APPARATUS FOR HEAT SEALING SHEETS OF THERMOPLASTIC MATERIAL BETWEEN TWO ELECTRODES

Abstract: An apparatus for impulse sealing thermoplastic sheets (4, 5). The apparatus includes two electrodes (1, 2), at least one of which is movable (1) and at least one of which has a deformable head. The deformable head includes a flexible tube (9) housed in a cavity (8) of one of the electrodes. Compressed air controls the deformation of the tube. A protective strip (13) overlays the deformable head and may be advanced when necessary to provide a new protective cover for the flexible tube (9).

Description: APPARATUS FOR HEAT SEALING SHEETS OF THERMOPLASTIC MATERIAL BETWEEN TWO ELECTRODES Field of the Invention The present invention relates to apparatus for heat sealing sheets of thermoplastic material between two electrodes. It relates more particularly to apparatus that can heat seal sheets composed of, for example, polyethylene or polypropylene, especially for the fabrication of flexible bags or containers holding a perfusion liquid to be administered to a patient intravenously. The apparatus according to the present invention is used for impulse heat sealing sheets of thermoplastic material along a horizontal or vertical line forming the bottom or the lateral wall, respectively, of a flexible bag. Description Of Related Art Presently utilized in the art for heat sealing sheets of thermoplastic material is a device comprising an electrode movable with respect to a counter-electrode which can be fixed or movable. Two sheets that are positioned between these two electrodes are to be heat sealed together. The movable electrode then moves to bear upon the counter-electrode, with the two sheets positioned in between, to heat seal the sheets of thermoplastic material to one another at the location where they are pinched between the electrodes. To achieve good impulse heat sealing it is important to cont r ol two parameters: the length of time of the heat sealing procedure and the pressure exerted by the electrodes on the sheets to be heat sealed. In the prior a the heads of the electrodes that came into contact with the sheets of thermoplastic material have a hard surface, so that the pressure between these heads cannot be regulated. Summary of the Invention An object of the present invention is to provide a heat sealing apparatus that can regulate the pressure exerted by the electrode heads for the duration of each heat sealing cycle, i.e. during the pressing of the sheets between the electrodes, during the heat sealing of the sheets together, and during the crystallization of the heat sealing band. According to the present invention, the apparatus for heat sealing sheets of thermoplastic material between two electrodes, at least one of which is movable with respect to the other, is characterized by one of the electrodes having a deformable part. The deformation of this part is regulated while the electrodes are in contact with the sheets of thermoplastic material to be heat sealed. In one embodiment of the invention, the deformable part is included in the counter-electrode, fixed or movable, that cooperates with an electrode that is movable with respect to the counter-electrode. The deformable part of the counter-electrode preferably comprises a flexible tube lodged in a cavity in the counter-electrode head which faces the head of the movable electrode. This flexible tube is connected to a compressed ai source which creates in the flexible tube a controllable pressure that prevents its deformation by the head of the movable electrode. The pressure existing in the flexible tube is, accordingly, a counterpressure to the pressure exerted by the head of the movable electrode on this flexible tube. Another feature of the instant invention is that the flexible tube, only one portion of which protrudes from the head of the counter-electrode, is protected by a strip that is displaceable step by step over the head of the counter-electrode. This protective strip is rolled at each end over a roller and is guided in grooves provided in the counter-electrode. Other features and details of the invention will become apparent from the following detailed description and drawings which illustrate, as an example, apparatus that is an embodiment of the instant invention. Brief Description of the Drawings Fig. 1 is a side view of the heat sealing apparatus that is the subject of the instant invention. Fig. 2 is a front view of the counter-electrode. Description of the Preferred Embodiment The heat sealing apparatus shown in the drawings comprises a movable electrode 1, a portion of which is shown in Fig. 1. Electrode 1 is moved back and forth in the direction of the arrows X and is positioned adjacent a counter-electrode 2 that is fixed in a support 3. Electrodes 1 and 2 are in the shape of bars, the length of which is approximately equal to the length of the heat seal to be formed between two sheets 4 and 5 of thermoplastic material. Sheets 4 and 5 are intermittently moved along within the apparatus in the direction of the arrow Y. The movable electrode 1 includes a head 6 that is moved into contact with the outer face of the sheet 4 of thermoplastic material, at the location where sheet 4 is to be heat sealed to sheet 5. The counter-electrode 2 includes a head 7 having a longitudinal cavity 8. Cavity 8 houses a flexible tube 9 of rubber, silicone or other similar material. A small portion of the exterior surface of flexible tube 9 protrudes from cavity 8 and faces head 6 of movable electrode 1. As shown in Fig. 2, flexible tube 9 is connected at each end to a junction 10, one of which is connected to a source of compressed air (not shown). The other junction 10 is connected to a conduit 11 having a gate valve 12 which allows for the regulation of the ai " pressure in the flexible tube 9. A strip 13 of a heat resistance material, such as polytetra- fluorethylene, extends from a roll 14, through guide grooves 15, to the front of head 7 of the counter-electrode. Strip 13 then passes through guide grooves 16 and ends at a roll 17. Strip 13 protects the flexible tube 9 from deterioration caused by heat. Strip 13 may be moved intermittently in the direction of arrows Z. r The heat sealing apparatus described above functions in the following manner. When sheets 4 and 5 of polyolefin material, for example, have been moved in the direction of arrow Y until that portion of the sheets to be heat sealed together is positioned between electrodes I and 2, the movable electrode 1 is displaced toward the fixed counter-electrode 2. During this displacement of the movable electrode 1, the pressure of compressed air in the flexible tube 9 carried by the counter-electrode 2 is regulated. The pressure in the flexible tube is a counterpressure to the closing pressure exerted by the movable electrode 1 on the counter-electrode 2. When the sheets 4 and 5 are pinched between the head 6 of the movable electrode and the portion of the protective strip 13 covering the flexible tube 9, electric current is sent between electrodes 1 and 2 to heat seal sheets 4 and 5 together. A predetermined air pressure is maintained in flexible tube 9 during the entire heat sealing cycle until the crystallization of the heat sealing band 18. This pressure may be somewhat higher during the heat sealing process than during the crystallization of the heat sealing band. The heat sealing apparatus taught by the present invention provides for a uniform pressure between the electrodes 1 and 2 during the welding of sheets 4 and 5, which results in an improved heat seal. The heat sealing apparatus with flexible tube 9, described herein, advantageously provides the regulating of the pressure in tube 9 as a function of the heat sealing and crystallization characteristics of the sheets 4 and 5, and as a function of the profile of the heat seal band desired. When the heat sealing is completed, movable electrode 1 separates from fixed counter-electrode 2 and the air pressure in flexible tube 9 is reduced to a predetermined level. As soon as electrodes 1 and 2 are separated by a sufficientdistance, sheets of thermoplastic material 4 and 5 are displaced in the direction Y to the next position where they are to be heat sealed. The cycle described above is then repeated. If necessary, protective strip 13 can be moved in the direction of one of the arrows Z, so that a new portion of strip 13 covers flexible tube 9. The control of the intermittent displacement of the protective strip 13 may be either mechanical or manual. The width of strip 13 corresponds to the length of the heat sealing band 18 and may be slightly less than the length of flexible tube 9. It should be understood that various changes and modifications to the embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departiny from the spirit and scope of the present invention and without diminishing its attendant advantages. It is, therefore, intended that such changes and modifications be covered by the appended claims.

