This invention relates to a method and an apparatus for continuously welding thermoplastic materials and in particular thermoplastic polymer composites.
Advanced thermoplastic composites are widely used, particularly in the aerospace industry. As the use of such composites has increased, the need for effective and reliable methods for joining the composites has continued to grow. Traditional methods of joining thermoplastic composite parts include adhesive bonding and mechanical fastening, both of which are tedious, labor intensive and costly. Extensive surface preparation, long curing times of adhesives and poor bonding properties between adhesives and the thermoplastic polymers make adhesive bonding undesirable. Mechanical fastening methods suffer from problems arising from stress concentration, galvanic corrosion, mismatch of coefficient of thermal expansion and damage to reinforcing fibers induced by drilling.
In the recent past, thermoplastic composite materials have been fusion bonded by induction welding, ultrasonic welding and, to a limited extent, by resistance welding. Resistance welding has been identified as a promising technique among various fusion bonding methods. Resistance welding is based on the principle of placing a layer of conductive material called a heating element between the surfaces of the parts to be joined. An electrical current is applied to the heating element to increase the temperature thereof as a result of resistance heating. The heat causes the surrounding thermoplastic polymer to melt. Under the application of pressure, molecular diffusion occurs at the interface and when the joint is cooled the polymer solidifies resulting in a weld. Resistance welding of small pieces, e.g. lap welding of coupon size pieces is a fast process with short welding times ranging from 1 to 5 minutes with little to no surface preparation. In addition, the welding equipment is simple and inexpensive and can be made portable for repair purposes.
However, resistance welding has not been fully developed for a variety of reasons including (i) non-repeatability and inconsistent performance of welded parts, (ii) pressure and power limitations for welding large parts, (iii) problems relating to preferential and local heating in weld areas, and (iv) the amount of time to produce a weld between large thermoplastic parts.
For example, U.S. Pat. No. 5,313,034 to Grimm et al teaches a process for producing long, continuous, thermoplastic welds between large structures. A series of tabs are used in pairs, and especially in alternating, overlapping pairs to effect resistance heating of a strip of material placed in a bond line. The resistance of the tabs is less than that of the strip of material. A continuous weld is produced by clamping electrodes to pairs of tabs and applying voltage and pressure. After the first weld cools, another section of the weld is made using a second pair of tabs to produce a weld overlapping the first weld. It will be appreciated that producing a continuous weld by this method is a much more lengthy procedure than the production of the same weld using a single heating and cooling operation.
It is readily apparent that a need exists for a workable system for resistance welding of thermoplastic composite materials.
An object of the present invention is to meet the above defined need by providing a relatively simple apparatus and method for continuously welding thermoplastic materials.
According to one aspect the invention relates to an apparatus for resistance welding of two thermoplastic articles comprising:
a support for supporting the articles in overlapping relationship;
a resistance heating element for positioning between said articles in an area of overlap along a length to be welded, said heating element having a width sufficient to span a weld area and extend outwardly beyond the area of overlap providing two exposed side edges of the heating element;
electrodes for connection to respective ones of said two side edges of said heating element;
a compactor for pressing said articles together in said weld area following melting of the articles to effect welding of the articles; and
a drive for moving at least one said electrode and compactor in synchronism relative to said articles along said weld area with said compactor behind at least one said electrode,
whereby said articles are pressed together in said weld area following melting of the articles to effect welding of the articles.
According to another aspect, the invention relates to a method of resistance welding two thermoplastic articles comprising the steps of:
placing a heating element between an overlapping area of said articles to define a weld area between overlapping side edges of the articles with the heating element extending outwardly beyond said side edges;
effecting relative movement between electrodes in contact with said heating element and the articles while applying current to the electrodes to effect localized heating of the weld area sufficient to create a moving, localized melt zone; and
applying pressure to said articles in the localized melt zone of the weld area immediately following creation of said localized melt zone to fuse the articles together.
The invention is described below in greater detail with reference to the accompanying drawings, wherein:
Referring to
Welding of the layers 2 and 3 is effected by applying current to the electrode 5 while moving the electrode and the pressure roller 10 along the top layer 2 adjacent to the side edge 11 thereof above the weld area 12, which is roughly the same width as the roller 10. The use of a roller electrode 5 ahead of the pressure roller 10 in the direction of travel of the rollers creates localized heating of an area 13 (
With reference to
The table 21 supports overlapping layers, in this case panels 24 and 25, formed of a thermoplastic composite during a welding procedure. The panels 24 and 25 are clamped in position on the table 21 by a pair of strips 26, which are slidably mounted for transverse movement on the table 21. Slots 27 in each end of the table 21 receive bolts 28, which permit sliding of the strips 26 transversely of the table. Nuts (not shown) beneath the table 21 are used to fix the strips 26 in position on the panels 24 and 25.
Posts 30 extending upwardly from the sides of the baseplate 16 support a pair of crossbars 32. A second drive assembly indicated generally at 33 is mounted on the crossbars 32. The assembly 33 includes a housing 34 (similar to the housing 18) with an open front end. The shaft 36 of a linear motor 37 mounted on the top end 38 of the housing 34 carries a slide 39 for effecting vertical movement at the slide. A compaction or pressure assembly is mounted on the slide 39. The pressure assembly includes a disc-shaped roller 40 rotatably mounted in a clevis 41 (
When performing a welding operation, the composite panels 24 and 25 are placed on the table 21 with their sides in overlapping relationship. A heating element 49 in the form of a strip of metal mesh (
One end of a coil spring 60 on the axle 56 is fixedly mounted in an ear 61 mounted on the axle 56, and the other end of the spring extends into the arm 55 beneath the bearing 57. Thus, the electrode 50 is biased downwardly against the heating element 49.
The second electrode 51 extends upwardly through a longitudinally extending slot 62 (
During a welding operation, the two electrodes 81 advance along the exposed sides of the heating element 78. By supplying electrical current to the element 78, a weld area is created between the electrodes 81. The compaction roller 86, which follows the electrodes 81 in the direction of the arrow 87 (
The apparatus of
The apparatuses described above can be used to weld most thermoplastic-based materials. The process is significantly faster than producing welds at overlapping areas in sequence, provides more consistent welds throughout long connections, and is simple, inexpensive and clean. Moreover, the process can be applied to large structures and to other topologies. The method described above permits continuous welding of overlapping portions of at least two thermoplastic or thermoplastic composite parts under well-controlled processing conditions. In the method welding occurs in a continuous gradual manner as opposed to welding the entire parts at once or welding individual segments of the weld one at a time. The system controls and provides excellent temperature distribution, minimizing discontinuities in the weld area. The production of the melt zone, and the power and pressure requirements for the system depend only on the width of the weld area and are independent from the length of weld area. These capabilities substantially increase the capacity of this process for welding large parts.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/CA2005/000529 | 4/8/2005 | WO | 00 | 10/4/2007 |