How to minimize pretreatment problems to maximize powder performance

Transcription

1 Pa a/ April How to minimize pretreatment problems to maximize powder performance Brad Gruss F remont Industries Never underestimate the power of a well-cleaned and pretreated substrate when it comes to successful powder coating application. The first step in the finishing process, surface preparation affects the outcome of each subsequent step, including the final one-product performance. This article s aim is to help you head off surface preparation problems early so you can get the best product perfomnce later. The article identifies soil and substrate types and explains several tests to determine substrate cleanliness. It also includes the advantages and disadvantages of recirculating spray washing, handheld spray-wand phosphatizing, and immersion cleaning. Finally, the article discusses the importance of proper rinsing and iron phosphatizing. S uccessful powder coating hinges on the proper cleaning of the metal substrate before the powder is applied. Ideally, pretreatment for powder requires a surface that is 100 percent organically clean. This means that the substrate should be free of oils, corrosion, oxidation, and scale. The substrate should also exhibit surface uniformity and have a consistent profile or lack of profile if you re considering abrasive blasting or mechanical cleaning. Furthermore, the conversion coating should be suitable for the base metal and shouldn t exhibit major variations, streaks, or a powdery film. Finally, the substrate should be moisture free before entering the powder booth. By now, you re probably saying to yourself, there s no way I can achieve this goal. Don t despair. You re not alone. In fact, very few substrates can be so thoroughly cleaned so consistently. The professionally made and pretreated test panels you buy are usually consistent, but they aren t representative of what happens in the manufacturing facility. This article is intended to give you solid background information to minimize pretreatment problems and to steer you to a more realistic goal-maximizing your powder s performance through pretreatment. Soil and substrate types Many types of soils and substrates exist. It s important, therefore, to do soil and substrate audits so you can select the appropriate cleaner and conversion coating for the parts you ll be pretreating. ~ Soils. Soils are generally shop dirt, smut, oils, metal chips, and drawing, stamping, and buffing compounds. The cleaning operation would be less difficult if the individual soils were understood more completely. Soils can be classified as organic or inorganic. Organic soils. These soils are oily, waxy fdms, such as mill oils, rust inhibitors, coolants, lubricants, and drawing compounds. Alkaline cleaners remove these soils from parts.

2 ~ 38 POWDER COATING, April 1994 Inorganic soils. These soils include rust, smut, heat scale, and inorganic particulates, abrasives, flux, and shop dust. Acidic cleaners most easily remove these soils from parts. It s ohn assumed that all soils can be easily removed. However, soils can be classified by the degree of dif iculty present in cleaning them from parts. Very difficult soils to remove. These include chlorinated lubricants, sulfiuized lubricants, heavy-duty rustinhibiting compounds, honey oils, buffing compounds, stearates, die-cast release agents, and oxidized soils. Very di.f lcult soils tend to be heat sensitive. Naphthenic, parafhic, chlorinated-paraffin blends, or soils containing waxes are generally heat sensitive. Moderately dijjkult soils to remove. These include fatty oils, waxy oils, heavy-duty hydraulic oils, mill oils, lapping compounds, and water-displacing rust inhibitors. Relatively easy soils to remove. These are soluble oilcutting fluids, synthetic cutting fluids, spindle oils, lightweight machine oils, mill oils, water-soluble oils, short-term rust inhibitors, and vanishing oils. ARer you ve completed a soil audit and chosen a suitable cleaner, you should make every effort to prevent the introduction of new soils into your finishing process without pretesting to see if your cleaner will effectively remove them. Substrates. The composition or chemistry of a part s base metal is one of the key limiting fadors in cleaner choice. The cleaner must be compatible with the metal. In multiple-metal cleaning lines, nonferrous metals are typically the limiting factor. It s important to choose a cleaner that either doesnt attack the metal or attacks the metal in a controllable way. Many chemical suppliers and manufacturers make the common mistake of conducting incomplete base-metal audits when selecting a cleaner for their customers. Most aluminum and zinc alloys, which have slightly different alloy contents, can vary widely in their ability to withstand alkaline or acidic cleaner attack. In some cases, such as those in which minute etch is desirable, slightly more or less cleaner attack is unacceptable. Substrates should be classified as follows to make cleaner choice easier. *Ferrous or iron-bearing: cold-rolled steel, hot-rolled steel, stainless steel, and ferrous castings I PROCESS OVENS BURN-OFF OVENS P YEARS OF EXPERIENCE Cure, Bake, Dry, etc... Gas or Electric... Small to Large... Batch or Continuous Standard Sizes... Prompt Quotes... Quick Deliveries...Temperatures to 650 F FAST 0 SAFE 0 DURABLE Rate of Temperature Change Control... Top/Down Heating... Attention to Detail... Quality Components... 5 Layer Wall Protection...FM,CGA & IRI Specifications Write in PC #29 Write in PC #30

