Welding apparatus

ABSTRACT

A dielectric welding apparatus for welding a plastic sleeve contains a solid state radio frequency generator linked via a matching network to a welding tool. The tool () includes a frame () with a handle (), the frame defining a slot in which are opposed welding electrodes (), the slot being open at one end and at the other end communicating with a wider aperture (). A pneumatic cylinder () can be actuated to urge the electrodes together, both the cylinder () and the RF generator () being activated by a trigger () on the tool (). The electrodes are coated with an electrically insulating material that is not dielectrically heated, and are readily removable, so they can be replaced, for example when the electrically insulating coating becomes worn.

INFORMATION

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings, in which:

FIG. 1 shows a perspective view of a dielectric welding apparatus of the invention, incorporating a welding tool and a cabinet;

FIG. 2 shows a side view of the welding tool of FIG. 1, partly in section, and partly broken away; and

FIG. 3 shows diagrammatically the components within the cabinet of FIG. .

Referring to FIG. 1, a dielectric welding apparatus includes a movable steel cabinet on casters , the cabinet enclosing a radio frequency generator, an air compressor and a compressed air reservoir. The top surface of the cabinet is slightly recessed, and covered with a non-slip mat . The front face of the cabinet carries control buttons and for the generator and compressor, and a gauge to measure the air pressure. In addition there are two tubular handles bolted to the front of the cabinet , and a support mast attached to the back of the cabinet extends above the cabinet , ending in a hook to support a welding tool . The tool is connected by a coaxial cable , an electric cable , and a pneumatic hose (linked together for most of their length) to the cabinet .

Referring now to FIG. 2, this shows the welding tool in longitudinal section. The tool comprises a housing with a tubular handle attached on its underside. The housing has a generally horseshoe shape at one end, from which upper and lower jaws extend parallel to each other, so that there is a slot between the upper half and the lower half of the housing , the slot being open at one end, and at the other end opening out into a wider aperture . The upper half locates a welding electrode , of stainless-steel spray-coated with PFA insulator, which locates in a recess and is held rigidly by two screws . In this example the lower face of the electrode is 10 mm thick (out of the plane of the figure) and 110 mm long. At the centre of the upper half is an eyelet to connect with the support hook . The lower half locates a similar welding electrode of stainless-steel spray coated with PFA insulator, 10 mm by 100 mm, and attached by a screw into a support block of electrically insulating material such as a ceramic, or delrin. The support block is connected to a pneumatic cylinder . Next to the handle is a trigger mechanism which acts on a pneumatic switch to control air supply to the cylinder (the connecting tubes not being shown), and also on an electrical micro-switch beside it (not shown) connected to the cable . Several screw holes enable a cover plate (not shown) to be attached to the housing .

The pneumatic hose , and the electrical cable and coaxial cable (indicated by broken lines) are fed into the closed end of the housing through couplings ; the pneumatic hose is connected to the switch in the housing . Similarly the coaxial cable is connected within the housing so that the upper electrode is earthed (the earth connection being connected to a tag connected to screw hole next to the electrode ), while the lower electrode is supplied with the radio frequency signal (this live connection being connected to a screw at the back of the insulating support block , in contact with a metal foil (not shown) extending along the base of the slot in which the electrode locates).

Referring down to FIG. 3, this shows diagrammatically the components within the cabinet . An air compressor pumps air through a non-return valve into a reservoir that is connected to the pneumatic line via direct acting 3/2 solenoid/spring valve . The compressor is arranged to raise the air pressure to 0.57 MPa (85 psi); during use the pressure gradually falls, and when it reaches 0.50 MPa (75 psi) the compressor is switched on again. If the pressure drops to less than 0.43 MPa (65 psi) this suggests catastrophic damage to the hose , and the compressor is therefore switched off, and the valve closes to prevent loss of air from the reservoir . A solid state radio frequency signal generator supplies a radio frequency signal via a coaxial cable to a matching network , from which the signal is supplied to the coaxial cable (and so to the tool ). The matching network is shown in more detail. The signal passes through a monitoring circuit (shown diagrammatically), a variable capacitor , an inductor , and a variable capacitor , and so to the cable . The monitoring circuit monitors the radio frequency current and voltage, and adjusts the values of the variable capacitors and using servo-motors and so that the impedance presented to the generator remains at a constant value such as 50 Ω. A capacitor connects the junction between the capacitor and the inductor to earth potential, and by switching in other capacitors and the effective capacitance between this junction and earth can be adjusted. This has the effect of finely adjusting the radio frequency voltage applied between the live electrode and the opposed, earthed electrode , and the RF current supplied. It thus controls the power that is actually supplied between the electrodes and .

Thus in use, when an operator wishes to seal a sleeve of PVC tubing, he places an edge of the sleeve between the electrodes and of the tool , and pulls the trigger so that the electrodes and are pneumatically squeezed together. After about half a second the electrical micro-switch, via the cable , activates the RF generator , so that the radio frequency signal (which may for example be at 27.12 MHz) is applied between the electrodes and . As a result of dielectric losses the PVC melts, and the sheets are welded together. This welding process typically needs less than 5 seconds, and a timer may be arranged to automatically switch off the generator after say 12 seconds. This process is repeated as the sleeve is moved step wise through the jaws, and with a wide sleeve it will be necessary to bundle part of the sleeve in the wider space .

The recess on the top of the cabinet provides a convenient place for the operator to rest the sleeve and its contents during this welding process.

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

CLAIMS

1. A dielectric welding apparatus for welding a plastic sleeve, the apparatus comprising a solid-state radio frequency generator, a matching network, and a welding tool, the tool comprising a frame with a handle, the frame defining a slot in which are opposed welding electrodes, and means for urging the electrodes together, the welding electrodes being connected via the matching network to the generator, wherein the matching network incorporates at least one variable capacitor and at least one servo-motor arranged to adjust said variable capacitor such that the impedance presented to the generator remains at a substantially constant value, and wherein the slot defined by the frame is a partly horseshoe-shaped slot, the slot being open at one end adjacent to which are the electrodes and communicating at the other end with a wider aperture.

2. An apparatus as claimed in claim 1 wherein the electrodes are coated with an electrically insulating material that is not dielectrically heated.

3. An apparatus as claimed in claim 1 wherein the electrodes are readily removable, so they can be replaced.

4. An apparatus as claimed in claim 1 wherein the radio frequency generator and the matching network are in a cabinet, and the welding tool is connected to the cabinet by a coaxial cable.

5. An apparatus as claimed in claim 4 wherein the cable is of a length that is a whole number of quarter wavelengths of electromagnetic radiation along the cable at the operating frequency.

6. An apparatus as claimed in claim 4 wherein both the coaxial cable and the means supplying power to urge the electrodes together are fed into the frame of the tool opposite the end at which the slot is open.

7. An apparatus as claimed in claim 1 wherein pneumatic means are provided for urging the electrodes together.

8. An apparatus as claimed in claim 1 wherein the welding tool incorporates a switch for activating both the means for urging the electrodes together, and the generator.

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