On electrostatic coating production lines, hangers are often considered "insignificant supporting roles"—after all, their core function is simply to support the workpiece and complete processes such as pretreatment, spraying, and curing. However, experienced professionals know that hangers are far more than just "supporting tools"; they are key variables affecting coating quality. Broadly speaking, hangers include hanging fixtures, hooks, brackets, baskets, etc., and their impact permeates the entire coating process, from coating thickness uniformity to surface cleanliness, from curing effect to production safety. Today, we'll break down various quality problems caused by hangers and share practical solutions to help you avoid this "hidden pitfall."
1. Poor Conductivity, Thick Coating
The core principle of electrostatic spraying is "electrostatic adsorption," which requires the workpiece's receiving resistance to be less than 1MΩ. Otherwise, powder cannot be effectively adsorbed, resulting in either a thin coating or no adsorption at all. For liquid paint, poor conductivity also leads to poor droplet orientation, making it difficult to achieve the required thickness. An excessively thick hanger coating is a major cause of poor conductivity.
The key to solving this problem is proper cleaning of the hangers: Regularly clean the hanging points using tools like pliers and a grinder until the metal substrate is exposed; this is fundamental to ensuring conductivity. If the overall coating on the hanger is too thick, complete delamination is necessary. Currently, there are three main delamination methods, each with its advantages and disadvantages: manual hammering is inefficient and ineffective, suitable only for small-scale cleaning; solvent immersion is clean and fast, but the solvent contains volatile components and halogenated hydrocarbons, making it difficult to remove the detached coating; and a thermal cleaning oven decomposes the organic components of the coating at high temperatures, producing harmless gases and solids, making this the most recommended method. However, it's crucial to note that magnetic hangers lose their magnetism at high temperatures and should never be used with this method.
Furthermore, poor grounding of the hangers not only affects coating thickness but can also generate electric sparks under high voltage conditions, posing a significant safety hazard, especially for electrostatic spraying with solvent evaporation. It is recommended to enhance the conductivity of the hangers from the design stage and perform regular delamination maintenance. Two special cases can lead to thin local coatings: First, if the sprayed surface is too close to the hanger, a "powder (paint) grabbing" phenomenon occurs; increasing the distance between them will solve the problem. Second, if the workpieces on the hanger are too sparsely arranged, powder (paint) will penetrate the workpieces or adhere to the back; adjusting the amount of powder (paint) dispensed will improve this.
2. Residual Impurities from Plastic Demolding
Many people's first reaction to particle defects on the coating surface is to clean the curing oven, even stipulating weekly cleaning, but they overlook the hanger, a significant source of impurities. To maintain conductivity, the hanger needs periodic plastic demolding, but regardless of the demolding method, some oxide scale and paint residue will remain. In the oven with forced hot air circulation, the high temperature causes these residual paint scale and rust to flake off, spreading with the hot air and contaminating the entire curing environment, eventually adhering to the coating surface and forming particles.
To eliminate these impurities, the key is to thoroughly clean and inspect the hanger after plastic demolding; this can be quickly determined by visual inspection or tapping. The following seven methods can be used to address the issue: blowing with compressed air, rinsing with a high-pressure water gun, applying a thin layer of powder (paint) without the workpiece attached and allowing it to cure, forcefully tapping or cleaning with a wire brush, and soaking in acid to remove rust. If using magnetic hangers, small iron filings will be attracted to the magnets after deploring; these should be cleaned with a bristle brush or copper wire brush. Avoid wiping with a cloth alone to prevent fiber shedding and contamination of the spraying and curing environment. Sandpaper is inefficient and not recommended as a priority.
3. Heated Hangers
Many people encountering problems such as low coating gloss, poor impact resistance, and localized peeling will prioritize checking the curing temperature and time, neglecting the influence of the hangers. When the contact area between the workpiece and the hanger is large and the hanger itself is heavy, the hanger will absorb a large amount of heat, leading to insufficient curing of the workpiece near the contact point and the aforementioned defects. If there are many heavy hangers on the production line, and the workshop temperature is low in winter, or the oven's heating capacity is limited, it may even cause all workpieces in the oven to fail to cure properly. This kind of "localized bottleneck" problem requires advance prediction of the rack's heat absorption and appropriate adjustment of curing parameters or optimization of the rack design.
