I have seen alot of posts about people looking to polish aluminum. I would like to make a post here so everyone can have something to fall back on.
Most aluminum parts that people want to polish, have some machine lines or burrs that need to be removed before the polishing step. It you do not have a smooth finishing before you polish you will not get the shine that you are looking for. Think of it this way. If you looked at the surface of the material under a microscope you would see peaks and valleys, would almost look like a mountain range. If you do not remove the peaks before you polish, you will only be polishing the peaks. The reason that some parts still look dull is because of the shadowing of the valleys by the peaks. I hope that makes since.
In most cases, you will need to start the polishing step by running the parts in a plastic media that contains a small amount of abrasive. Don't get to caught-up in the color because the color is just an identifier. Every manufacturer of media uses a different color chart. The most important thing is to have an understanding of the weight of the media and the cut rate of the meia. Typically plastic meda weighs about 65 lbs per cubic foot. Generally the heavier the media the faster it will cut. Think about it like sand paper, the harder you push down the more it will cut.
When you choose a media you dont want it to have to much cut because you might take off the peaks but you will create more valleys. I would recommend going on the lighter side first, even though it may take longer you will have less problems with the final step.
Media size and shape are also a very important step. You want to choose a media that has the most surface contact and is large enough not to get lodged in any holes. You will get more work done in a shorter time using a shape that a large flat surface if you are doing alot of flat parts, and if you have any corners you will want a media shape that has sharpe edges. The only precaution with sharpe eges is that if you have alot of parts and media in the machine you may get some parts that get damaged from the media, especially when running a soft material such as aluminum. Alot of people confuse damage from media with damage from part on part contact.
Once you choose a media and are ready to start running the parts you will also need to make sure that you have a good quality compound (liquid). This compound will be mixed with water with a consentration of about 1-2%. Vibratory compounds are designed to pull out any contaminents that are in the system, such as metal removed and media breakdown. This is a very important part of the process because if you do not pull out the contaminents you will 1) Have dirty parts with all kinds of stuff embedded in the surface of the metal, 2) Bad media because once again if you don't pull out the contaiminents they will get embedded in the media and the media will stop working. Think of it as sand paper again. Once you get the paper loaded up it stops cutting. This is called "glazing" of the media. You can tell if the media is glazed because it will have a "glossy" appearance or a shine to it. If your media looks like this there are several things that you can do to it to try and bring it back. The easiest is to add a some aluminum oxide, 220 grit should do, and run the machine without any parts over night.
In most small smaller job shops it is common to have some type of settling tank to let the soilds fall out and then re-use the water. This is not the best practice, but it is very common. There are several things to help make this set up more successfull. The tank should have 3 separate compartments so the solids have a chance to settle out. The line comig from the vibratory machine should enter the first compartment at the top and freely drain. Then the first compartment should drain into the second compartment over top of a divider, then this compartment should do the same into the third. Then the pump for the water and compound should suck water from about an inch below the surface of the third compartment. If you still have problems with not pulling out all of the solids you can also add a floculent to drop out all of the solids. The settling tank should also be dumped and cleaned on a regular basis.
Also note if you are using a settling tank you should make sure that the compound is designed to recirculate. Most standard type compounds will start to lose their effectiveness after a few cycles.
Now you are ready to run the parts in the plastic media. The cycle time in the media can vary greatly from under an hour to several hours. It all depends on the starting surface roughness of the material. The parts may have a galvanized look or cloudy appearance after running in the plastic media. You should check that the appearance is uniform and covers the entire piece.
After the parts have been run in the plastic media, you will need to run the parts in a second step. There are two options here. Either a "microbright" type media which requires a contolled environment including the right compound, or a treat organic material,such as corn cob, or walnut shells. The treated material is the easiest because you can load the parts and walk away without any monitoring. This is also a dry process which has less variables. The thing that makes this nice is that you can turn this process on and run it all night without any negative effects.
I hope that I have provided some good information here without confusing anyone. If you have any questions please either post here or send me a private message.
We have a vibratory bowl, which we bought used from a company. We would like to polish aluminium parts.
The bowl came with a urea, non abrasive media and a solution(unknown). The media does deburr the parts, but larger burrs tend to be rolled. Are we barking up the wrong tree and need to have an abasive media first, followed by the urea?
Then comes the polishing, we have acess to walnut shell, is there a particular mesh size that works better? I have seen reference to treated media. Is that iron oxide added to the shell, if so what ratio?
And lastly stainless balls for burnishing, does it work? I have seen references to it and heard mixed results and also cost, as we have a 7 cubic ft bowl.
Look forward to your response