), and a cupla forums that seem to like both diy setups. Apparently any oven will do.
Do you spray the items while they're still in the oven, or are they taken out, hung, and then sprayed? If the latter, I imagine you gotta be quick, eh? I notice that suppliers distinguish between the oven and the booth
) -- is this because the items are actually sprayed in the booth and not the oven?
About how hot are the pieces when they are sprayed? I imagine this varies with specific coatings.
If I fool around with powder coating, I will need to do 1/4" alum, up to 16 x 48", so I'll proly make my own oven out of various heating elements, brick, etc.
Holy shit.... talk about having nary a clue, eh??!! :)
So I guess you gotta be perty careful when transferring the sprayed stuff to the oven? Is the sprayed metal wet or dry before it goes into the oven? Either way, I'll bet loading the oven is the paint-equivalent to walking on egg shells.
The wiki article
does refer to spraying already-heated metal as a "common alternative".
A lot of drama here. Canned spray paint, esp. some of the new metallics, is much lower-ante bet, coating-wise. Ahm all for low low ante's.
In order to powder coat, the object to be coated is sand/beadblasted to an extreme state of cleanliness. It is then electrostatically charged. Then the powder, which is oppositely charged, is sprayed on to the object. At which time, the object is rolled into the oven which is then brought up to about 400 degrees F. The temperature depends on which powder is actually used, as they can have different melting temperatures allowing different color effects to be used. The powder then melts and flows uniformly. If done correctly, the coating thickness is about .020" thick. The object is then cooled and is ready to use. If you do not the facility to perform all of these steps, successful coating will not occur. Steve
The charge is actually IN the powder, and it retains it for quite a while. You can't touch the work, or bump it into stuff, but you can bang it around surprisingly much without knocking the powder off.
The powder is totally dry before baking, this is not a wet spray process, it is a dust process.
You do not remove the charge. The powder is not conductive. So even though it is in contact with the metal, only the small area that actually contacts the metal loses its charge. The rest of the powder is still charged.
"The powder will remain attached to the part as long as some of the electrostatic charge remains on the powder. To obtain the final solid, tough, abrasion resistant coating the powder coated items are placed in an oven and heated to temperatures that range from 160 to 210 degrees C (depending on the powder)."
A caution that probably won't apply to anyone here: people with pacemakers or ICD's might want to steer clear of the DIY powdercoat guns. They're not mentioned in Medtronic or Boston Sci literature but they employ high electric fields (up to 50KV) to ionize the powder.
I'll try to answer this as brief as possible. First just so you have some background I spent 14 years working in a coating plant that did both liquid and powder coating. Did every job in the place over the years from hanging parts on the lines to supervisor including driving the trucks to P/U and deliver the parts.
First powder coating as a process is not hard to do. The trick to the entire process is really simple. THE PART MUST BE CLEAN!!!! After that it's a cakewalk. Well maybe a few minor items are a PIA but money can fix those.
First your talking aluminum. I would rather coat my tongue than aluminum BUT... First off - How long do you want the coating to last? With aluminum this is the BIG problem. Aluminum Oxide forms VERY fast. This is what causes the coating to fail. So you need to come up with a cleaning method to remove it. In the plant we used a 5 step process, First a simple degreasing bath, then through a 4 stage wash rig. First stage rinsed off the panels with PURE water. Then into an acid wash and rinse then a phosphate bath. Then through a quick rinse and into the drying side of the oven. From there they could be handled WITH CLEAN COTTON GLOVES. Touch the part or let it get dirty and you start over.
Next you hang the parts. The easy way is to drill a hole or holes and use a steel wire to hang the part. Any tapped holes or unpainted areas get masked off (Just about anything that is easy to remove can be used, IF you remove it before curing the powder)
Now you coat the parts. Here you need a powder gun, The cheap guns work BUT if you plan on doing many of these items hit an industrial auction and get a REAL powder gun, Graco, Sames, Norton all make good machines. Make sure you get the COMPLETE unit, should be the control head, and a fluidized hopper you put the powder in, plus the gun itself and the cables and color hose.
Powder to match the characteristics you want. This can get interesting due to all of the different ones out there. ANY color you like can be made, from Clear to Black, High Gloss to dead flat, Textures, Hammertone, Mixed colors, Metallics, Candies, you name it and it's out there.
Powder booth to contain the over spray and reclaim the unused CLEAN powder. (easy to make one for the parts you list out of cardboard or light tin)
The racking for the parts, This can range from a single wire to a complete conveyor system. For many folks the common bakers rack with steel wheels works well.
