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Adhesive Injection

Adhesive injection for Posted Joints

With many space frame structures the design uses hollow structural members and posted joints. These are joints where one member slots inside another one with a tight fit. Many of the materials used are ideal for use with adhesives to produce strong very rigid joints. It is almost impossible however to apply adhesive to a posted joint and then assemble it without all of the adhesive being wiped from the joint. It is also true that to ensure structural integrity it is essential that the joint is filled with adhesive.

In order to get around this problem the decision was taken that assembly followed by injection of the adhesive was essential. The tolerances achievable by the extrusion process for the aluminium sections used means that the joint gaps to be filled by the adhesive can vary between 0.3mm and 3.5mm. A system has been created based on a commercially available adhesive gun which monitors the back pressure in the joint caused by adhesive flow and uses it to calculate the joint gap (5,6). The amount of adhesive dispensed is then set appropriately for each situation. Trials show that using this technique a circular adhesive pad can be achieved to a set radius within the tolerance band +1/-5%.

Currently this process has been applied to flat joints and has been optimised for one adhesive type. In addition it can handle tapers with no modification. Extension to other adhesives will require recalibration of the algorithms but this is simple to do using the procedures documented.

In order to make best use of this process it is important that the joints are designed correctly. It is possible to use features extruded into the sections to change the shape of the adhesive pad generated and to set minimum pad thickness for the joint. For long thin joints a method of sealing the edges to contain the adhesive has also been designed. The details of the design requirements have been produced in the form of graphical design guidelines.