Filtration System Improves Performance of Pre-Treatment and Electrodeposition Stages of an Automotive Assembly Plant

Transcription

1 Separation and Purification Sciences Division Technical Brief Filtration System Improves Performance of Pre-Treatment and Electrodeposition Stages of an Automotive Assembly Plant By Pauline Paul, Sales Application Support Specialist, Scientific Applications Support Services 3M Purification Inc. Reprinted with permission from the Society of Automotive Engineers, SAE World Congress 27, Advances in Coating Technology Session SP 274. In this re-print from the SAE Technical Paper series, novel filtration system and innovative filter elements both refer to the 3M DF Series Filter Element System. Compared to conventional filter bags, 3M DF series filter elements offers up to four times more service life, superior contaminant reduction efficiency, enhanced flow, and reduced costs associated with frequent filter changeout. The 3M DF series filter elements will easily retrofit most existing bag filter housings. Abstract A filtration study was performed at an auto assembly plant on phosphate and electro deposition systems. Information was collected on a) service life; b) weight of contaminants trapped per filter; c) filter integrity; d) quality average daily hood dirt counts; e) cost savings as well as operators comments. A filtration system lasted not only 3.2 times longer but also trapped 4.8 times more dirt than the standard bag/magnetic rod system when evaluated in the dip-cleaning stage. The superior performance of the innovative filter system was even more striking in the first city water, spray-rinse stage. For example, an increase of 5.3 times the life was observed and the weight of contaminants trapped during that time was 6.5 times the standard filter bags without the assistance of magnetic rods. Additionally, after six months the average hood daily dirt counts dropped by at least 87% once the new filtration system was installed. During the first year of conversion, the assembly plant attributed at least $16, in cost savings to the innovative filter system. Part of the savings came from the elimination of oil logs in the electrodeposition tank. This plant is now applying this filter system on all filtration stages in the pre-treatment and electrodeposition areas.

2 Figure 1: Filter Bag; poor support causes bags to burst Figure 2: Unique Flow Path of Filtration System Introduction It is commonly known in the automotive industry that many surface defects lie in poorly cleaned bodies prior to applying an electrodeposition coating. For example, oils and greases are known to cause craters and metal fines greater than 4µm are visible to the human eye resulting in automobile surfaces having to be re-worked. Re-working a vehicle is not only costly and labor intensive, but the look is never quite the same, particularly for metallic finishes, resulting in reduced customer satisfaction. Oils, greases and metal debris ( weld balls ) not only cause surface defects but can also cause serious corrosion problems if not reduced before applying a phosphate coating as the uniformity and weight of the coating is dependent on the metal being totally clean. 1 These problems, in turn, result in a higher level of warranty claims and subsequent reduction in profitability as well as quality reputation for the automotive companies. Filter bags inserted in canister filter housings are commonly used to reduce particulates from various fluids in the pre-treatment and electrodeposition areas. Many users encounter a number of problems with bag filtration systems in these areas. A major problem is lower efficiency (the ability of a filter to retain a contaminant, expressed as the ratio of the concentration of contaminant in the feed stream to the concentration of contaminant in the effluent 2 ) due to the stitched seam of the filter bags. For example, the filter medium may be rated as 1µm; however, the diameter of the sewing needle may be 1 15µm in diameter. Additionally, the standard filter bag is not fully supported as the bag does not conform exactly to the shape of the support basket. Since the standard filter bag is generally longer than its support basket, many times the unsupported part of the bag, i.e. the bottom area, can burst thereby releasing contaminants downstream. Figure 1 is a photograph of a burst bag which had been in service at an automotive assembly plant. Innovative Filtration System The filter system has a unique flow path. (Figure 2 is a cross-sectional diagram of the filter element.) It is comprised of two porous cylinders ultrasonically-bonded to a rigid top-plate as well as a rigid, lower seal-plate. The fluid enters through flow channels in the top-plate and then flows between the inner and outer cylinders, which are comprised of two sleeves of filter media. The fluid then flows through both sleeves into the closed chamber of the housing. The high-performance fibrous media of the innovative filtration system is comprised of an open porous pre-filtration layer integrated with a more efficient final layer. (Figure 3 on the next page, shows a photomicrograph of the layered filter medium.) This gradient-porosity medium has greater capacity/life (time in service before it plugs with accumulated solids 3 ) than an equally thick homogeneous medium. The two sleeve construction results in a 62% increase in surface area compared to standard filter bags. This, in turn, reduces the flux (volumetric flow rate/area of medium) which leads to a decrease in initial pressure drop, increased dirt-holding capacity 4 and improved efficiency. 2

