Troubleshooting for Electrocoating
Characterizing the type of defect is essential in identifying the root cause and eliminating its source...
Electrocoating provides a decorative and protective finish. Common coating defects may adversely affect these properties, so it is important that electrocoat users develop basic, effective troubleshooting skills.
Troubleshooting process. It is important to understand troubleshooting in order to develop sound, workable solutions to problems. There are five steps to troubleshooting.
1. Define the problem. Defining the problem is the first step in the troubleshooting process. It characterizes the problem by answering several questions:
Isolation—Is the problem in the operation of the system or appearance related?
Identification—What type of defect is occurring and does it occur on all parts?
Location—Does the problem move around on the part or is it always in the same area?
Timing—When did the problem start?
Duration—Is it constant or sporadic?
Problems do not occur without something having changed. Problems are not solved unless something is changed.
2. Identify the root cause. Identifying possible root causes is step two in the troubleshooting process. It is necessary to understand common causes for the defects. There could be several bath variables and process areas to investigate. Commonly asked questions include the following.
Correlation—What are the common causes for this type of defect? Have there been any recent changes made to the line? Is there a correlation between tank parameter test data and the appearance of the problem?
Mechanical—Is the testing equipment functioning properly?
Procedural—Have the testing procedures been followed?
3. Determine corrective action. Determining the appropriate corrective action is the next step in the troubleshooting process. This step is probably the most time consuming. Developing a logical plan that tests one variable at a time is crucial to identifying potential solutions. Answering the following questions should help develop a practical action plan.
- Which variables can be tested on line quickly?
- Which variables can be tested with little disruption to production?
- Which variables can be tested on line in the paint lab?
- Which variables need technical support from the suppliers?
4. Implement corrective action. Implementing corrective action is the success step in the troubleshooting process. As each variable is tested, it will either be eliminated or identified as a possible solution. There may be multiple solutions and in rare cases there may be no solution. In the cases where no solutions have been found, it is necessary to start the troubleshooting process again by redefining the problem.
5. Follow-up. Follow-up is the last step in the troubleshooting process. It involves determining what permanent changes are needed to prevent the problem from recurring. This troubleshooting model provides guidance for identifying, reacting to and solving problems and if properly followed and documented, it can provide a faster response for future problems.
Preventive action methods. Troubleshooting should be more than a reaction to problems. Preventive maintenance is often overlooked as a tool to guard against problems and defects. It is not possible to list every housekeeping item or maintenance program associated with an electrocoat system, but there are essential mechanical, chemical and substrate considerations.
Tank agitation. Agitation of a paint tank is necessary for paint suspension, filtration and removing excess heat generated from pumping and coating. Proper agitation is dependent on the header/eductor system and the circulation pump. Broken or misaligned eductors can cause appearance defects in production and dead zones in the paint tank. A malfunctioning pump can cause poor bath circulation, foam and appearance defects. Excessive tank agitation can cause parts to fall. The velocity of the paint in recirculation lines should normally fall between seven and ten fps. Semi-annual tank cleanings should be performed to check agitation, and weekly cleaning of the pump screens will prevent blockage.
Oven. The oven is critical to the final coating appearance and the desired performance properties. It is recommended that the oven buildup be monitored and cleaned out when necessary. Oven temperature recorders should be run semi-annually to ensure correct oven balance and dehydration zone temperatures.
Rectifier. The rectifier should be checked yearly for ripple. It should not exceed five pct under anticipated load conditions. The amperage and voltage displays should also be checked to ensure calibration accuracy. There should be no stray voltage to the paint tank during non-production hours.
Electrodes. The electrodes should be checked weekly for degradation, proper operation of supply and return flows and to ensure electric leads are connected. Periodic checks of each electrode's amperage draw can be used to monitor anode performance. It is important to maintain a 4:1 or lower ratio of coating surface to electrode (based on a two-minute immersion time).
