Weighing of Resin and Hardener.
An article in the current Europa Flyer points out the importance of
accurate weighing for cured strength and the major source of error from
the residue left in one cup when using the Europa two-cup balance. This
can be avoided by only using one cup. An epoxy pump does this of course
anyway, but you can do it with homebuilt scales as follows.
Make up a pair of weights, one for the hardener and one for the resin.
Put the small one in a matching cup on the other side from the mixing cup
and balance it with the hardener. Add the larger weight to the first, and
top up the mixing cup with resin to balance. A full cupful weighs around
300 gm so with a 3:1 arm ratio the weights are 20gm and 80gm for 1:4 mix
ratio. Note that the arm ratio no longer represents the mixing ratio,
just the mix weight relative to the sum of the weights. So you can mix
different quantities by moving the pivot point along the arm. (This is
easy if you make a u-shaped cradle for the pivot).
Quite accurate weight ratios can be obtained by using the right number of
readily available objects, like nuts (not the squirrel variety), and you
can use the balance (or a more accurate one) to check them.
For these mount a vertical bolt through the end of the weight arm,
trimmed to balance an empty mixing cup and any mismatch due to the
different arm lengths, and drop the weighing nuts over it.
Or if you have the material and facilities, you can make weights with
very accurate ratios by cutting brass or steel bars or rods. Assuming you
can cut the ends square (turn them if you cant !) its easy to get the
lengths accurate to well under .5% with a micrometer or gauge. In steel,
100 gm is about 1" dia by 1" long and you dont have to get the absolute
weight accurate, just the ratio.
The only drawback of the one cup method is that you must not overshoot
the deliveries into the cup. As the balance starts to swing in advance,
(depending on how far the pivot is above the beam axis) you get advance
warning and can use a dropper or syringe for the last trim if you are
nervous.
Because you can easily set up the same balance for a different mix ratio,
it is possible to use it on the dreaded gooey Redux mix. Dump the
required quantity of the thick component first, adjust the position for
balance, add the extra weight for the hardener and syringe it in.
Although this is in principle backwards, i.e. larger quantity first
(because that's the viscous one), the ratio is not so extreme, and it
probably beats any other method.
Graham Clarke b/n 83
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