3 Case Study An auto assembly plant wanted to improve the quality of pre-treatment fluids, in particular the dip-cleaning and city-water, spray rinse which is located after the cleaning tank. Figure 4 below, is a schematic of a typical pre-treatment area in an automotive plant. Baseline data was collected for the existing filter-bag system. Magnets were inserted in each filter bag to boost efficiency and extend filter service life. After the baseline data was collected, the innovative filter elements were installed under the same process conditions except no magnets were used. Figure 3: Graded Porosity of Innovative Filter Media Trial Conditions Baseline data was collected on the filter bags used in the dip cleaner recirculation system. Similarly, baseline data was also collected on the first city-water, rinse spray recirculation system. (This system immediately follows the dip-cleaner stage as is shown in Figure 4.) The innovative filters were also installed for the same period of time in the same locations. Both filter systems were run continuously, 24 hours a day, Monday Friday. Change out differential pressure: 1 12 psid. Stainless steel basket was installed to fully support each innovative filter element. Standard stainless steel basket was used to support each standard filter bag. No magnetic rods were installed in the housing vessels when the innovative filters were installed. Magnetic rods were installed in each competitive filter bag while in service. Dip Cleaner Fluid: alkaline immersion cleaner, blended with city water. Housings: #2-size bag housings, with filter baskets in each of the housings. Filter Micron Rating: 1µm, nominally-rated, polypropylene filter. Magnetic rods (filter bags only): At least one inserted in each filter and two inserted in the upstream fluid within the housing. Rinse Wash Rinse Phosphate To ED DuoFLO Filters Water Figure 4: Schematic of Pre-Treatment Area in an Automotive Plant (*initial trials performed at these stages) 3

4 Service Life (Hours) Micron Innovative Filter Bag Filter with Magnetic Rod Figure 5: Dip Cleaning Comparison of Service City Water Spray Rinse Fluid: City water Housings: #2-size bag housings with filter baskets inserted in each of the housings Filter Micron Rating: 1µm, nominally-rated polypropylene filter Magnetic rods (for filter bags only): At least one inserted in each filter and two inserted in the upstream fluid within the housing Procedures Information was collected on a) service life; b) weight of contaminants trapped per filter; c) filter integrity; d) average daily hood dirt counts and e) ease of use as well as operators comments on the difference between both filtration systems. Specific details on the procedures used are outlined below. Service Life The end point of the service life was defined as the point when the differential pressure across the housings reached 1 12 psid. The differential pressures were periodically recorded (once per hour). Determination of Weight of Contaminants Trapped per Filter Prior to installation, each filter was weighed and indelibly marked for future identification. After the end point of the service life, i.e psid, the spent filters were reduced from the filter housings. The metals fines from each of the magnetic rods inserted in each of the bag filters were scraped into its corresponding bag. The filters were fully dried and reweighed. The difference between the weight of the unused filter and the dried spent filter was recorded as the dirt-holding capacity. Filter Integrity When reducing the filters, the operators would document if filters burst while in service or if there were any problems in insertion or reduction. Quality Average Daily Hood Dirt Defects The hoods of each vehicle exiting the electrodeposition (e-coat) oven were inspected by an experienced inspector. The total amount of hood dirt counts per day was divided by the number of vehicles processed on that day. The average daily hood dirt count was recorded for a period of six months, i.e. three months baseline (bag filters installed) and three months having the innovative filtration system installed. The average daily hood counts were normalized, i.e. 1. was assigned to bag filter s average daily hood count value. 4

5 Service Life (Hours) Micron Innovative Filter Bag Filter with Magnetic Rod Figure 6: City Water Comparison of Service Life Weight (Grams) Micron Innovative Filter Bag Filter with Magnetic Rod Figure 7: Dip Cleaning Contaminants Trapped Results and Discussion Service Life The innovative filters lasted 3.3 times longer than the standard filter bags in the Dip-Cleaning Stage. Figure 5 is a histogram of the comparison in service life. The innovative filters lasted on average 21 hours whereas the bag filters only lasted on average 6.4 hours. The innovative filters lasted 5.3 times longer than the standard filter bags in the First City-Water Rinse Stage. Figure 6 is a histogram of the comparison of service life. The innovative filters lasted on average 5.8 hours whereas the bag filters only lasted on average 9.6 hours. The longer service life exhibited by the innovative filtration system can be attributed, in part, to the lower flux through this filter as reducing flux, increases life exponentially. 5 Another factor contributing to the longer service life of the innovative filter is the graded-porosity construction of the media. This type of construction increases dirt-holding capacity over homogeneous, [non graded porosity] material. 7 The media in the filter bag used in the collection of the baseline date was of non graded porosity construction. Weights of Contaminants Trapped per Filter Even though the innovative filter elements did not have magnets inserted while in service, more dirt was retained. Figure 7 is a comparison in the amount of contaminants trapped per filter for the Dip-Cleaning Stage. The innovative filter elements trapped 4.8 times more dirt on a weight basis. They trapped on average grams per filter whereas the bag filter/magnet setup only trapped on average grams. There was an even more striking difference in contaminants trapped per filter in the First City Water Rinse Stage. Figure 8 on the n is a comparison in the amount of contaminants trapped per filter. The innovative filter elements held 6.5 times more dirt on a weight basis. They trapped on average 37.2 grams per filter whereas the bag filter/magnet setup only trapped on average 47.4 grams per filter. The city-water rinse fluid was kept cleaner as the innovative filters were able to trap more debris than the bag filter/magnet system. The contaminant-reduction rate (weight of contaminants/time) was 6.6 g/hour whereas the contaminant-reduction rate for the filter bag/magnet system was only 4.9 g/hour at this stage. Filter Integrity Although not included in the data analyses, several of the standard filter bags burst during the baseline-data collection phase. This indicates some of the filter bags could not sustain either start-up conditions when the flow to the housing is suddenly introduced after a filter change-out - or when differential pressures got too high and seams burst. In contrast, no innovative filter elements burst as its design allows for the basket to fully support the media. 5

