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Mixture of milled glass & epoxy not setting

Lemmingman

Well Known Member
I came into a situation earlier this week that needed me to fill in a mistake I had made. I had drilled a hole too large and wanted to fill it in and try again.

I decided that a mixture of epoxy and milled glass (flox) was the way to go. I had a pound of the glass in an airtight container in a cabinet. I purchased it from Aircraft Spruce in April. I decided to shoot for a peanut butter consistancy which I slowly mixed into an already mixed cup (1/1 pump) of epoxy. Once I had the desired consistancy I spread it over the area, filling the holes. After 36-hours it had still not set. The cup of mixture was not set either. It was still just like it had been when I mixed it.

Yesterday I tried again, this time shooting for mayonaisse consistancy and after 24-hours I have a similar situation. It is harder than the peanut butter was after 24 hours, but still mushy. BTW, I mixed a control cup of 1 pump ea with no filler and it is hard as a rock this morning. My garage is about 73 degrees and it has been rainy here in north Texas this week. I cant find anything on the West site.

Is this normal? With a filler added to the epoxy is the setup time drastically longer than no filler? Clearly I am doing something wrong. I dont remember having this issue at the Sport Air class.

My purchase history says that I bought milled glass, not flocked cotton from AS. Looking at it with a jewlers loupe I am certain it is glass, so I dont think it is the wrong product.
 
Hi Gil,

Neither the addition of the milled glass nor its age should impact the cure of your epoxy resin. (I have a bag of milled glass that is more than 25 years old and still use it.) I don't trust pumps and elect to mix resin and hardener, though your control experiments would suggest that is not the problem. (I might make a batch by weight, anyway, though.)

If you are convinced the resin/hardener mix is not to blame, perhaps you have to consider that your glass somehow became contaminated?

Do you have another filler that you can test with a new batch of resin?

Good luck,
 
While it shouldn't matter here, milled glass fibers is not flox. Flox is a mixture of milled cotton fibers and mixed epoxy, I think.

See Spruce's page here.

Dave
 
Sounds like a case of mis-mixed resin/hardener. Are you using West System? The ratio is supposed to be 1 part hardener to 5 parts resin. I'd check the pumps you're using to make sure they are the correct ones for the can they're in.
 
That's pretty weird. I've got some West system stuff with the slow hardener. I'm an almost total klutz with fiberglass (hence the slow hardener), and I've done just about everything 'against the rules' you can do with it and it just works. I've even mixed small batches with guesstimated ratios (partial pump strokes) with good results. Don't panic; this isn't structural stuff. But it is an example of how forgiving it is. My milled glass isn't 25 years old, but it's a lot older than a year. I've mixed in plastic cups, a slice of milk jug, paper index cards, etc, all with good results. I just plugged some small indexing holes in a radiator duct a few days ago with a milled glass mix like you describe.

Any chance you got a batch of waxed cups for mixing?

Charlie
 
Did you thoroughly mix the epoxy before adding the filler?

That is the only way to ensure a homogeneous mix of the resin. Adding the filler before thoroughly mixing will inhibit proper mixing and could explain the situation that you have.
 
If all else fails, mix up some epoxy minus the milled glass.

Set it aside and see if it sets up.

From all I have ever learned working with this stuff over many years, the glass fiber should not be causing a problem.
 
I'm surprised this hasn't been mentioned yet, but get a digital gram scale and use it for measuring proportions. The total weight, including the resin is 1.2 times the weight (minus the weight of the cup of course) of the resin only. West System assumed.

Easy to do and it allows you to make very small amounts for small patch jobs. Reason I mention this is that there is an assumption built into the discussion that your pumps are accurate.
 
Did you thoroughly mix the epoxy before adding the filler?

That is the only way to ensure a homogeneous mix of the resin. Adding the filler before thoroughly mixing will inhibit proper mixing and could explain the situation that you have.

But even if it is not homogenous wouldn't some sections harden, creating lumps?

Perhaps it cooled down too much and putting it in a hot place for a day might see if it will eventually harden.
 
I have been doing glass work on boats, planes and RC models for decades. Even with resin that is 10 yrs old and I have never had something like this happen. I have used aluminum powder, graphite powder, chopped glass, cabosil, microballoons, you name it and it has never affected the cure. While it is true that scales are better than the pumps, the West system, if that is what you are using, is measured 5:1 in volume, not weight. The densities of the 2 parts are not necessarily the same, so the proportion by weight could be different. In that case there are other options - small graduated cups, or large veterinarian syringes work well.

