This is a guide for the layman and it sets out in simple non-technical wording what Powder Coatings are. If you read it and can remember the basic facts at the end of each section it will provide you with everything that you need to know about Powder Coating.
If you are a designer or specifier the better you understand the ins & outs of Powder Coating the more relevant your chosen specifications for a particular product will be.
For buyers and purchasing departments understanding Powder Coating will help you to choose more cost effective solutions for components that require finishing and not be bamboozled by the excuses that your coating subcontractor or in-house facility will give you when parts don’t come out right and you miss deadlines for delivery due to re-work.
It’s Dry not Wet;
That is the first thing to remember, powder coating is exactly that ‘powder’.
It is dry and it is applied dry.
But before going into any detail about powder coating it is a good idea to understand basic facts about liquid or wet paint.
Paint is a mixture of various compounds and chemicals that are suspended in a ‘wet’ solution so that it can be applied by brush, roller, dip, or spray to a surface that needs to be a particular colour.
Going right back to caveman days our ancestors who wanted to record important events on the inside of their dwellings gathered up pigments, berries and roots, and then mixed & ground them into various coloured pastes to produce the first primitive paints.
Paint continued to be made in much the same way until the early 1970’s, a liquid carrier containing a mixture of pigments, resins and other chemical agents usually in a spirit or oil base such as turpentine or linseed oil and later on thinners. It was applied by various means; spray, dip, roller or brush and then either allowed to air dry or force dried in ovens to produce a smooth even coat of colour.
The process of painting industrial goods was quite labour intensive, in order to produce a durable finish that would withstand corrosion it was often necessary to apply 2 or even 3 coats of paint, a first or primer coat was applied and then dried off, then a second and sometimes a third. This was time consuming and rather polluting to the atmosphere as the thinners (which is derived from oil) in the paint had to be dried and released into the atmosphere through the drying oven chimneys adding to atmospheric pollution.
Around the 1970’s Paint Chemists developed a process by which they took all of the ingredients of paint; Resins, Pigments etc in dry form and ground them into a powder, after this applying heat and melting the dry powder so it all mixed together and resulted in a brittle toffee like substance being produced that was a homogenous mixture of all the ingredients of paint. This pre-mix material was then re-ground a second time into a fine powder, each grain of powder being about the thickness of a human hair and containing exactly the same proportions of all the ingredients that made up wet paint except there were no liquids.
Basic facts, Powder Coating is the application of a dry powder containing all of the ingredients of paint to metal components in one step.
Electrostatic Application, it’s a Wrap.
The next process in the development of powder coating was electro-static spraying. This was a well-known industrial process that had been used for many years to apply wet paint. The paint being applied was charged with static electricity causing it to be attracted to an earthed component being sprayed.
This was a much more effective method of coating parts as the static electricity resulted in the phenomena of ‘wrap around’ the charged paint would (depending on the shape and size of the parts being coated) wrap around up to 50% of the unseen side of the component due to the static attraction, resulting in more efficient use of materials as more of the paint adhered to the work piece rather than being exhausted to atmosphere or adhering to the spray booth sides and due to wrap around some of the work was being done by the static attraction so much less time was taken to coat components.
Equipment based on the electrostatic equipment used for paint spraying was developed that was adapted to spray the newly developed ‘powder paints’.
The powder coating equipment was made up of a hopper that held the powder, a generator that produced the static charge and a spray gun that was used to apply the powder to the components that required coating.
When the trigger on the gun was activated the hopper fed powder through a tube to the spray gun where by the application of compressed air it was atomised into a fine cloud and at the same time charged with static electricity produced by the generator. The operator then moved the gun around the work piece and applied an even coat of powder. For many parts such as tubular structures and small fabrications the charged cloud of powder ‘wrapped around’ the work piece and it was possible to coat most of the part by spraying just from one side.
Powder coating had many advantages, the main one being that in one pass the operator could apply 60-80 microns of powder. This was the equivalent of spraying 3 coats of paint onto the part in about the same time it would have taken to apply just one coat of wet paint the old fashioned way.
There were no volatiles, solvents or thinners in the powder paints that needed drying off.
Working conditions in metal finishing workshops improved dramatically with less fumes and a much safer & cleaner working conditions for the operators.
After coating the parts were placed in an oven and heated up to 180 degrees centigrade and ‘cured’ at that temperature for 10-12 minutes. The powder coated surface would firstly melt and then flow into a smooth coating at around 130 degrees centigrade and then the resins in the powder mixture would cross link and harden after curing for 10-12 minutes at 180-200 centigrade.
