Patent Application
20090250440
The invention relates generally to welders, and more particularly to welders configured to weld along a predetermined pattern.
In electrical welding machines, welding elements, such as electrodes or heating elements (e.g., electrically heated resistance wires or coils), are used to transfer heat to workpieces to be joined. For example, pulse heat welding, which is commonly used to weld polypropylene (PP), welds by passing pulses or bursts of electrical energy through the heating elements, such as nickel-chromium resistance wires, such that the heating elements transfer heat at a very high temperature for short periods of time.
For welding along a predetermined pattern, heating elements are arranged in the corresponding pattern. When heating elements are arranged in an intersecting or overlapping pattern, however, the electrical contact of intersecting heating elements can short circuit the heating elements, which can stop the welding, at least in certain parts of the welding device, or cause the current level to increase, often to the ignition point of the workpieces, which can cause a fire or other dangerous conditions and damage equipment.
To prevent short circuiting, intersecting heating elements are electrically insulated from one another. For example, U.S. Pat. No. 5,451,286 discloses providing insulation of intersecting pulse-heat wires with electrically insulating and heat-conducting layers and strips of polytetrafluoroethylene (PTFE) such as TEFLON® tape, manufactured by 3M, or polyimide such as KAPTON®, manufactured by DuPont. It is difficult, however, to provide insulation that is thin enough not to increase the height of the intersection and strong enough to withstand the high frequency and high pressure of the pulse heat welding production cycle. Other disadvantages of using insulation includes uneven welds due to the thickness of the insulating material in between intersecting heating elements or slimming of the insulating material, complex and expensive equipment tooling, complex temperature control, limited sources for insulting materials, and increased manufacturing costs due to the expense for insulating materials equipment setup. Further, when the insulting material is be broken or worn out, a short circuit can develop.
Alternatively, individual intersecting heating elements can be fired separately, in multiple steps, with electrical current to each heating element being supplied by a separate supply circuit. This process, however, results in a prolonged welding time.
Thus, there is a need for an improved welding process that provides a simplified welder design and operation while avoiding short circuit.
In an embodiment, the invention relates to a method for electrically welding two workpieces. The method comprises: placing a first and second welding elements (e.g., heating elements such as metal wires or coils) in association with first and second portions of the workpieces, respectively, for heating and welding the workpieces; and powering the first and second welding elements out of phase from a common power source. The welding elements can be powered out of phase by alternatingly directing a current through each of the first and second welding elements for causing the first and second welding elements to weld the workpieces substantially simultaneously.
The current can be alternatingly directed through the first and second welding elements by applying a potential difference alternatingly across ends of the first welding element and ends of the second welding element, and/or by providing a source current that has a waveform and cyclically directing first and second portions of the waveform through the first and second welding elements, respectively. For example, an alternating current can be provided as the source current, and positive and negative portions of that waveform can be directed through the first and second welding elements, respectively. Current directors, such as diodes, can be used to alternatingly direct the waveform portions through the welding elements.
In a further embodiment, a power factor of the waveform directed through the welding elements is controlled with a power factor controller that is connected between the power source and the current director. The power factor controller can comprise a phase controller, e.g., a triode or two silicon-controlled rectifiers joined in an inverse parallel configuration, that is configured for conducting a fraction of the waveform portions.
The present method can be used to weld workpieces made of any suitable material, including thermoplastic, such as polypropylene. In an embodiment, the method is used to weld thermoplastic sheets to make a folder or binder cover.
The invention also relates to an electrical welder comprising first and second welding elements, a power source connected to the welding elements, and an electrical circuit configured to conduct a current out of phase from the power source to the first and second welding elements.
The invention will be better understood with reference to the attached drawings illustrating preferred embodiments, wherein:
The embodiment of
The welder 10 includes a platform 20 supported on a support 22. For making binders, the platform 20 is preferably substantially planar and is configured to receive thereon a welding member 30 that is configured to provide heating to weld the workpieces. The welder 10 also includes a pressure member 40, which is movably mounted on the welder 10 and is configured to operably engage with and exert a sufficient pressure on the welding member 30 during welding operations. In preferred embodiments, the welding member 30 and pressure member 40 are molds configured to cooperatively weld a predetermined pattern. Preferably, the welding member 30 and pressure member 40 are upper and lower molds configured to cooperatively weld a predetermined pattern.
