A Complete Guide to Powder Coating

History and Evolution of Powder Coating

Discover the fascinating history of powder coating, from its early development to its evolution as a revolutionary finishing process. Learn how it differs from traditional wet paint and how it transformed the coating industry.

Basic facts, Powder Coating is the application of a dry powder that contains all of the ingredients of paint.

Read more about the history of powder coating

Electrostatic Powder Coating Process

Explore how the introduction of electrostatic powder coating dramatically improved production efficiency and material usage. Understand how this process offers faster application and cleaner, safer working conditions.

Basic Facts; Electrostatic Wrap around improved production and material usage , 60-80 microns of coating applied in one go, less labour and energy input, no volatiles or atmospheric pollution.

Read more about electrostatic powder coating.

Preparing Surfaces for Powder Coating

Surface preparation is critical for achieving a durable and high-quality powder coat finish. Learn the essential steps for prepping various substrates, from steel to aluminium castings, for optimal results.

Basic facts; Vapour degreasing is best for cleaning, aqueous washes are satisfactory but ensure that no trapped liquid remains in recesses/joints, solvent wiping to be avoided unless absolutely necessary. Pre -Treatment by Phosphating or Chromating cleans the surface and improves corrosion resistance and adhesion.

Read more about surface preparation for powder coating

Cleanliness is next to Godliness

This saying is nowhere more true than when applied to Powder Coating. If your facility or the facility of your sub-contractor is not clean and tidy then it is unlikely that you will receive consistent good quality powder coated work.

If the person managing a Powder Coating facility cannot be bothered to keep the place tidy then he/she is very unlikely to care about or fully realise the importance of good housekeeping and will be not be able to produce high quality work.

Cleanliness of the facility is a very important part of the preparation of metal parts prior to Powder Coating. You can prepare, degrease & pre-treat parts perfectly but if your facility is not clean then your coated parts will for one reason or another not be up to standard.

Floors should be free from dust and any other residues/rubbish, preferably they should be cleaned with an industrial vacuum cleaner every day at the end of production.

Have plenty of receptacles for rubbish and empty them every day.

Benches and tables that are used for preparing work prior to coating should be clean and tidy. It is a good idea to put a fresh sheet of brown paper or similar on the tables and fix with tape. A clean dust free work bench is vital.

Hooks, jigs & other fixtures should be organised and kept in containers so that they can be found easily when required.

Boxes of Powder should be kept in a Powder Store and not left hanging around the production area.

The production area should be kept free from dust and as part of a maintenance schedule machinery & plant should be vacuumed regularly. Particularly the ovens & conveyor systems must be kept spotlessly clean and dust free. A well organised Powder Coating Facility will set aside time every week to maintain all of the equipment and keep it dust free and clean. In essence a few hours a week cleaning and maintaining plant & equipment pay for themselves many times over by reducing the number of parts that need to be re-worked due to poor quality because of dust & inclusions in the surface of the coating. See example Maintenance Sheet at the end of this article.

Basic Facts; Keep it Clean ok.

Now that we have our parts degreased or pre-treated what comes next;

Masking; What a Cover Up:

Many manufactured parts have areas where no coating is required. Prior to any further preparation the first step is to mask these areas. Masking parts has moved on from the days where a person armed with a roll of sticky masking tape and a Stanley Knife would crudely attempt to cover areas or holes where coating was not required.

These days there are masking material manufacturers who produce all manner of masking materials;

High temperature tapes that leave no sticky residue.

Silicon bungs and caps of all sizes and shapes.

Die cut masks that exactly cover intricate shapes.

These are used in a variety of different situations such as;

Parts that need to fit together and have tight tolerances such as two tubes that form an assembly.

Parts where Powder Coating on mating surfaces would impede them fitting correctly and result in wasted time/effort cleaning the mating surfaces.

Holes or recesses that have bearings fitted into them; Bearings are usually a ‘press fit’ and the inside diameter of the part that accepts the bearing must be the correct dimension.A build-up of Powder Coating can interfere with the fit and residues of powder that are removed/loosened during the fitting process can block the bearings and stop them running freely.

