How many of you just mix your 2 part epoxy and hope it sticks two parts together.
Over the years I have built some very high stress (36G) structures, bonding all kinds of materials together, Balsa, Ply, Glass cloth along with Aluminium and epoxy based mouldings. Now having seen more than 1or2 models come apart at the local flying site, I thought it was time passed on some of what I have learnt and do practice. Now don’t go thinking epoxy has some kind of magic power and will bond anything together no matter what.
- 1. Buy the good quality epoxy for me it’s got to be Zap, Deluxe Materials for normal applications, if temperatures are likely to exceed 100deg C look for Loctite Hysol 9462 (Aeropoxy) I won’t use anything else.
- 2. Mix your Resin and hardener to instructions normally at 50-50, also check the working temperature and try to work at the top end even if it means turning on the central heating on, it will mix and cure far better, but do read the instruction sheet.
- 3. Start by mixing a small test sample on to a piece of plastic/something flexible so that you can remove the cured epoxy in one piece to test it for its flexibility. A good epoxy should flex slightly, if it cracks try again with less hardener, if you can flex it like rubber try a little more hardener. Epoxy does age in the bottle, by doing this test I find that a bottle set which normally lasts 2 years will require 2or3% more hardener by the end.
- 4. You will see from doing the above test epoxy peels easily from smooth surfaces so don’t try sticking it to Solafilm, canopies and anything plastic it just won’t stick, find something else.
- 5. Next everything must be spotlessly clean, a light sanding and a wipe down with acetone should see any mouldings ready, wooden items if after sanding look fragile lightly wipe with cyano that should make a good base to bond to.
- 6. It is very rare that I will mix epoxy on its own to make a joint. Whilst you can make most things stick together for a time, add some additive such as glass microfibers and the joint will be much much stronger. Micro balloons will make it much lighter (epoxy is very heavy) great for making fillets around tail planes and matching up mouldings etc. Kevlar pulp, lighter and stronger than micro fibres but very difficult to sand. Colloidal Silica, Thixotropic agent great for thickening and controlling the viscosity of your mix.
What next, start by buying a small quantity of microfibers and micro balloons, mix up some epoxy, make 3 test samples 1 without 1 balloons and 1 with Fibres, so that you can see the effects you will never go back.
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