I have met my match

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jttheclockman

jttheclockman

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Joined
Feb 22, 2005
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NJ, USA.
Well I have stumbled upon a problem that I am wrestling with. I will throw it out here in hopes someone may have encountered the problem and has overcome it.

First let me state all the things I have done so we get them out of the way. What I am trying to do is cast resin to aluminum and a brass tube. I am using fresh Silmar41. I have sanded the aluminum to give it some tooth. I have wiped it down with acetone and waited for it to dry (next day) I have mixed the resin with some black dye powder. I heated the resin to eliminate air bubbles. I added 8 drops per ounce of hardener. I stepped that up instead of 6 because of the dye. I let the blank cure for 3 days and then hit it with some heat in a toaster oven for 1/2hour. Turn the next day. Plenty of time for the blank to cure.

The one thing I have not done is put it under pressure but from what I see the blank has no bubbles and there is no air gaps. I go to turn the blank and the resin peels away from the aluminum easily.

My question is has anyone done this and have you encountered this problem?? Is this one of those situations that just is not going to happen. Is there a chemical makeup preventing the resin from sticking to aluminum.

It takes quite a bit of doing to make these blanks up so with 2 failures now I am pressed to try it again. I probably will but this time use pressure but I just do not think that is the answer. I am at a loss. What say you all???
 
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Is it possible that the heat created by the setting of the resin is enough to expand the aluminum then when it cools the resin pulls away? Have you been successful casting with aluminum before? Are you roughing it up really well?
 
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Two thoughts, one the aluminum is oxidizing between the time of cleaning and resin coating allowing the resin to separate. Two the heating in the oven is causing the aluminum to expand faster than the resin and its separating. You might try a trial piece where you sand and clean a piece of aluminum and cover it in resin immediatly and see if it adheres.
 
Another possibility is to coat the aluminum with a thin layer of epoxy prior to casting. Aluminum is one of those materials that is hard to get a good bond. Commercially a pretreatment is used, but that may be impractical for small samples. Scuffing the oxide off should provide a good enough surface for the epoxy. The PR resin should adhere to the epoxy as well.

Or maybe send it to Brad for anodizing:)
 
There is a big difference in coefficients of linear thermal expansion between Aluminum and PE, that may be your problem.

Aluminum = 12.3 (10-6 in/in oF)
Polyethylene (PE) = 111 (10-6 in/in oF)

PE expands just a bit over 9 times more than aluminum.

I'm with some other folks here, don't put it in your easy-bake oven and see how it goes.

Tom
 
Al oxidizes almost

immediately when in contact with air. This does prevent further oxidation and protects the metal, but the oxide will form a barrier that will make adhesion very difficult.
 
OK let me address some of the suggestions. One that was mentioned a few times is heat. I do not put these in an oven before they have cured which like I said was 2 days later. I did that basically to get rid of the stickyness. Now if the resin is heating the aluminum that much I can not see that. Silmar does not get that hot when curing.

I have never done this process before so it is a learning experience.

As far as the aluminum oxidizing seems odd. I have used aluminum in many segmented pens shown here but I had epoxyied those pieces in. In this case I am gluing them to the tube and casting around them. I use CA to glue the aluminum to the tubes which works well. Cast after several days of outgassing so no that is not the problem.


Being heat seems to be the common factor to most people's answer I will try the next set with no heat at all and this time I will put them under pressure too. I will let you know the results.

I may also try powdercoating before casting
 
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as you KNOW, I am really a rookie. What casting I have done has worked well for me. When the Simlar cured, even the next day it was still sticky. What I did was set it in the sun for about 4 hours and that fixed that part. How this bit of info may help.........Jan
 
I haven't cast the materials all at once, but I have made the combination with wood, drilled and glued the tube in. They are at best...a huge pain in the friggen butt!! Pardon my french, but they really are!!
 
John I have to agree that you should omit the toaster oven part. Just let the blanks cure in the shop for a few days and see where that gets ya. Regardless of the resin curing, the added heat can effect the final result of the blank.
 
Hey Brad

Where is our local chemist when you need him???:) He is probably in his lab coming up with a solution for me. Yea that has to be it. :biggrin:

Someone put out a Bat signal for Brad the chemist. Calling Brad.
 
I think both solutions need to occur, dont apply the extra heat and prime the aluminum immediately after abrading and cleaning, because aluminum does oxidize very quickly and the oxide layer prevents good bonding. When we weld aluminum in the aircraft industry, we typically have a 1 hour window after preping to complete the weld or we have to reclean.
 
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