HC Surface Treatment Network: The surface of a passenger car body is usually bright, eye-catching, and radiant. The general gloss requirement is greater than 90, with good results reaching about 95, almost approaching a mirror-like effect. On such a glossy surface, any defect becomes clearly visible, especially dust particles, which are extremely conspicuous. Dust particles are omnipresent and pervasive. The sanding work during the body coating process continuously generates dust particles, making dust the great enemy of high-quality coating and a difficult enemy to defend against. Major automobile plants have exhausted every means and devised various methods to prevent dust particles during the coating process, in order to reduce the harm of dust particles, improve surface quality, reduce rework rates, and save costs.
This article will discuss the harm of dust particles during the coating process, dust particle monitoring, analysis methods, classification and sources of dust particles, as well as dust prevention during planning and production.
I. Harm of Dust Particles
A few protruding dust particles on the painted surface of a passenger car body will have a very adverse effect on surface quality and leave a poor first impression on customers. If the dust particles are on the surface layer of the paint and the particle size is not large, they can still be treated by sanding and polishing before inspection and acceptance. Otherwise, rework is the only option. Foreign automobile plants stipulate a rework rate of 25 percent, including full-vehicle and partial rework. The rework rate for export vehicles is about 50 percent. Each vehicle rework consumes an additional 8 kg of paint, wastes production rhythm and energy, and the waste in human and material resources is considerable. Typically, reworking one vehicle wastes 600 yuan in coating materials, and with labor and energy, the total cost exceeds 1,000 yuan. Some parts plants supporting whole vehicles have even higher rework rates. A bumper plant had a first-pass acceptance rate of less than 10 percent due to excessive dust particles, and less than 40 percent even after sanding and polishing, causing great difficulties in supply and significant economic losses.
To control overall vehicle quality, major automobile plants have a whole-vehicle quality scoring process, called audit inspection in Germany. The general paint audit quality standard is 70-80 points, and a single dust particle deducts 10 points. Sanding marks and polishing marks also result in point deductions.
Dust particles on the electrodeposition and mid-coat surfaces are removed by sanding. With fewer dust particles, the sanding work is naturally reduced, and the chance of sanding marks is greatly diminished. Dust particles on the topcoat surface are sanded with 2000-3000 grit sandpaper and then polished. With fewer dust particles, the consumption of sandpaper, polishing compound, wool pads, etc. is reduced, saving both human and material resources. Moreover, the chance of point deductions due to polishing marks during whole-vehicle quality scoring is also reduced.
II. Monitoring of Dust Particles
To understand the degree of impact of dust particles on coatings, large automobile plants have dedicated personnel monitoring dust particles, including visual counting of dust particles on the body surface and sampling analysis of dust particles. Some plants even have 2-3 people dedicated to this work, demonstrating the level of importance attached.
Visual counting of dust particles on the body surface is conducted on vehicles after electrodeposition, mid-coat, and topcoat, with five vehicles sampled per stage per day for counting to understand the pattern. Some factories also count dust particles in the body cavity. Generally, there may be 30-50 dust particles on the body surface after electrodeposition, 8-20 after mid-coat, and about 4-8 after topcoat. The author compiled statistics at the topcoat line of the Volkswagen Mosel plant in Germany: the topcoat dust particle count was 2-8. The plant requires about 4 dust particles on the topcoat surface, which is very difficult to achieve.
In addition to counting, sampling analysis of dust particles is also necessary. Generally, five vehicles are sampled daily for electrodeposition and mid-coat, with three dust particles taken from each vehicle for analysis. For topcoat, random sampling is used for analysis. Data and charts related to dust particles are compiled daily and reported to relevant departments, and posted on the dust particle control board so that all employees can stay informed about dust particle trends. Weekly and monthly summary analysis reports are also produced. At the weekly quality meeting, the causes of defects, sources of dust particles, and preventive measures are analyzed and reported.
In newly built paint shops or after replacing new models of filter materials, dust particle counters can be used to test the air inside the workshop and spray booth to understand the quantity of dust particles of different sizes, thereby determining the effectiveness of air purification.
III. Sampling of Dust Particles
To analyze and study dust particles, sampling is the first necessary step. Dust particles are very small and difficult to sample. Typically, a surgical scalpel can be used to cut open the dust particle, exposing it for identification.
