Application of epoxy resin in aluminum parts

[roberto1brazil]

Hello everybody.
I have the following case and if someone could give me a light I would be very thankful:
At my company we are trying to perform a repair on some small casings made of Aluminum. Because of some parts display areas with corrosion, the following process is applied:
1-Clean the casing
2-Smooth all corroded areas with fine grade abrasive paper
3-Mask where it is necessary and carry out dry sand blasting with Aluminum oxide grain size 50µm on all corroded areas
4-Clean the reworked area with a cloth soaked with acetone.
5-Mix the two parts thoroughly for 2 minutes to get a homogeneous mixture.
6-Apply the prepared mixture onto the areas to be reworked, using a putty knife. Press the mixture well into the bottom of the cracks or pits
7-Remove the casing from the oven and allow it to cool to ambient temperature
8-Clean the excess non polymerized cement using acetone
9-Smooth all the treated areas with fine grade abrasive paper.
10-Do a machining where it is necessary.
NOTE: The room where the process is done has a controlled temperature between 21°C and 24°C.
Despite of all cares that the process requires, many times when finishing the casing, we can watch the origin of blisters (around 0,2-0,3 mm diameter (.008 - .012 inches)) in the epoxy surface. These blisters are not permitted according to our inspection. See photo attached. In order to eliminate this problem, we added the following operation between # 5 and # 6
- Do a pre heating of the mixture – 5 minutes at 60°C in a furnace
The problem did not disappear completely but for sure the quantity of non conformances was reduced
Information:
The product LOCTITE 3475 Hysol must be stored and protected from heat and humidity at a temperature between 8°C and 21°C.
LOCTITE 3475 HYSOL is a two-part epoxy cement charged with aluminum which polymerizes at ambient temperature. Its mixing ratio is of 1/1 with a low contraction ratio, it does not get rusty.
Please, has someone of you an extra advice to be applied on the process described?
Thanks

Roberto

 

[Screwman1]

Sounds to me like there is residual acetone down in the bottoms of those cracks and pits that is out gassing. Have you tried heating the parts for a period of time and letting them cool prior to coating.
Also; why are you having to clean off uncured material in #8? If you have a proper mixing protocol you should end up with no uncured material.

 

[TVP]

I was thinking the same thing as Screwman. Heat the casing after application of acetone. Probably 5-10 minutes at 100 C would be sufficient.

 

[roberto1brazil]

Hello Screwman1 and TVP.
Thanks for your tips. We have not tried heating the part before coating. Next time we will try it and for sure we will test the process without clean with acetone, as indicated to check the results. I will let you know about the results.
Best regards
Roberto

 

[SnTMan]

You may be outgassing the epoxy itself. You may be introducing air when you mix it.

Regards,

Mike

 

[Compositepro]

I agree with the above advice. You should not have any uncured epoxy on your casting. If you do that is a major problem by itself. It means incomplete mixing. A good procedure is to first mix in one container then transfer to a clean container and finish mixing. This gets rid of the layer of unmixed resin on the walls of the first container, which is difficult to completely mix in.
Your blisters are very likely simply air bubbles from the mixing process. A degassing operation in a vacuum chamber is very helpful to removing bubbles. Even then it is still possible to introduce bubbles while applying resin to the casting. Application technique is important. Applying resin to a preheated casting would be helpful.

 

[arunmrao]

Usually, vacuum impregnation technique is adopted , so that all pores get filled and there are no blisters or leakage under pressure.

 

[tbuelna]

roberto1brazil-

Hysol recommends de-greasing the substrate before and after grit blasting/sanding. If there is any grease/oil contamination on the surface when you grit blast, the grit blasting will simply drive the grease/oil deeper into the surface making it more difficult to completely remove. Also, follow the good advice from TVP. If you look at the linked TDS for Hysol 3475 you'll note that it has extremely poor resistance to acetone. You should not use acetone anywhere near the adhesive during application or after curing.

The advice given by Screwman1 is also good. You need to be careful about measuring the two component quantities and make sure they are thoroughly mixed. It is usually good practice to avoid preparing very small batches of 2 component adhesives, since very small batches have little tolerance for measurement errors. If you contact your Hysol rep they will give you recommendations on minimum batch size for your adhesive.

Finally, it is somewhat difficult to achieve optimum adhesion with epoxy adhesives on a bare aluminum substrate due to the oxide film that quickly forms on bare aluminum surfaces. It helps if you apply the adhesive (or a primer) as soon as possible after preparing the surface.

http://tds.loctite.com/tds5/docs/3475-EN.PDF

Hope that helps.
Terry

 

[berkshire]

Roberto
If you need to use small batches of epoxy , some manufacturers will give you pre proportioned resin batches in pop up containers.
The containers have a small ridge in the package that you push out when you are ready to use the epoxy, this allows the separated hardener and resin to mingle, ready for mixing.
B.E.

You are judged not by what you know, but by what you can do.

 

[roberto1brazil]

Hi all.
Sincerely I am so grateful for all of you for the advices and recommendations. Probably during this week we will introduce the procedures suggested one by one in order to have some idea about the influence of the changes in the process. I believe that it would be the best way to controlling the changes to be introduced. I have understood that small but important procedures/changes can cause in the final results.
Thanks again and best regards
Roberto