mixing epoxy by weight

[Dana]

I saw an interesting idea in the book “Canoe Paddles” regarding the measurement of resin and hardener. Graham Warren measures them out by weight which he claims is much simpler and more accurate. It sounds like a good idea to me and I can’t see any drawbacks to it. Any comments?

[Glen Smith]

Measuring epoxy and hardener by weight is the more accurate method of measuerement. These products are actually sold by weight and not by volume. On my can of West 207 hardener it states: "Dispensing without mini-pumps: Combine 3.3 parts 105 Resin with 1 part 207 Hardener by volume measure (3.7parts resin to 1 part hardener by weight)."

Some builders use accurate electronic scales to weigh the components and some have made a set of balance scales for weight measurement.

The main problem with using pumps is that they "burp", expelling air which throws off the ratio.

[Erik, Belgium]

I switched from measuring by volume to weight during the building of my first canoe. 2 boats further now, and I still don't complain about the decision: much easier, cleaner, more exact and quicker.

Another one.
A similar switch from what I understand is normal practise. Wetting out the glasscloth with a squeegee wasn 't satisfying to me. I 'm using rollers since then, and that 's way easier, and most of all: very quick.

PS. I just glassed the hull of a new kayak http://users.pandora.be/kano-kayak/DiskoBay.htm

happy building,
Erik, belgium.

[Its Me]

I used John Miichne's double cup method of volume measuring and it work fine for me.

I used Raka epoxy and it seemed pretty forgiving. A little over the line one way or another didn't seem to effect the cure time. Maybe under controlled testing there would be a perceived strength change but maybe not.

Actually as write this I wonder; does a differential change in the mixture ratio effect both cure time and strength?

[Glen Smith]

This info is from the Raka User Manual
: "I want to emphasize the importance of proper mixing.
Epoxies strength and desirable properties come from the
complete mating of the resin and hardener molecules in
the correct ratio. Too little hardener produces brittleness,
and too much hardener makes your epoxy softer with the
accompanying loss of strength. If you make an error, you're
safer to use a little less hardener, than too much. Generally
speaking you can have a 20% error tolerance on the minus
hardener side and 10% on the plus side and still get an
acceptable cure. Regardless, your cure will have less
strength than its ultimate potential!"

[AlanWS]

Measuring by volume is easier, but measuring by weight definitely can be more accurate. A reasonable compromise (if you are concerned about accuracy) is to measure by volume and check the weight. If it's off, you can fix the ratio by weight, and if it's OK, just use it.