One or multiple welding members 30 can be mounted on the platform 20. The welding member 30 is preferably movably mounted on the platform 20. For example, when multiple welding members 30 are mounted on the platform 20, the welding members 30 can be configured to alternately slide under the pressure member 40. In the embodiment shown in
To make a cover of a ring binder 300 as shown in
The welding member 30 of this embodiment can be made of any suitable electrically non-conductive, heat-resistant material that can withstand the high temperatures of pulse heat welding, e.g., thermoset plastic, metal, and ceramic. Preferably, the welding member 30 comprises a mold made of a thermoset plastic, such as a thermoset phenolic resin, e.g., Bakelite. The welding member 30 can include a single layer or multiple layers of such thermoset material, and preferably includes at least two layers 32,34 of thermoset phenolic resin as shown in
The pressure member 40 is preferably made of a non-corrosive or corrosion-resistant metal having thermal conductivity sufficient to transfer heat therethrough. Preferred examples of such suitable metals include copper alloys, brass, bronze, aluminum alloys, and stainless steel. For welding plastic, the pressure member 40 can be heated to keep the plastic workpiece from sticking thereto. For example, the pressure member 40 can be heated to about 60° C. to 140° C. Alternatively, the pressure member 40 can be coated with a non-stick material (e.g., PTFE such as TEFLON®).
The pressure member 40 can be configured as desired and suitable, depending on the welding configuration and the configuration of the welding member 30. For welding a typical ring binder or folder, the pressure member 40 can be a mold including a substantially flat steel base plate of a square or rectangular shape. The mold can include relatively thin walls or protrusions around the edges of the plate.
In a preferred embodiment, the pressure member 40 comprises first and second parts 42,44 that are spaced apart from each other. The first and second parts 42,44 are preferably made of material having thermal conductivity, preferably non-corrosive metal having sufficient thermal conductivity to transfer heat therethrough. Preferably, a filler 46, which is preferably a compressible material, at least partially fills the space between the first and second parts 42,44, to expulse excess air trapped between the first and second parts 42,44. The filler 46 preferably entirely fills the space between the first and second parts 42,44. The filler 46 is preferably a relatively soft foam material, e.g., soft rubber foam, that can withstand heat of at least up to about 140° C.
The surface of the pressure member 40 that contacts the thermoplastic material to be welded during operation can include embossed or textured patterns as desired, to provide embossment or patterns on the welded portions of the thermoplastic material. Also, the embossing or patterns can be at least partially covered with a thin heat-resistant tape or film (e.g., PTFE such as TEFLON®) to soften the effect of embossing and to provide a smoother, more even texture to the thermoplastic material.
The pressure member 40 is preferably mounted to the welder 10 so that it can be vertically moved, e.g., pneumatically. For example, the pressure member 40 can be attached to the welder 10 by mounting members 50, such as slide rails or pneumatic cylinders. In preferred embodiments, the pressure member 40 is configured to exert a pressure of at least about 20 psi, preferably at least about 25 psi, and at most about 60 psi, preferably at most about 45 psi on the welding member 30 that is placed thereunder. It will be appreciated, however, that the pressure exerted by the pressure member 40 can be varied depending on the size of the pneumatic cylinder and welding areas of the welder 10.
The welding member 30 includes first and second welding elements 102,104. In a preferred embodiment, the first and second welding elements 102,104 each comprise a plurality of first and second welding elements. The welding elements 102,104 are preferably heating elements that conduct an electrical current from one end to another. The welding elements 102,104 are made of conductive material, such as metal wires or coils that generate heat when a current is passed therethrough. In a preferred embodiment, nickel-chromium resistance wires are used. Such wires can transfer very high-temperature heat in a short period of time, and therefore are suitable for various electrical welding, including pulse heat welding. The welding elements 102,104 preferably include end portions 115 configured to be retained with a retention member, such as a fastener. Each welding element 102,104 also preferably includes a stretcher, such as a spring 117, proximate each end portion 115 to maintain the welding element 102,104 straight during the thermal expansion and contraction during welding cycles. The first welding elements 102 are sufficiently long to extend at least the length 35 of the welding member 30. Similarly, the second welding elements 104 are sufficiently long to extend at least the width 33 of the welding member 30. Preferably, the welding elements 102,104 are at least about 1 inch longer than the respective length 35 or width 33 of the welding member 30. The thickness of the welding elements 102,104 are preferably uniform and can be selected as suitable. In an example, the thickness is about 0.1 mm to 0.5 mm, but other dimensions can be used in other embodiments.