In any case, mix some more up and divide it into 2 lots. Leave one lot and take the other and mix in your filler. Both should cure. The one left in the pot should cure faster, because it will generate more heat, but they should both cure in a reasonable time at 73F. If the unthickened resin cures and the thickened resin doesn't then you have some sort of very strange chemical interaction going on, which is something I have never seen or heard of.

The stuff you have applied that has not cured should be cleaned off - it is no good.

Putting a warm incandescent light on the work piece after laying it up will help accelerate the cure. Each 10F cuts the cure time in half.
 
Putting a warm incandescent light on the work piece after laying it up will help accelerate the cure. Each 10F cuts the cure time in half.

Lots of experience to the contrary, but I too have had some changes in cure times with West 105 and slow hardener. Weighed every time, but using micro balloons not glass fiber filler. It may just be too dry, you could try cabosil additive to thicken it for application rather than just more fibers.

My micro took a long time (days) to set (used as filler) and I used heat. The pants were covered with plastic and set in the sun. Measured temps with infrared thermometer to 110F. That seemed to speed up and fully cure the mix.

I called West and they also had never had this happen - or maybe just the person who answered.
 
While it is true that scales are better than the pumps, the West system, if that is what you are using, is measured 5:1 in volume, not weight.

Offhand, I recall using only one epoxy without a weight ratio in the instructions, FPL-16A. Probably others, but West isn't one of them.

Copied from the website:

To measure 105 Resin and 205 or 206 Hardener by weight or volume, combine five parts resin with one part hardener.

To measure 105 Resin and 207 or 209 Hardener by volume, combine three parts resin with one part hardener (by weight, 3.5 parts resin-1 part hardener).


Throw away the pumps and get a scale.

There was one time when I didn't get a cure with West. Luckily I was able to determine that the problem was beer. Apparently I could not divide by 5 after the second one.
 
Based on the conversation I decided to check my pumps. One pump of resin is dispensing 20.5g. One pump of hardener is dispnesing 1.5g. By my math I am getting a 20:1 ratio, not the 5:1 that is required by West. I have a pump issue, it seems.

I'll play around with it some more and let you know what I find, but for now I may just do as Dan suggests and weigh the material rather than trust the pumps. I'll try another test like I did last night and see what happens.
 
I always mix by weight and start with the smaller portion, then multiply by 5 to set the target for the larger portion. I figure if I miss measure the larger component by a half gram, I'll end up with a more accurate ratio than if I miss the smaller portion by the same amount.

Amazon carries several appropriate digital scales with at are function, which comes in handy.
 
I always mix by weight and start with the smaller portion, then multiply by 5 to set the target for the larger portion. I figure if I miss measure the larger component by a half gram, I'll end up with a more accurate ratio than if I miss the smaller portion by the same amount.

Amazon carries several appropriate digital scales with at are function, which comes in handy.

I have a nice one that got me through my fuel tanks. I just made up the batch 20g resin 5g hardener and the filler. The consistency is somewhere between catsup and mayo. We'll see how it goes. I feel like this is the solution.

What I dont understand is why my other mixes with no filler came out just fine. Maybe the pump just started failing? Possible...I may give West a call tomorrow and see what they say.
 
I have a nice one that got me through my fuel tanks. I just made up the batch 20g resin 5g hardener and the filler. The consistency is somewhere between catsup and mayo. We'll see how it goes. I feel like this is the solution.

What I dont understand is why my other mixes with no filler came out just fine. Maybe the pump just started failing? Possible...I may give West a call tomorrow and see what they say.

I've seen pumps fail before. I had one that physically broke, with the body of the pump falling down into the gallon of epoxy. :eek: I've also seen 'em fail where one loses its seal and starts drawing air, leading to gross errors in mix ratio.
 
Check Valve

What I don't understand is why my other mixes with no filler came out just fine. Maybe the pump just started failing?

So, if you're talking about the WEST pumps, they have very simple ball check valves inside. Typically you will find that the ball gets blocked, either by crystallized resin or by some other muck that go in. It makes it so that when you push the pump down some resin comes out, but the rest gets pushed back into the container. The result is that you have an off-ratio mix.

