Powder Coating Questions and Answers

Here is a synopsis of the top powder coating questions and answers we have been asked over the past 15 years about the industry

Powder coating is increasingly accepted as the preferred finishing process for many applications. Increasingly stringent environmental regulations, rising costs in all areas, and demands by consumers for better quality and more durable products are among the challenges facing today’s finishers. Powder coatings provide a solution to these challenges and others. Powder coating is the technique of applying dry paint to the component.

The powdered paint is normally applied by using a powder feed system and gun to electrostatically charge and spray the powder onto the part. For some applications, the part being coated is dipped into a fluidized bed of powder. The coated part is then heated in an oven, or via infrared panels, to melt and cure the paint. During the curing process, a chemical cross-linking reaction is triggered and it is this chemical reaction that gives the powder coatings many of their desirable properties.

We know that powder coating can seem intimidating and overwhelming for the new powder coater. For you, we have compiled this section with the top questions asked by new powder coaters as well as information for setting up your new powder coating system.

Powder coating questions and answers: Why should I use powder coating?

The use of powder coatings vs. liquid solvent-based coatings results in significantly fewer emissions of volatile organic compounds (VOCs) and hazardous air pollutants (HAPs). They can also reduce energy consumption, can be more cost-effective, and provide better properties.

How long have powder coatings been around?

Powder coatings have been applied in the US since the mid-1950s when they were being used to coat pipes for corrosion protection and electric motor parts for insulation. These powders were applied using a fluidized bed into which a heated part was dipped. These coatings were functional coatings and did have the greatest appearance.

What is the difference between Thermoplastic and Thermoset powder coatings?

Thermoplastic powder coatings will melt and flow when heated but do not change chemically. They will be remelt when reheated. Thermoset powder coatings will melt and flow when heated and then chemically ‘crosslink’ or react. These will not be remelted when reheated and tend to have better physical properties than Thermoplastics. Most powder coatings are Thermoset.

Powder coating questions and answers: What Powder Coating Chemistry should I buy?

It all depends on your application. Most chemistries can be custom formulated to meet your specific needs. For most applications, the best bet is a TGIC Polyester, these can be used on parts that are outside and have good physical properties. For interior applications, a Hybrid powder coating is cost-effective and has good properties.

An Epoxy powder coating has great properties and excellent corrosion and chemical resistance but is also only for interior applications. A Urethane Polyester has good flow and physical properties, is for exterior applications but can be expensive, and has a higher oven temp requirement than TGIC coatings. When in doubt, go with a TGIC. Remember, never use Epoxy or Hybrid for outdoor applications. They will fade.

What about Porcelain enamel Powder coating?

Although Porcelain is applied with similar equipment as standard powder coatings, it is very different. Porcelain powder is called Frit and is much more expensive a process that requires an oven temperature of 1,200 F to fuse the Frit (powdered glass) to the metal surface. Sinks, bathtubs, and some appliance parts are coated with Frit.

Most coatings need some degree of pretreatment in order to achieve acceptable coating properties and appearance. With powder coatings the pretreatment is critical. Pretreatment refers to cleaning parts and leaving a ‘conversion coating’ for the paint to adhere to. At a minimum, parts need to be clean and dry with no residual oils or chemicals on the parts.

Depending on the final use of the part additional processing may be required before powdercoating. Interior applications, noncorrosive environments ( chairs, furniture, decorative finishing): For these applications, you may be able to get by simply degreasing the parts. The finish may remain for a number of years, or come off in sheets after a few months. Interior applications, corrosive or heavy use environments (washers, auto under hood, equipment) You must at a minimum, degrease and apply an iron phosphate pretreatment.

A one-step cleaner/phosphatizer may work for basic applications. Exterior applications or highly corrosive environments. Do not even consider simply degreasing the part. The coating will fail, sooner than later. A minimum of a three-stage pretreatment is required. Your customers will eventually be very disappointed if you do not do this

How do I spray Powder? What makes it stick?

Powder coatings are generally applied via an electrostatic spray gun. Compressed air moves the powder coating through the feed tube from the box, drum, or hopper to the gun. It is possible to move the powder with air because the air ‘fluidizes’ the powder making it act like a liquid. The gun then applies a charge to the powder particle by the use of a high-voltage, low amperage electrostatic field which it generates between the gun electrode and the grounded part.

