FR2854832A1 - Plastic sleeve dielectric welding apparatus especially for sacks of hazardous waste, has semi-conductor high-frequency generator, matching network and welding tool - Google Patents

Abstract

Welding apparatus consists of a high-frequency (HF) generator, two facing welding electrodes (40, 44) and a mechanism (48) to push the electrodes together. HF generator is based on semi-conductors, and the apparatus includes a matching network and a welding tool (30) with a housing (35) and handle (36). Welding apparatus consists of a high-frequency (HF) generator, two facing welding electrodes (40, 44) and a mechanism (48) to push the electrodes together. HF generator is based on semi-conductors, and the apparatus includes a matching network and a welding tool (30) with a housing (35) and handle (36). Housing has a slot giving access to the electrodes, which are coated with an insulating material that is not heated dielectrically, and leading to a larger aperture (39). The electrodes are connected via the matching network to the HF generator, and they are easy to remove for replacement.

Description

WELDING APPARATUS

  The present invention relates to an apparatus for welding together plastic sheets, in particular, but not exclusively, for welding and sealing plastic bags containing waste or a dangerous material.

  During the treatment or handling of small quantities of radioactive material, it is known to use a glove box, the glove box protecting the operator against radiation and incorporating gloves mounted in openings in the wall of the box so that the operator can manipulate objects inside the box. Objects can be transferred outside the box through an opening in a plastic tube or sleeve, and the plastic sleeve can be sealed to form a bag containing the objects.

  Therefore, objects can be removed while remaining sealed from the environment. A similar procedure can be used by removing objects from a hot cell in which they are handled by a manipulator. The use of a dielectric solder (which can also be indicated as radiofrequency heating or high frequency heating) for welding such a tube or plastic bag has been known for many years. In this process, the two films of thermoplastic material (such as polyvinyl chloride) are positioned between opposite electrodes (or an electrode and a base plate), the electrodes are pressed against each other and a tension high frequency is applied between the electrodes. This process is applicable to materials which have a significant dielectric loss index, for example greater than 0.2, at high frequency. This can be between 1 MHz and 200 MHz, usually between 10 MHz and 100 MHz, although in practice it can be imposed by regulations, for example at 27.12 MHz or 40.68 MHz.

  However, these tools as used hitherto are no longer acceptable since the operating frequency varies with the electric charge during the welding process and thus the tool transmits radio waves in the vicinity at frequencies other than those authorized. for dielectric welding.

  According to the present invention, there is provided a dielectric welding apparatus for welding a plastic sleeve, the apparatus comprising a high frequency semiconductor generator, an adapter network and a welding tool, the tool comprising a frame with a handle, the frame delimiting a slot in which there are opposite welding electrodes 20, the slot being open at one end and at the other end communicating with a larger opening, and means for pushing the electrodes 1 one against the other, the welding electrodes being connected via the adapter network to the generator.

  Preferably, the electrodes are coated with an electrically insulating material, for example PFA (perfluorinated alkoxyalkane). Preferably, the electrodes are easily removable, so that they can be replaced; this can happen because the electrically insulating coating is worn out or alternatively the electrodes could be replaced by electrodes of a different shape, although preferably of the same surface.

  Preferably, the high frequency generator and the matching network are in a box and the welding tool is connected to the box by a coaxial cable. The cable is preferably of a length which is an integer number of quarter waves of electromagnetic radiation along the cable at the operating frequency. The means for pushing the electrodes against each other 10 can be for example a pneumatic cylinder and can be under the control of a trigger. The coaxial cable is preferably linked to the cable or tube bringing the energy to push the electrodes against each other, for example the pneumatic tube if a pneumatic cylinder is used, and the coaxial cable and the power cable both are brought into the tool frame opposite the end at which the slot is open. This helps to balance the tool during use.