WEST 105/205 is really tolerant to being off-ratio. But if you were at 20:1 it is never going to cure. For all of my resin batches I save the cup and put it on the work bench. I come back and verify that the resin passes the scratch test. If it does not I look into it, as it can mean scrapping the parts.
 
Cleaning pumps

Hot water works well to clean the pumps. Gelled resin easily clears when the assemblies are given a hot soaking and pump through the water to free everything like new
 
So, if you're talking about the WEST pumps, they have very simple ball check valves inside. Typically you will find that the ball gets blocked, either by crystallized resin or by some other muck that go in. It makes it so that when you push the pump down some resin comes out, but the rest gets pushed back into the container. The result is that you have an off-ratio mix.

WEST 105/205 is really tolerant to being off-ratio. But if you were at 20:1 it is never going to cure. For all of my resin batches I save the cup and put it on the work bench. I come back and verify that the resin passes the scratch test. If it does not I look into it, as it can mean scrapping the parts.

That is a good point, Kevin. I'll check the inside of the pump and see what I can find. BTW, I do the same as you. I leave the cup on the work bench to make sure it hardened correctly. Thats why I know the other batches were A-OK.
 
Solved.

The hardener pump not pumping the correct ammount was the problem. This morning the area I fixed and the pot of left over are hard as a rock. So future builders beware that you may not be able to trust the pump even if you are getting a solid setup.
 
Yes you have, and now I will just consider them convienent lids used to dispense product into a cup that I will measure on a scale.
 
Offhand, I recall using only one epoxy without a weight ratio in the instructions, FPL-16A. Probably others, but West isn't one of them.

Copied from the website:

To measure 105 Resin and 205 or 206 Hardener by weight or volume, combine five parts resin with one part hardener.

To measure 105 Resin and 207 or 209 Hardener by volume, combine three parts resin with one part hardener (by weight, 3.5 parts resin-1 part hardener).


Throw away the pumps and get a scale.

There was one time when I didn't get a cure with West. Luckily I was able to determine that the problem was beer. Apparently I could not divide by 5 after the second one.

ok, that's highly useful - didn't now that. Well, I knew about reduced competence with beer, but not the other bit. I would never drink while working on my airplane - I screw up enough sober! Or perhaps I should start :eek:
 
One other little recommendation I have is to create a spreadsheet that you can use to quickly determine target total weight of the resin, hardener and cup.

For example, 95% of my fiberglass work is with the small 3oz mixing cups that ACS sells. Empty, they weigh about 0.75 grams. So it's a simple matter to create a spreadsheet which can be printed and posted by your workbench with pre-calculated total target weights. I would grab an empty mixing cup, squirt an amount of resin into it, based on how much I thought I needed, put the cup and resin on the gram scale, note the weight, look it up on my spreadsheet printout and see what the target total was...then add hardener until you get to the total weight. Boom done and you can even be drinking beer and get it right.:D
 
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One other little recommendation I have is to create a spreadsheet that you can use to quickly determine target total weight of the resin, hardener and cup.

For example, 95% of my fiberglass work is with the small 3oz mixing cups that ACS sells. Empty, they weigh about 0.75 grams. So it's a simple matter to create a spreadsheet which can be printed and posted by your workbench with pre-calculated total target weights. I would grab an empty mixing cup, squirt an amount of resin into it, based on how much I thought I needed, put the cup and resin on the gram scale, note the weight, look it up on my spreadsheet printout and see what the target total was...then add hardener until you get to the total weight. Boom done and you can even be drinking beer and get it right.:D

It's a lot more accurate to start with hardener and add the resin. Weigh it, multiply by 5, tare the scale then add the resin.
 
That's pretty weird. I've got some West system stuff with the slow hardener. I'm an almost total klutz with fiberglass (hence the slow hardener), and I've done just about everything 'against the rules' you can do with it and it just works. I've even mixed small batches with guesstimated ratios (partial pump strokes) with good results. Don't panic; this isn't structural stuff. But it is an example of how forgiving it is. My milled glass isn't 25 years old, but it's a lot older than a year. I've mixed in plastic cups, a slice of milk jug, paper index cards, etc, all with good results. I just plugged some small indexing holes in a radiator duct a few days ago with a milled glass mix like you describe.

Any chance you got a batch of waxed cups for mixing?

Charlie
Same here. I have practiced rebellion with most of the rules and don't remember having problems . Half shot of resin half shot of hardener. You guys move to Mississippi and your problems go away. :D
 
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