This charge is usually negative. There are guns available that do not use an electrode to generate the charge, they use a special insulator/conductor inside the gun which generates a charge through the use of friction. This is a Tribo gun and it generates a positive charge.

Most guns used are Corona guns. The powder sticks to the part the same way a magnet sticks to metal or lint to a static-charged shirt. This charge will diminish or decay over a few hours. Most parts enter an oven as soon as they are powder coated. In the oven, the powder melts, flows, and crosslinks.

Powder coating questions and answers: Why do I need a spray booth? How is the powder reused?

The main function of the spray booth is to safely contain the powder coating so that the overspray generated does not go into other areas of the plant. You must ensure that the airflow through the booth is sufficient to contain all the overspray into the recovery chamber without disrupting the powder depositing on the part. A general rule of thumb is to maintain a face velocity of 100 ft/min across all openings.

Since there is no solvent in the coating, the exhaust air can be safely circulated back into the plant. The powder can be reused by simply blending the overspray with virgin, or new, powder. The reclaimed powder is of value only if it is free of contamination from other coatings. Color changes in a reclaim-type booth can be simplified by the use of separate recovery chambers.

What about curing ovens?

The main types of ovens used for baking, or curing, powder-coated parts are Convection, infrared, or a combination of the two. Convection ovens are gas or electric, which heat air and circulate the heated air in the oven. Infrared ovens are also gas or electric and emit radiation energy in the IR wavelength band. This energy is absorbed and causes the powder to cure.

Powder coating questions and answers: What is Transfer Efficiency?

Transfer efficiency is defined as the ratio of powder applied to the parts to the total amount of powder sprayed. Reclaimed, or recovered powder is not considered in this calculation.

Factors that affect transfer efficiency include the operator skill, shape of the part, booth design, powder quality, spray techniques, grounding of the parts, and condition of the equipment. The high transfer efficiency rates and ability to reclaim overspray contribute to the large economic advantage powder holds over liquid

Can I recoat parts? How?

Powder-coated parts are routinely recoated for a variety of reasons. To recoat a part with the same product it was coated with you usually can simply rehang the part, turn the gun voltage down, respray it and cure as needed. Certain chemistry powders cannot be recoated, such as high chemical resistant epoxy powders. These products need to be lightly sanded to obtain the proper level of adhesion.

Powder coating questions and answers: Can I strip Powder coatings? How?

Powder coatings can be stripped with Mechanical methods, Thermal/Bake methods, Chemical methods. The method used will depend on the speed, degree of stripping, effect on the parts, and environmental concerns. Mechanical methods included sanding or blast stripping the powder coating with abrasives or fluidized aluminum oxide (sand).

Thermal methods may adversely affect the hardness of the parts stripped due to the operating temperatures required. This is however often used for noncritical parts. Most of these systems are batch-type systems in which the oven is ramped up to 750 F and the organics in the coatings are volatilized.

These ‘VOCs’ are usually destroyed by an afterburner in the exhaust. Chemical methods included hot or cold chemical solutions of aqueous caustic or non-caustic alkalis. Most solvent systems such as Methylene Chloride are no longer viable due to environmental and health concerns.

Care of Powder Coated Parts

Properly cured powder coatings are resistant to most chemicals, however, DO NOT allow MEK or Trichloroethane to come in contact with the powder as it will attack the surface. You can wax powder-coated pieces if you want to but it’s not necessary.

Whenever possible, use washers beneath both nuts and bolt heads. Excess powder can be removed with either an oiled tap/die for threads or with a die grinder affixed with an abrasive wheel or cone. Unless using a high heat powder, do not expose cured powder coatings to temperatures over 400 degrees for an extended period of time. Use care in packaging powder-coated parts, they are durable but do not like cardboard packing.

Powder coating questions and answers: How do I change over from one powder to another without contamination?

