adjacent strand.

A method of making a plastics material mesh structure, comprising:

• providing a plastics sheet starting material which has holes in a regular pattern, which holes define potential strand-forming zones extending between respective holes and which on stretching the starting material in one direction would stretch out to form oriented strands;
• forming depressions in and thereby weakening some but not all said potential strand-forming zones without material removal when the plastics material is at a temperature below the lower limit of its melting range, said depressions defining a regular pattern, wherein in said one direction, said depressions are formed in every other potential strand-forming zone;
• and
• applying a stretch in said direction so that the weakened potential strand-forming zones form oriented strands but the non-weakened potential strand-forming zones do not form oriented strands though some stretch may be applied thereto and whereby the mesh structure so produced is not that that would be produced from the starting material without said depressions.

A method of making an oriented plastics material geogrid, comprising:

• providing a plastics sheet starting material which has holes on a rectangular grid whose axes extend in a first direction and in a second direction substantially at right angles to the first direction, thereby providing first rows of holes extending in the first direction and second rows of holes extending in the second direction, and which starting material has weakened zones between alternate pairs of adjacent holes in each first row, the weakened zones being staggered as between adjacent first rows so that a weakened zone in one first row is adjacent a non-weakened zone in the adjacent first row on either side;
• applying a stretch in the first direction to stretch out strand-forming zones between adjacent holes in each second row to form oriented strands from such zones;
• and
• applying a stretch in the second direction to stretch out the weakened zones to form oriented strands from the weakened zones without stretching out non-weakened zones between adjacent holes of the first rows to the same extent as the weakened zones are stretched;
• whereby the non-weakened zones form junctions each of which interconnects six of the oriented strands.

A method of making biaxially oriented plastics material mesh structure which has oriented strands which extend at an angle other than 90° to the first and second direction of stretch, comprising:

• providing a plastics sheet starting material which has holes in a regular array;
• applying a stretch in a first direction to stretch out respective strand-forming zones between adjacent holes and form oriented strands from such strand-forming zones;
• applying a stretch in a second direction substantially at right angles to said first direction to stretch out other respective strand-forming zones between adjacent holes and form further oriented strands from the latter strand-forming zones, whilst applying restraint to the material in the first direction;
• subsequently discontinuing said restraint;
• and
• subsequently allowing the material to relax in the second direction.

A method of making a biaxially oriented plastics material geogrid, comprising:

• providing a plastics sheet starting material which has holes in an array of hexagons of substantially identical shape and size so tat substantially each hole is at a corner of each of three hexagons, there being within the hexagon no holes of a size greater than or equal to the size of the first-mentioned holes, the vertices of the hexagons being aligned in a first direction, the vertex pitch of each hexagon being less than the diagonal pitch;
• applying a stretch in the first direction to stretch out strand-forming zones between adjacent holes on sides of the hexagons and form first and second oriented strands from such zones, the first and second oriented strands extending in different directions to each other;
• and
• applying a stretch in a second direction substantially at right angles to the first direction to stretch out strand-forming zones between adjacent holes on the sides of the hexagons and form third oriented strands from the latter zones, whereby centre portions of the hexagons form junctions interconnecting the oriented strands and triangular meshes are formed each by a first oriented strand, a second oriented strand and a third oriented strand, the first oriented strands entering a junction being substantially aligned and the second oriented strands entering a junction being substantially aligned.