3 ~ POWDER COATING, April *Nonferrous: aluminum, sheet, coil, castings, extrusions, zinc castings, galvanized, terneplate, and zinc-plated.yellow metals: copper and brass *Mixed metals: combinations of the above.composites: mixtures of metals with nonmetallic materials Surface cleanliness tests What is a clean surface? A clean surface is h e of oil and other unwanted contaminants. The degree of cleanliness required may differ, however, depending on the operation or the process parts are subjected to. Manufacturers who use cell or work-station cleaning between process steps, such as welding, do not require the same degree of cleanliness that is required for final powder-coat preparation. How can you tell if a part is as clean as you require? Following are several tests you can do to determine substrate cleanliness. Water-break-he test. A water-break-flee surface tells you that all of the organic soils have been removed. The parts exiting the last rinse stage before drying will show uniform sheeting of the rinse water, indicating organi- cally clean surfaces. A water-break-flee test has been a long-standing determiner of cleanliness. The key to the test is using fresh uncontaminated rinse water. Detergent additives or rinse aids in a final rinse can hide poor cleaning. Contaminated rinses due to poor overflow can also mask poor cleaning because of the surfactantk wetting ability. If a part isn't suf6ciently cleaned, it will exhibit a beaded surface that resembles a fleshly waxed car arer a good rain. Poor cleaning is most often found on or near weldments or in areas that receive poor spray impingement. White towel test. Wiping a white towel across clean and dry surfaces will indicate the effectiveness of inorganic soil removal (see figure 1). Check flat surfaces and those areas that don't receive direct spray impingement. Tape-pull test. In the tape-pull test, apply a clear tape to a clean and dry surface, remove the tape, and place it on a white piece of paper. This will also indicate the effediveness of inorganic soil removal because the contrast between the tape and the paper allows you to easily identify remaining soils. Tape-pull nonferrous test. "his quick test is for finishing lines that pretreat multiple metals or nonferrous GET SUPERIOR FINISHES FROM POWDER COATINGS! 0 FAST CURING HEATS FROM INSIDE OUT 0 PIN POINT HEAT CONTROL driquik Infra-Red Ovens Foc fast, precise heat and bp qualt~ curing d powder ccntings, use driquik InfruRed Ovens Offered in both the durable ceramic generators and the quick response gold plated quartz tubes Write. Call a FAX for latest cataloal d08@18~ driquik Oven Division DRY CUME UMP CORPORAWN An Omni Groy, Company P.O. Box 146, Greensburg, IN Phone. (812) Fax: 18 12) ASK ABOUT FREE IN-PUNT TEST! Write in PC #31 Write in PC #32