The first issue is to consider the compatibility of the two powder coatings. A compatibility test of the two powders can determine if problems will result when the two powders are mixed. Generally your powder coating supplier can run this test for you. The test consists of blending the two powders in ratios of 90:10, 50:50 and 10:90, then spraying a test panel with each blend. If the two powders are compatible (compatible is when minimal gloss change and no craters occur) the change over can be rather simple:

• Run the existing powder as low as possible. Blow out hoses, guns, transfer and powder pumps and your sieve. Squeegee down the booth and dispose of the powder. You should also consider:
• For cyclone systems, air purge for 15 minutes. Blow off and wipe down the bottom 2 feet of the cyclone interior. Remove and vacuum clean the collection canister.
• For cartridge filters, which hold up to 35 pounds of powder in each filter, it could take several hours or even days until the majority of the powder is back pulsed through the filters. Back pulsing will not completely remove the prior powder from the cartridge filter. This could leave some doubt if any contamination quality issues arise. Consider replacing, and asking your new powder supplier to assist with the cost of, these filters. If the products are not compatible, all electrostatic equipment (including the booth) will need to be cleaned and some items replaced. The proper steps to follow are:
• Run the existing powder as low as possible in the coating booth.
• All powder should be removed from all electrostatic spray equipment such as transfer pumps, hoses, powder pumps, sieves, feed hoppers and spray guns.
• Cyclone ductwork should be cleaned with compressed air and where possible wiped down. If necessary, plastic shot blast media can be run through the ductwork to ensure all traces of powder are removed.
• Powder in the booth should be squeegeed into the collector module and then wiped down with a diluted alcohol (IPA) solution. Interior walls of the feed hoppers and the sieve should be wiped down with this solution. Do not get any of this or other solvent solutions on the fluid bed membrane.
• Replace equipment on the coating line such as all hoses, venturi tubes and the sieve screen.
• The bulk feeding apparatus should be completely cleaned.
• Cleaning the collector module (reclaim) involves the removal of cartridge filters, vacuuming out all powder, wiping down the walls with a diluted alcohol solution. Once the system is completely cleaned and the new cartridge filters are installed, they will need to be seasoned. A properly seasoned cartridge filter will prolong its life expectancy and help maximize the coating booth’s operating efficiency. Seasoning of the cartridge filters takes anywhere from 2-8 hours.
• Seasoning instructions are provided by your booth manufacturer. They generally consist of operating your system without electrostatics turned on and is complete when predetermined booth face air velocities are reached at specific air settings.
• Your system is now ready to go!

What maintenance does a powder system need?

We have put together a complete maintenance checklist for your use. This is available at the end of our manual.

Powder coating questions and answers: Tell me about grounding and powder coating

The inadequate ground is the most frequent problem encountered in powder coating systems. A sufficient ground path must be provided for all equipment associated with powder applications for safety and efficiency reasons

Safety Reasons: Grounding is necessary to carry the charge away from the application equipment to prevent static discharge or sparks which provide an ignition source and may lead to a dust explosion.

This is one of the most serious safety hazards associated with powder coating. All automatic powder systems must be equipped with spark detection equipment to shut down the spray guns in the event of a discharge. The grounding of the operators helps to prevent shocks as well as sparks. Conductive shoe soles and grounding straps can help prevent shocks from static build-up in operators.

Efficiency reasons for grounding. Good ground is required to carry away charge which is delivered to the parts by the powder or through ionized air. This allows more powder to be deposited and better penetration into Faraday areas. The main reason is that good ground improves transfer efficiency.

As powder builds up on the surface of the part, charge separation begins to occur in the part. That is, in the case of a negatively charged powder, positive ions in the part flow toward the powder coated surface and electrons carrying a negative charge flow to ground. In order to continue powder attraction to the part the ground circuit must stay intact. Thus ground maximizes the powder charge holding capacity of the part.

What causes poor ground? Most ground problems are the result of poor design or inadequate maintenance. For some powder booths, there may not be enough air the movement to capture the over-sprayed powder. This powder may drift out of the booth and settle on the conveyor or load parts. The powder is a very effective electrical insulator and it will interfere with the ground if deposited at contact points.

This is especially true if coated hooks or hangers go through the oven and the powder is cured in place. An important design feature involves hook/hanger configuration. S-shaped hooks may provide only a single contact point and allow the part to change position. V-shaped hooks, for example, can offer at least two contact points and hold the part more firmly in place to prevent undercoating with powder. When possible, the hooks/hangers should be designed to shield the contact point from the coating.

Hooks made from square stock should be turned on an angle to have a sharp contact point. It is also important to maintain metal-to-metal contact at all points. This is usually done by cleaning the hooks/hangers, via burn-off, chemical stripping or blasting. For the following burn-off, it is also important to insure that any residue or ash is removed. This ash may also be an electric insulator and as such it will interfere with the ground.