Internal or external threads; if a thread is coated with powder this will cause problems during the assembly process. Removing powder coating from tapped holes or threaded surfaces is extremely difficult and can waste a huge amount of time and slow down the assembly process after coating.

Gasket surfaces; many machined components that have to be fitted together have surfaces that require gaskets or seals between the two components. The mating surfaces have to be kept free of Powder Coating to ensure that no leaks occur and that the components fit together precisely.

Electrical Earthing; All metal electrical components or enclosures require an ‘earthing point’ This is an area of the component surface that has no coating applied so that an earth tag or connection can be fixed to the surface of the part ensuring the part can be used safely.

Basic facts; there are a wide variety of manufacturers who produce everything needed to ensure that Powder Coating only goes where you want it to go.

Wipe it and blow it!

Final Preparation before coating;

The word for bits of dust or particles in a Powder Coated surface is Inclusions. These are dust or other residues that are on the surface of the metal after degreasing or pre-treatment or they are airborne dust and particles that have landed on the work in the Powder Coating Facility at some point between Degreasing/pre-treatment and getting to the production line.

It is advisable to remove these prior to coating if you want to achieve a smooth inclusion free powder coated surface.

The usual way is to wipe the parts with a clean cotton rag, form the rag into a small pad by folding the edges in because bits of cotton can come away from the edge of the rag.

Wipe the component that you are going to coat all over, with parts that have been degreased there is sometimes a layer of light dust on the surface, wiping will remove these small particles.

After wiping if you are not going to coat them straightaway then make sure you place them on the preparation bench with the most important faces that need the best finish facing upwards, placing what is called the A face ( i.e most important face ) onto the bench can result in residues off the bench sticking to the surface and causing inclusion problems after coating.

Just prior to powder coating the part to be coated is either hung up using a wire hook or in the case of longer/larger items more than one hook. If not hung up individually parts are sometimes loaded onto Jigs, these are metal frames that have a series of hooks or contacts welded to them so that more than one part can be hung up.

It is advisable just prior to coating when the part/parts are hanging in the spray booth to blow off the parts with a fairly light jet of compressed air. You have to be careful not to dislodge the parts or cause to much turbulence in the spray booth and disturb dust or contamination that may land on the work. A light blow off on each component will remove any final bits of dust and contamination that may be remaining on the surface.

 

Basic facts; don’t skimp on final preparation it will pay it makes the difference between a mediocre finish and a great finish.

Are you pointing that gun at me?

The next part in the process is applying the powder to the components. The Powder Coating Machine consists of a hopper that holds the powder, a generator that creates the static electricity to charge the powder and a gun through which the powder is sprayed.

The hopper has a semi porous membrane in its base, air is passed through this to ‘fluidise’ the powder in the hopper, basically to slightly ‘puff up’ the powder so it becomes a mixture of powder and air. A venturi/nozzle in the hopper feeds the powder to the gun when the trigger is pulled and the generator sends static electricity to the tip of the gun to charge the powder.

The Powder Coating Machine has two controls, both of these must be in balance, firstly the purging air that passes through the venture nozzle in the hopper, this determines the amount of powder that reaches the tip of the gun and secondly the level of static electricity applied to the tip of the gun. If these two are not in balance then a variety of faults can occur which will be dealt with in the Trouble Shooting Section later in this article.

The key to producing a high quality finish is that the Powder Coater has ‘feel’ for the spray gun. He must be ‘in tune’ with the flow of powder coming out of the gun and apply it evenly all over the component, if the operator does not concentrate then it will result in an uneven coat of powder onto the part, some areas getting more powder and some not getting enough.

More on this issue in the Troubleshooting section.

The Bake Off

The next stage after coating the parts with powder is applying heat to the parts so that the powder coating ‘cures’. In this sense the definition of Curing is the cross linking of the resin structures within the make-up of the powder so that the powder coated film hardens.

