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LureMaker1000

Alumifoam Cracking Problem

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Hi All, 

I bought a batch of Alumifoam from Alumilite last season. I made a number of lures--4"-9" sized striper lures mostly all round basically e.g. spooks. I spraypainted most with Rustoleum 2X Ultracover rattle cans and then topcoated with Devcon 2 ton epoxy. Just about all of the painted and epoxied plugs have developed crossectional cracks (running around the circumference). Of these plugs, the ones that I used a lot ended having these cracks develop into a full split of the plug body. These are all throughwired plugs(308L TIG wire). 

 

I made a few that I neither painted or epoxied. No cracks on these.

 

Some of the plugs I painted right away, some I waited a week. I would usually wait a week before epoxying them. 

 

I am curious if anyone else has had this problem with the cracking?  

 

  

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I was googling about expanding foam cracking and I guess in the insulation spray foam world you get this sometimes. Cracks in foam or even wood framing being distorted when foam shrinks. Causes of the shrinkage were suggested as 1) a bad mix from the factory 2) applying too much foam at a time leading to improper curing conditions (too hot and fast). 

 

So maybe the epoxy or the wire harness or both in tandem are preventing the foam from shrinking and the plug basically tears(shrinks) itself apart.

 

I am trying to get alumilite on the phone about this will report back with any news. 

 

 

 

 

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I think you may have identified the problem in information in your post!  Polyurethane expands with heat (shrinks with cold) quite a bit.  I wonder if the epoxy-coated baits are creating stress (tension) in the polyurethane foam when the lure is cooled down from the cool water...just a guess.  I'd think the effect might be worse for larger baits.

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22 hours ago, Anglinarcher said:

I have not had that problem.

Thanks Anglin. Are you epoxying your plugs with D2T

 

I've reviewed some of your other posts on alumifoam and if I recall correctly you use pretty stiff molds for this stuff. I wonder if you are packing the foam at allso it cures to a higher density which perhaps means smaller cell size and maybe greater cell strength and possibly less shrinkage or sufficient resistance to shrinkage to prevent this cracking stuff to happen. 

 

 

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I used Etex instead of D2T on my bigger baits.

D2T is a glue epoxy, designed to be hard and rigid, so it can't move with the lure body if it expands or contracts.

Etex is a decoupage epoxy, designed to cover wide wood surfaces like bar and table tops.  It stays flexible, so it moves with the lure body's thermal expansion and contraction.

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Can you post pictures, including close-ups of the foam/coating interface as well as the foam/wire interface if possible? That will give a lot of clues as to the force distribution prior to the rupture.

Definitely sounds like residual cure shrinkage stress issues, but odd that the coated lures cracked, as I wouldn't expect the foam to be able to build up enough stress to cause the epoxy skin to rupture. Foams do experience a greater degree of thermal expansion/contraction, but they're also much softer/less stiff, so they usually just tear themselves apart due to thermal gradients in the foam during the cure process.

Working guess would be that the (presumably) stiff epoxy was well bonded to the foam, preventing the foam from shrinking radially, which would cause the foam to also want to contract axially, which led to the circumferential cracking as the weaker foam failed before the stronger foam/epoxy interface did (or foam/wire but that still eludes me). You see this kind of failure frequently, but usually with very stiff, very brittle ceramics.

Another useful set of data points would be to measure the length and depth of the mold cavity, as well as the length and diameter of the uncoated and coated foam bodies. That would help identify where the shrinkage is, and if simply changing to a flexible epoxy will solve the issues or if there are other process changes that need to be made. Thankfully, those shouldn't be too difficult either, but no sense overcomplicating things until the root cause is identified.

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I think the simplest solution is to use an epoxy designed to move, like a decoupage epoxy, instead of a glue epoxy, which is designed to cure extremely hard and rigid.

I found that using D2T epoxy over the larger surfaces of my swimbaits resulted in cracking upon impact with anything hard, and the epoxy peeling off in large chips.