The welding elements 102,104 are arranged to correspond to the welding pattern of the welded product. For example, the first and second welding elements 102,104 can respectively be connected in parallel with each other as shown in
The welding elements 102,104 are provided on the welding member 30 in any desired pattern. For example, the welding elements 102,104 can simply be laid in a desired pattern on a surface of the welding member 30 and secured to the welding member 30 or to external retention members, adhesively (e.g., with tape strips), with fasteners, or by any other suitable means. In a preferred embodiment, holes 36,38 are provided on the top and side surfaces of the welding member 30 to extend the end portions 115 of the welding elements 102,104 therethrough and secure them to external retention members 122,124, such as clamps. The end portions 115 of each welding element 102,104 are preferably inserted into the top holes 36 of the welding member 30, pulled out through the side holes 38, and secured to retention members 122,124 with fasteners 120, such as screws and nuts. For clarity, only one of the corner retention members 122 is shown in
In a preferred embodiment, portions of the welding member 30 immediately below the welding elements 102,104 can be removed to form channels 100 that substantially correspond to the shape of the welding elements 102,104, so that the welding elements 102,104 are substantially flush with the welding member 30. A barrier layer having a higher heat resistance than the welding member 30, e.g., ceramic, can be provided between the welding member 30 and the welding elements 102,104 to protect the welding member 30 from the high temperatures of electrical welding. For example, barrier layers in the form of ceramic stripes can be placed in the channels 100. The barrier layers can be configured to partially or entirely replace the welding member material that is removed to form the channels 100. The barrier layers can be attached to the welding member 30 in any suitable manner, such as with an adhesive. In addition to or alternative to the barrier layers, a layer of heat-resistant, non-conducting material, such as PTFE or polyimide tapes or sheets (e.g., TEFLON® or KAPTON® tapes or sheets), used on top of heat-conductive metal elements/strips/inserts, can optionally be provided under the welding elements 102,104, to form a heat sink for excess heat generated during welding cycles. No electrical insulation is needed, however, between intersecting welding elements, i.e., between horizontal welding elements 104 and vertical welding elements 102.
Referring to
The first and second welding elements 102,104 are powered out of phase from each other, preferably from a common power supply. In the embodiment shown in
In an embodiment, the first welding elements 102 and second welding elements 104 are powered by alternatingly directing a current through each of the first and second welding elements 102,104 for causing the first and second welding elements 102,104 to weld the workpieces substantially simultaneously. The current can be alternatingly directed by applying a potential difference alternatingly across the ends of the first welding elements 102 and the ends of the second welding elements 104, and/or by cyclically directing a first portion of the waveform of the source current through the first welding elements 102 and a second portion of the waveform of the source current through the second welding elements 104. When portions of the source current waveform are conducted through the first and second welding elements 102,104, the voltage conducted through the welding elements 102,104 corresponds to that of the conducted waveform portions. For example, when first and second half portions of the source current waveform are conducted through the first and second welding elements 102,104, the voltage conducted therethrough is about half the voltage of the source current.
In the embodiment shown in
In a preferred embodiment, the current directors 212,214 are diodes that are capable of directing a selected portion of the waveform of the source current. For example, where the source current is alternating current having a traditional sinusoidal waveform or another suitable waveform, the first current directors 212 can conduct a first portion of the waveform through the first welding elements 102 and the second current directors 214 can conduct a second portion of the waveform through the second welding elements 104, such that the first and second portions of the waveform are cyclically directed through the first and second welding elements 102,104. In a further embodiment, the current directors 212,214 can be configured to direct the current through a first portion of the circuit during a first portion, e.g., a positive portion, of the waveform to direct the current through the first welding elements 102 and to direct the current through a second portion of the circuit during a second portion, e.g., a negative portion, of the waveform to direct the current through the second welding elements 104.
Referring to the embodiment shown in FIGS. 4 and 5A-5B, the power source 200 supplies an AC source current having a traditional sinusoidal waveform 220 shown in
Further advantageously, welds formed without insulation according to the invention have been found by the inventors to be generally more uniform and even compared to welds formed by conventional processes using insulation material between intersecting welding elements, which usually causes weld protrusions at the intersections of welding element intersections and uneven welds due to the slimming of the insulating material. Also, because the entire current travels through each welding element 102,104 in the selected fraction of the waveform cycle, such as each half cycle, it has been found that there is no significant increase in welding time, which remains substantially the same as conventional pulse heat welding that does not use any current director. For example, the welding time is about 2 seconds for welding a polypropylene film of about 100 μm along two directions of weld lines.
