Patent ES-303474-A1 (ES)
Title: Method for the production of open-work structures
IPC: B29C55/16
Filed: 1964-08-26
Abstract: Openwork structures are made from sheet thermoplastic materials, e.g. polyethylene, polypropylene and its copolymers, and polyesters, e.g. polyethylene naphthalene-2,6-dicarboxylate, in which at least one surface of the material is embossed with small solid bosses or raised figures arranged with spaces between adjacent bosses in longitudinal and transverse rows, said material being subjected to stretching or stressing in two directions at right angles to one another simultaneously until the material takes a degree of permanent set and splits in various positions or zones of the thinned material located between the raised figures, to leave, upon the cessation of the stressing, bands integral with and interconnecting the various adjacent raised figures, to constitute the openwork structure. In Fig. 1, the material 28 is stretched transversely (arrows 31) in a tenter frame (not shown) as it passes from rollers 29, 30 to rollers 32, 33. As the transverse stretching would normally longitudinally shorten the material in the tentering zone, simultaneous longitudinal stretching would be effected even if the rollers 32, 33 had the same peripheral speed as rollers 29, 30. Rollers 32, 33 are however rotated at a higher speed than rollers 29, 30 to increase the extent of simultaneous longitudinal stretching. In Fig. 2, the material 34 passes from rollers 35, 36 to rollers 37, 38 having a higher peripheral speed. The material would normally become narrower but is restrained from so doing by providing gripping means (not shown) to prevent this so that the material is automatically transversely stressed (arrows 39) while being longitudinally stretched (arrow 40). Thus, material as shown in Fig. 3 (not shown), with hexagonal bosses, takes up the form shown in Fig. 4 (not shown), with six thinned bands 41 proceeding from each hexagonal raised figure 42 to the six adjacent hexagonal raised figures. Similarly, material as shown in Fig. 5 (not shown), having elongated rectangular projections takes up the form shown in Fig. 7 (not shown). In general, the bosses may be polygonal and are preferably regular. Triangular, square, pentangular, hexagonal and heptagonal bosses are specified. The stretching force in one direction may be equal to, greater than, or less than that in the other direction, and each such stretching force may be limited in its action and only remain effective to produce a certain percentage or actual elongation in the particular direction, whatever the percentage or actual elongation to be effected in the other direction. The applied force of one of the two simultaneously applied forces may be uniform or regularly or otherwise varied during its calculated continuance. Although the simultaneous forces are in action simultaneously in some cases one may be started more or less a short time before the other or one may be calculated to end more or less a short time before the other. In some cases the net-like sheet material obtained may be submitted to a second simultaneous stretching method according to the invention. Thus the material of Fig. 8 (not shown), is made by softening the material of Fig. 4 (not shown), by heat and simultaneously stretching it both longitudinally and transversely.