That never happened with Etex. 

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Posted (edited)

I think I will be trying Etex. I wasn't aware of the decoupage vs. glue epoxy distinction. I just need to rig up a turner.

But still interested in investigating the source of this foam cracking issue. Here are the requested pictures. 

I numbered the lures. Here are some notes on the process. Alumifoam cast in a silicone mold. Spray paint usually 3-7 days after casting. Epoxy some at least 1 day after painting but sometimes much longer.

All the red circles indicate splits in the plug all the way through. 

On 1 the split on the right occured when catching a fish and that split was at the hook hanger. The split on the left was when I was testing the lure for other weak points by trying to bend it. 

On 2 and 4 I never caught a fish and hardly fished them. 

On 3, this is interesting, the paint and epoxy started flaking off almost immediately. I believe tried dipping that one in primer immediately after demolding. That split didn't happen though for several months. 

DSC-4734-Copy.jpgDSC-4735-Copy.jpg

 

 

 

Edited by LureMaker1000
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Alumilite emailed me the following thing to try: 

"One trick of the trade is to add 10% by volume of Microballoons to the Alumifoam. When it cures, it provides a much more consistent cure and porosity throughout the foam. It comes out tan like the RC-3 and all the bubbles inside the foam are very similar in size. It does continue to shrink as it fully cures even though 98% of the hardness and movement happen within the first 4 hours or so. "

 

I have no idea what 10% volume of microballoons would weight and I do everything by weight. With Amazing Casting Resin I can do between 10-15% by weight. 

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hi.we use urethane foams and wired thru. have you considered foam density.try 16ld for stregnth thebreak inbody is from leverage versus density. as for painting.we hot water wash  with dawn dish soap. primer 2x rustoleum. finish coats are envirotex. or automotive clear. hope this helps you out.

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On 5/20/2022 at 10:02 PM, LureMaker1000 said:

Alumilite emailed me the following thing to try: 

"One trick of the trade is to add 10% by volume of Microballoons to the Alumifoam. When it cures, it provides a much more consistent cure and porosity throughout the foam. It comes out tan like the RC-3 and all the bubbles inside the foam are very similar in size. It does continue to shrink as it fully cures even though 98% of the hardness and movement happen within the first 4 hours or so. "

 

I have no idea what 10% volume of microballoons would weight and I do everything by weight. With Amazing Casting Resin I can do between 10-15% by weight. 

For every 10g of mixed Alumifoam add 2.2g of microballoons. (per 1 mL of mixture, 0.1 mL microballoons, 0.9 mL resin; 1.2 g/mL for resin, 2.35 g/mL for balloons, 0.1*2.35=0.235, 0.9*1.2=1.08, 0.235/1.08=0.218 g microballoons/g mixed resin)

Overall I would tend to agree with Mark that if the uncoated lures didn't give you problems a flexible epoxy will probably solve most of your problems. That said...

Looking at the images, particularly the cracks in lures (2) and (4), and the right-side crack of (1), I'm seeing mostly signs of failure from flexural loading. (2) shows chipping in the coating right at 12 o'clock and then a straight crack down, while (4) shows a chipped coating at 6 o'clock and then crack propagating the rest of the way. Those are pretty common crack initiation site features, which would suggest those surfaces were in tension prior to failure. The right-hand crack of (1) appears to be pretty flat and straight down below the wire and rough at an approximately 45deg angle to the wire, which would suggest tensile and shear failure, respectively. Put together, that also looks like a flexural failure.

That makes me less inclined to believe it's the result of a cure shrinkage/thermal dialation issue and more mechanical. Guessing the epoxy is too stiff and is working its way into the pore structure of the foam to an appreciable amount, which is causing the foam to fail completely, something along those lines, though I'm not fully convinced of all of that for things I'll save for now, more off the cuff speculation.

