Patent GB-1067871-A (GB)
Title: Improvements in and relating to apparatus for stretching sheet materials
Inventor: BROMLEY KEITH
IPC: B29C55/16
Filed: 1964-04-17
Abstract: 1,067,871. Films. BAKELITE-XYLONITE Ltd. July 19, 1965 [April 17, 1964], No. 16064/64. Heading B5B. [Also in Division D1] In a tentering apparatus for simultaneous biaxial stretching of films the distance between successive film grippers (12) in the diverging portion (4) of the film track is increased by engaging the grippers with the spokes and a rotating wheel or wheels. In Fig. 1 a film 34 is fed from a feed roll not shown, and gripped by grippers (12) at point A. The grippers are opened and closed by cam means not shown on the star wheels 5a and 5b. Grippers (12) are passed along guide means (1) by other grippers fed round behind them and by the action of star wheels (5) and run along platforms 32. While grippers (12) are contacting platforms 32 springs (26) (Fig. 3, not shown) are compressed inwardly so that when grippers (12) reach the end of the platforms 32 the pins (22) spring outwardly to contact wheel (30) at a point between two ribs (31). Each gripper (12) is engaged by a rib (31) and is hurled outwardly along the divergent portion (4) of its associated guide means (1) at an ever-increasing velocity. WHen grippers (12) reach the edge of the wheel (30) they fly off and are picked up by the endless belts (33) which carry them along portions (3) towards rubberrimmed wheels (6). Grippers (12) release the film 34 at point B and are opened at that point by cam means not shown associated with wheels (6). The clearance between the rubber rimmed wheels (6) and guide means 1 is such that grippers (12) are forced against wheels (6) and are therefore carried round the semicircular portions of guide means (1) by rotation of wheels (6). The grippers (12) are then returned to star "wheels (5) to begin another circuit by the, action of gravity. This is effected by arranging that the rubber rimmed wheels (6) are on a higher level than the star wheels (5) so that when the grippers come up to engagement with the wheels (6) they run back towards the star wheels (6). The construction of the grippers is shown in Fig. 3, not reproduced. The grippers 12a that run on guide means 1a are mounted in the position shown in Fig. 3, i.e. with the pins (22) extending downwardly and grippers, (12b) running on guide means 1b are mounted in the inverted position with the pins (22) extending upwardly. Hence the spokes 31a of wheel 30a engage grippers 12a, and the spokes 30b of wheel (31b) engage grippers (12b). Wheel 30a is located below the plane of guide means 1a and rotatable wheel 30b is located above the plane of the guide means (1). The action of platform 32a is shown in detail in Fig. 2. If desired the part of the apparatus comprising wheels (30) and divergent portion (4) of guide means (1) may be enclosed within an oven, not shown, and an annealing zone and/or a cooling zone may be provided at the end of the divergent portion (4) of guide means (1). In the apparatus of Figs. 1 and 2 the ribs (31) on wheels (30) are straight and radial but the ribs (31) may have other shapes as shown in Figs. 4-7 (not shown), depending on the velocitytime relationship desired for the grippers. These wheels may replace wheels (30) shown in Figs. 1 and 2, the rest of the apparatus remaining the same. Both wheels (30) used in the apparatus, however, must be of the same form. Fig. 8 (not reproduced) indicates a form of apparatus similar to that of Figs. 1 and 2 except that portions of the guide means (1) instead of being straight as in Fig. 1 are curved. This has the effect of straightening out the velocity-time curve of the grippers and tending to produce a more linear acceleration. This form of apparatus also includes means for returning the grippers to star wheels (5) comprising an endless chain having means (35) engaging with the grippers as they leave the coaxial wheels (30). The chain is driven by wheels (36) and wheels (6). The apparatus of Fig. 9 (not reproduced) is the same as that of Fig. 8 except that wheels (30). are of the form shown in Fig. 5. With this form of wheel the acceleration imparted to the grippers increases as the grippers approach the circumference of the coaxial wheels (30), thus flattening out the velocity-time curve for the grippers, so that in this form of the apparatus the acceleration tends to be linear until near the circumference of the wheel (30) where it gradually decreases. In Fig. 4 the ribs (31) are offset from the radii of wheel (30) so that the paths at the centre of the pin (22) of each gripper, (12) remain on the same radius as the wheel (30) as that gripper proceeds along itsguide means (1). In Fig. 5 ribs (31) have much the same form as those of Fig. 4, except that at the outer ends they curve away from the radii of the wheel (30). In this case the acceleration curve of the grippers (12) flattens out as grippers (12) approach the circumference of wheel (30). This has the advantage that the change in acceleration as the grippers (12) leave wheel (30) is reduced. In Fig. 6 the ribs (31) lie on non-radial chords of wheel (30) and in this case the acceleration curve of grippers 12, although being of the same general shape as the acceleration curve of the grippers accelerated by wheels (30) shown in Fig. 1, is flatter. In Fig. 7 ribs (31) initially diverge from the radii of wheel (30) but then curve round to cross the radii and finally again curve round towards the circumference of wheel (30). Using this type of wheel the velocity-time curve tends towards a straight line, provided the grippers first contact wheels (30) at a point after ribs (31) have started to curve round to cross the radii of wheel (30). By suitable adjustment of the guide means and rotatable members exceedingly high longitudinal stretch ratios may be obtained and the apparatus is thus especially suitable for materials such as polypropylene requiring a stretch ratio of about 7:1. High rates of stretch are possible as the stretching zone can be made short. The apparatus is also suitable for stretching materials requiring much lower stretch ratios, for example polyvinylchloride and polystyrene.