  In the preferred embodiment, the electrodes are pushed against each other by a pneumatically actuated mechanism and the cabinet incorporates a compressor and a reservoir for compressed air. The compressor is arranged to start if the pressure is below a lower exercise limit and to stop when the pressure reaches an upper exercise limit. However, if the pressure drops below a threshold value (below the lower exercise limit), this indicates a significant leak in the pneumatic system and the compressor is stopped.

  The invention will now be described later and more particularly, only by way of example, and with reference to the accompanying drawings, in which: FIG. 1 represents a perspective view of a dielectric welding apparatus of the invention, incorporating a welding tool and a case; 2 shows a side view of the welding tool of Figure 1, partially in section and partially exploded; and FIG. 3 schematically represents the components inside the box of FIG. 1.

  Referring to Figure 1, a dielectric welding device 10 comprises a movable steel box 12 on wheels 14, the box 12 containing a high frequency generator 15, an air compressor and a compressed air tank. The upper surface of the box 12 is slightly hollow and coated with an anti-slip mat 16. The front face of the box 12 supports control buttons 18 and 19 for the generator and the compressor and 20 an instrument 20 for measuring the pressure of the air. In addition, there are two tubular handles 22 bolted to the front of the cabinet 12 and a support rod 24 fixed to the rear of the cabinet 12 extends above the cabinet 12, ending in a hook 26 for 25 supporting a welding tool 30. The tool 30 is connected by a coaxial cable 32, an electric cable 33 and a pneumatic hose 34 (linked together over most of their length) to the box 12.

  Referring now to Figure 2, this 30 shows the welding tool 30 in longitudinal section.

  The tool 30 comprises a housing 35 with a tubular handle 36 fixed on its lower side. The housing 35 has a substantially horseshoe shape at one end, from which upper and lower jaws extend parallel to each other, so that there is a slit between the upper half 5 37 and the lower half 38 of the housing 35, the slot being open at one end and opening at the other end into a wider opening 39. The upper half 37 positions a welding electrode 40, made of spray-coated stainless steel with an insulator 10 PFA, which is housed in a cavity and rigidly held by two screws 41. In this example, the underside of the electrode 40 has a thickness of 10 mm (outside the plane of the figure) and a length of 110 mm. In the center of the upper half 37 is an eyelet 42 15 for connection with the support hook 26. The lower half 38 positions a similar welding electrode 44 made of spray-coated stainless steel with PFA insulation, 10 mm by 100 mm , and fixed by a screw 45 in a support block 46 of electrically insulating material 20 such as a ceramic or Delrin. The support block 46 is connected to a pneumatic cylinder 48. Next to the handle 36 is a trigger mechanism 50 which acts on a pneumatic switch 52 to control the supply of air to the cylinder 48 (the connection tubes 25 not being shown) and also on an electric microswitch next to it (not shown) connected to the cable 33. Several screw holes 56 make it possible to fix a cover plate (not shown) on the housing 35.

  The pneumatic hose 34 and the electric cable 33 and the coaxial cable 32 (indicated by dashed lines) are brought into the closed end of the housing 36 via couplings 54; the pneumatic hose 34 is connected to a switch 52 in the housing 36. Similarly, the coaxial cable is connected to the interior of the housing 36 so that the upper electrode 40 is grounded (connection to the earth being connected to a label connected to the screw hole 57 near the electrode 40), while the lower electrode 44 is supplied with the high frequency signal (this connection crossed by 10 a current being connected to a screw 58 to the rear of the insulating support block 46, in contact with a metal sheet (not shown) extending along the base of the slot in which the electrode 44 is positioned).