After cooling the parts were ready for assembly or packing.
From the 1970’s onwards Powder Coating transformed the way in which metal components have a paint type/coloured finish applied to them.
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.
Failure to Prepare is to prepare to Fail.
In order to get the best out of Powder Coating it is very important to understand that the condition of the surface that Powder Coating is applied to is very important.
Like any surface finishing the quality of the finished article is determined by the preparation of the underlying surface.
It is very easy to fall into the trap of applying a coat of powder and ‘covering up surface defects’. This will hide the defects and problems and they may not become apparent for many months or years.
Before addressing Preparation the first and most important issue is the surface or substrate that you are going to Powder Coat.
Usually it will be Steel or Aluminium, Castings can be made from Aluminium or Zinc. & Stainless Steel is sometimes coated for cosmetic purposes as well as Brass & Copper parts.
Steel comes in 3 basic forms;
Hot Rolled Steel;
the steel is rolled to shape and size in its semi molten form. This process leaves a scale on the surface, sometimes referred to as mill scale. It is made up of iron oxides and is about 0.1 mm thick. Although it protects the steel for a while in its raw state it breaks off quite easily along with any coating that is applied to it. Hot Rolled parts should always be pickled or shot blasted before applying a powder coat finish. Some steel mills supply hot rolled material that is Pickled & Oiled after being produced, this removes the scale with an acidic wash and after rinsing it is dipped in light oil. If the hot rolled steel is not pickled and oiled it should always be shot blasted prior to Powder Coating. Otherwise the adhesion of the applied powder coating film to the substrate will only be as good as the adhesion between the iron oxide mill scale and the surface of the steel. Which can result in premature corrosion of the part and costly remedial work or possible failure of the product.
Cold Rolled or Cold Reduced Steel;
In this case Hot Rolled Steel is subjected to further processing that improves the dimensional tolerances and strength of the product.
The main issue that relates to Powder Coating & finishing of this type of steel is that the surface of Cold Reduced Steel is smooth and scale free meaning that it can be powder coated without having to remove any scale or surface corrosion.
This is cold reduced sheet steel that has had a very thin coat of zinc applied to the surface. If the product being coated requires long term corrosion resistance then Zintec is the ideal substrate. Although the product costs more than basic cold rolled sheet it can be Powder Coated without any form of pre-treatment thus saving time and money processing the product. Also for products that have unseen areas such as the inside of parts or a reverse side that fits to a wall or to another part then savings can be made because these areas do not need to be coated because the zinc coating will provide basic corrosion resistance, resulting in less powder being used and faster coating times. These savings can in many cases outweigh the extra cost of the Zintec material.
Basic facts; Hot Rolled Steel, usually cheaper but not suitable for powder coating without pickling and/or shot blasting. Cold Rolled steel, dearer than hot rolled but a much better surface for powder coating. Zintec, the best material to powder coat as the thin layer of zinc on the surface provides very good corrosion resistance and it can also provide cost savings due to some areas not requiring coating.
Aluminium is used in a wide variety of industries either in sheet metal, tubular form or other extruded shapes and is usually either anodised or powder coated to protect the surface from corrosion.
Aluminium has a layer of oxide that forms on the surface which can affect the adhesion of the Powder Coated film to the surface this is usually removed by chemical means.
Aluminium is cast in three ways; Sand casting where molten Aluminium is poured into a sand mould to produce a desired shape. Gravity die casting, here again molten Aluminium is poured into a metal mould or die to produce the shape and finally Pressure Die casting where Aluminium is forced under pressure into a metal die. The main issue that can affect Powder Coating is porosity within the metal. When the molten metal is poured (or forced in the case of pressure die-casting) into the die it displaces air that is within the die, this air can get caught up in the general turbulence of the process and become trapped within the resultant die cast part. When the part is powder coated this trapped air can expand and force its way out of the surface of the part causing porosity blisters that can be unsightly and result in parts being rejected. Another common fault is that is caused by less than optimal design of the die. When the molten metal enters the die it is important that it metal flows into the die quickly and spreads into all of the areas of the die before cooling. If cooling of the molten metal happens to early it produces ‘cold spots’ that invariably contain a lot of porosity that cause surface defects when powder coated. There will be further details about this in the Trouble Shooting section later.
Is also used to make Die Castings and many of the faults/issues noted above for Aluminium Castings are the same for Zinc Castings.
