Technology Interchange— If You Can’t Measure It

This edition of Technology Interchange is all about calculations. In our daily life as powder coatingprofessionals, it is essential to quantify everything from quality to cost to color to size and beyond. A wise person once said (probably more than once), “if you can’t measure it, you don’t know it.” Let’s get started.

Physical Properties

Let’s start with film thickness. It’s a pretty simple measurement that is quoted either in Imperial or metric terms. The most common Imperial convention is using mils, which are thousandths of an inch. So 2.0 mils is 0.002”. Nearly every country other than the United States uses the metric system for film thickness. For powder coatings, this is expressed as microns, or 10^-6 meters. 25.4 microns equal one mil. Table 1 provides a comparison of the two systems.

The specific gravity (S.G.) of a powder coating is very important because it influences the coverage of a given weight of powder. Also known as density, S.G. is the relationship between the weight and absolute volume of a material. For ease of definition, S.G. is described as grams per cubic centimeter (aka milliliter). From a formulator’s perspective, we take the formula by weight of a powder and convert each ingredient into volume by using each constituent S.G. The sum of the weight is divided by the sum of volume to determine the S.G. Table 2 provides an example of a generic formula.

The S.G. of a powder coating is used for the critical calculation of theoretical coverage of a given powder coating.

The equation used to calculate theoretical square feet coverage per pound is:

((192.4 ÷ Powder Coating S.G.) ÷ mils) x Transfer Efficiency = Square feet per pound

Table 3 demonstrates how powders with differing S.G.s yield significantly different coverage. The old question, “how far will a pound go” is captured in this calculation and should influence all product selection decisions.

Particle size is another aspect of the physical nature of powder coatings. Nearly all powder coating manufacturers use laser diffraction particle size analysis, which delineates particle size from less than 1.0 micron to upwards of 1000 microns. From a practical standpoint, typical industrial grade powders should range in size from 1.0 to around 100 microns. Particle size distribution is actually more important than merely “particle size,” which is typically quoted as the median or D50 particle size. Average particle size is also quoted; however, particle median is arguably a better measure. The other data I find helpful is percentage below 10 microns and percentage above 100 microns. Too much below 10 microns (> 7%) causes fluidization and electrostatic problems, while too much above 100 microns causes undue orange peel and texture.

Transfer Efficiency

Transfer efficiency is basically calculating the amount of powderdeposited on a part versus what left the spray gun. It’s a really good measure of how well your guns are operating and how good a powder transfers, charges and is attracted to a part. It can be determined by measuring gun output at given conditions (feed air pressure, voltage and current). Determining it can be accomplished by fixing a vacuum cleaner bag to the business end of the spray gun and triggering it for 15 to 30 seconds. Weigh the net contents of the bag and you can approximate the rate of powder delivery per minute. To determine deposition, weigh a part before and after it is coated. Then divide the amount deposited by the amount of powder estimated to exit the gun.

The equation used to calculate system efficiency is:

Powder (grams) Deposited on Part ÷ Powder Exiting Spray Gun (grams) = % Transfer Efficiency
Powder Output Rate (grams per minute) x time (fraction of minute) = Powder Exiting Gun

First pass transfer efficiency describes the percentage of virgin powder deposited on a part. If overspray is reclaimed and reintroduced into the feed hopper, then the more generic term “transfer efficiency” applies. Transfer efficiency tends to drop somewhat as more reclaim is added because the “best particles” (>10 microns, <100 microns) transfer at the highest rate, whereas the concentration of the less transfer efficient particles (<10 microns, >100 microns) increases as reclaim is introduced.

System efficiency is probably more important than transfer efficiency. This measurement captures the ratio of powder used versus powder deposited on the part. It takes into account losses encountered anywhere within the finishing system. Things like coating on hangers and racks, powder hung up in reclaim filters and contaminated powder that was disposed are measured. Calculating system efficiency takes a lot of time and data. It’s best to track the amount of powder coating used, the approximate amount of powder applied to parts and the number of parts produced.

The equation used to calculate system efficiency is:

Amount of Powder Applied to Parts ÷ Amount of Powder Introduced into System = % System Efficiency

Some shops track the amount of powder purchased per month less what is left in inventory and compare it to number of parts or units produced for the same time period to gauge system efficiency.

Cost

Finishing costs affect a business’ bottom line. We’ll take you through an analysis of some of the factors that influence cost.

Formula Raw Material Cost

It is a formulator’s responsibility to determine and understand the raw material cost (RMC) of a formulation or product. Calculating RMC takes into account not just the nominal cost of each component but should also capture the delivered cost of a raw material. Delivered cost will not only include the cost per pound (or kilogram) of a material but also any taxes and shipping charges. Some purchasing managers agree to volume-based rebates with vendors so these discounts should also be included.

Raw material cost is part of the equation, of course, with selling price. Manufacturing and packaging costs have to be included as well as fixed overhead costs and profit margin.

Cost per Coverage

As previously mentioned, coating coverage per pound (or kilogram) is an essential facet of the operation of a finishing shop. Cost per coverage is critical in understanding the final cost of fabricating a part. This calculation takes into account the cost of the coating, the efficiency of how it is applied, the S.G. of the powder and the average film thickness applied.

As you probably know, these calculations only capture the material cost. It is important to also account for labor, energy, utilities, maintenance and depreciation costs in calculating the overall cost of finishing a part.

Color

Color and color difference is important in determining if a finish meets a customer’s specification. The most common color system employs a CIELAB (Commission Internationale de l’Elcairage) color space model. The color is described using the coordinates indicated in Table 7.

Using this system, one can set a specification and also the acceptable range of variation. Ranges are defined as deltas such as Δb* or Δa*. The most common description of overall color difference takes into account all the color coordinate deltas and is referred to as ΔE: Color Difference = (ΔL2 + Δa2 + Δb2)-2 = ΔE

This is described as the “square root of the sum of the color coordinate differences.” Depending on the demands of the industry and color space, the acceptable color difference (ΔE) can be specified from 0.3 for shades of white to as high as 2.5 for saturated colors such as deep reds or blues.

These are but a few of the most common calculations used in powder coating technology. Quality control experts and statisticians undoubtedly have a host of many more calculations that you can use to further define quality, costs and performance. Remember, “if you can’t measure it, you don’t know it.”