The cutting method involves making a diagonal incision on the coating surface, then gradually cutting off several thin slices to expose the center of the dust particle, and then analyzing the cut dust particle directly on the surface of the coated object.
Another method is to cut a 10 micrometer thin slice from the incision, transfer the slice to a slide, and analyze it under a laboratory microscope.
The third method is to make a horizontal cut to expose the contents of the dust particle. Analysis can be performed directly on the coated object, or the cut film can be transferred to a slide and brought back to the laboratory for analysis of its backside with a high-magnification lens.
IV. Analysis Methods for Dust Particles
A simple analysis method for cut dust particles is direct observation with a 30-40x illuminated magnifier, which provides a rough understanding of the structure but makes detailed analysis and research of dust particles difficult.
A better method is:
Using a high-power optical stereo microscope for observation. This instrument can use different light sources for illumination, and coaxial reflected light illumination makes it easier to distinguish and identify the internal structure and composition of dust particles compared to ordinary diffuse light illumination. This method is intuitive and clear.
A scanning electron microscope can magnify microstructures by about 200,000 times, naturally allowing observation of finer structures of dust particles. This method is generally only used by research institutions for in-depth studies and is typically not possible or necessary in factories.
Without cutting the paint coating, we also have methods to analyze its internal structure to some extent. For example, X-ray micro-spectroscopy analysis, micro-Raman spectroscopy analysis, etc. Under a Raman microscope, the laser spot is focused on the paint dust particle, and analysis is performed layer by layer from the surface downward, with a measurement depth of about 100 micrometers. When the longitudinal spectrum of the paint area is significantly different and shows no C-H vibration signal, it can be determined that the particle is an inorganic component, such as rust, welding slag, etc.
When Raman spectra at different depths all show C-H vibration signals, it indicates that the dust particle is an organic substance, such as paint or PVC particles.
The United States produces an instrument specifically for dust particle analysis in automobile plants called the MicroView, with a magnification of 72x, equipped with a computer processing system. It can conveniently and intuitively analyze, photograph, and compile statistical results of dust particles sampled on-site daily, and print various statistical charts. It is very convenient to use, though the price is relatively expensive, at about seventy thousand US dollars per unit.
V. Classification and Sources of Dust Particles
Under a high-power microscope, the morphology and material of dust particles can be analyzed clearly, which allows further tracing of the sources of dust particles. Dust particles affecting coatings can be roughly classified into the following categories:
1. Atmospheric dust: Mainly soil dust, also including various types of debris. Compared with developed countries, China has less ground vegetation and forest coverage, so there is more dust and suspended particles in the air.
2. Metal shavings: Mainly from body sanding in the assembly workshop, including iron shavings, zinc powder, etc. In addition, there are iron shavings and brass-like foreign matter from mechanical wear in the paint workshop, such as wear of hinges, conveyors, and skids.
3. Rust: The surface of cold-rolled steel sheet may have slight rust, or trace filamentous or spot-like rust may occur during pretreatment due to line stoppage, tank solution parameter fluctuation, or poor spraying.
4. Welding spatter beads: Spot welding in the assembly workshop can produce a large number of welding spatter beads, some of which adhere to the body and cannot be removed during pretreatment.
5. Residual PVC sealant: Splashes from robotic or manual PVC sealant application that are not wiped clean will remain on the body surface.
6. Multi-color paint particles: During the spraying process, colors are frequently changed, and paint mist drifts and adheres to equipment and filters. After drying, it falls onto the coating surface. Such paint particles have a multi-color interlaced layered structure.
7. Overspray agglomerates: Overspray agglomerates can drip onto the paint wet film from spraying equipment such as spray gun nozzles, spray cups, or clothing. If vortexes form in the spray booth air, paint can also agglomerate in the air and then fall onto the wet film. Overspray agglomerates from basecoat tend to form spherical structures.
8. Sanding dust particles: Sanding of electrodeposition and mid-coat produces many dust particles, including paint particles and sandpaper grit and other debris.
9. Fibers: Fibers come from work clothing, gloves, and air filtration materials. Cotton fabrics emit significantly more fibers than synthetic fabrics. Under a microscope, cotton fiber structures appear as flat ribbon-like shapes, sometimes partially twisted. Synthetic fibers have various cross-sectional shapes, but the fiber growth direction structure is the same. When synthetic fibers are damaged, they also emit large quantities of fibers that contaminate the air.
