i glassed a board with epoxy resin and messed up with the measurements of how much hardener to resin ration. what is the best scale to get or way to measure this.
I just bought a cheap (AU$20) digital gram scale.
I’m a cheapskate, but it works just fine with the bote cote epoxy i’ve been using.
I had the same problem. Spent $70 NZD on some digital kitchen scales that are accurate to a gram. Problem solved.
Hi Guys
Is it better to mix by weight rather than liquid volume?? as in oz/mls??
Cheers
It’ll be more accurate if you have a good set of scales.
Neither.
It’s best to mix according to the instructions, that’s how they have been designed and tested.
If it’s a by-volume epoxy, mix it that way.
If it’s a by-weight epoxy, mix it that way.
I know, I know. The difference probably isn’t that much. But it’s not worth the risk of hotcoating a board and coming back two days later to find the whole thing is still tacky and coverred in dust.
Ignore me if you wish.
When you buy you scales put some plastic over them right away
Am I right guys? Who here has a nice set of very sticky scales?
You always think you’re gunna be really careful with them, but they’ll be covered in it in no time
Hey Hoppa,
This reminds me of an old post on this topic. Something to keep in mind.
So I’m day dreaming about the board I’m gonna build. I’m stupid enough to try a swirl on my first go 'round and that got me thinking about a bunch of small cups of colored epoxy and hardener. Gotta keep proportions correct. Me thinks, okay, I’ll use a precise scale and do them by mass. Simple enough.
Hmmm…the proportions are 2:1 by volume. What if the resin and hardener are different densities? That could screw me up if I’m weighing them. I’ve heard that you’ve got to keep the proportions accurate or you’ll be in trouble. I don’t want trouble.
I checked the MSDS from RR for specific gravity and sho’nuff the resin is about 12 % more dense than the hardener. I don’t know how critical a 12 % volume error is but it don’t want to chance it.
FYI - The SpG of the resin is 1.10 and the hardener is 0.98. Mulitiply your desired volume in CCs by those numers to get the grams you need to weigh up.
In other words.
Resin: 1000 cc x 1.10 = 1100 g
Hardener: 500 cc x 0.98 = 490 g
This will maintain your 2:1 ratio by volume.
Ryan
Thanks for the tip on the volume. I have little Post Office scale I got off of ebay and measured 2:1 by weight on my first board. Came out fine, a couple of the coats took quite a while to harden… now I see - Volume not weight is the key.
From your post I did a little poking around on Greg’s site - he recommends 2:1 volume or 100:45 by weight http://resinresearch.net/_wsn/page8.html
If your doing Resin Research its a 45%:55% ratio by weight. The hardner is slightly lighter than the epoxy. So just use a tad more epoxy than hardner when measuring by weight.
If you do it by weight at 1:2 you’ll end up with too much epoxy, and not enough hardner for your next job. Anyhow the measurments on RR epoxy should be accurate, but they don’t need to be scientifically measured to the mil-gram.
The most important thing here is the temperature your working at.
Yep, that’s right. The volumetric ratio is 1:2 or 0.5:1. Due to the difference in density, the mass ration is 0.45:1. Hardener is 0.98 g/cc, resi is 1.10 g/cc. Calc’s like this 0.98/(1/10*2) = 0.445 or 0.45.
The way my brain works is that I think “I’m gonna need about 150 cc for this fin box” or whatever because I visualize by volume. Volumetrically that 100cc resin and 50cc hardener. Since I don’t want to eyeball small amounts in large cups, I’d weigh it. I was curious if the weight was the same. Turns out it’s not. I never saw the 0.45:1 on the RR site and didn’t think of it that way. That’s a simpler way to do it.
Pour out 100 g of resin x 0.45 and I need 45 g of hardener to get the 2:1 volume. Pour out 1,231 g of resin x 0.45 and I need 554 g of hardener. One simple calculation to keep the ratios precise. Resin x 0.45 easy peasy. I mean, how hard is that? If you’ve got a scale and you’re going to weigh it, why not do one multiplication on a pocket calculator to make sure you’re on target.
Hey
I just use 14oz disposable graduated cups.
Measure carefully and have never had any problems.
Rick.
Digital kitchen scales that will weigh 6+ pounds and have a high degree of accuracy are easy to come by. You can generally find them at a supermarket, drug store, online, eBay, etc.
One general thing I have found is the guys who really push the strength to weight ratio in composites, make high stress parts, or build small parts measure by weight. The formula for mixing epoxy by weight varies by manufacturer and is usually easy to obtain on a website or in their instructions.
As said before, cover your scale in plastic before you use it. The 3M ‘Super Strength’ packing tape works great for this. You can pull it off with zero residue or mess, months or years later. That only goes for that particular tape which is about $5 US a roll ( also works amazing for repairs), other tapes leave a mess.
I personally measure by weight because I find it most convenient and accurate. I don’t like the air in the resin and confusion associated with pumps (‘uh, was that 3 pumps or 4?’)
I use one cup, drop it on the scale and hit the tare button, pour to my target part A, close the container and open part B, the number is still sitting right there on the display so I don’t have to worry about forgetting it, pour in B topping it off nice and slow to hit the perfect weight, done. No bending down, no extra cups, no mixing more then I need so I can hit some arbitrary line, etc.
Didn’t mix enough? Drop your container down, it the tare button and mix more in the same container.
If you are mixing epoxy regularly a scale will save you money and time. I journal how far the epoxy goes for different things I build, so when I mix a new batch very rarely do I have more then 10 grams of resin left over.
Curing problems were relatively rare when I mixed by volume, but they did happen. They are nonexistent when I measure by weight.
For anyone interested in making this really easy try the sticky stuff despenser from michaelengineering.com It’s the easiest and best way. Costs a couple humdred dollars but is well worth it.
Im with rickrick on this
keep it simple
use the cups with oz incriments
10oz resin then add hardner to 15oz as an example
allways remember to stir slow and for about 5 minutes
no brainer
Ive never had a problem…yet…knock on wood!
Whoa.
How long do those last? Ever clog up?
Rick
Hey Ryan,
I always go by weight now because I usually heat the resin, which can change its volume. X amount of resin by weight will have different volumes at 70 deg and at 100 deg. I don’t know if the volume change is noticeable, but I have come to not trust the marks on the containers I use after I ran out of hardener once with some resin left in one batch I was using…
JSS
hey thanx for all the info guys. hopefully my next board will turn out better than the first. good luck to all
Quote:
When you buy you scales put some plastic over them right awayAm I right guys? Who here has a nice set of very sticky scales?
You always think you’re gunna be really careful with them, but they’ll be covered in it in no time
Listen to Kit!
Otherwise you are gonna end up cleaning it with DNA over and over… Don’t be like me and use acetone when you run out (forcing me to wear gloves and thorw the rag away afterwards). DNA is a better option, platic a better one still.
These days I just use a bit of glad wrap and replace as needed.
medicine cups
no air in the resin i use
it doesn’t bubble and doesn’t require heating and sands reasonably well
also has a long pot life, 30 minutes at least and low temp post cure
costs about 1/3 of the price of other board building epoxys in new zealand