present invention a continuous roll of flexible plastic interlayer material such as polyurethane or plasticized polyvinyl butyral is unrolled from a roll 10 to form a continuous ribbon 12 the ribbon 12 is cut into interfitting alternate sheets of trapezoidal shape 14 and 16 while the ribbon 12 is smoothly supported in an unwrinkled condition a typical apparatus for treating a continuous ribbon of clear plastic is disclosed in u s pat no 3 455 197 to richardson it is also feasible to wind the continuous ribbon about a special mandrel such as disclosed in u s pat no 3 467 322 to bachman in the latter apparatus a plurality of layers are wound in unwrinkled condition around the periphery of the mandrel and each circumference of the ribbon is cut at approximately two diametrically opposite locations to form a pair of interfitting trapezoidal sheets from each layer after the sheets 14 and 16 of trapezoidal shape are cut and separated from the continuous ribbon 12 the alternate sheets 14 are stacked in one stack 15 in one common orientation and the alternate sheets 16 are stacked in another stack 17 oriented in another common orientation then each sheet 16 in turn is removed from stack 17 and applied in a desired orientation while at a uniformly but lightly stressed condition merely sufficient to avoid.