Q&A: Film Thickness Control

By: Rodger Talbert 22. May 2013

 

Q. We are having some trouble controlling the thickness of the powder on our parts. We have some light coat areas in recesses and edges, but we also get orange peel and heavy coating in some other areas. What can we do to control the thickness better? P.T.

 

A. Coating thickness is controlled by the use of a standard set of operating variables that affect film build. There are many variables involved but the critical controls include:
• Gun-to-target distance
• Electrostatic settings (voltage and micro-amps)
• Flow rate and powder velocity
• Number of strokes or dwell time in front of the gun

 

To develop an optimum set of controls you need to do some experimentation and verification. Try full voltage and amperage to start with a medium to low setting on flow rate, average distance (6 inches for manual guns and 12 for automatic guns) two to three passes. Use medium to slow strokes with some overlap of your pattern.

 

For complex shapes, reduce the amperage and voltage and coat difficult areas first. Observe the spray pattern to see how much velocity you have (high velocity is bad) and note how much overspray is generated. If the overspray is low and the coverage is good you are fine. Increase flow if you need more powder, decrease flow if you have a lot of overspray. Establish a standard set up and be consistent.

 

In addition to the gun settings and optimum control, the atmosphere can affect film thickness control. If the humidity is too high the powder will not fluidize or flow very well. If the humidity is too low the powder will behave differently when exposed to an electrostatic charge and potentially produce more rejects for lumps and light coating. The optimum range for humidity is 40 to 60 percent RH.

 

Control of the reclaim powder is important, too. If the powder is recycled in a cyclone recovery system, it will gradually move the particle size distribution higher due to the fact that the cyclone classifies out some of the finer particles. In a module system, the impact on reclaim is the opposite: the particle size distribution will gradually move to a finer overall mean as a result of the accumulation of finer particles. A cartridge module system does not remove any fines and history has demonstrated that 100 percent reclaim will have a finer grind size than virgin powder. To control this, make sure to consume the reclaim and do not let it accumulate. Keep the First Pass Transfer Efficiency at or above 50 percent to make sure that fines do not accumulate.

 

One final and very important control item is the earth ground. If you want film uniformity, control the ground at all times. Clean hooks are imperative to film build control. Control the gun, control the powder and control the atmosphere surrounding your application area. This will give you the best possible film build control. 

 

To read more of Rodger Talbert's answers to readers, click HERE

Powder Coating Q&A: Discoloration of Powder

By: Rodger Talbert 15. November 2012

 

Q. In order to powder coat a grey nylon furniture part, I preheat the part, powder coat it with a white coating, cure the white coating, and then apply a powder clear coat over the white and cure that. The issue I have is that when the part comes out of the oven it appears pink. I have applied the white and the clear coatings on other nylon parts from another company, but they don’t come out pink. What could be the cause of pinking in this scenario? J.S.

 

A. Nylon is a thermoplastic material that can be affected by heat. I suspect the heat of the oven is causing some breakdown of the nylon during cure and that is creating the discoloration. There is a difference between the composition of the nylon that is showing the pink color and the other parts you have sampled. See if you can find out something about the nylon composition and how the two different products compare. Also, check film thickness and make sure you are getting enough to hide the substrate.

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Powder Coating Q&A: Powder on Aluminum Extrusion

By: Rodger Talbert 12. September 2012

 

Q. We have been using liquid paint for our architectural aluminum extrusions for about 20 years, but have considered using powder coatings to reduce our VOC and save cost. We have seen some successful projects with powder, but we also hear stories about the UV resistance of powder when compared to high-performance liquid coatings and other performance issues. Can powder coatings meet AAMA 2604 and 2605 standards? What are the strengths and limitations of powder versus liquid coating? B.A.

 

A. There are two critical qualifiers that are important to the discussion. First, the surface has to have the right treatment to ensure good adhesion and corrosion resistance. For architectural aluminum, this means using chrome, chromate, or a high-quality, non-chrome conversion coating. Second, the coating must be a high-quality material with the necessary properties to meet the specification. If the surface treatment is good and the coating is a superior coating with the needed properties, liquid and powder can both be effective.

 

The AAMA standards describe treatments in some detail and recommend minimum performance levels and rigorous testing to meet the specification. These specifications include minimum coating weights for the chrome and severe adhesion testing to confirm success.

 

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Ask The Expert: Streaks in Powder Coating

By: Rodger Talbert 15. June 2012

 
Q. We apply a medium-gloss gray powder on our parts at around 3 to 4 mils thickness. Sometimes our operators leave streaks on the parts when they apply the powder, and some areas are darker than others before cure and clearly visible after cure. What is causing the streaks, and how can we fix the problem so we do not get them? J.K.
 
A. The streaks could be caused by stains associated with your pretreatment process. If this is the case, the streaks will be clearly visible on the part before application and show through the coating after cure. If they are, you need better cleaning, better rinsing and better drainage to avoid stains on the part.
 
If the part is clean, the streaks are in the film and the most likely cause is application technique. When the gun is moved closer to the part surface it will change the voltage/amperage ratio and more current will flow to the part. The high current in a localized area causes variation in appearance. The operators need to concentrate on maintaining as consistent a gun-to-target distance as possible. If the gun has to be moved in close to the part to get coverage, they need to take care to avoid excess film build and texture. Keep the flow rate down and the amperage down.

ASK THE EXPERT: Powder Coating Color Change

By: Rodger Talbert 27. April 2012

 

Q. Are color changes for powder that are under one minute really possible? I spray all manual in one booth, but I have automatic guns in the other one, and we do reclaim some of our colors, but not all of them. M.G.

A. A single powder gun can be color changed by one person in less than one minute. It requires the right equipment to be able to quickly purge the gun interior and switch over to the new color, but it is possible. You can use a box feed system and clean the gun up, change the box and then start the new color. Or, you can use a manifold with a switch system and a series of dedicated feed hoppers for even-faster color change times.

Keep in mind that the actual color change time is partly dependent on how close the parts are racked. You may be able to physically change the gun over, but you cannot discharge a color too close to a part that is not supposed to be that color. If you are cleaning the gun and next part is only a couple of feet away, you will get some of the powder on that fresh part. Or if a coated part is still leaving the booth, you could get some of the new color on the trailing edge. Realistically, you can change the gun in around 20 or 30 sec, but the spacing is most likely going to be greater due to the risk of cross contamination.

Now, if you have the clean the booth walls, or if you have automatic guns, or if you intend to recover the colors for reuse, the game changes. Automatic guns can be purged automatically using a manifold system and a color-feed center. Booth walls can be cleaned more quickly if the booth construction is designed for fast color change. In a best-case scenario, you can go from reclaim color to reclaim color in 10 to 15 minutes with automatic guns. You can go much faster if you do not reclaim. These booths use cyclone recovery systems to avoid the time involved in moving collectors for reclaim colors. The cyclone itself does not necessarily speed up color change time unless it is fitted with the right features.

It all depends on what you need in terms of reclaim and what equipment you buy. You can get some very fast times if you invest in the right equipment.




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