6 Weight (Grams) Figure 8: City Water Rinse Stage Contaminants Trapped per Filter Micron Innovative Filter Bag Filter with Magnetic Rod Innovative Filter.129 Bag Filter with Magnetic Rod 1. Figure 9: Hood Defectivity Rate Normalized Quality Average Daily Hood Dirt Counts There was a striking reduction in hood dirt defect counts after the innovative filtration system was installed. For example, defect counts dropped precipitously and they remained extremely low throughout the remaining three months of the trial. There was over 87.1% reduction in the average daily hood dirt counts while this innovative filtration system was in service as shown in Figure 9. Additionally, even though the filter bags had very short service lives, they were less efficient at reducing dirt than the innovative filtration system. The innovative filtration system was able to consistently maintain cleaner tank fluids which, in turn, accounted for at least a seven-fold reduction in hood dirt counts. Quality was also improved by not having to shut down the filter system as often since the innovative filters lasted much longer between change-outs. During filter change-outs, vehicles are still depositing contaminants into cleaning tanks which, in turn, temporarily causes a rise in the fluid contaminant levels. Besides reducing more particulates from each stage, the innovative filter elements also trapped crater-causing oil. Cost Savings Upon reviewing the results of the six-month trial, the plant adopted the innovative filtration system in all of the filtration stages in both the pre-treatment and the electrodeposition areas. They discovered within a matter of a few months the filters lasted longer between change-outs which in turn reduced not only labor costs but also filter disposal costs. Additionally, they were able to reduce filter pore sizes for various stages to reduce defect rates even further while still not increasing labor costs. In fact they saved over $16, in the first year using the innovative filtration system. 8 Labor Instead of frequently changing out filters, the operators now spend their time making the manufacturing process more robust, i.e. cleaning nozzles, ovens, etc. Since the innovative filtration elements are more predictable and last so much longer, shift scheduling is easier allowing for certain maintenance work to be performed during the week rather than weekends. Reduced oil logs In the past the plant would use oil logs in the e-coat tank. These logs would absorb oils from this tank. Once they began using the innovative elements they had no need to purchase these expensive items. Reduced filter use Significant savings resulted from reduced filter usage as the innovative elements lasted over five times longer. Additionally, less inventory is required, freeing up storage space and reducing inventory costs. 6

7 Conclusion Cost savings (over $16, in the first year of service) were achieved as the innovative filtration system not only reduced labor costs but also caused filter, disposal, and inventory costs to sharply drop. The innovative filter elements were more efficient at reducing particulates from tank fluids. This, in turn, accounted for a dramatic reduction (87.1%) in hood dirt counts. Defects caused by these particulates can be a source of future warranty claims. As a result the savings realized by converting to the innovative filtration system was probably greater than $16, in the first year. The innovative filtration system was so successful at reducing crater-causing lubricants that oil logs are no longer needed. The contaminant-reduction rate (grams/hour) in the city water spray rinse was 23% higher when the innovative filter system was installed. Bodies are cleaner in each pre-treatment stage and dirt drawn into the electro deposition (e-coat) bath is minimized. No innovative filters burst during the trial whereas several standard filter bags did. The innovative filter element is fully supported minimizing the risk of burst elements and sudden release of contaminants back into the fluid. The innovative filters lasted up to 5.3 times longer than the standard filter bags. Instead of changing out filters, the operators now spend their time making the process more robust. Overall, the innovative filtration system reduced operating costs, optimized process efficiency and contributed to a significant improvement in first-time pass. References 1. Zbiegien, J. Preparation and Pretreatment of Painted Parts Electrocoat 2 Conference Proceedings, p.71 (2) 2. Schroeder, H.; G, Simonetti, J. A.; and Meltzer, T. H. Prediction of Filtration Efficiency from Integrity Test Data, Fluid Filtration Liquid, Vol II, ASTM Special Technical Publication 975, Philadelphia, PA, p.27 (1986) 3. Johnston, P. R. Fundamentals of Fluid Filtration, First Edition, Tall Oaks Publishing, Inc, Littleton, CO, p. viii (199) 4. Purchas, D. B. Dirt-Holding Capacity, Handbook of Filter Media, First Edition, Elsevier Advanced Technology, Oxford, England, p.34 (1996) 5. Repetti, R.V. and Ponchick, A.R Selecting the Most Economic Cartridge Filter, Chemical Processing, pgs (February, 24) 6. Uberoi, Tony Effectively Select In-Line Filters, Chem. Eng. Prog., 88, p.75 (March, 1992) 7. Johnston, P. R. Life or Capacity of the Filter Medium, Fundamentals of Fluid Filtration, First Edition, Tall Oaks Publishing, Inc, Littleton, CO, p.63 (199) 8. Auto Assembly engineer in charge of project Private communication with author May 25 7

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