4 40 POWDER COATING, April 1994 castings. Apply clear tape to a flat area on a casting before the casting enters the pretreatment washer. Wait until the casting exits the washer (before the dry-off stage), then manually blow-dry the tape. Now, remove it from the casting. This gives you a good, quick way to visually compare cleaned and micro-etched areas with a raw, or untreated, area that has been insulated from pretreatment by the tape. Ultraviolet detection. This test requires inspecting the part under ultraviolet light (see Figure 2). The degree of substrate cleanliness can be quantified by a numbering system. This is accomplished through photoelectron emission or reflectance. The higher the reflectance, the cleaner the surface. Fluorescence test. In this test, a fluorescent dye is added to the soil. After the part is cleaned, it s inspected under ultraviolet light. Contact-angle test. This test is conducted after a part is dried. Droplets of water (0.05 milliliter) are placed on the part. Cleanliness is measured by how the droplets flatten out. The greater the droplet diameter, the cleaner the part. Application methods and equipment The parts you coat and the overall quality you require will dictate the type of surface preparation equipment you ll need to accomplish consistent cleaning performance. A surface preparation system can become very complex and costly depending on your requirements. It is critical, then, to review carefidly all the design proposals you receive. The most commonly used chemical cleaning methods are recirculating spray washing, spray-wand phosphatizing, and immersion cleaning. Recirculating spray washing. Recirculating spray washers are the most popular systems used for powdercoat surface preparation because of their high efficiency in continuous-production lines. Figure 3 shows a typical multistage recirculating spray washer. As the name implies, a recirculating spray washer includes pumps and pipes that transfer chemical solutions from a reservoir to a set of risers. Nozzles attached to the risers spray a jet or swirl of solution onto the parts that physically, as well as chemically, removes soils or produces a conversion coating. Following are key elements to consider when selecting a system. *Nozzles: how many, type per stage, placement, pounds per square inch of pressure, and construction material *Risers: how many per stage, spacing, construction material, and pumps.tanks: gallon capacity, construction, construction material, and design *Drain vestibules: length, shielding, and pitch *Heating: available temperature at load, type, construction Typical recirculating spray systems operate between 10 and 30 pounds per square inch of nozzle pressure. Impingement on the part is what gives these systems the high efficiency needed in powder coating pretreatment. The system s recirculating action also keeps solutions fresh. In addition, process times, solution temperatures, and chemical concentrations can be reduced, resulting in energy and chemical savings. These systems also have some disadvantages. For one, they require higher capital costs than the other systems. For another, the chemistries available are limited to those that don t support foaming. Because recirculating 1 White towel test reveals smut on substrate An ultraviolet-detection test shows the degree of substrate cleanliness

5 ~ * 9' POWDER COATING, April spray systems are more complex than the other systems, they also require more upkeep, which increases costs. Furthermore, troubleshooting requires more expertise than it does in the other systems because of the variables involved, such as nozzle alignment, spray pattern, and drag out between stages. Spray-wand phosphatizing. Handheld spray-wand phosphatizing (see Figure 4) can produce excellent results at affordable prices for shops with low to medium production runs and a variety of part shapes and sizes. Typically, handheld spray-wand systems are best suited for powder coaters who handle (1) large, heavy, or bulky fabrications used in the agricultural, transportation, marine, and structural industries or (2) large postassembled products, such as electrical enclosures, cranes, hoists, blowers, and other products used in the manufacturing, transportation, and maintenance industries. As with any system, a handheld spray-wand phosphatizing system has limitations. Because handheld systems are operator dependent, they are limited by how well trained and conscientious the operator is. There are also some factors to consider when selecting a system:.surfaces that come in contact with acidic phosphate should be made of stainless steel *Heat capacity should be ample enough to deliver 180 F solution at the wand tip to facilitate cleaning, phosphatizing, and rapid drylng *Pressure and volume should be 1,000 pounds per square inch at 4 to 5 gallons per minute to supply the minimum cleaning and impingement needed Immersion tank cleaning. This method involves a tank or a series of tanks that hold solutions. Parts are dipped into the tanks in batches or processed continu- ously via conveyor. Immersion systems can be static or agitated. Agitated immersion systems are preferred because they accelerate removal of contaminants and provide clean, or fresh, chemical to the parts' surfaces. Agitation can be achieved by turning the solution over with a recirculating pump, air sparging, mixing, or physically moving the parts within the solution. Immersion tanks have several advantages. They have few mechanical parts, are simple to maintain, and are relatively inexpensive. Prolonged contact time in the solution is also possible. In addition, multicodgured parts come in complete contact with the chemical solution. As foaming is usually not a problem, more flexible chemistries can also be used. This type of cleaning method also has some disadvantages. Processing time, temperature, and chemical concentration are higher compared with spray-type systems because less mechanical and physical force is used to remove the soils in immersion systems. This method is also more suitable for batch cleaning than continuousproduction cleaning. Rinse-water quality A part is no cleaner than the quality of the rinse water used. The purpose of effective rinsing may be one or all of the following: 0% flush remaining wetted soils from the substrate *To neutralize or dilute remaining alkalinity after the cleaner stages.tb maintain a wet substrate between stages *To flush nonadherent phosphate or conversion coating from the substrate A multistage recirculating surface preparation system is suitable for ai:omated lines An operator uses a handheld spray-wand phosphatizing systen? to dear! large palts