4. Rack Rotation
Rack rotation may seem like a minor issue, but it can cause two major problems: First, the workpiece rotates during spraying, causing the angle between the sprayed surface and the spray gun to deviate from 90 degrees and resulting in uneven distance. This leads to significant differences in coating thickness, with larger workpieces exhibiting more pronounced thickness variations. Second, in the pretreatment spray channel, the rotating rack can cause the workpiece to collide with the channel, leading to workpiece deformation or falling, or even workpiece jamming in the channel, potentially breaking the conveyor chain and causing production stoppages.
The key to solving these problems is optimizing the rack design: If the existing spraying equipment's racking method is not suitable for the specific workpiece, or if the racking mechanism is prone to deflection, modifications must be made first. These modifications could include adding fixing clips or adjusting the racking angle to ensure the workpiece remains stable during transport and spraying, preventing rotational deviation.
5. Workpiece Drop
Workpiece drops not only result in defective or scrapped products but also threaten operator safety. Common causes include: damaged hanging points, unsuitable hook angles, and insufficient strength of the hanger material. These problems are more easily exposed when the conveyor chain is going uphill, downhill, or turning. Workpieces with process holes are relatively stable and less prone to falling, but the hanger's condition should still be checked regularly, and damaged hanging points and hooks should be repaired promptly.
Magnetic hangers are a "good helper" for small workpieces—they don't rely on process holes and loading and unloading are fast. However, a key issue to note is that the magnet's magnetism weakens during heat curing, potentially causing workpieces to fall. Therefore, when using magnetic hangers, the magnetic parameters must be strictly controlled to ensure sufficient magnetic force to hold the workpiece in place during baking. Additionally, magnetic hangers are relatively heavy, have a large contact area with the workpiece, and absorb a lot of heat, which can easily lead to uneven curing. These factors need to be carefully considered when using them.
6. Other Easily Overlooked Fixture Issues
Besides the core issues mentioned above, fixtures can also affect coating quality in some details, and overlooking these can lead to problems:
6.1 Waste of Pretreatment Solution
Fixtures made of round steel carry less pretreatment solution, while fixtures made of angle steel, square steel, or steel pipe are prone to leaving solution residue, resulting in ineffective consumption of the bath solution and even frequent scrapping. It is recommended to prioritize round steel when designing fixtures; if other materials must be used, pay attention to adjusting the fixture angle to facilitate smooth solution flow.
6.2 Upper Workpiece Contamination of Lower Workpieces
When suspending multiple workpieces, pay attention to the direction of the dripping solution from the upper workpieces to prevent unclean liquid from dripping onto the lower workpieces. These liquids will leave impurities after drying, causing defects such as watermarks, bubbles, and coating peeling after spraying and curing.
6.3 Causes workpiece deformation
Protruding weld points on the hanger can compress thin steel workpieces, causing deformation. The weld points need to be smoothed beforehand. If there is coating adhesion between the workpiece and the hanger, forcibly removing it will also cause workpiece deformation. It is recommended to thoroughly remove any residual coating when cleaning the hanger.
6.4 Affects phosphating quality
During tank immersion phosphating, if the workpieces are packed too tightly in the basket, causing surface-to-surface compression, it will hinder the flow of the treatment solution, resulting in uneven phosphating film and yellowing. If necessary, spacers can be inserted between workpieces to ensure that the treatment solution fully contacts the workpiece surface.
7. Summary
In fact, coating defects caused by hangers all have obvious characteristics. As long as the root cause is identified, targeted solutions can be implemented. From a production practice perspective, effective rack management requires focusing on three core aspects: First, careful consideration during the design phase, prioritizing materials and structures with good conductivity, easy cleaning, and minimal residual solution to suit the different workpiece mounting requirements; second, thorough daily maintenance, including regular cleaning of mounting points and periodic deplasticizing to ensure good conductivity and a clean surface; and third, frequent inspections during use to promptly identify rack damage, rotation issues, and insufficient load-bearing capacity, preventing potential quality problems and safety risks.
Don't underestimate the importance of racks—they may seem simple, but they directly determine the stability of coating quality. Proper rack design, maintenance, and management not only reduce defects such as uneven coating, particles, and peeling, lowering rework rates, but also improve production efficiency, ensure operational safety, and ultimately create greater economic benefits for the company.