Anolyte/catholyte. The anolyte/catholyte is needed to remove excess acid/base from the paint bath and should be checked for proper liquid level, proper functioning of the conductivity set-point indicator and probe, water supply availability and that the supply and return flows are operational.
Racking. Racking includes design, part loading and cleaning. There should be specially designed racks available for odd-shaped parts and spring-loaded hooks for small parts. It is recommended to have one contact point for part grounding and to handle them in a way as to eliminate liquid pooling, air pockets, falling off and contact with other parts. Maintaining clean racks and contact points will help prevent rack buildup and coating failures. The grounding system should be checked yearly for wear and good contact.
Rinsing. Ensure the rinse system is operating monthly without any plugged or misdirected nozzles. Make sure spraying is contained within the tank, and the recirculated rinse flow is balanced from stage to stage. Check for a buildup of paint solids and parts that have fallen in the tank. The headers should be cleaned periodically to remove any settled material from the piping and pressure maintained between five and ten psi.
Filtration. Filtration of the electrocoat tank includes both bag filters and ultrafilters. Bag filters should be changed when the pressure differential inlet to outlet is five psi. If oil absorbent media is being used, it should be changed frequently. The ultrafilter flux rate should be checked daily using the site gauges and cleaner per the manufacturer's recommendations. Most manufacturers recommend a cleaning at 70 pct of the stabilized flux rate, or membranes can be irreversibly fouled, shortening life span and making them more difficult to clean. The ultrafilter pump should be monitored by pressure gauges.
Cleaning process. Proper cleaning parameters will allow for removal of substrate contaminants such as stamping oils, surface dirt, fibers and weld smut. Whether an acid or alkaline cleaner is used, the concentration and the process time must be maintained and monitored daily. This includes any auxiliary cleaning steps such as shot blasting and pickling. A clean substrate is necessary for proper pretreating and coating. Dirty or contaminated substrate surfaces will cause final appearance and/or performance problems leading to rejected parts and rework.
Pretreatment process. Pretreatment functions as a conversion coating for improved paint adhesion and performance. It is crucial that this process be closely monitored and followed per the supplier's recommendations. The four basics to optimum pretreatment chemistry require monitoring of time, temperature, pressure and concentration. Routine testing for proper coating weights and crystal morphology should also be performed because the final appearance and performance of the electrocoated part is only as good as its preparation.
Water. High-quality water is essential for operation of an electrocoat system. Quality water is characterized by low conductivity, less than 10 micro mhos/cm for deionized or 20 micro mhos/cm for reverse osmosis, low silica levels and microbe free. It is recommended to have a minimum of two water sources, either two alternately functioning units or one unit plus water storage capability. Resin bed cleanings and regeneration procedures should be followed.
Substrate. Substrate quality is the first consideration in achieving a quality finish. There are many types of metals used in electrocoating ranging from aluminum to cold-rolled steel and from galvanized to heat treated metals. Using the first in, first out rule will keep substrates clean. Protective storage conditions will aid in the prevention of flash rusting and other surface defects.
Operating parameter effects. All systems, from cleaner and pretreatment through electrocoat, have specifications that recommend optimal ranges of operation. Understanding how each specification affects the appearance and performance of the coating will allow for corrective adjustments. Also, through accurate testing and charting, a historical picture of the system can be built and the occurrence of defects minimized.
Bath solids. This includes the pigments and non-volatile components of paint. Low bath solids cause lower film thickness, decreased throwing power, higher rupture voltage and higher ultrafilter flux rate. Low bath solids occur from normal excessive replenishment additions of paste.
Pigment to binder ratio. Low pigment to binder ratios cause higher gloss, decreased throwing power, less hiding and more cratering. Low pigment levels occur from excessive replenishment additions of resin and settling in the tank. High pigment to binder ratios cause lower gloss, increased throwing power, settling in the bath and rinses, and it makes the film more sensitive to water spotting. High pigment levels occur from excessive additions of paste.