To prevent powder build-up on hooks/hangers ensure they are fully loaded to prevent empty contact points from being coated. Maintenance of proper gun placement and aiming as well as booth capture air flows will also prevent a build-up of powder on hooks/hangers. Cleaning excess powder off of the conveyer is also a good practice. Ensure that your gun control panels and booths are properly grounded using ground cables instead of depending on connections to the control cabinet.

Powder coating questions and answers: How can I test for sufficient ground?

Testing for sufficient ground is preferably done with a Megohm meter or megger. It is important to follow the entire ground path. For parts, it is best to start with the conveyer and check every link in the chain to the part (from the conveyor to load bar, load bar to the rack, rack to hook, hook to part). This will tell you if and where any problems may exist.

The NFPA Bulletin 33 states that less than one megohm is needed to minimize the chance of sparks, for efficient coating resistance of 0.5 megohms or less is best While testing, if a wire lead is attached to the ground and directly to a clean part, the effect of good ground can actually be seen by coating the part with this connection in place. If the noted improvement in the deposition/thickness or penetration is dramatic then the benefits of good ground become more clear.

If the improvement is slight, but resistance readings are one megohm or higher, chances are still good that better ground will improve harder-to-measure characteristics such as transfer efficiency and consistency.

Obtaining good ground:

A good ground starts with the earth connection. Using water pipes may provide you with reasonable success. The optimum ground, however, is achieved by driving a copper rod into the soil to a depth of 8 to 10 feet. This should then be connected to the conveyer at multiple points for insured good contact. Minimizing connections between the conveyer and the part can be very helpful in optimizing ground.

A solid rod has fewer contact points to worry about than a chain of multiple connections. The addition of a well-grounded rub bar or brush above the booth helps to insure contact with fixtures in the event that the conveyer is not providing adequate ground due to a build-up of lubricants and contaminants. Modify hook designs to encourage contacts and frequently clean hooks

Can I use powder as a primer for liquid coatings? How?

Powder coatings are frequently used as basecoats or primers for liquids. The reasons for this can be:

• To provide improved corrosion protection from the powder coating
• To provide improved edge coverage from the powder coating
• To provide appearance or special effects available only in liquid topcoats. Most powders work well as a primer coat, however, some additives, such as gloss control agents, will interfere with topcoat adhesion. Some may float to the surface of the coating or co-react to give a film that is very solvent resistant. Epoxies may be the best for corrosion protection and are the most popular base coat. They are not good for clear coating though, since UV light can be transmitted through the topcoat and cause the epoxy to chalk. The composition of the liquid topcoat is also critical. Most solvent-based liquids offer the best results because they “bite” into the powder. Lacquers
  and enamels work well due to their high solvent content. High solids and two-part systems can also be used, but compatibility becomes even more important since the solvent content is decreased.
  It is important to always test the appearance and adhesion of a liquid over a powder coating. Test intercoat adhesion using the crosshatch method (see our quality section).
  Some issues you may encounter include
• Lack of intercoat adhesion: Change to a liquid with a more aggressive
  solvent composition or solvent wipe the base before coating. You can also scuff sand the base coat.
• Color bleed through Incomplete cure of basecoat, try to cure the basecoat for a longer time.
• Lack of adhesion to part: Poor cleaning or the base coat is underscored.

Can I touch up powder coatings?

It is sometimes needed to touch up areas left by hangers or hooks or to repair damage resulting from abuse of the coating during handling or assembly. Most room temperature cured epoxy coatings can be successfully used to touch up the powder with excellent results. These epoxy coatings generally consist of two parts: A base and a hardener which you mix together before using. You can spray or paint this on.

Powder coating questions and answers: Where do I begin? What do I need to powder coat?

To powder coat parts you will need the following:

• A powder coating gun. The inexpensive $80 to $150 dollar guns are worthless for anyone who wants to go into this for business. For hobby work they may suffice – for a while. A good powder coating gun will cost you around $3,000.
• A powder coating booth. Basically, this is an enclosure with a vacuum to collect the oversprayed powder. I have seen old liquid waterfall booths used for this as well as plywood enclosures. I have also seen plants burn down using plywood enclosures with ordinary vacuum cleaners collect the overspray because sparks from the vacuum motor ignited the powder cloud.
• A curing oven. For a hobby powder coater, an ordinary oven will probably work for you as long as the parts fit. Do not plan on cooking food in the oven once you cure the powder inside of it. Powder cure does cause some nasties to come off and these will collect in that oven.
  For anything other than hobby powder coating you will need a real industrial oven capable of holding around 380 degrees F. They can be a basic ‘batch’ oven which is basically a very large oven in which you can wheel a rack of parts, a large panel constructed oven with a conveyor passing through it or an elaborate Infra Red Oven. Your parts and production goals will dictate what you need. With these three items, powder gun, powder booth, and cure oven you can powder coat parts successfully provided they are completely clean. For the
  hobby coater, it will suffice to clean parts such as valve covers, fishing lures or knick knacks in a pail with a solvent such as MEK or Acetone. For a professional powder coater, you will need, at a minimum a system to clean and pretreat your parts. If you want to powder coat for business, it is imperative you understand pretreatment and sell this feature to your customers. Without good pretreatment, powder coating is worthless! I cannot stress this enough and trust me, valuable customers know this.

