From: "Marvin Kaye" <marv@lancaironline.net>
Subject: Re: [LML] Re: Epoxy pump AR-100 
To: lml
Date: Thu, 10 Apr 2003 00:21:48 -0400
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Posted for "Paul Davis" <pdavis@bmc.com>:

Several folks (Marv and Don come to mind) have recommended scales in 
lieu of epoxy pumps.  Appeals to me, so I've been shopping.

Marv seems satisfied with the Ohaus Scout (300G capacity with 0.1 
resolution).

I'm also looking at the Ohaus CS200 (200G capacity with 0.1G 
resolution and 0.1G accuracy (for $68.00).

I think most of these scales do "tare to capacity by subtraction"
which I understand to mean you can tare up to the capacity of the
scale in multiple steps.  So I could put an empty container on the
scale, tare to 0, weigh resin, tare to 0, weigh hardener, all without
removing the container from the scale -- so long as the total of
container, resin and hardener is less than the capacity of the scale.
Is that the way "tare to capacity by subtraction" works?

We have the fast-build Legacy and did the one week workshop so most
of the big volume steps are complete.  And of course we could always 
just measure several cups individually and then mix.

My question is what capacity scale do we really need?
My instinct is to go with "just big enough" on the assumption that 
I'll get best accuracy.  Any thoughts, recommendation or regrets?

-------------------
Potest ex casa magnus vir exire
A great man can come from a hut
Arguments are repl Aviation/Lancair/List /home/pdavis/Mail/drafts/332


(Just a couple things... first of all, the easiest method I've found for 
measuring the epoxies is to put the container on the scale, zero out the tare, 
put in the required amount of resin, multiply its weight by 1.22 and add 
catalyst until you reach the number on the calculator.  For Hysol, do the same 
thing but multiply the resin weight by 1.445.  Couldn't be simpler.

If you do decide to purchase a scale, tape some clear plastic (pieces of 
baggies work great) over the control panel and wrap the platform with plastic 
as well.  No sense gooping up a nice piece of equipment.

While we're on the subject of mixing epoxy, just a few other important things 
to keep in mind... don't ever forget that mixing the two components together 
starts a chemical reaction, and if you mix a large quantity and leave it in a 
compact mass it's going to exotherm...  think smoke <g>.  Any time you mix 
more than an ounce or two you should try to transfer it to a larger container 
so it spreads out into a thin layer.  The folks at Jeffco Products suggest 
using a tinfoil lined cookie sheet, and that works for me.  You won't be 
mixing lots of large quantities, thankfully, but when the time comes don't 
forget about this.

Trying to figure out how much epoxy to mix for a given layup can also be 
problematic... when in doubt, weigh the glass or carbon fiber you're going to 
lay up and then mix a similar amount of epoxy.  They say that the ideal ratio 
of reinforcement to resin is about 60:40, but that's hard to achieve unless 
you're vacuum bagging.  Using as much epoxy as reinforcement (in our rather 
crudely controlled layups) insures that you get a good wet out, and you can 
draw out the excess by stippling through a paper towel before applying the 
peel ply.  Additionally, don't draw further resin out of the layup into the 
peel ply... stipple it down with some additional resin on top so you don't 
starve the layup.

I really enjoy working with composites, as the possibilities are so great. 
 Try several different methods of doing layups and use whichever works best 
for you.  As in most things, practice makes perfect.  

      <Marv>          ]