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Resin mixtures by weight

Posted by Anonymous User 
Anonymous User
Resin mixtures by weight
May 14, 2014 07:06PM
West System resin and hardener have the same density so you mix them by weight accordingly.

From Raka -- other manufacturers will be different

2:1 epoxies 100/43 for 1 weight unit of resin add 0.43 weight units of hardener
4:1 epoxies 100/23
5:1 epoxies 100/18

You would need a digital postal or kitchen scale to make use of this information. And a pocket calculator. Pumps are easier but less accurate.

There are several ways to think about this. One way is to put an empty Yogurt container on the scale and then to pour resin into the cup. See how much it weighs.

For 2:1 Raka mixtures multiply the weight of the resin by 0.43 to get the required hardener weight. Pour and measure that in a separate container. Now mix the two.

For 5:1 West System mixtures consider the densities as equal. So you want exactly five times as much resin weight as hardener.

Take two. Pour resin into an empty cup. Weigh it. Multiply by 0.43 to get the weight of the needed hardener. Add the two weights. Now pour hardener into the resin cup slowly, watching the digital scale. Stop pouring hardener into the cup when the combined weight is the sum of the separate resin and hardener weights. That's the lazy man's technique. So that's what I do.
Re: Raka Resin mixtures by weight
June 03, 2014 07:33PM
Thanks for the post. I will kept bookmarked.

Hoping my cast comes off tomorrow (surgical repair of torn tendon which happened in February, completely ruining any plans to work on the boat and or rowing this year) and can get back working on my shop. It's first before the boat work.

Thanks Workman's Comp for delaying the surgical repair so long. But since the initial MRI was delayed 41 days from date of injury, I wasn't expecting much.


"Trout don't live in ugly places."
Re: Raka Resin mixtures by weight
June 05, 2014 01:58PM
In the leadin post I said "pumps are less accurate." They're plenty accurate at first. Perhaps forever if you live in a warm place. But resin and hardener sometimes crystalize slightly, if you live in a cold place. When even small amounts of hard resin or hardener crystals get lodged inside the pump mechanism you can start to get bogus mixtures. And it's hard to tell. Sticky and messy to clean up.

Weighing resin takes a little fussing and practice at first. And then it becomes automatic. And it always works.
Re: Resin mixtures by weight
December 10, 2019 03:44AM
I always use 205 or 206 resin for WEST, but their boutique 207 and 209 hardeners are 3.5-1 by weight.

The thing about measuring epoxy is that the success of the method varies a little relative to the quantity needed. So mixing in a 5 gallon bucket is different that mixing 3 cc .

My favourite small batch method is mixing with o-ring syringes, often called animal feeding syringes of PD dose syringes. These syringes use O rings in place of the rubber plugs. They can stiffen up a little and still remain usable for years. They would probably work indefinitely if one retrofitted with the o-rings that are epoxy resistant.

Mostly I use conventional pumps. I always promise myself that I will never take on another large job without buying a proper metering system but I have one that one needs to build from plans, and I keep putting it off. I have pals who have built 80 foot sailboats and never invested in a proper pump, yet we live in a world of 700 dollar drill drivers.

Weighing is cool. I just make up a table for standard batches and hang it over the postal scale. So it would be like 100g = 143 g. I just zero the scale so the pot is out of it, and dispense the epoxy from squeeze bottles.

I saw this cool site where the guy had built his own scale, it was a balance where one beam held the pot, and the other beam was a long sawzall blade with the notches up. You put one weight in position to net out the pot. You had a second weight that has two parts. The first part was washers, or whatever, proportioned to the weight of the resin, and a second part proportioned to the hardener. So you would pour the amount of resin you wanted in the pot, then you would zero it with the resin weight. Then you would dangle the second weight from the first weight, and add the hardener till it was level. The great thing about it is that it is well able to deal with the rough and tumble of a workshop, and once it is set up, there is no math. But a very wide range of batch sizes are possible.
Re: Raka Resin mixtures by weight
December 10, 2019 03:48AM
The chrystals should come out with heat. Direct hot sunlight seems best. But a heat gun used with great care might be OK. I haven't had a problem with my pumps for a while. But that is part of the reason I want to make a proper set with quality O-rings and machined parts. It would be user serviceable, which you rightly point out is probably messy, but at least I would always have a pump that worked like a champ at the start of every season.
Re: Resin mixtures by weight
December 10, 2019 01:36PM
I'm too impatient to be scientific. I use Raka's pumps and pump into a graduated cup for a second control. I never had an issue except on my first boat when I forgot to add the hardener on a transom (what a mess).
I might be more cautious if I were to mix larger batches but at this point, for the small craft I've built, I never use more than 9oz at a sitting. This is little enough to rebound from if I screw up somehow.
Re: Resin mixtures by weight
January 18, 2021 01:16AM
I tried pumps but I have a hard time counting to 100. I mixed by volume on my last project which worked well for 20 oz at a time. I just switched to measuring with a postal scale. Resin research is 100:45 so I made a spread sheet of grams.
Re: Resin mixtures by weight
January 18, 2021 09:42PM
Yes I measure all resin with a digital postal scale now, from small to large. I keep a calculator next to the scale.

About his wife George Foreman once said "I do what ever she says. It works every time." The postal scale and calculator does too.
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