So yeah, maybe try a flexible epoxy and see how they behave.

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Thanks for these posts, folks. I have not tried etex yet as my understanding is I will need a turner. I will get to that at some point. 

 

I tried 32g resin with .05 microballoons mixed in and poured a 9" 3/4" diameter plug. 48 hours later painted. 72 hours after paint epoxied. Fished the next day. Today is next day and I have my first crack it's on side of plug. I gave it a flex and the foam has separated inside. Crack looks same as the ones above--crossectional crack.

I just poured another plug and will fish this one without sealing it with epoxy--for now. This will test if unepoxied plugs don't crack. 

 

Later I'll try the etex once I procure some and build myself a turner. 

 

Thanks again! 

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On 5/21/2022 at 11:57 PM, RideHPD said:

For every 10g of mixed Alumifoam add 2.2g of microballoons. (per 1 mL of mixture, 0.1 mL microballoons, 0.9 mL resin; 1.2 g/mL for resin, 2.35 g/mL for balloons, 0.1*2.35=0.235, 0.9*1.2=1.08, 0.235/1.08=0.218 g microballoons/g mixed resin)

Overall I would tend to agree with Mark that if the uncoated lures didn't give you problems a flexible epoxy will probably solve most of your problems. That said...

Looking at the images, particularly the cracks in lures (2) and (4), and the right-side crack of (1), I'm seeing mostly signs of failure from flexural loading. (2) shows chipping in the coating right at 12 o'clock and then a straight crack down, while (4) shows a chipped coating at 6 o'clock and then crack propagating the rest of the way. Those are pretty common crack initiation site features, which would suggest those surfaces were in tension prior to failure. The right-hand crack of (1) appears to be pretty flat and straight down below the wire and rough at an approximately 45deg angle to the wire, which would suggest tensile and shear failure, respectively. Put together, that also looks like a flexural failure.

That makes me less inclined to believe it's the result of a cure shrinkage/thermal dialation issue and more mechanical. Guessing the epoxy is too stiff and is working its way into the pore structure of the foam to an appreciable amount, which is causing the foam to fail completely, something along those lines, though I'm not fully convinced of all of that for things I'll save for now, more off the cuff speculation.

So yeah, maybe try a flexible epoxy and see how they behave.

 

RideHPD, 

So I just poured a new one and got a crack within 36 horus of epoxying and painting it. Crack is on one side of the plug and is propagating around to the other side as I fuss with it. Do you think the plug could be shrinking unevenly and putting one side under tension as the other side shrinks at faster rate? 

 

The crack is located at an area where I glued on some pancake weights to the wire harness.

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By the way, while I don't know if this qualifies as flexural testing, I tried bending the new 9" plug with the new crack and while there is definitely some flex in this thing I did not observe any cracking. I didn't try to break it but I did flex it pretty good. 

 

The yellow plug #1 in the photo above broke while catching a fish on it, just fyi. 

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So, some things as they come to mind:

This lure that broke in 36hrs, was it fished at all, or just resting that entire time?

Definitely possible that different areas cured at different rates, and not just radially, that's standard.

In the image, were lures (2) and (4) fished at all?

Have you tried bending an uncracked one to see if it would crack? Bending the cracked one may or may not yield any usable info, as that crack formation would relieve a lot of the stress that was built up prior.

Overall I'm getting the gist that these lures are failing without being fished, or otherwise loaded, and that those cracks may just be initiating from the largest defect in the interface.

At this stage I'd start recording data on every lure. Date, time, temp, humidity, relative pressure, what resin, how much mixed, etc. and record when they fail and how, including making some in each batch to just rest and watch for cracks, some to bend, etc. The more you can record, the better you can fall back on the numbers, as it sounds like you may also be seeing process changes that could be due to environmental/seasonal effects. Room temp cure thermosets are pretty sensitive to these things, so you could just be outside of the use envelope or something like that. If the flex epoxy isn't expensive it may be worth applying it to a sample without a turner just to see what happens as a test article. Might find that it doesn't fix anything and save you from buying the equipment. Best thing I learned as an engineer is that it's way faster and better to just get up and try something than burn time behind the desk researching (unless you have a really, really specific problem and you've pretty much proven what the solution is).