Bath pH. High bath pH for cationic systems can cause tank settling, dirt, a decrease in ultrafilter permeate rates and sensitivity to streaking. High pH can occur from excessive anolyte purges, excessive replenishment and caustic contamination from carryover or deionized water. Low pH occurs from excessive acid levels and can cause redissolution. Potential causes of low pH are deficient anolyte purges, anolyte leakage in the paint tank, insufficient membrane surface, membranes surface plugging, excessive acid additions and acid contamination from carryover or poor quality deionized water.
Low bath pH for anionic systems also can cause tank settling, dirt, a decrease in ultrafilter permeate rates and sensitivity to streaking. Low bath pH occurs from excessive ultrafilter purge, excessive replenishment, and acid contamination from carryover or deionized water. High pH occurs from insufficient ultrafilter purges, excessive amine additions and caustic contamination from carryover or poor quality deionized water.
Bath conductivity. Low bath conductivity can cause poor throwing power, low film build and roughness. Low conductivity is caused by excessive ultrafilter purges and low bath solids. High bath conductivity can cause rupturing, high film build and roughness. High conductivity is caused by high bath solids and ionic contamination from carryover or poor quality deionized water.
Solvent. Low solvent levels can cause low film builds, higher rupture voltages, sensitivity to streaking (phosphate mapping), lower gloss and poor flow or orange peel. Low solvent levels are a result of inadequate solvent additions and excessive ultrafilter purges.
High solvent levels can cause high film builds, lower rupture voltages, higher gloss and poor throwing power. High solvent levels are a result of excessive solvent additions.
Common Electrocoat Defects. Coating defects are numerous and this section will not address every one, but it will provide some common defects related to the electrocoat tank, potential causes and their solutions.
Cratering. Craters are bowl-shaped depressions with material in the center and raised circular edges. They are usually caused by contamination of the bath, rinses or substrates with particulates or incompatible oils. These contaminants can be from the substrate forming process, greases or lubricants and other processes that allow airborne contaminants to enter the system. Craters may also be caused by post tank contamination of parts. This can come from chain oils, conveyor drips and blow out of contamination in the oven. Often it is difficult to identify the cause of cratering without close investigation of the line. A permanent solution to cratering must be to identify and eliminate the source. Short-term solutions are using oil absorbent media inside of bag filters, increasing the pigment to binder ratio and in some severe cases diluting the contaminant with fresh feed.
Rupturing is the bursting of the deposited film by an excessive generation of heat (anodic) and electrical sparking/gassing at the film/substrate interface (cathodic). Rupture defects are caused by excessive voltage, excessive ripple, high film build, electrode/counter electrode in close proximity and bath contamination. By racking the parts according to substrate type, size and weight, the voltage can be adjusted as necessary. Rupture can also be due to high bath temperature, solvent levels and bath solids. The cathode or anode should be a safe distance away from its counter electrode. Bath contamination by ionic species can be removed by ultrafiltering to drain and replacing with deionized water.
Roughness is indicated by patches on a cured film that exhibits an alternately non-uniform and smooth appearance. Patchy roughness can be due to ionic contamination, low solvent levels and substrate irregularities. Ionic contamination is typically brought into the bath through part carryover, poor water quality and anolyte/catholyte malfunctions. Adding solvents can also help smooth the overall coating appearance. Using clean, high-quality substrates and controlling pretreatment will minimize the non-uniformity of the cured electrocoat film.
Redissolution is where all or part of the electrocoat film washes off or dissolves. Redissolution can seriously limit the high transfer efficiency of an electrocoat system. It occurs in the paint bath or post rinses and can be caused by excess solubilizer, high solvent levels and line stoppages. Excess solubilizer and high solvent levels in the bath lead to aggressive permeate post rinses that dissolve the deposited coating during rinsing. This can be eliminated by maintaining proper bath pH and solvent levels. The amount of time the ware is in the bath and rinse stages during the line stoppages should be minimized.
