Mixing epoxy

I get asked all the time - how do you mix epoxy? I always deflect that question and say - refer to the epoxy manufacturer that you are using.

Every epoxy system has its own specification, so it is EXTREMELY important that you follow what the manufacturer of YOUR epoxy system states - not this blog post.

In this post, I will explain how I mix my epoxy for the system that I use (primarily Aeropoxy PR2032 resin with PH3660 hardener). The idea is that you note the takeaways as to point out things to look out for... do NOT treat this as an instruction manual.

Most epoxy systems consists of two parts: a resin and a hardener. In order for the epoxy to meet its manufacturer's published mechanical properties, it is vital that the proper mix ratio be used. Why is this important?

Let's talk a little more about mechanical properties:

Mechanical properties are physical properties that a material exhibits upon the application of forces. Examples include: tensile strength, elongation, hardness, etc. You can think of these as: for a given load, how much does the material deform? How much load can it take before it cracks? How does temperature effect the material? Luckily, we live in a universe where the laws of physics don't change... and so if we test a specific material for these properties, they will always behave the same.

So when an engineer designs a part (e.g., a wing spar, a control surface, a control linkage), he or she uses these known properties of the material being used to make sure the part won't fail or break under expected loads. In other words, the shape of the part and the number of fiberglass plies is specifically designed based on the mechanical properties and the expected loads.

You can now see the danger of building a part out of a material that might have different mechanical properties than what the engineer expected... the part might fail! If the epoxy mix ratio is wrong, the epoxy won't have the mechanical properties published by the manufacturer. This would be REALLY bad, as the airplane can have a catastrophic structural failure even if operating under known safe conditions.

Ok, so how close to "perfect" do we have to be when mixing? Aeropoxy states that for PR2032 and PH3660, the ideal mix ratio is 100:27 (parts by weight), and that they like the mix ratio to be between 100:25 and 100:29 in order for the epoxy to obtain the physical properties they state on their technical data sheet.

Understanding how important it is to have the ideal mix ratio, I have (and will always) use a digital scale to mix my epoxy. Many builders use pumps so that they don't have to go through the trouble of measuring every ounce of epoxy used... but what if a small clog develops in the resin tube? Or the hardener tube? How will you know if the correct mix ratio is actually being pumped into the cup? This uncertainty scares me enough to take the time to measure each and every individual ounce of epoxy that I use on my airplane.

Calculating the parts:

The math to calculate how much resin and how much hardener needs to go in the cup can be a little daunting. Here, I will explain the process I use.

Given a mix ratio of 100:27 means that a total of 127 units of weight (100 resin + 27 hardener) will be in the cup. So if we divide the resin part (100) by the total weight (127), 100/127 we get 78.74%. This means that 78.74% of the total weight is resin. Similarly, if divide the hardener part (27) by the total weight (127), 27/127 we get 21.26%. This means 21.26% of the total weight is hardener.

How do we apply this? If I want 5 ounces of epoxy, I multiply 5 by 78.74% to find out how much resin I need (3.94 oz)... and multiply 5 by 21.26% to find out how much hardener I need (1.06 oz). Notice how 3.94 oz of resin + 1.06 oz of hardener gives me a total of 5 oz.

Rather than calculate this for EVERY cup of epoxy that I mix, I created a table for easy reference that I keep next to my digital scale:

Do I pour the resin into the cup first? Or the hardener? Does it matter? Well, it does if you go over by 0.1 oz. Let me explain:

To prove my point, lets say I want to mix a 4 oz batch of epoxy. This means I would need 3.15 oz of resin and 0.85 oz of hardener.

Let's say I pour the resin in first (3.15 oz)... and then pour the hardener into the cup until the scale reads 4.0 oz, but instead accidentally go over by 0.1 oz for a total of 4.1 oz. This means I have 0.1 oz more of hardener than I should. Is this ok? Do I need to scrap the batch and start over? Let's see:

Having 0.1 oz of extra hardener would mean that I have 0.95 oz of hardener and 3.15 oz of resin. This means my mix ratio is 100:30 (0.95/3.15 x100 = 30). Recall that Aeropoxy wants the ratio to be between 100:25 and 100:29... so we would be OVER the allowable range and have to scrap the batch!

On the other hand, let's say I poured the hardener in first (0.85 oz) and then poured the resin into the cup until the scale reads 4.0 oz, but accidentally go over by 0.1 oz for a total of 4.1 oz. This means I have 0.1 oz more resin than I should. Is this ok? Do I need to scrap the batch and start over? Let's see:

Having 0.1 oz of extra resin would mean that I have 3.25 oz of resin and 0.85 oz of hardener. This means my mix ratio is 100:26 (0.85/3.25 x100 = 26). We are inside the allowable mix ratio range and do not have to scrap the batch... saving time and money!