The oven. This is the part that kicks most people in the guts. "any oven can be used" Well yes/NO First you need an oven that holds the internal temp at the cure point within a few degrees for 20-40 minutes. The common home gas oven doesn't do that very well. An electric beats it hands down. Then you need a way to vent any smell/fumes (yes curing powder does STINK). For the parts you have the easy way out would be an oven built out of tin with electric oven elements and fiberglass insulation with a shell of tin. OR you could go with infrared heaters and a simple insulated tunnel.
As for the process itself.
Hang and clean the part. OR clean it and handle it GENTLY while you hang it up.
To coat it the part gets grounded to a ground strip through the hanging wire. The powder goes out of the hopper and picks up a positive electrostatic charge when it goes through the charging section of the gun (usually at the very exit point). The powder goes through the air and settles on the part due to the different charges.
This is the point where the pro gun beats all the home guns. CONTROL, both in the powder stream, speed of discharge, amount of electrostatic charge (the typical Sames gun we used put a 150KV charge on the powder, if you got it close to a ground it would throw a 4" arc!!!) and powder coat mil thickness. Rule of thumb is that the higher the charge the better the powder sticks, BUT it also will apply the powder THICKER, which isn't good. This is where the controls come in. On a good gun you can adjust the entire process.
YES you can cause all the same things with powder that you can do with liquid, gobs stuck in the paint, dirt, bugs, runs, drips. BUT prior to you actually curing that powder you can do one thing you cannot do with ANY liquid paint, REMOVE THE PROBLEM. Simply grab a VERY filtered air line and blow the powder off the part and recoat. (once you get the hang of it you can even do spot touch-ups, blow off JUST the offending item and even create paint designs with just the air gun.
Now you have a clean part, hanging on a wire, with a nice even coat of powder on it. PERFECT. OOPS you sneezed....
OK you didn't sneeze this time... Believe it or not it takes a LOT of force to knock that charged powder off that part if you use a good gun.
Now you transfer the part to the rack and GENTLY roll it into your oven. OR transfer the part to the rack until you have enough coated to fill the oven (you want room between parts if they move some but not a lot)
The oven will be PRE-HEATED to the cure temperature. Open the door, load the oven and close the door. WHY pre-Heat? Because you want those parts to come up to temp FAST so the powder cures properly and bonds to the aluminum. Remember that metals expand when heated, if the powder is already starting to gel as the expansion occurs it will move with the part. If the powder isn't starting to gel and the part expands you get micro-fissures in the paint that let air/moisture in. NOT a good thing. Now for cooling you let them cool together. The powder will again follow the metal. It will actually stay in a semi-plastic state for quite a while when it is warm.
DO NOT TOUCH THE WARM PARTS!!!
Use the hanging wires to move them out of the oven to cool. Why not leave them in the oven till it cools? Because powder CAN also be over cured. Just like baking cookies, there is a point where the dough softens, flows out and sets. Then they start to brown. If you leave them in too long what happens? You get CHARCOAL COOKIES... Powder does almost the same thing at a molecular level. Properly cured powder will move with the substrate, I have taken .020 test panels, coated them, bent them over. Flattened the bend down with a mallet and not had the coating fail on properly cured powder! OVER cure that same powder and it will flake off and fail.
You asked about spraying hot parts as well. YES you can do that and we did that a lot on HEAVY cast parts or on parts where the customer needed a specific thickness of coating for another purpose. For instance the E-One company (
) had us coating the pump housings and parts inside many of the sewage grinder pumps they built. They wanted a VERY thick coat that would then get machined flat and used as a gasketing surface. This kept all of the cast iron housing covered and made those pumps last. To give them the proper coat we would clean the parts and then pre-heat them up to cure temperature. Then spray on the powder to build up the thickness. The heated parts would instantly cause the powder to begin curing and then we baked them a while longer till we reached a full cure.
This process also works on porous items like cast aluminum or iron because it out-gasses the part and allows the powder to gel without having gas bubbles in the coating. On 1/4" it isn't needed.
Very good write up. I would make one addition. Use a good respirator when working with the powders. They are toxic until they are cured. Cheap dust masks don't cut it. Use a vacuum to clean up the area if not recycling powder overspray. Same goes for your clothes.
Thats why a booth is used as a controlled area. Powder goes back into the can or into the filter.
Also, if you make your own oven, check the cure temperature of the powder you plan to use. Harbor Freight powders need a cure temp of 400 degrees F for 20 minutes after powder glosses over.
Not unless your stupid enough to test the voltage by sticking it over the pacemaker!
I used to demonstrate the voltage on the guns to folks visiting the plant by arcing it to ground in the booth with the powder OFF. Most of the better guns allow you to control the voltage, current, powder deliver, air flow to the hopper from the front panel.
Powder coating without spray gun, charges, or oven:
I watched a CalTrans crew today repaint street markings in front of an intersection. They laid down metal stencils, then used a weedburner type torch to heat the asphalt inside the letter cutouts. Then white powder was dusted over the stencil.