A joined up future

TWI has been developing two distinctly different joining technologies – adhesive bonding and plastics welding. The resultant processes are termed AdhFAST and ClearWeld respectively.

Let’s look at AdhFAST

AdhFAST is a hybrid joining system, where adhesives and fasteners are combined to maximise the benefits of each technique, i.e. rapidity and ease of use of the fastener coupled with the sealing ability and high fatigue resistance of the adhesive.

Conventional hybrid joints are formed in a continuous linear process, where an adhesive is applied to the surfaces, the joint closed and a fastening system used to secure and hold the structure together while the adhesive cures.

By adopting an innovative approach, the process can be broken down into two stages, i.e. a ‘dry’ assembly stage followed by an adhesive injection stage. In reality, this means that these operations could be done in different geographical locations or at different times depending upon production and manpower resources. With the correct selection of surface pre-treatment, where a bonding window of days or weeks is possible, the storage of dry assembled parts ready for bonding is possible, with disassembly and re-use should an order be changed or amended.

What makes this process different is the addition of new functionality to the fastener employed. The traditional fastener has two primary functions, location – holding the components in the correct position, and retention – fastening the pieces together.

The AdhFAST fastener controls the spacing between the materials to be joined, so enabling the adhesive to be easily injected and defining the final thickness of the adhesive in the joint. This second factor aids the calculation of the mechanical properties of the bonded joint. And it injects – accomplished either through a central hole or down features on the side of the fastener. The fastener design is completely flexible and can take many forms, from nuts and bolts through blind riveting systems to the wood screw.

The fastener element can take a range of forms, incorporating a central hole with exit ports or a modified head combined with vertical grooves or flats along the length of the fastener. The adhesive can therefore be pumped through the fastener, filling the joint from the inside out.

Many industry sectors can benefit from this technology. Complex joints can be filled in one go enabling more efficient better-designed structures to be made. The aerospace industry consumes hundreds of thousands of rivets per aircraft. Aluminium is still the primary structural material but with increasing use of composites and other materials within the fuselage, structural adhesive bonding will become a necessity presenting opportunities for the AdhFAST system.

Bridge stiffening and strengthening is often accomplished by bonding, bolting or welding steel plate to the underneath of the structure. The processes although well established, are messy and time consuming where adhesives are used; not so effective in the case of bolting and labour intensive where welding is required. Using an AdhFAST fastening system could simplify the process considerably and remove the need for welding.

The marine and shipbuilding industries are also likely candidates for AdhFAST, either in the fabrication of bulkheads (composite structures) or for the attachment of secondary structures. The specialist automotive sector is increasingly looking at ‘new’ materials and adhesives to join them, for better, lighter, faster cars. High levels of quality control are required to maximise consumer confidence.

TWI has filed a patent on AdhFAST and is in detailed discussions with a global fastener manufacturer. Both parties are assessing essential applications to establish this enabling technology within the manufacturing world.

So what’s ClearWeld then?

Another process developed at TWI is aimed at laser welding plastics with an infrared absorbing medium, creating a joint almost invisible to the human eye.

Typically, carbon black would be used as the absorbing medium for the laser light; however, this new approach enables two similar clear (or coloured) plastics to be joined with a minimal mark weld line.

Welding may be carried out using a Nd:YAG laser (1064nm wavelength), or using the relatively new high-power-diode lasers (typically 808nm and 940nm wavelengths). This process has been termed ClearWeld.

In the conventional transmission laser welding technique, a transmissive plastic material is used for the upper section and a carbon black loaded plastic for the lower layer. The carbon black absorbs and heats in the laser beam to generate a weld at the interface between the two pieces. The process is limited by the fact that one side of the component has to be black.

The nature of the ClearWeld medium means the laser wavelength is absorbed with high efficiency, thus requiring relatively small amounts of the medium at the interface between the two components to be welded.

When the incident laser light is absorbed, the medium molecules dissipate the absorbed energy principally as heat to the medium molecules and their local environment.

The medium can be incorporated into a thin film which can be placed at the interface of the plastic pieces to be welded, or the medium can be introduced into the bulk of the polymer. A medium laden film may also be added as a mould insert to coat a moulded article.

The surface of the solid or fabric material may be coated in medium from solution, either by dip coating, medium infusion, painting, spraying, printing, dry burnishing, paste application, etc. The material to be welded can also be co-extruded with polymer containing the medium.

The new welding process can be applied to films and fabrics or coated fabrics.

The welds produced are cosmetically appealing and the upper and lower surfaces of the material are unaffected by the process. The welding process is efficiently achieved using compact diode laser sources that are commercially available, and the process itself lends itself easily to high levels of automation.

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