Powder coating questions and answers: Powder Application Rooms

The purpose of application rooms, or environmental rooms, are multi-purpose rooms designed to isolate the application process from the harsh, dirty plant environment as well as to provide consistent temperature and humidity control to provide the optimum application efficiency of the process. In addition to Federal and local fire codes (which typically include a 1-2 hour fire rating) incorporate the following ideas into your design:

• The walls, ceilings and surfaces of the application room should be made of smooth, easy to clean materials which should be sealed to ensure they do not introduce airborne contaminants into the room. Do not use materials such as unpainted sheetrock, unsealed wood paneling, or plasterboard which will shed fibers. Typical construction methods include using common lumber yard materials, masonry materials such as concrete blocks or purchasing a prefabricated room from a supplier who specializes in such designs. These prefab rooms are generally insulated, powder-coated steel panels with tongue and groove construction.
  • The room should be large enough to allow workers to clean, maintain and operate the system with ease. Keep in mind how much room is needed to roll booths offline for cleaning and maintenance if setting up a multi-booth system. It is also important to have powder coatings condition at the temperature it will be sprayed at so size the room for a 1 to 2 day supply of powder. If using fork trucks or hand trucks, ensure you have enough room for these. Do not however use the room for general powder storage, set up another area for long-term storage.
  • Ensure that the operators can enter and exit easily while minimizing cross-through traffic. Use tacky walk-off mats to prevent dirt and contaminants from entering the room. Some operations even use air chambers that blow the operators off with ionized air. Incorporate at least one overhead door to make booth maintenance and delivery by fork truck possible.
  • Powder suppliers recommend that the application areas be maintained in the range of 65 to 75 degrees F with 40 to 60 percent humidity. These ranges ensure consistent film builds without the need for constant adjustment and reduces equipment wear by keeping moisture away from the reclaimed powder.
  • Ensure the room is positively pressurized to prevent outside
    contaminants from entering through the openings in the room for the parts and doors when open. Ensure that the air vents do not interfere with the airflow in the booth. The recirculated air should also be filtered to remove airborne contaminants as small as 2 microns and have a velocity of less than 200 FPM. These systems should have a two-stage filtration system on the return air side, with a washable prefilter and disposable after the filter.
  • Good lighting is a necessity in the application area. The lighting fixtures should be flush mounted to eliminate any dust traps for a powder that may (and it will) escape the powder booth. Remember that according to the NFPA 33 standard, any light fixture within 5 feet of a booth opening must be rated for Class II, Division 2 locations.

Presuming your system has been properly set up, which for a hobby coater is as simple as:

1. getting a box for a booth
2. plugging in your gun and oven
3. connecting the ground strap from the gun to the part
4. trigger the gun and spray the powder on the part
5. put the part in the oven for the cure time
   These same basic steps would be followed for the job shop and OEM coaters who spray millions of pounds year

Powder coating questions and answers: What can I powder coat?

The basic rule of thumb is that you can powder coat anything which you can bake in an oven. Current powder technology allows a minimum cure cycle of 15 minutes at 250F. This means you can powder coat:

• Virtually anything made of Aluminum, Brass, or Steel
• Magnesium parts – including die-cast and Thixomolded
• MDF (medium density fiberboard) (certain toilet seats are powder coated)
• Glass (for many years Polo cologne bottles were powder coated)
• Many grades of plastics. (certain Bic pens are powder coated)
  You may ask, how do I powder coat wood, glass, and plastic if the powder is applied electrostatically? The answer is easy, simply preheat the part to get it up to around 200 degrees F and spray the powder on – it will stick right to it. Then you bake it out as usual.