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Comment by dieselgate | Hacker News Reader

Interesting to see the connection of moles and epoxy putty but wish this article was more in depth. The two parts of putties are typically different colors so one can visually tell when they're sufficiently mixed. It would be nice if epoxies in general (putty and liquid) data sheets had more information on their constituents. For example if an epoxy is 1:1 by volume and you have a scale available what would an equivalent 1:1 by mass/weight be? It's possibly to find out somewhat visually/by trial but would be nice if we could more easily calculate the mass/weight ratios

edit: fwiw in recent experience with 1:1s there's not much of a difference between equal weights/volumes but it could made a large difference on industrial scales

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Having mixed, and used, quite a bit of 2-part epoxies, I concur that measuring by weight is far more satisfactory vs. attempting to measure volumetrically. Most industrial epoxies are mixed at ratios of 2:1 to 5:1 (resin/hardener). The 1:1 varieties are mainly consumer products. As a rule if the 1:1 ratio is a bit off it doesn't noticeably affect results.

Some manufacturers/suppliers provide volumes and weights for proper mixing. For example, West System instructs mixing 1 quart/2.4 lb of their "105" resin with .43 pint/.47 lb of "205" hardener. (Ratios: by volume: ~4.6, by weight: ~5.1) Of course the savvy user can mix smaller/larger quantities proportionately.

In some cases measurement by weight isn't mentioned by the producer. However it can be inferred from product density. The info is usually given in the product's SDS. The "205" hardener SDS states it weighs 8.85 lb/gal, whereas "105" resin is 9.6 lb/gal. With this info on hand, it's straightforward to compute the required quantities of the components.

Manufacturer of epoxies explicitly advise not to alter the mix ratios. IMO measuring by weight is the easy way to mix epoxy accurately.

i concur, though you seem to have more experience with epoxies than i do

it's easy and cheap to get an electronic strain-gauge balance that measures weights to ±0.05%, though weighing things out that precisely is quite challenging, especially if they are viscous liquids. (note that the rounding error in the densities you give is probably ±½%)

but measuring small amounts of viscous liquids by volume i think you'll probably get errors more like ±10%

powders, like for cooking or for resin fillers, are even less precise to measure by volume

> "...though weighing things out that precisely is quite challenging, especially if they are viscous liquids."

The "error tolerance" in mixing epoxy is probably at least 1% so inexpensive scales are quite adequate. The "secret" to mixing epoxy is measuring the resin and hardener in separate containers, then combining by pouring the hardener into the resin. I also put some of the mixture back into the container used to measure the hardener, mix thoroughly and add it to the main portion of mixed epoxy. This procedure assures all of the hardener is mixed with all of the resin.

BTW I use my kitchen scale every day. As you say, measuring powders, like flour, by volume isn't reliable. Then again, liquids, even water, are easier to weigh out. Besides, measuring cups aren't very accurate. Then again being only as precise as necessary for the job is what efficiency is about.

The reputable brands of liquid epoxy have data sheets included or available on their site with weight mixing ratios. The best way to mix is to use a paint scale (mettler toledo and sartorius make some) which lets you get both amounts precisely. They are made for automotive paints which require very precise ratio measurements (like 30:20:1).

Source: I mixed up about 20 gallons of various brands/ratios last year.

He says the 25-fold was the point of visual homogeneity but the putty didn't reach full strength until 60 folds.

Some epoxy manufacturers make it pretty easy. For example, West System sells special hand pumps[1] for their resin and hardener, which attach directly to the product's cans and provide the right ratio for you--just use the same number of pumps from each product and you're good. Maybe a lot of manufacturers offer their own ratio pumps--I'm only really familiar with West.

1: https://www.westsystem.com/the-105-system/metering-pumps/300...

Those pumps are awful. If you get an air bubble in the line or need a small amount you risk messing up the ratio and having a poorly cured batch. Plus the hardeners react with the air so if its been sitting a little while the first part of your pump is bad hardener. Highly recommend everyone use a scale.

That's a good point. I've found the pumps to be handy for West System especially the 1:5 (or whatever odd ratios they have) but don't like how they are messy and tough to transport. Have had the hand pumps get pressed in the back of the truck and stuff and it can be a nuisance - but they can also be quite convenient when working as intended.

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