Metal Finishing Guide Book

2012-2013

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A: Magnesium alloys are a tricky substrate to powder coat unless you know how to do it. Most magnesium-fabricated products are cast, resulting in a certain degree of porosity on its surface. Cleaning the substrate is a great idea; however, the cleaners/pretreatment can remain harbored in the pores. Indeed, even without cleaning, air resides in the pores. As the powder melts and flows, the cleaners and air escape from the pores. Most powders are curing at this point and can���t recover or reseal the holes caused by the volatiles. The result is pinholes, low gloss, and unsightly surface disruptions. My advice is to continue cleaning as you are doing at present, but run the parts through a relatively high-temperature dry-off before you apply the powder coating. It���s preferable to coat the parts very soon after the dry-off, even while they are still warm, so they don���t re-absorb ambient moisture. As for dry-off temperature, 400��F for 10 minutes is a good place to start. You should also be aware that many powder suppliers offer product lines that are better suited for porous substrates such as magnesium. It may be best to use one of these with a well-controlled dry-off process. MEASURING POWDER COATING DENSITY Q: Is there any method, equation, or software program to calculate powder density? A: I am aware of two methods used to determine powder density. Both are covered in detail in ASTM D5965���02(2007) Standard Test Methods for Specific Gravity of Coating Powders. One uses the volume displacement of the powder into a fluid (kerosene or hexane) with a known density. The weight of the powder is known, so the relationship between weight and volume can then be calculated. This method involves introducing the fluid into a graduated cylinder. The volume and weight of the fluid is recorded. Next, a given weight of powder is mixed into the fluid and the displaced volume is determined. It is essential that you eliminate all air pockets in the mixture to obtain a reasonably accurate measurement. Please be aware that this method doesn���t easily account for the surface porosity common with most powder coatings and typically results in a lower-than-true specific gravity. Nonetheless, it can be used as a decent tool to compare powders. A much more accurate method, based on the Ideal Gas Law, utilizes a gas pyncometer instrument that measures volume of a known weight of powder by gas displacement. These are relatively expensive instruments and are availPowder-Specific Gravity able from a number of commercial = instrument suppliers. Each instruWeight of Powder (g) ment is slightly different���some Final Volume ��� Original Volume (ml) measure volume; others can measure volume and density. You would have to consult the specific procedure provided by the instrument manufacturer to successfully measure specific gravity of powders. I recommend you use the simpler fluid method, but always run a control sample of know specific gravity along with your samples to be evaluated. 618

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