6 I._. 42 POWDER COATING, April 1994 \, *To cleanse excess water hardness and salts before dry Off powder coat, is easy to control, has low- or non-sludging characteristics, and is economical. Proper and adequate rinsing is critically important in powder surface preparation when an accelerated corrosion specification is required. The key fadors affecting sound rinsing are listed next: *Original water quality *Water volume *Immersion or spray *Contact to part *Part configuration or design.solution contact time Rinse overflow rates can range between 3 and 20 gallons per minute, depending on the fadors listed. Table l shows the typical cleaner drag out you can expect from various part configurations. Initial raw water quality varies from municipality to municipality. The existing water source should be analyzed for unexpected or changing water conditions throughout the year. Table 2 shows the differences in quality between raw water and water that has gone through softening, deionizing, or reverse osmosis. Obviously, rinse-water quality, volume, and application must be at some minimum to be effective. Use above that minimum leads to waste and higher costs in treatment, handling, and disposal. The ph value and the amount of total dissolved solids (TDS) in the rinse water are fadors in determining how effective rinsing will be. These factors can be monitored either by handheld measuring devices or by totally automated systems with sensing probes. The probes sense when to increase the overflow rate or reduce TDS by activating a drain and refilling the rinse tank with fresh water. Automated systems maintain water quality without regard to part volumes, shape, size, and so on. They have three major pitfalls, however: (1) The probes must be maintained and cleaned to be accurate, (2) the automatic controllers must be continuously recalibrated, and (3) care must be taken that the false sense of security that an automated system can create doesn t lead to poor quality control. Iron phosphate coatings are usually derived from solutions that contain very little iron. They are produced on ferrous metals through the combined use of acid phosphate salts, free phosphoric acid, and accelerators. For nonferrous metals, such as aluminum and zinc, a microetched surface and a combination alloy phosphate are produced in place of a normal phosphate coating in the range of 40 to 70 milligrams per square foot. Operating ph will vary widely with the type of phosphate compound. Some will favor a ph in the range of 3.5 to 5.0; others will favor a ph in the range of 4.8 to 6.0. It s more economical to use ph adjustment acid concentrate than to change or add phosphate compound. In most instances, the ph will tend to rise in operation. For a cleaner and iron phosphate combination, the cleaning ability of the chemical formulation is critical. Quality phosphating will not take place until the surface is sufficiently void of all organic soils. In many instances, operators and managers stress coating weights and salt-spray requirements for three-stage systems when they should be more focused on soil control, cleaning ability, and system upkeep. Multiple-metal lines, or systems treating ferrous and nonferrous metals through an iron phosphate system, are generally faced with a major trade-off: Maximizing salt-spray performance is very difficult when running both ferrous and nonferrous substrates through the same line at the same time. You should evaluate and test the substrates before equipment installation. If you require high corrosion protection, you should consider two pretreatment lines for the best performance. Typical cleaner drag out expected when processing various part configurations Part type Drag out in ga1/1,000 ft* Vertical parts, 0.4 well drained Vertical parts, poorly drained Vertical parts, very poorly drained Horizontal parts, well drained Iron phosphate coatings Iron phosphatizing is the workhorse of the general finishing industry because it delivers a quality surface to Horizontal parts, very poorly drained Cup-shaped parts, very poorly drained to 24 or more