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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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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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