Better than new: Epoxy fillets Part II

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Epoxy fillets can make a bond between components even stronger than the components themselves, ensuring that loads can be transferred between structures and panels effectively, with no risk of stress fractures or other structural issues. In Part I of this blog, David Johnson explained what they are and how they work. Here, we overview the five steps that are the hallmark of a perfect fillet.

The filleting process

1. Surface Preparation

Surfaces should be clean, dry, dust free, well abraded and free from contamination (see the WEST SYSTEM® User Manual for special treatment for difficult substrates such as oily woods). Make sure the joints are smooth without bumps of cured epoxy, as this will upset the shaping action of the mixing stick.

2. Priming (two-step bonding)

Priming the area where the fillet will be applied, using the resin/hardener mix, is what we would call ‘best practice’. This allows the mixed epoxy to wick into the surface wood fibres and ‘tack’ to create more accurately formed and structurally robust fillets.

3. Applying the epoxy to create a fillet

Applying the filleting mix neatly reduces clean up. Best results are achieved by piping the filleting mix into the angle between components using a plastic food bag with a hole cut in the corner. This allows you to fill right into the angle between components with zero air voids. You will need to apply a little more of the mix in the corner (see shaping fillets below).

4. Shaping the fillet

The fillet can be shaped using mixing sticks with rounded ends.

Care should be taken to draw the mixing stick to bisect the angle of the two components coming together and to hold the stick perpendicular to the work. This creates a symmetrical shape that will work efficiently to transmit loads between panels. Drawing the mixing stick along several times ensures the fillet has filled the joint.

When you come to a corner (three angles coming together) you will need to practise your technique, but essentially you just carry the fillet around the first angle, rotating the stick as it passes the angle, then come back and starting at the top of the fillet you have just created, draw the mixing stick up and away from the corner. Practice makes perfect.

Our WEST SYSTEM 804 reusable plastic mixing sticks have a radius of 10mm and are ideal for creating smooth symmetrical fillets . Larger fillets can be made using wood or plastic with rounded ends. Some CNC plywood kits have a selection of rounded-end sticks pre-machined as part of the kit.

5. Cleaning up the fillet

It is possible, at this stage, to apply glass reinforcement over the fillet.

For the most robust structure we would suggest a biaxial fabric reinforcement.  These fibres can be draped across the fillet joint at a sympathetic angle with both orientations of fibres, albeit slightly off axis, contributing to tensile or compressive strength.

Woven tape has fibres at 0°/90° and as such will always have one redundant (warp orientation) run of fibre.

To reduce time sanding and removing dust and amine blush, WEST SYSTEM Peel Ply Tape, available in 50mm or 100mm widths, should be applied over the shaped fillet. Pre-cut this into short overlapping strips, this will make application very easy. Once the fillet is cured, the peel ply can be pulled away, leaving a fair, graded finish that is an exact impression of the weave of the peel ply. Because of this textured finish there is little or no sanding required. This has the additional advantage of removing any amine blush with the peel ply.

David Johnson is Technical Operations Manager at West System International. He can be contacted at here.

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