The process of Powder Coating sounds very simple;
You buy the powder, spray it onto your components using a special type of electrostatic gun, stick it in an oven & hey presto you have a perfectly coated item in your colour of choice!
Does that sound too good to be true? If you answer yes you would be correct, it is too good to be true.
The Powder Coating Process contains many steps and elements that need to be understood before you start work.
Firstly the items that you want to coat must be free of grease, oils and any rust or scale. To achieve this you should first of all degrease the items. The best way of doing this is with a Vapour Degreasing Tank. This contains a chlorinated hydrocarbon that is heated up and produces a hot vapour, your components can be placed in the vapour and the condensing action of the ambient temperature parts coming into contact with the hot vapour (around 80 degrees C) causes a condensing action that washes away grease and oils. As soon as the parts have reached the same temperature as the vapour the degreasing action stops and you can remove the parts.
There are other methods to remove grease; you can use an aqueous type wash facility that cleans the components with a detergent based solution and then dries them, this has some shortcomings in that with complicated components moisture can get trapped in joints or welds that can ‘bleed’ out during the coating process and cause rejects.
It is also possible to wipe the components with a solvent rag and then dry them off, this is the least efficient way as residues of grease can get left on the parts causing loss of adhesion of the powder coated film.
Whichever way you choose be careful to fully remove grease, oil & other residues.
If the parts are made of hot rolled steel that that have a layer of scale or rust on the surface, this should be removed by shot blasting. This involves a stream of steel shot in a jet of compressed air that literally ‘blasts’ the rust or scale off the surface.
Some people will tell you that they use wire wool or abrasive pads to remove rust or scale, this does not work. If the surface of the metal is not clean then the powder coating film will not adhere correctly and corrosion problems will appear sooner than they should.
If the parts are aluminium then they should be degreased and then have a pre-treatment applied. This is usually a dip process that converts a few microns of the surface to Aluminium Chromate. This is in essence a crystalline structure that allows the powder coated film to adhere more strongly to the surface of the metal.
After the preparation of the metal surface we can now move to the preparation of the parts prior to coating.
The first step is to remove any dust or residues that may be left on the surface.
This can be achieved by wiping the parts with a clean lint free rag and then blowing the parts with compressed air to remove airborne dust particles.
At this point you are now ready to spray the parts. You will need your gun & powder hopper to be loaded with the correct powder and your spray booth to be cleaned and free from any other powder residues from previous jobs that have been sprayed.
The secret to good spraying is to apply the powder slowly, adjust the gun settings so that the purging air and the powder volume settings are in balance. Too much air and not enough powder causes the powder to not adhere to corners as it is effectively ‘blown off’. Too much powder and not enough air causes ‘surging’ , the powder is not properly atomised and lumps together causing the coated surface to be lumpy and not smooth.
Apply a smooth even coat, you do not have to coat the whole component in one go, apply a first pass, check where the powder has & has not coated and then apply a second pass until the surface looks even and consistent.
When coated the items must be cured, i.e have heat applied so that the powder fuses/melts and cures hard. A major fault amongst powder coaters is not giving parts enough heat. Sometimes this is because components are made of differing thicknesses of metal. It is common for parts to be predominantly made of say 4mm steel but with flanges or brackets made of 6mm or 8mm material.
You must remember that the items must be completely heated up to the recommended curing temperature. The heavier items will need more heat. If the instructions on the box of powder say 180 degrees C for 10 minutes that means that the 10 minutes time commences at the point when all of the workpiece or component has reached this temperature and in the case of heavier parts this can take many minutes. For instance a part may take 10 minutes to reach temperature and then it will need a further 10 minutes for the powder coated film to harden.
After the curing process allow the parts to cool down fully and then pack or wrap them to avoid damage in transit or while moving them to another department or process.
This is a brief overview of the Powder Coating Process and as you can see there are many issues to take account of. If you follow the basic rules laid out here then you should be able to produce good quality work.