As long as the surface is free from grease, oil or waxes then Stainless Steel can be Powder Coated successfully. It is important to ensure that the parts have the correct curing after coating (i.e. given enough heat/time in the oven). There are many manufactured parts that are normally produced from Stainless Steel and sold in self colour stainless steel, but at some point customers sometimes require the parts coating in a particular colour. An example of this is Flues for boiler or stove chimneys, usually made out of stainless steel but often there is a requirement for them to be coated in Black or other colour. Powder Coatings Ltd have successfully powder coated Stainless Steel Flue Components without any adhesion issues.
Brass & Copper.
No problems with powder coating these metals. The main issue again is ensuring the surface is clean and free from Oil, Grease & Waxes. We have successfully coated parts that have a small brass casting attached to a copper disc without any issues for over 20 years. One minor issue can be that pastel shades can show a ‘bleed’ of the copper surface, i.e. the white or pastel shade shows a discolouring when the copper is coated. This does not happen with all powders and can vary with different alloys of copper so sometimes a certain amount of trial & error has to be undertaken to find a powder that does not show the discolouration.
Basic facts; all metals can be Powder Coated as long as care and attention is paid to the preparation of the parts
Keep it Clean please.
Now that we have dealt with the most popular metals and the various forms of those metals we can move on to the preparation of the surface or substrate of the parts.
The most important first stage to preparing metal prior to powder coating is called Degreasing.
This involves removing, grease, oils, waxes and other surface contaminants that may be on the surface.
This can be carried out in three ways;
This process involves a tank containing in its lower half a reservoir with a quantity of a chlorinated hydrocarbon solution (non-flammable) which is heated usually by gas but the best way is electric to its boiling point of around 80 degrees centigrade. When boiling a hot vapour is produced that slowly rises up the tank. The parts that need degreasing are placed into the tank. The rising hot vapour condenses on the cold work pieces and in the process causes a washing/cleaning action to take place. The washing action ceases when the vapour has heated the parts up to the same temperature as the vapour. This is by far the best way to degrease metal parts. The hot vapour is very efficient at removing oils, greases and waxes from the surface of the metal parts. Because the solution is non aqueous there are no water based residues that can become trapped in welded joints and cause future corrosion. Also as the parts are heated during degreasing they do not need any sort of drying process. Powder Coatings Ltd have been using Vapour Degreasing to remove surface contaminants from metal parts for over 40 years and find that this method is far and away the most efficient way to prepare metal parts for powder coating.
Aqueous Wash Cleaning
Another method used to degrease metal parts prior to Powder Coating them is an Aqueous Wash. This type of machine is in reality similar to a large dishwasher. The parts that need degreasing are loaded into the machine. In the base of the machine is a reservoir containing hot water and usually some sort of strong detergent. The machine is closed and turned on. The parts are then washed by a series of jets that spray the hot detergent solution over the parts for a number of minutes. They are then rinsed and dried and the cycle finishes.
This type of cleaning process has certain drawbacks; firstly the parts inside have to be stacked so that they drain correctly to ensure that there are no areas where liquids can be trapped. Secondly if there are joins in the parts where they have been welded then water can become trapped and the drying cycle may not fully remove them. This is not so important with non ferrous metals as over time the joints will dry out and pockets of moisture can be dried with pieces of rag but with steel parts unless the residues are dried immediately the entrapped liquids very soon turn rusty which defeats the whole object of coating them in the first place.
Immersion Wash Cleaning.
This method involves a series of tanks containing a hot cleaning solution and a hot rinse and a cold rinse. Sometimes a heated tank at the end with lids that acts as a type of drying oven.
The parts that need cleaning are usually loaded into baskets and by means of a hoist are loaded into the tanks, usually in the heated cleaning solution first for a number of minutes and then the cold rinse followed by the hot rinse and finally the drying oven. This process can suffer the same drawbacks as Aqueous Wash Cleaning in the case of ferrous parts due to trapping of liquids in recesses of the parts that are not fully dried.
This method is still used in situations where the parts to be coated are too big for the cleaning plant that is available. It involves a solvent, usually thinners applied to a clean rag. The part that requires degreasing is wiped with the wet/damp solvent rag. After this a dry rag is used to remove the damp solvent/dissolved grease or oil from the surface. This method of degreasing is not recommended as it has many drawbacks. The solvent rag needs to be changed regularly otherwise after wiping a few oily parts with the rag it is effectively just wiping dissolved oil over the surface of the parts. Similarly the dry off rag is collecting oil residues and needs to be changed regularly to avoid spreading oil/grease over the surface.
If done properly it can be effective but basic but if not it leaves a film of oily residue on the surface of the parts that affects the adhesion of the powder coated film.
