Patent GB-647054-A (GB)
Title: Apparatus for stretching films
IPC: B29C55/08
Filed: 1947-02-18
Abstract: <PICT:0647054/IV (a)/1> Apparatus for the continuous lateral stretching of film material comprises an oven for heating the film, the oven having top, bottom, and side portions for the whole of its length, and gripping devices at each side of the oven, and outside thereof, for conveying the film through the oven and stretching it, the gripped side edges of the film passing by the oven exteriorly thereof.  The oven may be formed of two parts with sufficient <PICT:0647054/IV (a)/2> <PICT:0647054/IV (a)/3> <PICT:0647054/IV (a)/4> <PICT:0647054/IV (a)/5> <PICT:0647054/IV (a)/6> clearance between them for the film to pass into and from the oven.  At least part of the oven may be of fan shape in horizontal section, and the gripping means may travel in paths which are parallel to the diverging walls of the oven.  Heated air may be passed through the oven and adjustable baffles may be provided to control the direction of the air currents.  The stretched film may be laminated with other film layers while it is still hot from the stretching operation.  The film material may be rubber hydrochloride, a cellulose ester, or a polyvinyl compound.  As shown in Figs. 1, 2, 3, the film is taken from roll 1, passed through the oven 3, and finally wound on roll 4. Hot air is forced through the oven by fan 5.  It enters the oven through conduits 7, 8.  Baffles 12, 13 control the circulation of the hot air in portion 11 of the oven.  The air leaves the oven through holes 14 and the gap 15 between the upper and lower halves of the oven.  Baffles 16, 17, pivoted on rods 18, 19 direct the hot air into the space between them so that this space is at a higher temperature than the lateral zones.  The film is stretched laterally by gripping devices such as tenter clips which are moved farther apart as the film passes through the oven.  The clips are linked together to form two chains, one on either side of the oven.  Bars 33 at the front of the machine open the latches of the tenter clips so that the edges of the film may pass into the clips.  The latches shut when the bars 33 are passed and the film is then gripped laterally.  The gripping pressure may be initially provided by springs but as the lateral tension in the film increases the gripping force also increases. Bars 34 at the rear of the machine lift the latches and release the film.  As shown in Fig. 2, the film 30 is gripped across its width by heated roll 40 supported on and driven by rolls 41, 42 at a surface speed at least as great as the speed of the tenter chains.  The film may be laminated with another sheet either of the same or different material.  The other sheet may be brought from above round roll 40 or from below round roll 41.  Two or more rubber hydrochloride films may thus be united under heat and pressure.  By the time the film reaches roll 43 it has cooled considerably, e.g. to room temperature. Between roll 43 and the wind-up roll 4, the film is allowed to contract.  In the case of a plasticized film retraction to the final dimensions may readily be completed before roll 4 is reached.  In the case of unplasticized film more time is required and the film may be passed over a festoon before reaching roll 4, or after being wound on roll 4 may be rewound on to another roll soon afterwards.  The surface speed of roll 43 is preferably slightly higher than that of roll 1 so that the film is stretched longitudinally as well as laterally.  The film is trimmed at the edges by knives 50 and the trimmings 49 are wound separately.  The film passes over a tensioning roll 53 which may move along rack 54 to take up slack.  Guide rolls 56 convey the film to wind-up roll 4.  Roll 57 may have a sponge rubber surface to remove surface tackiness from a rubber hydrochloride film.  Rolls 58 are rubber covered and are set at a slight angle to keep the film taut.  If desired, the hot pressing roll for laminating the film with other films may be situated at a point where the stretched film has been allowed to retract somewhat. To avoid curling, it is preferable to laminate three or more layers, and if the transversely stretched film is sandwiched between two longitudinally stretched films the tension in the two outer layers should be the same at the time of application.  As shown in Fig. 11, the gripping members consist of pairs of tongue-and-groove belts 71, 72 which grip the film 74 and are guided in channels 73.  Another form of gripping device is shown in Fig. 13.  The belts 76, 77 are forced together and guided in channels 79, 80, provided with rollers 81. The gripping device shown in Fig. 14 comprises two chains and guide channels 89.  The chains comprise inter-engaging links 90 and blocks 91, 92.  Rubber hydrochloride film may be heated to 75-110 DEG C.  If desired, the film may be laminated with other film layers before the transverse stretching.  Specification 647,059 is referred to.