The Powder Coating Oven works!

As Jill was folding the first batch of fascia cladding, Ric built a booth to house the equipment we will use to spray the powder on to the aluminium. It was wider than 2.5m, so it could take full-length sections of cladding. He had a shelf underneath to hold the compressor and the electronics for the spray gun. He got it set up to his satisfaction, then we broke into one of our 25kg bags of powder. This happened to be the grey epoxy primer. We put a couple of cm into one of our two spray canisters, he set the pressure at 1 bar, pressed the foot pedal to get the static charge and tested it out on a manky, rough piece of waste aluminium. He got us along to spray the other side. My first pass was too light, with aluminium still showing through, the next pass finished the job.

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At this point we decided we may as well fire up the oven and see if it really worked. We had acquired an oven thermometer that we stuck through the lid and into the top of the oven, to measure oven temperature rather than thermocouple temperature i.e. the temperature of the air being blown out from the fan. We hung the aluminium on a piece of wire, clamped the lid on and turned the power on. It was still set at 200c from our last trial run, Ric got it back to 180c and we watched it as it heated up. The oven thermometer reacted surprisingly (to me) quickly to the increasing temperature indicated by the thermocouple, but soon started to diverge by a few degrees. The question was, was this because there was thermal lag i.e. it was taking a long time for the heat to raise the temperature in the oven, or was it because the thermometer was rubbish? The difference increased right up to 22c at 180c, with the thermometer indicating 158c, at which point it stopped rising. It stayed like this for the 10 minutes we ran it at temperature.

We turned the whole thing off and after another 10 minutes lifted the lid off. The powder had fused and had left a not-bad finish, except that Ric had not cleaned the aluminium, so there were corrosion pits and bits of sawdust faithfully trapped in the primer. It answered the question about the thermometer – it is rubbish and the oven does in fact work as advertised! The heating cycle was about 40 minutes total.

When we have an area dry enough to have the oven on the floor next to the spray booth, we will get going on the cladding panels. At present we plan to scrub the outer surfaces down with scouring pads and soapy water, clean them with acetone and hang the panels in the spray booth on a rack one at a time for spraying the primer on, then lift the rack en-masse into the oven for 10 minutes at 180c. Then back on the rack in the spray booth for the final coat and back into the oven for another 10 minutes at 180c.

Powder coating – Oven

The first use of the metal folder was to make a powder-coating oven, using three sheets of 0.7mm aluminium. Ric folded a sheet to form the long sides and bottom, with false bottom below that. This housed a 2.5kw cooker element and a fan that drew air down from the middle of the oven, through the heating element and along the false bottom. He ducted the heated air into the oven at each end and folded a lid to go completely along the top. He used a digital controller and thermocouple to control the heating element. The electrics worked first time and we got the thing up to 50c for a sustained period. Ric wrapped it all with 100mm loft insulation wired on to the outside and, at the second firing, just got up to a sustainable 200c. It leaked a lot of heat out around the lid, so we bought a set of clamps to close up gaps. We await our electrostatic spray gun so that we can try it out for real. We have bought 25kg of primer and 25kg of our chosen RAL 6004 powder.

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Powder coating – Aluminium folder

We need a folder that will take full-length sheets of aluminium up to .7mm thick. Buying one would cost thousands, so Ric decided to make his own. We got him two lengths of I-beam (Universal Beam) in two different sizes, a length of angle iron and steel tubes for legs. He used M20 bolts drilled at each end to join the UB together, with  fillet of wood. The top, lighter beam lifts up to allow a sheet of metal to slide between and bolts down to hold it in place. A length of angle iron runs along one side of the lower beam and is hinged to it at either end. The angle iron can be lifted with a handle and bends the metal sheet to up to 90 degrees, with a nice reasonable sharp corner. He even put trampoline springs at either end to make it easier to lift the upper beam when feeding the sheet through.

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