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Posted (edited)
On 5/29/2022 at 7:40 PM, RideHPD said:

 

So, some things as they come to mind:

This lure that broke in 36hrs, was it fished at all, or just resting that entire time?

Thanks for your reply Ride and your thoughtful ideas. 

That one broke on day 5 or 6. I fished it on day 5 and discovered the crack on day 6. I waited two days to paint. Then waited another 72 hours to epoxy. Fished it throughout next day. Basically discovered the crack 36 hours after applying the epoxy but it could have happened while fishing. Didn't catch any fish on it.  

On 5/29/2022 at 7:40 PM, RideHPD said:

 

In the image, were lures (2) and (4) fished at all?

Yeah. All lures fished. But 2 and 4 I never caught a fish on them. OF course I probably dragged in some seaweed at some point on them. 

On 5/29/2022 at 7:40 PM, RideHPD said:

Have you tried bending an uncracked one to see if it would crack? Bending the cracked one may or may not yield any usable info, as that crack formation would relieve a lot of the stress that was built up prior.

That big yellow lure #1 the crack on the left developed when I tried flexing it after the crack on the right had happened while catching fish. Other lures have multiple cracks in epoxy but not all of them seem to have developed to split stage. 

 

I made another version of the yellow one about five days ago. I spray painted it, but no epoxy. I was flexing it last night. Quite a bit of flex in this foam. I didn't try to break it but I flexed it a good bit. No cracks.  I'll try some others. 

 

 

On 5/29/2022 at 7:40 PM, RideHPD said:

Overall I'm getting the gist that these lures are failing without being fished, or otherwise loaded, and that those cracks may just be initiating from the largest defect in the interface.

It's sort of a mix. I fished just about all of these. But a lot of them that cracked never had a fish on them and I don't think many of them got snagged either. They all did go through a fair amount of heating and cooling cycles while just sitting in my car for weeks. 

On 5/29/2022 at 7:40 PM, RideHPD said:

At this stage I'd start recording data on every lure. Date, time, temp, humidity, relative pressure, what resin, how much mixed, etc. and record when they fail and how, including making some in each batch to just rest and watch for cracks, some to bend, etc. The more you can record, the better you can fall back on the numbers, as it sounds like you may also be seeing process changes that could be due to environmental/seasonal effects. Room temp cure thermosets are pretty sensitive to these things, so you could just be outside of the use envelope or something like that. If the flex epoxy isn't expensive it may be worth applying it to a sample without a turner just to see what happens as a test article. Might find that it doesn't fix anything and save you from buying the equipment. Best thing I learned as an engineer is that it's way faster and better to just get up and try something than burn time behind the desk researching (unless you have a really, really specific problem and you've pretty much proven what the solution is).

Great ideas. I will see if I can do the flex epoxy without a turner. I just heard back from alumilite and they gave my a 25% off coupon so I might order some new foam, try again with old pcess, and see what happens. 

I am about to run about of devcon 2-T and they don't sell in local stores anymore. Maybe now will be good time to swtich to etex for a while just to see if it easier to use, makes better end product anyways.

Edited by LureMaker1000
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I picked up the .5g mb/32g resin lure that developed the crack so soon after making it and tried flexing it at the hook hanger. Broke apart fairly easily--I think the weight of a big fish could lead to breakage there if pulled the fish by the lure up the bank. I then tried flexing it between the two existing cracks. Took a good deal more flexing before breaking. 

 

I'll have to find some other ones to break ha ha.  

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No problem, this is the side of engineering I actually enjoy, and it's much more fun when it's not your headache to solve.