On the electrodeposition coating surface, approximately 87 percent of particles are composed of welding spatter beads and metal shavings. This is followed by PVC contamination. Fibers are the main contaminant on mid-coat and topcoat surfaces. On the basecoat surface, this type of contamination accounts for about 58 percent. This is followed by sanding dust particles and multi-color paint particles. On the clearcoat surface, fibers, multi-color paint particles, and overspray agglomerates are the main contaminants.
VI. Prevention of Dust Particles
1. Prevention during Planning:
To reduce coating dust particles, great importance must be attached from the planning stage of the paint workshop, otherwise the consequences will be unimaginable. To prevent dust contamination, various aspects need to be fully considered during planning. The main aspects are as follows:
(1) The entire workshop building should be fully enclosed, with as few windows and doors as possible. If there are windows, they should be double-glazed and non-opening. Doors should also be double doors, and doors frequently used by personnel should be automatic air shower doors. The air shower channel should have sufficient length with winding height variations, and the entire workshop should maintain positive air pressure to prevent outside dust from being drawn in. The air supplied to the workshop should also pass through at least two stages of filtration.
(2) The entire workshop should be divided into several zones, including general clean areas, high-cleanliness areas, and non-clean areas. Non-clean areas such as waste residue treatment, skid washing, and wax injection should be isolated by walls.
(3) The spray booth is the cleanest area in the entire workshop and should also have isolation walls to separate it from other areas. Entry and exit must be through a forced air shower area for cleaning before entering. The forced air shower time should be greater than 30 seconds, otherwise the door to the spray booth will not open. The air entering the spray booth is typically filtered through four stages: EU3, EU5, EU7, and EU5, at a velocity of 0.3-0.5 meters per second, ensuring thorough filtration of dust in the air.
(4) An automatic cleaning station consisting of compressed air, ostrich feather dusters, and ionized air should be set up before spraying. Ostrich feathers are soft, have good anti-static properties, and the barbed tips are very effective at removing dust.
(5) Thoroughly removing dust particles brought by the body-in-white during pretreatment is very important. The body-in-white has various contaminants such as oil, iron shavings, welding slag, adhesive, rust, and dust, especially iron shavings and welding slag, with 80 percent distributed on the body floor. Whether these contaminants are thoroughly removed greatly affects electrodeposition quality.
To remove these contaminants, a manual high-pressure water gun cleaning station can be set up before pretreatment to preliminarily clean the body surface crevices, which can reduce dust particles by about 30 percent. If a pre-cleaning station can be added directly in the body workshop, the effect would be even better.
Using high-flow nozzles to spray approximately 1000 liters per minute of degreaser and rinse water through the vehicle windows into the body cavity, combined with a 45-degree immersion angle, can significantly reduce body dust, thereby reducing dust particles in the electrodeposition film by 45-50 percent.
(6) Remove contaminants from pretreatment and electrodeposition tank solutions as much as possible. For example, installing bag filters in the circulation pipelines of pre-degreasing, main degreasing, and water rinse tanks to filter out contaminants. Installing cyclone separators and magnetic filters can capture 95 percent of iron powder particles larger than 20 micrometers. Electrodeposition tank solutions can use 25 micrometer or even 10 micrometer filter bags to filter out impurities that affect the paint film surface.
(7) The bottom of the phosphating tank should be designed in a hopper shape to facilitate the settling of phosphate sludge. Using intermittent discharge to a phosphate sludge filter for further concentration and pressing into sludge cakes, the sludge content in the phosphating solution can be controlled below 300 ppm. This can achieve complete clarity and transparency, thereby avoiding phosphate sludge contamination.
(8) In the pretreatment and electrodeposition tanks, the entire body is completely inverted. This helps improve coating quality and is also beneficial for removing the impact of dust particles, especially reducing dust particles on upper surfaces. This is a process invented in the world.
(9) A new process involves immersing the entire body in deionized water after mid-coat sanding, followed by rinsing, drying, and then topcoat application. This can thoroughly clean dust inside and outside the body, greatly helping to reduce topcoat dust particles.
(10) The passage from mid-coat sanding to the topcoat spray booth should be completely sealed to prevent outside dust from settling on the body. Water tanks should be set under the grating plates of the sanding area to prevent sanding dust from becoming airborne.
(11) Cleaning stations should be set up before mid-coat and topcoat spraying, where operators use compressed air to blow the body from inside out, and then wipe the entire body with tack cloth.
