PDA

View Full Version : Calculating epoxy coverage



gazzer
08-14-2009, 07:23 PM
Epoxy has gotten quite expensive, so I don't want to mix up too much for a particular task. Too much in the glue joint makes for much extra work cleaning up, as well. Mixing up not quite enough and having to rush an extra batch is always fun (stress builds character, right? Shoulda bought the slow hardener)

So, does anyone have any wisdom about calculating how much epoxy would be necessary for a particular glue joint? It would be straight forward if one did not have to consider absorption of the epoxy into the wood. For instance - 1 cubic centimeters of epoxy would glue an area of 100 square centimeters with a intrajoint width of 0.01 centimeters (assuming no absorption).
Considering absorption, would the effective glue area only be half? or less?

Lot of variables: Wood species, temperature, viscosity of epoxy, clamping pressure, ????

I reckon that there are no perfect answers. Perhaps the strategy should be to determine coverage as I go along (means keeping records and thinking ahead), but the backup plan would be to always have something else ready that needs gluing.

All insight is appreciated.

-G

JimConlin
08-14-2009, 07:30 PM
THis has been beaten to death and the body is not yet cold.
http://www.woodenboat.com/forum/showthread.php?t=100728

gazzer
08-14-2009, 09:21 PM
Saw that thread, but as it related to lay up of glass or other fabric, it didn't cover my interest: gluing wood to wood. My understanding is that epoxy-wood joints don't like to be starved by either not enough glue or over clamping. Is there an optimum cured interjoint epoxy thickness to shoot for?

-G

Bob Adams
08-14-2009, 09:38 PM
I always try to have somewhere useful to absorb any overage. IE: whatever googe does not go in the joint, I'll have the fillers required at hand tomix quickly and use the extra fairing something, ect.

kc8pql
08-14-2009, 09:45 PM
It just takes a little practice. You soon get a feel for how much glue a given joint will take. Pre-coating the joint with unthickened epoxy first is always best practice and removes any concern about a starved joint because of excessive absorption. The thickness of the glue line is controlled by the fit of the joint, not by the viscosity or quantity of glue. With epoxy it should be a very easy fit and clamped only tight enough to hold the joint together, using enough glue to get a little squeeze out. The epoxy just needs to be viscous enough not to run out of the joint on it's own.

Dennis Rioux
08-15-2009, 09:24 AM
I will second the "it takes practice" advice. Not sure of the scale of your project, but I have found for my kayaks and small boats that a digital scale that reads to tenths of a gram allows me to reliably mix "sub-pump" quantities of resin and hardener. More than paid for itself in reducing waste.

Dennis