The most important factor to take account of is that of Metal Temperature. The curing instructions on the powder manufacturer’s data sheet will say something like ‘Cure for 12 minutes at 180 degrees centigrade. This does not mean put the parts in the oven for 12 minutes at 180 degrees C. It means that first the component has to reach curing temperature and then it has to cure for the required amount of time. The most common fault encountered with powder coating is under curing of parts.

If parts are not given sufficient heat for the correct amount of time the powder coated surface does not harden correctly and will chip or flake off. Special account must be taken regarding the thickness of the materials making up the component, it not unusual to see parts these days made of a variety of thicknesses of metal, certain parts can be 4mm, others 6mm and even thicker at 8mm or 10mm. In these instances care must be taken to cure the parts to the thickest material as this will take longer to reach curing temperature.

To be absolutely sure of the time taken to heat parts up to the curing temperature there are Meters available that have probes attached to them. The meter is put into a heat proof container and the probes attached to various parts of the component. The uncoated component can then be put into an oven with the Meter and heated up for a period of time.

When cooled the meter can be retrieved and it will have recorded how long the various areas of the component took to heat up to curing temperature. So for instance it may show that the 8mm thick parts of the component took 6 minutes to heat up to 180 degrees centigrade, so the curing of these parts would be 6 minutes plus 12 minutes, 18 minutes in the oven in order for the curing to take place.

There are various types of oven in which curing can take place; the two main types are Box or Static Ovens and Conveyorised Ovens.

Box Ovens are as the name implies a box type structure with doors and some means inside it to hang work from. It will usually have a gas process heater attached that blows heated air into the oven. There will also be thermostats and controls so that a desired curing temperature can be chosen and the time for parts to be in the oven. Powder Coated parts are hung in the oven, the temperature is set and the timer, the doors are closed and the curing cycle started, normally some sort of bell or alarm will sound when the desired time has elapsed so the oven can be turned off, doors opened and the parts can cool.

Conveyorised Ovens are larger structures with an open aperture at each end and a motorised overhead conveyor running through it in a loop. Normally a Spray Booth will be located at some point on the conveyor. The parts to be coated are hung from the conveyor and moved through the booth at a pre-determined speed, sprayed with powder in the booth by one or sometimes two spraying personnel or by automatic reciprocators with the powder spray guns attached.

Reverting to the Curing Time Issue with conveyorised ovens there are two factors determining the speed of the conveyor, firstly the speed at which the parts passing through the booth can be sprayed and two the time that the parts need in the oven to effect a cure. In some case the parts can be sprayed faster and there is a temptation to increase the speed of the conveyor to increase production throughput. This can cause problems as although production is increased it will be at the cost of properly cured parts.

That being said Powders these days are resilient to higher temperatures so for instance 12 minutes at 180 degrees C curing is equivalent to 8 minutes at 210 degrees C. This means it is possible to increase the temperature inside the oven and lower the time that parts must remain in the oven, always remembering that the metal must heat up to the Curing Temperature so that parts can be fully cured.

An Inspector Calls

The next stage is in the writers opinion the most important part of the Powder Coating Process;

Inspection of the Coated Parts.

The first point to take on board is that nobody can produce perfect work all of the time, everybody at some point in the process at some time will do something wrong and cause rejects.

The three reasons for Inspection are one to provide immediate feedback to the other members of the team who are involved with other processes/actions that there are quality issues so that they can be remedied quickly and secondly to ensure the customer does not get sub-standard parts or rejects arriving at his factory as this will cause him delays and costs. Finally if the feedback provided to the rest of the team is acted upon and processes are altered and improved then rejects and reworks will be reduced.

The Most Common Powder Coating Faults

There are many faults that can occur when powder coating and over the years we know exactly how to avoid them. Read about faults including inclusions, uneven coating, and under-curing. The best way to avoid faults is to leave to the professionals. So, why not get in touch and get a quote?

Click here to read in depth about the most common faults.