The most important anodizing tank
A typical type III aluminum process has about 5 different types of tanks for cleaning, etching, deoxidizing (or desmutting), anodizing and rinsing. If you look at a type II process you can also include a dye tank and a seal tank. If you want to look at the complete process, you need to also include incoming receiving, unpacking and inspections, racking, unracking, final QA, packing and shipping. Given all of that, the anodizing process can easily encompass a dozen steps or more. All of which need to be done correctly to provide the customer with the quality service they expect. But of these processes, which ones are more important? The short answer is none of them.
The anodizing process begins when part arrive in your receiving department. They get unpacked, inspected, and a job traveler gets created. These are all very important steps. If parts get dropped or damaged otherwise when they are unpacked the anodizing process will typically enhance the damage. Likewise if they were damaged during shipment and processed anyway. In either case, the customer will blame you for the damage. If in the unfortunate case, the case job traveler is created incorrectly, the parts could undergo the wrong process. Any one of these thing can lead to an unhappy customer and are candidates for the most important step in the process.
After the parts leave the receiving department, they head off to racking. Racking involves physically attaching the parts to a fixture of some kind so they can be suspended in the process tanks. How parts are racked and where those parts actually contact the racking fixture is extremely important. Racking can be considered a destructive process in that there will be no anodic coating at the rack’s contact points, and in most cases, the anodizer and the customer will agree where the point will be on the part. If the parts are not racked using the correct contact points, those point will have not be anodized and would probably be rejected by the customer. Not only is it important to rack the parts in the correct places, but the parts need to be racked firmly as well. A loose part on a rack could fall off during the process and be lost at the bottom of one of the tanks. Worse than that, a loose part can stay on the rack and arcing can occur between the rack and the parts. This can ultimately lead to the destruction of the part. So is racking the most important step in anodizing? Maybe.
Assuming the parts have gotten through receiving and racking, the core anodizing process can begin with the cleaning, etching and deoxidizing of the part. Simply put, parts must be thoroughly cleaned to be processed correctly. For the etch tank to work properly, the chemistry needs to react with the parts. If they are not cleaned properly, the parts may etch unevenly or they may not etch at all. After etching, the parts will typically be covered with smut. Smut is the byproduct of the etching process. It is the “ashes” of etching if you will and needs to be removed. If it is not removed, it acts as a barrier in the anodize tank and can impede oxide growth. This can lead to uneven oxide growth or worse in can lead to part destruction. If the rectifier is set to run the part at 12 ASF, and half of the part cannot be anodized because of excess smut (or improper etching), the part will run at twice the current density it should. In short, the part will burn. Given all of this, it is easy to see just how important cleaning etch and deoxidizing the part are.
That brings us to the anodizing tank, and intuition would say that this is the most important part of the parts. Indeed that anodize tank is important. It is the tank that is responsible for most of the part destruction that can occur. As mentioned, if the part are unclean, under etched or not deoxed properly, burning is likely. If the rectifier has no automated controls, an operator will be responsible for setting the voltage and current. For type III processes, the operator will be (should be) making adjustments every 3-4 minutes as the coating builds. For these manually operated rectifiers, it is nearly impossible for any single operator to control the rectifier consistently from run to run. When there are multiple operators working multiple shifts and consistent results become difficult. But it’s not just rectifier control that is at issue in this tank. Bath temperature and agitation are also very important. Run the parts too cold and they will become darker. Run them to hot or without adequate agitation and they will burn. Electrical contacts are also important. Loose parts can fall off, connections between the rack and the work bar, or between the work bar and the rectifier can become corroded and create large voltage drops. If the process is running under voltage control, the voltage drop from bad connections can lead to oxide coatings that are too thin. This is without question an very import tank in the anodizing process.
Dyeing & Sealing
For type II processes, the parts may be dyed and sealed. These are obviously important steps, because they affect the part in the most apparent way; by how it looks. Obviously a part that should be red needs to go into a red dye tank, but if it doesn’t you will need to strip the parts and start over. Not only is getting the part in the right tank important, but it needs to stay in it long enough for the dye to be taken up by the anodizing pore structure. Also, let’s not forget that the dye temperature and pH are important as well. Once the parts is dyed, it must be sealed correctly otherwise the dye will stay in the pore. From a cosmetic view point, the dye and seal tank a very important steps.
Then there is rinsing. In and of itself, rinsing is not a very important step in the anodizing process. It doesn’t add to the part and (with the exception for improper sealing) it doesn’t take away from the part. However, rinsing is extremely important for the overall health of the process line. Improper rinsing leads to bath contamination from chemical drag-out. For example, etch tanks are typically strong bases where as the next tank (deox) is typically an acid. If the parts are not adequately rinsed between these tanks, the deox will begin to neutralize causing it to be less reactive. The same reasoning can be applied for any tank in that if the parts are not rinsed well enough, you will eventually end up with a very ineffective process line.
Finally the process concludes with the final QA and shipping step. The importance of the final QA should go without saying. It is the last chance you have to find problems before your customer does, and it should include coating thickness measurements and a visual inspection at the very least. From there, the parts go to the shipping area where they have the last contact with you. After spending all the time and energy processing the parts to this point, it should be obvious how important it is to package them well enough as to not be damaged in transit.
So now it should be clear that out of all of the steps taken to anodize a part, there is no one step that is more important than another. If you fail the see damaged parts coming in, or if you damage them in receiving, your customer will blame you. If the parts are not racked correctly, they will either be anodized wrong or not at all. If the parts are not cleaned correctly, they may not etch correctly. With improper etching or deoxing, they may not anodize well. Improper anodizing can ruin the parts. With incorrect dyeing and/or sealing, the part will not look good, and finally if you don’t package them well, the can be damaged in transit. So every station and tank the parts visit during the process has catastrophic potential, and for every part that is done processed correctly will have the customer blaming you.