In preferred embodiments, a power factor controller 230 can be connected between the power source 200 and the heating elements 102,104, for controlling the power factor, i.e., the voltage magnitude, of the current transmitted to the heating elements 102,104. For example, the power factor controller 230 can be configured to conduct the current by the full power factor, i.e., 100% of the current voltage, or by a reduced power factor, i.e., less than 100% of the current voltage. The power factor controller 230 is preferably configured to apply a preselected power factor to the current transmitted therethrough. Any suitable and desired power factor can be selected. In an embodiment, the power factor controller 230 is configured to conduct about 5% to 90% of the current voltage, preferably about 10% to 80%, and more preferably about 15% to 60%, but other percentages can be used in other embodiments.
The power factor controller 230 can comprise a phase controller that is capable of selectively conducting a fraction of the current waveform portions conducted therethrough. The phase controller 230 is preferably configured to conduct a preselected fraction of the current waveform portions conducted therethrough. Any suitable and desired fraction can be selected. Preferably, the phase controller 230 is a triode (also known as TRIAC, meaning triode for alternating current) or two silicon controlled rectifiers (SCR) that are joined together in an inverse parallel configuration, but any other suitable device capable of selectively conducting a fraction of the current waveform portions can be used.
Preferably, a power factor controller 230 is connected between the power source 200 and each welding element 102,104 as shown in
In the embodiment shown in FIGS. 4 and 5A-5D, the power source 200 can provide an AC source current having the traditional sinusoidal waveform 220, and the current directors 212,214 can be configured to selectively conduct positive and negative portions 222,224, respectively, of the source current waveform, as described above. Phase controllers 230 are provided between the power source 200 and current directors 212,214 to control the power factor/phase of the current conducted to the current directors 212,214. The phase controllers 230 can be configured to conduct the source current in full phases, such that the voltage of the source current is unchanged. Alternatively, the phase controllers 230 can be configured to selectively conduct a fraction 226 of the current waveform conducted therethrough, such that the positive and negative portions 228,230 of this waveform fraction 226 are conducted to the welding elements 102,104 through the current directors 212,214.
Advantageously, the power factor controller 230 does not require a complex system setup and does not cause power dissipation. Thus, the power factor controller 230 achieves the desired voltage with ease and high power efficiency. The power factor controller 230 also allows welding by firing the electrical power in one-time firing, wherein welding is achieved by turning on the power source 200 for a short period of time. For example, for welding polypropylene sheets, for example to make a conventional polypropylene ring binder, the power source 200 is turned on for less than 2 seconds, more preferably less than 1 second.
Preferably, the power factor controllers 230 are connected to an electronic regulator 240. The electronic regulator 240 is configured to regulate the timing and power factor of the current transmitted through the power factor controller 230. The electronic regulator 240 controls the welding time and power factor by controlling the operative parameters of the power factor controller 230. The electronic regulator 240 is preferably a microprocessor controller that is capable of regulating the timing within the step of 0.05 seconds, and more preferably within the step of 0.01 second. In preferred embodiments, the electronic regulator 240 is set to provide the current to the power factor controller 230 in pulses of about 0.2 to 6 seconds, preferably about 0.3 to 4 seconds, and more preferably about 0.5 to 2 seconds, with rest periods of about 0.1 seconds between pulses.
Other suitable and desired circuit components and devices can be included in the pulse heat welding circuit according to the invention. For example, the circuit embodiment shown in
The present electrical welding process and device can be used with any suitable electrical welding process. In an embodiment, the electrical welding is pulse heat welding, in which electrical energy is conducted to the welding elements 102,104 in pulses. The present welding process and device also can be used to weld any suitable type of workpiece materials. One suitable type of material is plastic, including thermoplastic. In an embodiment, the workpieces are thermoplastic binder covers, such as polypropylene binder covers. An example of a thermoplastic binder cover 300 and a finished 3-ring binder 350 made therefrom are shown in
As used herein, the term “about” should generally be understood to refer to both the corresponding number and a range of numbers. In addition, all numerical ranges herein should be understood to include each whole integer within the range. While illustrative embodiments of the invention are disclosed herein, it will be appreciated that numerous modifications and other embodiments may be devised by those skilled in the art. For example, the features for the various embodiments can be used in other embodiments. Therefore, it will be understood that the appended claims are intended to cover all such modifications and embodiments that come within the spirit and scope of the present invention.