  Referring next to FIG. 3, this diagrammatically represents the components inside the box 12. An air compressor 60 pumps air through a non-return valve 62 into a reservoir 64 which is connected to the pneumatic line 34 via a 3/2 direct-acting spring / magnetic valve 65. The compressor 60 is arranged to increase the air pressure to 0.57 MPa (85 psi); during use, the pressure gradually decreases and when it reaches 0.50 MPa (75 psi), the compressor 60 25 is restarted. If the pressure drops below 0.43 MPa (65 psi), this indicates a serious deterioration on the pipe 34 and the compressor 60 is consequently stopped and the valve 65 closes to prevent loss of air from the tank. 64. A high frequency semiconductor generator 66 provides a high frequency signal via the coaxial cable 67 to an adapter network 68, from which the signal is applied to the coaxial cable 32 (and thus to the 'tool 30). The adaptation network 68 is shown in more detail. The signal passes through a monitoring circuit 70 (shown diagrammatically), a variable capacitor 72, an inductor 73 and a variable capacitor 74 and thus to the cable 32. The monitoring circuit 70 monitors the current and the voltage at high frequency and adjusts the values of variable capacitors 72 and 74 using servomotors 76 and 77 so that the impedance presented to generator 66 remains at a constant value such as 50 Q. A capacitor 78 connects the junction between capacitor 72 and the inductor 73 to earth potential and, by switching to other capacitors 80 and 81, the effective capacitance between this junction and earth can be adjusted. This has the effect of fine-tuning the high frequency voltage applied between the electrode through which a current 44 passes and the opposite electrode grounded 40 and the high frequency current supplied. This thus controls the power which is actually supplied between the electrodes 40 and 44.

  Thus, during use, when an operator wishes to seal a sleeve of PVC tubular, he places an edge of the sleeve between the electrodes 40 and 44 of the tool 30 and pulls the trigger 50 so that the electrodes 40 and 44 are pressed against each other by pneumatic action. After about half a second, the electric microswitch, via the cable 33, actuates the high frequency generator 66, so that the high frequency signal (which can be for example at 27.12 MHz) is applied between the electrodes 40 and 44. As a result of the dielectric losses, the PVC melts and the sheets are welded together. This welding process usually requires less than 5 seconds and a timer can be arranged to automatically stop the generator 66 after, for example 12 seconds. This process is repeated when the sleeve is moved step by step through the jaws and, with a wide sleeve, it will be necessary to bundle part of the sleeve into the wider space 39.

  The cavity 16 on the top of the cabinet 12 provides the operator with a suitable location for placing the sleeve and its contents during the welding process.

  It will be appreciated that a dielectric soldering device may differ from that described above while remaining within the scope of the invention.

Claims (8)

  1. Dielectric welding apparatus for welding a plastic sleeve, apparatus comprising a high frequency generator (66), opposite welding electrodes (40, 44) and means (48) pushing the 5 electrodes against each other another, characterized in that the generator (66) is a high frequency semiconductor generator and in that the apparatus comprises an adapter network (68) and a soldering tool (30), the tool ( 30) comprising a frame (35) with a handle 10 (36), the frame delimiting a slot in which the welding electrodes (40, 44) are located, the slot being open at one end and at the other end communicating with a larger opening (39), the welding electrodes (40, 44) being connected via the adapter network (68) to the generator (66).   2. The apparatus of claim 1, wherein the electrodes (40, 44) are coated with an electrically insulating material which is not dielectrically heated.   3. Apparatus according to claim 1 or claim 2, wherein the electrodes (40, 44) are easily removable, so that they can be replaced.   4. Apparatus according to any preceding claim, wherein the high frequency generator (66) and the adapter network (68) are in a cabinet (12) and the soldering tool (30) is connected to the box (12) by a coaxial cable (32).   5. The apparatus of claim 4, wherein the cable (32) is preferably of a length which is an integer number of quarter waves of electromagnetic radiation along the cable (32) at the operating frequency.   6. Apparatus according to claim 4 or claim 5, wherein the coaxial cable (32) and the means (34) supplying the energy for pushing the electrodes (40, 44) against each other are both brought in the frame (35) of the tool (30) opposite the end at which the slot is open.   7. Apparatus according to any one of the preceding claims, wherein pneumatic means (48, 34, 60) are provided for pushing the electrodes (40, 44) against each other.   8. Apparatus according to any one of the preceding claims, wherein the welding tool (30) incorporates a switch (52) for actuating both the means (48, 60) for pushing the electrodes (40, 44) one against the other and the generator (66).