Fastening & Joining

5-2000

Design Application

Difficult materials joined in fast bondage

Tom Shelley looks at novel ways of quickly and inexpensively bonding substances that are normally hard to join

New techniques are constantly being developed to join even the most intractable materials. Most new processes are variations on old ones but it is always worth looking at all possible techniques.

If there seems to be no suitable adhesive, consider mechanical fasteners. If there are no suitable fasteners, consider fasteners assisted by adhesives. If fusion welding will not work, consider brazing, soldering, and diffusion bonding. There is always something that will work better than a process which has proved to be problematical.

One process that is proving increasingly popular is low temperature welding, which allows you to weld substances that are not normally weldable. This is usually done by introducing another substance, which melts more easily and forms the bond. The same can be said of soldering and brazing. But whereas soldering and brazing use a low melting point intermediary which wets the surfaces to be joined, low temperature welding processes achieve a more intimate bond between the welding phase and the component surfaces. The nature of the bond can be by mechanical interlocking, if the surfaces to be joined are initially porous, or an intermingling on the atomic scale of the joining substances and the components surfaces to be joined.

Low temperature welding can be used with substances that are totally impossible to weld using conventional techniques. A particularly striking recent example of low temperature welding is a particularly neat new process to join wood.

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The idea in this case is to drive ultrasonically excited plastic stakes into the pieces of wood to be joined, which then melt and spread into the porous structure of the wood and along the interface between the pieces. The process is being developed and promoted by Swiss company WoodWelding, partlyowned by Elmar Mock, one of the inventors of the Swatch watch.

Mock personally demonstrated the technique to Eureka at the recent Hannover Fair, joining plywood to blocks of softwood in a few seconds. He then split the block open to show how well the plastic had penetrated into the grain of the wood. He said that it is not normally necessary to drill any pilot hole. Because the plastic flows into the wood grain, it produces a joint much stronger than can be achieved by a screw. An additional advantage over screws, and also nuts and bolts and other mechanical fasteners, is that there is opportunity to vibrate loose. Compared with use of hot-melt glues and other adhesives, there is no need to clamp pieces together while glue solidifies or cures. Furthermore, the stakes add strength within the structure of the wood. There is no way a WoodWeld joint can be pulled apart at or near the join since it is being held together both by the strength of the wood and the plastic stake. Applying the edge of a wedge to a joint or performing tensile tests on hot melt and other adhesively bonded constructions usually leads to failure immediately adjacent to the bonded zone.

He said that he expected the technique to mainly be used to make furniture, but that it was suitable for any combination of porous substances, including difficult to join materials such as gypsum and structural foams. Plastic stake materials experimented with so far have been made from ABS, polyamide and polypropylene reinforced with mineral fibres. More information can be found at www.woodwelding.com

French connection

Marcel Boursier, managing director of French company BMTS, says that the ability to thermally weld flexible polyurethane is the cornerstone of his company’s special purpose conveyor and industrial belts. Information about exact process details is not available, but as well as being able to weld the belts themselves, the company can also weld on spigots and other features in order to assist in the handling and processing of mass produced food products. Combined with moulded on toothed tracks on undersides, it is possible to ensure, for example, that jam is squirted exactly into the centre of doughnuts every time, without having to sense where they are. Polyurethane is, incidentally, normally regarded as a thermoset but it can also be made thermoplastic, and so weldable, hard or soft, depending on the amount of crosslinking.

One low temperature plastic welding process where details are available is that used in the design of ‘Easycarton’ pull tabs to go on the side of milk and drinks cartons.

The goal of the project was to find a way whereby consumers could open the cartons without having to insert a finger in the contents. Cost had to be very low, yet the tab had to work every time and not come off when used.

The tab went through some 80 design iterations to perfect both the shape and the bonding process. It now consists of 60 to 75Ám of polyester, melting point 260 deg C, adhesively laminated to 25Ám of polyethylene. The carton too has an outer lamination of 14Ám of polyethylene, and it is then possible to weld the polyethylene layers to each other. Welding is by using a heated profile block, and takes 0.25s. Cost is about 0.12p per tab.

Russian researchers, from the Russian House for International Scientific and Technological Co-operation, have also developed industrial processes for bonding combinations of higher temperature materials as difficult as brass and ceramic, glass and copper, and stainless steel and permanent magnet.

They describe their processes as ‘low temperature diffusion welding’. The main process parameters are the welding temperature of 530 deg C and above, and compression forces of 5 to 20 MPa. The relative low pressures and temperatures protect parts to be joined from possible phase transformation changes and plastic deformation. The key technology is the use of intermediate gaskets made up of very fine dispersions of metal powders. In the course of their work, they have established a computer database of recommendations of optimum gasket compositions for a wide range of possible materials to be welded.

Others who have worked to achieve the same goals have used joining gaskets, which vary in composition across their thickness. Hence, to join glass to metal, the gasket is mostly glass on the glass side, so it fuses to the glass, and mostly metal on the metal side so it forms a diffusion bond, between metal and metal. The problems of ensuring a correct microstructure in the middle can sometimes be most formidable.

Small vacuum diffusion welding units for the Russian process are commercially available but the developers are also looking for possible opportunities for joint ventures, turnkey developments and consultancy. Applications are seen in instrument and unit engineering, machine building and tooling. The process is inherently cleaner than either gluing or brazing.

More information:http://www.bmts.fr E-mail: bmts@wanadoo.fr

http://www.easycarton.com E-mail:  jonathan@easycarton.com

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