Apparatus for stretching sheet material

Claim 2

In apparatus for stretching sheet material which comprises a plurality of clamps equally spaced from one another along the periphery of the sheet material for gripping the sheet material at locations around the area of the sheet material to be stretched, and means for moving the clamps collectively from a first relatively closer position prior to a stretching operation to a second relatively farther position after a stretching operation for stretching sheet material, the improvement comprising means for spacing the clamps with respect to one another in the first position a distance which provides a stress factor in the sheet not exceeding 0.35 over at least 80 percent of the area of the sheet material at all clamp positions between the first position and the second position.

Claim 3

In apparatus for stretching acrylic sheet material which comprises a plurality of clamps equally spaced about the area in which the sheet material is located and having clamping pads for frictionally engaging the sheet matErial at locations spaced inwardly from the outer periphery of the sheet material, and means for moving the clamps collectively from a first relatively closer position prior to a stretching operation to a second relatively farther position after a stretching operation for stretching sheet material, the improvement comprising means for spacing the clamping pads with a free space therebetween at least enough to provide a reduction in thickness of the sheet material between adjacent clamps during the stretching operation which is no more than the reduction in thickness of the middle portions of the sheet.

Claim 4

In apparatus for stretching acrylic sheet material which comprises a plurality of clamps equally spaced about the area in which the sheet material is located and having clamping pads for frictionally engaging the sheet matErial at locations spaced inwardly from the outer periphery of the sheet material, and means for moving the clamps collectively from a first relatively closer position prior to a stretching operation to a second relatively farther position after a stretching operation for stretching sheet material, the improvement comprising means for spacing the clamping pads with a free space therebetween at least enough to provide a reduction in thickness of the sheet material between adjacent clamps during the stretching operation which is no more than the reduction in thickness of the middle portions of the sheet.

Claim 5

The apparatus of claim 2 wherein the width of the respective clamping pads is not more than 55 percent of the distance between the centers of adjacent clamping pads in the first position along any line disposed parallel to the edge of the sheet to be stretched and passing under the respective pads.

Claim 6

In apparatus for stretching plastic sheet material which has been preheated, the improvement which comprises: a. pairs of opposed corner clamping pads for engaging the sheet material at the corners, the corner clamping pads applying forces to the sheet material along perpendicular directions extending parallel to the corner edges of the sheet, and b. a plurality of pairs of opposed clamping pads equally spaced along the edges of the sheet material between the corner clamps, the respective pairs of opposed clamping pads being operable to frictionally engage the preheated sheet material with regions elongated along the direction that force is to be applied to the sheet material and grip it without requiring the application of additional heat through the pads.

Claim 7

In apparatus for stretching plastic sheet material which has been preheated, the improvement which comprises: a. pairs of opposed corner clamping pads for engaging the sheet material at the corners, the corner clamping pads applying forces to the sheet material along perpendicular directions extending parallel to the corner edges of the sheet, and b. a plurality of pairs of opposed clamping pads equally spaced along the edges of the sheet material between the corner clamps, the respective pairs of opposed clamping pads being operable to frictionally engage the preheated sheet material with regions elongated along the direction that force is to be applied to the sheet material and grip it without requiring the application of additional heat through the pads.

Claim 8

The apparatus of claim 4 wherein the corner clamping pads are pie-shaped and have two sides that are perpendicular to one another and a curved third side.

Claim 9

An apparatus as defined in claim 4 wherein the width of the respective opposed clamping pads is not more than 55 percent of the distance between the centers of adjacent opposed clamping pads along any line parallel to the edge of the sheet material and passing under the respective pads.

Claim 10

An apparatus as defined in claim 6 further comprising: means for moving the clamping pads collectively from a first relatively closer position prior to a stretching operation to a second relatively further position after a stretching operation for stretching sheet material; and means for spacing the clamping pads with respect to one another in the first position a distance which provides a stress factor in the sheet not exceeding 0.35 over at least 80 percent of the area of the sheet material at all clamp positions between the first position and the second position.

Claim 11

An apparatus as defined in claim 1 wherein the portions of the clamps in contact with the sheet material are elongated along the direction that force is to be applied to the sheet material.

Claim 12

An apparatus as defined in claim 2 wherein the clamping pads are elongated along the direction that force is to be applied to the sheet material.

Claim 13

An apparatus as defined in claim 3 wherein the clamping pads are elongated along the direction that force is to be applied to the sheet material.