fiber and the polyurethane prepolymer the knit patterns that may be used in the manufacture of the substrates of the present invention are numerous generally the fabrics are knitted using a three bar knitting machine one bar for the elastic thread and two bars for the fiberglass in the knitted substrates of the present invention the elastic yarn must give the fabric stretch in the length direction of the fabric the elastic yarn may be in bar 3 of a 3 bar or possibly bar 4 of a 4 bar raschel knit construction fabric as previously indicated the elastic yarn in the present invention should not be in the chain stitch the elastic yarn may be intermittently spaced in the fabric there need not be an elastic fiber for every needle employed in knitting the fabric the elastic fiber need only be present in the fabric in a sufficient amount to give the desired compaction to the fabric when the fabric is removed from the knitting machine the third bar and the fourth bar if used could be used to lay in fiberglass in a zig zag or a sinusoidal pattern which would increase the crush strength of the final cast by comparison to transversely laid in yarns in these positions the elastic yarn may be in bar 3 in 3 bar knit or in bars 3 and or 4.