There are two processes that clean metal parts and also Pre-treat them;
Another form of metal cleaning that converts a few microns of the substrate to a crystalline structure providing a barrier to corrosion under the powder coated film and also improving the bond between the substrate and the powder coating. There are two types of Phosphating; Iron Phosphating and Zinc Phosphating. Both of these are technically called Conversion Coatings, i.e. the action of the solutions used ‘converts’ a few microns of the surface of the metal to either an Iron Phosphate or to a Zinc Phosphate. In the writers view Iron Phosphate is not as good as Zinc Phosphate. Phosphating is done either by immersion into various tanks, generally a cleaning solution followed by rinses and then immersion in the Phosphating Solution followed by a by two cold rinses a hot rinse and sometime Demineralised Water Hot Rinse to ensure that all salts and chemicals have been removed from the surface. If not done by the Immersion Method then the Phosphating is done by the Online method. In this way the parts are hung onto a conveyor that moves through a long covered enclosure containing tanks with the various solutions and nozzles that spray the moving metal parts with the rinses and phosphating solutions and finally moving into an oven that dries the parts.
This is a similar process to Phosphating that converts a few microns of the metal surface into a chromate coating. It is mainly used on Aluminium and Zinc Diecastings and can be carried out using the Immersion Process or Online Spray Process.
Phosphating & Chromating not only clean the metal parts but they also convert the surface into a crystalline structure that improves the corrosion resistance of the coated parts and the bond of the powder coated film to the substrate.
However as the solutions used are basically aqueous in nature they can suffer from the same downside as the other water based cleaning methods; firstly the parts have to be placed or hung in a way that does not cause liquids to be trapped in recesses and cavities of the parts and secondly the parts must be thoroughly dried at the end of the process or rust can begin almost immediately.
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.
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 sortof bell or alarm will sound when teh 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 faults;
Inclusions; Small particles of dust or residues that are visible on the surface of the part. This will be caused by poor preparation of the part prior to coating. Components should be wiped with a clean lint free rag and then just before they are coated blown off with a stream of compressed air. N.B. sometimes around welded areas there is welding splatter that can be mistaken for inclusions. The way to determine this is to scratch them with a screwdriver and see if they dislodge, if they do then they are inclusions, if they will not then it is welding splatter and feedback should be provided to the Welders. There are various solutions available that can be sprayed onto an area to be welded that lessen the effect of welding splatter.
Uneven Coating; This is a Sprayer and/or Gun set up problem. The process of applying an even coat all over a part depends very much on the concentration of the sprayer applying the powder. It is important firstly to have the gun set up correctly so it is discharging powder that is sufficiently dispersed that it can be applied evenly, if too much powder is arriving at the tip of the gun and there is not enough purging air to disperse it into an even cloud then you will get what are called ‘surges’. These result in a build up of powder in one particular area where the gun has effectively ‘spat’ an uneven unbalanced amount of powder out of the tip of the gun tip causing an unsightly build up of powder in one area. This also results in more powder being used and affects the profit/costs of a job.
Also where intricate parts are being coated the sprayer must move the gun in a continuous motion ensuring that the gun does not dwell in a particular area for any reason. If it dwells it is applying a greater amount of powder to an area that will result in an ‘Orange Peel Effect’ which can look unsightly especially when compared to other areas that have a smooth even coat applied.
Areas not coated; when spraying intricate parts it is easy to miss particular area, this is not so apparent when spraying the part in the booth as the area will have some powder applied but not enough to fully flow and cover the component. This again is a sprayer issue, the key word here is ‘repeatability’, the Sprayer should learn the movements and timings needed to spray a part evenly all over and concentrate so that each part is sprayed by repeating the actions of the one before, the same movements, the same amount of powder applied to all of the areas necessary.
Masking not applied accurately; This occurs when the masking is not to the correct specification. It pays in these cases to have a drawing of the component with the areas where powder is not required highlighted so that the person in charge of masking knows exactly where the masking is to be applied and can refer to the drawing if in any doubt.
Under curing of parts; It is not easy to spot under curing and it can easily be missed. The first sign is that the coating looks very glossy. If powder coating is cured correctly it may be glossy but will have a slight ‘bloom’ to it so that it is not un-naturally glossy. Also look for parts of the component that are made of a heavier material, are they more glossy than other thinner parts. A rather crude way is to take a component and hit it sharply with a hammer, perhaps in an area of the part that is not important, if the powder coated surface chips or come away from the substrate then you know that it is under-cured.