In a root cause analysis you try to isolate variables you think are responsible so you can eliminate them or identify their role in the failure. In this case there are way too many variables and not enough data points to identify a single area of focus.

Was the one that cracked in 36hrs subjected to heating in your car?

There's a couple things that stand out, namely the heating cycles in your car and whatever loading is occurring while fishing. Have any of these cracked that have just sat around, without being thermally or mechanically loaded in any way? That's the first thing I'd want to know; is it occurring just from the processing itself? It's hard to really gauge what's going on without having the material in my hands to see what their relative stiffnesses are, but it sounds like there's some residual stress being introduced from heating or loading that are being released during cracking, and my first bet is that it's thermal. Pour a batch of 6 lures, all identical. Get a cheap toaster oven (do not put plastic in your kitchen oven) and soak 3 of them at 150F-ish for a couple hours, then once they've cooled coat all of them in epoxy. Then leave them your car as normal. I'm thinking the foam is curing in your car at higher temp while expanded and that's what's introducing the residual stress (upon cooling). Then repeatedly going between hot car-cold water, etc. is fatiguing that interfacial bond.

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I am on luremaking hiatus currently. My last casting was of the 9" plug and I spray painted it but did not it epoxy it. It has been sitting in plug bag in car and is getting fished. Haven't caught on it. But no cracks. I flex it every once in a while. There is a quite a bit of flex. And no cracks showing. 

 

RideHPD: 1) the 36hrs cracked one was not subjected to car heating it went into car on the same day as I went fishing and it was a cool day to start with. 2) All the plugs have spent a number of days in car, some more than others. 3) I love your analysis ideas. I will refer to them should the etex thing and trying a new batch of the resin does not result in the problem being resolved. I'm ordering some more foam soon. Alumilite kindly offered me 25% coupon. 

 

Here's the picture Anglingarcher requested. The foam was purchased directly from Alumilite, not via Amazon. 

Screenshot-2022-06-12-152200-Copy.jpg

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I copied this off the site.  I removed some items not applicable to the conversation.

________________________________________________________________________________________________________________

TECHNICAL SPECIFICATIONS

Product:
 
AlumiFoam Color: Brown
Mix Ratio:  1:1 by weight or volume
Pot Life:  45 seconds
Rise Time: 75-90 seconds
Demold Time (100 gram mass): 10-15 minutes
 
3-5x expansion
Flex Modulus (psi): 50-1200 flex strength
_____________________________________________________________________________________________
 
The pictures of the bottles match; I kind of figured you were using a different foam, like the 320 or 610.  
 
One thing that did not seem right is that your foams seem different color(s) compared to what mine come out as, a brown or dark tan.  I see the specs say it is a brown also.
 
The ones I mold will not flex, just like it suggest when it says it takes 50 to 1200 psi.  In fact, when the stuff first came out I did a mold and took the lure and hammered it, yes, with a hammer, and it did almost no damage.  I had a video on YouTube a long time ago but my wife took my channel down.  I then took the same lure and tied it to about 5 feet of line, and the end of a fishing rod, and repeatedly smashed it into my concrete patio.  It took several smashes to break it.  This lure blank did not even have a wire frame in it.
 
Recently I made some floating jig heads, without hooks, for a friend so he could cut a grove, insert his hook, and glue it in.  He could not get his saw to cut it (I use a Dremel cutoff wheel).
 
So......... what is the difference?  I am not sure, but I have to believe it is one of a very few things.
1) The bottles are not properly mixed.  Sometimes shaking it is NOT enough.  I have used a bent wire on a drill to mix it.  Don't worry about bubbles, you will make more.
2) User error, not mixing correctly.  Follow the directions exactly, and if necessary, have a friend come over and do it and watch them to see if it comes out different.  Do not instruct them, have them do it on their own.
3) Last option, and until the first two have been completely verified by competent and experienced people I won't accept it; you might have got bad bottle(s).
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