Dirt. Dirt has three sources, process, environmental and oven. Process dirt develops within the bath or rinses from inadequate solubilizer levels, pump shear, altered circulation and improper filtration. In the early stages, dirt appears on a horizontal surface, but in severe cases it can affect all surfaces. Environmental dirt is caused by airborne particles that fall into the bath or settle on the ware. Electrocoat areas exposed to vehicular traffic, ventilation fans, and grinding/sanding operations are susceptible. Oven dirt is caused by condensation of electrocoat by-products that flake off when drying. On a cured part, oven dirt is more surface oriented, while process and environmental dirt is somewhat imbedded in the paint film.
Streaking. Streaking in an electrocoat film can be due to pretreatment, rinsing and racking. Pretreatment variations can cause differences in ware surface conductivity. This defect is usually a telegraphing or mapping of the pretreatment through the electrocoat film or an electrocoat film phenomenon. Rinsing issues include low solvent or solubilizer levels in the rinse stages and clogged or misaligned rinse nozzles. The greater the length of time from paint bath to post rinse can increase drying of the dragout, making it difficult to rinse. Dirty racks and improper racking also can be sources of drips or spots.
Pinholing/outgassing is a pattern of relatively small, random volcano-like holes in the electrocoat film that penetrates to the substrate. Pinholing is primarily seen on galvanized/zinc-coated substrates, but can be caused by poor metal quality and rectifier problems. Galvanized and other zinc-coated substrates may inherently have surface microvoids. These microvoids may allow for the gasses normally generated in the electrocoat process to be trapped under the electro-coating. During curing, the gasses blow out through the electrocoating, leaving a volcano-like hole. Poor metal quality, metal that cannot be pretreated evenly and voltage spikes from an unfiltered rectifier can cause rapid electrodeposition. This does not allow the normal gasses generated in the process to escape, therefore holes result.
Foaming/air entrapment. Foaming is typically caused by pump problems, poor tank circulation and improper part loading. Cavitating pumps allow for aeration of the electrocoat bath and poor tank circulation does not allow gas to dissipate. Odd-shaped ware entering the electrocoat bath at an angle or through surface foam can also be a reason for air entrapment.
Gloss variations can be caused by several factors, including pigment to binder ratios, solvent levels and solubilizer levels. Pretreatment variations cause gloss differences not only part to part, but also on one part. Cure time and temperature also affect the final gloss.
Color variations can be caused by iron contamination, improper cure and poor tank agitation. Iron contamination can cause a yellowing or browning of the coated film. Oven problems can discolor cured films. Poor tank agitation can cause pigment pooling that can cause a streaked or blotched discoloration on products.
Throwing power. Poor throwing power is usually related to low voltage, low bath solids, low conductivity, high solvent levels and insufficient deposition time. By increasing some or all of these variables and decreasing solvent and bath temperature, throwing power will increase. Throwing power also can be impacted by the addition of auxiliary electrodes close to areas where more film build is needed.
Thin coating. These coats may be caused by poor contact, a faulty rectifier, inadequate electrode surface, high part loading, low voltage and low bath temperature and inadequate deposition time. Clean hooks, proper electrical supply and maintaining the proper coating surface to electrode ratio is essential to proper film build. High part loading can cause an overall film-build decrease. Film build can be increased by increasing voltage, bath temperature and deposition time.
Orange peel is related to iron contamination and low solvent levels. Iron contamination can be caused by fallen parts, exposed mild steel and leaking anolytes. This type of contamination, although ionic, cannot be ultrafiltered from the bath. Coating out, adding fresh feed to dilute the contamination and eliminating the source are the recommended solutions. Increased solvent levels can improve the flow characteristics of the electrocoating, eliminating the orange peel.
The "Big Four" electrocoating troubleshooting areas include the troubleshooting process, establishing preventive maintenance schedules, controlling operating parameters and classifying common electrocoat defects. Troubleshooting electrocoating will help users develop effective skills needed to promote optimum electrocoating.
Choosing the right conveyor system, coating technology, and ancillary equipment.
This paper is a peer-reviewed and edited version of a presentation delivered at NASF SUR/FIN 2012 in Las Vegas, Nev., on June 13, 2012.
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