(12) Dedicated dressing rooms and passages should be provided for spray painters, so that after changing clothes, they do not come into contact with the outside environment on their way to the spray booth.
(13) All pretreatment materials should be liquid materials to prevent dust from powder addition of degreasers, surface conditioners, etc.
(14) Baffles should be installed on both sides of the body in the underbody sealant booth, with circulating air flowing from top to bottom between the baffles and the body, thereby reducing PVC splashed onto the body during underbody coating.
There are many more aspects to consider in planning and design, too many to enumerate. The more detailed the planning, the more beneficial it is for subsequent production. There are also many measures to be taken during the production process.
2. Prevention during Production:
Strict and persistent production management is an important part of reducing dust particle contamination. The slightest lapse can cause a significant increase in rework rates.
(1) First, a complete set of effective rules and regulations must be established and strictly enforced from the very beginning to develop good habits.
(2) The paint workshop strictly prohibits entry of all unrelated personnel and does not allow tours or visits. Dedicated security personnel must be stationed at the entrance. All personnel entering and exiting must use the air shower channel.
(3) All personnel entering the paint workshop must wear dedicated work clothing, work shoes, and work caps. This rule is not easy to consistently enforce and have everyone comply with. Work clothing should be coverall-style, without hoods, and made of anti-static synthetic fabric. Wearing damaged work clothing is strictly prohibited, as damaged edges will emit large quantities of fibers.
(4) Regular equipment maintenance and servicing should be performed to keep equipment in good condition. The filter pressure differential in the spray booth should be checked daily, and when the pressure differential reaches the specified value, indicating that the filter needs replacement, it should be replaced promptly.
(5) Hooks, bars, and skids should be cleaned promptly. Cleaning should be performed outside the paint workshop or in a dedicated isolation room. The grating plates of the spray booth should be washed with a high-pressure water gun at least once a week, taking care to prevent splashing from contaminating the spray booth. The VenTuri water sliding plates should also be cleaned regularly to ensure spray booth air volume balance.
(6) A cleaning schedule should be established for pretreatment and electrodeposition tank solutions and strictly executed as required to ensure that the cleanliness of each tank always meets requirements.
(7) Mid-coat sanding can adopt spot sanding to reduce dust generation. Electrodeposition and mid-coat should use wet sanding whenever possible, and towels should be washed and changed frequently. A type of high-efficiency cleaning cloth is very beneficial for dust wiping.
(8) The cleaning of the entire workshop should be contracted to a professional cleaning company. Cleaning work should mainly be carried out during night shifts and holidays. Only professional companies can accomplish this work satisfactorily.
(9) The interior walls of the spray booth should be coated with tacky coating. This non-drying coating can trap dust. Every Sunday, it can be washed clean with water and then reapplied.
(10) The baking oven must be cleaned once a week. During initial commissioning, a painted wet vehicle body can be run through the oven to remove dust particles from the circulating air. Under normal circumstances, when not in production, the oven should be kept in a heat-retention state to prevent outside dust from blowing in.
(11) High-speed electrostatic rotary bells (ESTA) and reciprocating automatic spray guns (SprayMate) should be cleaned regularly. Typically, the spray cups, spray guns, and associated mechanical equipment are cleaned every two hours during two production stoppages. If robot spraying is used, spray nozzles should be replaced every 30 minutes. When ESTA applies basecoat, automatic cleaning is performed for almost every vehicle, using solvent to wash paint buildup off the rotary bell before spraying the next vehicle. Insufficient cleaning of the automatic spraying system will bring a large amount of overspray agglomerates to the paint film.
(12) The air velocity and pressure of the spray booth should be checked regularly. Magnetic tape strips should be hung at the spray booth entrance and exit, tilted outward at 15 degrees to maintain positive pressure in the spray booth and prevent dust from being drawn in. The air volume balance at various points in the spray booth should also be checked to prevent vortex formation.
VII. Conclusion
Almost every step in the coating process can cause dust particles to affect the coating. To reduce the impact of dust particles, it is essential to raise the dust prevention awareness of all employees. Mobilize everyone to investigate, section by section and stage by stage, all aspects that may cause dust particle impact, and eliminate them one by one. This must be done persistently, with attention to dust prevention at all times. Only in this way can high coating quality and high production efficiency be consistently maintained.
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