Claims: I CLAIM: 1. Apparatus for heat sealing sheets of thermoplastic material between two electrodes, at least one of which is movable with respect to the other, characterized in that (2) of the electrodes (1, 2) has a deformable part (9) which is subjected to the action of a means regulating its deformation when these-electrodes (1, 2) are in contact with the sheets (4, 5) of thermoplastic material to be heat sealed together. 2. Apparatus according to Claim 1, characterized in that the above-mentioned deformable part (9) is made in a counter-electrode (2) cooperating with an electrode (1) movable with respect to this counter-electrode (2). 3. Apparatus according to either of claims 1 and 2, characterized in that means regulating the deformation of the deformable part (9) is constituted by a pressure due to a fluid such as a gas. 4. Apparatus according to any of the preceding claims, characterized in that the aforementioned deformable part is composed of a flexible tube (9). 5. Apparatus according to claim 4, characterized in that the flexible tube (9) is connected to a source of a compressed fluid, such as compressed air. 6. Apparatus according to Claim 5, characterized in that the pressure of the compressed fluid is regulatable. 7. Apparatus according to either of claims 4 and 5, characterized in that the flexible tube (9) is lodged in a cavity (8) made in the head (7) of the counter-electrode (2) facing the head (6) of the movable electrode (1). 8. Apparatus according to any of claims 4 to 7, characterized in that it comprises a means of protecting (13) the above-mentioned flexible tube (9). 9. Apparatus according to claim 8, characterized in that the protective means is composed of a sheet (13) covering a part of the flexible tube (9) protruding from the head (7) of the counter-electrode (2). 10. Apparatus according to claim 9, characterized in that the protective sheet (13) is interchangeable. 11. Apparatus according to claim 10, characterized in that the protective sheet (13) is composed of a band movable step by step over the head (7) of the counter-electrode (2). 12. Apparatus according to claim 1, characterized in that the protective band (13) is rolled at each of its ends on a roller (14, 17) and is guided into grooves (15, 16) provided in the counter-electrode (2). 13. Apparatus according to either of claims 11 and 12, characterized in that step-by-step displacement of the protective band (13) is mechanically controlled. 14. Apparatus according to any of claims 4 to 12, characterized in that the flexible tube (9) is of rubber or of an analogous material. 15. Device according to any of claims 8 to 13, characterized in the means of protection (13) of the flexible tube (9) is composed of a band of polytetraflourethylene.

Inventors: SOUBRIER PIERRE P, SOUBRIER, PIERRE, P.
Applicants: BAXTER TRAVENOL LAB, BAXTER TRAVENOL LABORATORIES, INC.
Application year: 1987
Main classification: B29C65
IPC codes: B29C65