Mixing by weight

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Scot2640
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Mixing by weight

Postby Scot2640 » Sat Jan 28, 2017 4:01 pm

This is just not clicking I my small brain.

Instructions for mixing adhesive call for mixing by weight (not volume so no need for specific gravity mumbo jumbo) in the ratio of 100 to 20. 100 for the adhesive, 20 for the hardener.

So if I use 16 ounces of adhesive, do I use 3.something ounces of hardener? It just doesn't seem right.

I wish I could just mix 100 ounces of adhesive, that I could figure out!
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mrintense
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Re: Mixing by weight

Postby mrintense » Sat Jan 28, 2017 4:20 pm

Scott, divide the adhesive by 5 to get the amount of hardener (if the ratio is 100 to 20). So instead of 16 ounces, use 15 and then 3 ounces of hardener
.. However, keep in mind that large quantities of epoxy will set off quicker unless it's spread out or kept cooler in some fashion.

what I did for my build was have the epoxy pre-measured in cups of approximately 5 ounces. Then a helper would measure out 1 ounce and mix another cup as I got close to running out of the other cup. Or as more often happened, I would wait until it was cooler and then do the same thing, except I was working by myself. In this case it gets a bit stressful because you have to mix and then resume work. Plan your epoxy work well ahead and have everything ready to go. Also, don't try to do too much at a time unless you can get help.
Carl
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slug
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Re: Mixing by weight

Postby slug » Sun Jan 29, 2017 4:01 am

I have been using epoxy for years, and have always mixed by volume....the simplest method. I have NEVER had any issues. On these last two builds I've switched to MAS epoxies due to sensitivity issues. It is slower to kick off. Sometimes an advantage, sometimes not.

Most popular brands have regulated pumps that make the mixing simple in any quantity....use them.

For larger quantities, I use cheap mixing cups available at any automotive paint supplier.

Contrary to most on this forum, I clean up with lacquer thinner.
Works for me.

Doug

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Re: Mixing by weight

Postby JimmY » Sun Jan 29, 2017 4:44 am

Hi Scott,

You are right, 16 oz. of adhesive will need 3.2 oz. of hardener.

Think of it this way....

100 oz of adhesive to 20 oz of hardener
10 oz of adhesive to 2 oz of hardener.
1 ox of adhesive to 0.2 oz of hardener

You will have trouble pouring exactly 16 ox of adhesive. I have a small digital scale (<$20 on amazon) and I tare (zero off) the weight on the mixing cup, then pour the adhesive and read the weight and calculate the weight of the hardener (divide by 5), then tare the weight again, and finally pour the hardener to the calculated weight. I have a junk calculator (free from some bank) that I don't mind getting glue on. Keep in mind that most epoxy mixing ratios are tolerant of some error. I'm using System 3 and Poxy-Grip and I think the spec is +/-5% so you don't have to be super accurate. On 3.2oz of hardener, +/-0.16 oz would be fine.
-Jim
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hoodman
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Re: Mixing by weight

Postby hoodman » Sun Jan 29, 2017 6:10 am

By weight or volume? Just do whatever the manufacturer recommends. The last thing you want is a bad batch!

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steveh41
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Re: Mixing by weight

Postby steveh41 » Sun Jan 29, 2017 1:34 pm

JimmY wrote:
You will have trouble pouring exactly 16 oz of adhesive. I have a small digital scale (<$20 on amazon) and I tare (zero off) the weight on the mixing cup, then pour the adhesive and read the weight and calculate the weight of the hardener


I agree with Jim; would only add that once you know how much resin you have in the cup, you can use your calculator to multiply by 1.2 ( or whatever the recommended ratio ) to get the total weight, then you can add hardener to reach the total without having to retare. With smaller batches you need to be sure to mix all the material together and don't neglect any around the edges. With System 3 Gel Magic, the ratio is 100 to 44 resin to hardener by weight, so I add whatever amount of resin is needed to the tared container, multiply that value by 1.44 and add hardener to reach the total. Then mix like crazy... never had a bad batch! (knock on sapele :lol:)

Steve
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Scot2640
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Re: Mixing by weight

Postby Scot2640 » Mon Jan 30, 2017 9:50 am

Thanks guys. I was working with Rescirnol so I was just unfamiliar with mixing in that fashion. I got lazy with epoxy using the metered pumps.

Everything worked out good, I maintained the temperature in my shop for about 30 hours under very high clamp pressure and things appeared to have set up properly. I took a dried piece of the squeeze out and soaked it in a cup of water for 24 hours to see how it would respond and it remained unchanged. This stuff sets up and turns to what seems like plastic.

Still not sure why they have the ratio as 100 to 20, wouldn't 5-1 be the same thing?

Anyway, it worked out so thanks for the explanation.
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Bill Edmundson
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Re: Mixing by weight

Postby Bill Edmundson » Mon Jan 30, 2017 10:24 am

I just don't think the mix ratio is that critical. The manufacturer has designed in a margin of error. Most of the hardeners are mostly solvents that evaporate. This is so they are easier to mix. The only time that I've had trouble my wife read 3:1. She mixed calibrated pumps 3 pumps of resin and one pump of hardener for a 9:1 mix. Getting it off was easier than explaining it to her without hurting her feelings. I had to scrape as much as I could off. Then I painted the rest with plain hardener. It all worked out. But, the anxiety was not good for me.

Bill
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