Of course, this only applies for the scenario where I accidentally go over by 0.1 oz. Don't ask me how many times that happens! So if I have the choice to pour resin or hardener first, I choose to pour the hardener first, knowing that if I accidentally over pour the resin by 0.1 oz, I can still use the batch.

Keep in mind that the extra "0.1 oz" number doesn't always work... and is NOT something special to Aeropoxy and/or other epoxy systems. It has to do with how much extra resin (or hardener) is poured compared to the TOTAL batch. In other words, if I were mixing a 1,000 oz  batch, 1 oz of extra hardener or resin would be negligible (literally a "drop in the bucket").

Which brings me to the next point - is there a minimum amount of epoxy I can mix? The answer depends on how accurate the digital scale is. Standard digital scales found in office supplies stores are accurate to the nearest 10th of an ounce (0.1 oz).

So lets say I want to mix the smallest amount possible that my scale can read and I put 0.1 oz of hardener. This means I need to put 0.37 oz of resin for an ideal total weight of 0.47 oz. But my scale can only read to the nearest 10th... so it would read 0.5 oz. This means I might have 0.03 oz of extra resin in my cup. Is this ok? Let's see:

If I have 0.1 oz of hardener and 0.4 oz of resin, that would mean a mix ratio of 100:25, which is allowed. However, if I go over by 0.1 oz of resin, I now have a mix ratio of 100:20 which is NOT acceptable

Taking all these things into account, I've expanded on the table above to let me know if: 1) is the scale accuracy limiting, and 2) if I go over by 0.1 oz of resin, can I still use that batch?


Ok, so we know how much hardener and resin to pour into the cup. Now what? It's very important to mix the two properly so that the batch becomes homogeneous (just a fancy word to state the the mixture is uniform). As you are mixing, imagine dividing the cup into thousands of little pieces... would each piece have the appropriate mix ratio? In other words, if you don't mix the batch thoroughly, you could have parts of the batch that don't have a proper mix ratio (and so would not hold its mechanical properties). This is why properly mixing the solution is just as important!

Take mixing seriously. For every batch, I start off by mixing the solution as I would stirring chocolate milk (or cream in coffee) for about 5 seconds, gently revolving the mixing stick in circles. I then use the mixing stick to "scrape" the bottom of the cup. Remember I poured hardener into the cup first, so I want to make sure all that hardener comes off the bottom of the cup and mixes in with the rest of the solution (for 5 seconds or so). Next, I use the mixing stick to scrape the walls of the cup. I want to make sure any hardener or resin stuck to the walls mixes in well with the rest of the solution (again, for about 5 seconds or so). I then slide the mixing stick along the rim of the cup to make sure any resin or hardener stuck to the mixing stick blends in well with the rest of the solution. I then repeat the whole process 4 or 5 times.

This process sounds daunting, but after you do it a few times, muscle memory kicks in and you don't even think about it.

Key takeaways:

1. The mix ratio is EXTREMELY important and should be taken very seriously. Not doing so can have catastrophic consequences.

2. Every epoxy system is different and will have a specific mix ratio particular to that system. When switching systems (even if it is manufactured by the same company), remember to create new mix ratio tables with the proper corresponding mix ratio.

3. When creating your own mix ratio tables, pay attention to the rounding error based on how accurate your digital scale is. No matter what epoxy system you use, there exists a minimum amount you can mix based on how accurate your digital scale is.

4. Given the choice, pour hardener into the cup first. Depending what your ideal mix ratio is (and assuming there is more resin than hardener), you might be able to get away with accidentally pouring extra resin into the cup. How much extra depends on the ideal mix ratio and the total amount of epoxy you are trying to mix.

5. Remember to ALWAYS tare your scale AFTER you place your clean empty cup on the scale. A paper cup can weigh 0.2-0.5 oz.. so not taring the scale would give you a large mix ratio error - deeming your batch unusable!

6. Always test your scale before each use. I've developed a habit of placing a small cube that I know weighs 3.5 oz before every use. This is not a calibration by any means, but at least lets me know the scale hasn't lost its mind!

7. Properly mixing the solution is just as important as pouring the right amount of resin and hardener! Be certain you have thoroughly mixed the solution and are using proper mixing techniques stated above.

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