Powder Coating Defects in High-Pressure Aluminum Die-Cast Lighting and Motor Parts and Solution Recommendations

Electrostatic powder coating method is known as the application of various powders electrostatically from a reservoir through a hose to the part to be coated with a high voltage generator. The working principle of powder coating is the creation of an electrostatic field between the grounded workpiece and the spray gun tip, at the part surface. This application provides lower coating costs compared to other paint systems, fewer production risks, and advantages against environmental pollution. Powder coating facilities are frequently encountered in industry and are facilities where risk analysis must be conducted and explosion protection documentation must be prepared in accordance with ATEX regulations. This article will analyze powder coating errors and solution proposal processes on high-pressure die-cast lighting components.

1.1. General Information on Powder Coating

Developed in parallel with global environmental awareness, a painting method emerged based on the principle of metals and powder coating material being charged differently in a magnetic environment, and as a result, the powder material covering the metal surface through magnetic effect. The metal surface coated with powder coating is cured by baking at high heat of 180-200°C, and thus a painted surface that becomes part of the metal is obtained.

Electrostatic powder coating, today's leading environmentally conscious product, proves this by the fact that 98% of the coating amount used during application adheres to the metal. The application is the transfer of coating particles by loading with very low 0.7 mA and very low direct current onto the grounded part. There is no risk of electrical shock in the system as in other electrical hand tools (drill motor, etc.). The coating gun charges negatively as it exits. The coated and grounded part surface follows hook-shaped force lines as it travels.

Coating particles are drawn onto the part according to the principle that opposite poles attract each other. However, since they have the same pole among themselves, they repel each other, ensuring homogeneous distribution. Thus they reduce the likelihood of sagging and dripping to negligible levels. The coating formed after polymer powders are electrostatically dispersed onto the parts to be coated and then melted is called powder coating. Electrostatic powder coating is a solvent-free surface coating method. The coating material is very fine powder coating particles that form the final coat layer. Powder coating is applied in the paint booth using special paint guns. As it passes through the gun, electrostatically charged powder coating particles adhere to the material to be coated in the booth, and the coating process is completed. For the powder coating to adhere properly to the material surface, the material must also be grounded very well. Excess applied coating is collected through the paint recovery system in the booth and put back into use. The paint recovery system is one of the greatest economic advantages of electrostatic powder coating technology. After the material is coated with powder coating, it enters the curing oven. The oven temperature of 200ºC causes the powder coating to melt and adhere to the material. As a result, a very durable, economical, environmentally friendly, wide color range and glossy surface coating is obtained.

The absence of solvent, surface quality, durability, paint recovery system, economy and environmental sensitivity make electrostatic powder coating a more attractive alternative compared to traditional painting processes.

Recent continuous development of powder coating technology applied on aluminum and steel parts has brought about a significant breakthrough in the field of industrial surface coating. Powder coating technology, which easily provides solutions to many needs, has numerous advantages over traditional paints such as coating, appearance, quality, durability, economy and environmental protection [1-4].

1.2. Advantages of Powder Coating Technology;

• Perfect Coverage, • Maintenance Coating, • Environmental Protection, • Economical, • Return on Investment.

1.3. Powder Coating Application Techniques

The application of powder coating is based on an electrostatic principle. It can be described as powder coating particles being sent to the spray gun with air at a certain pressure and electrostatically charged and sprayed onto previously grounded material. An attraction occurs between the powder coating and the surface due to the negative (or positive) charging of the coating, and the coating adheres electrostatically to the material. It then melts in a 200ºC oven and adheres to itself and the material. Based on this principle, research has shown that coating particles can generally be charged by 2 different methods. These are; Corona (ionization) and Tribo (electrokinetic) techniques. a) Corona Technique: This technique is based on the principle of the cascade located inside the gun body sending high voltage (max 100 kV) to the needle at the tip of the gun, thereby charging the coating exiting the gun with electricity. High voltage disrupts the neutral state of air creating negative ions. These ions attach to each other or the nearest object, namely powder particles. Thus, powder coating particles are also charged and sprayed onto the previously grounded surface. Subsequently, the material enters an oven at a temperature between 180ºC - 220ºC, causing the coating on it to melt and adhere firmly to itself and the surface. The greatest advantage of the Corona method is that it can be charged easily and is successfully applicable with every type of thermosetting powder coating currently produced. b) Tribo Technique: In this system, after air and coating reach the gun, they pass through specially designed paths and are electrostatically charged through the effect of friction. In the Tribo system, there is no cascade; powder particles are kinetically charged through friction. This mechanism is activated by the contact of two substances with different charges (one inclined to accept electrons, the other to give electrons) with each other. The specially designed charging paths inside Tribo guns are made of PTFE, or Teflon, which due to its mechanical properties does not allow excess powder coating accumulation and enables rapid charging without adhesion to the surface.

1.4. Applications of Powder Coating

Powder coating has a very wide field of application from automotive to construction, from white goods to bicycle industry. Additionally, all types of aluminum sheets, steel doors and elevator systems and all types of metal materials are places where powder coating is used. As can be seen, powder coating has a very wide field of application.

2. Powder Coating Application Errors

Various problems arise during powder coating application depending on many factors. This study will explain problems encountered during powder coating application and their solutions. In the electrostatic powder coating process method, error types can occur depending on many different factors such as apparatus design, gun position, air flow rate, environmental factors, amperage and voltage values. To prevent these errors, the problem must be correctly defined, root cause analysis must be performed for the error using problem-solving techniques, and necessary actions must be taken and results monitored. In this study; errors encountered during the powder coating process of lighting and motor body parts produced in the high-pressure aluminum injection die-casting process and solution methods will be examined (Table 1). Table 1. Errors encountered during the powder coating process of parts produced by high-pressure aluminum injection die-casting method and solution methods

3. Error Analysis and Solution on Sample Components Used in the Lighting Sector in the Powder Coating Process

In this section, root cause analysis will be performed using problem-solving techniques for powder coating errors encountered on a sample lighting part produced by aluminum high-pressure injection method in the lighting sector, and data on results obtained after improvements will be shared.

3.1. Current State Analysis

During the powder coating process of aluminum body parts used in the electronic pump sector, many errors were encountered, and an error tally form was created and data was collected from the field to perform current state analysis. As a result of the collected data, from the error tally graph (Figure 1), error types and encounter frequencies were defined in percentages on a Pareto diagram. All errors corresponding to 80% of total errors must be addressed. In the chart, the error types comprising 80% of total errors were identified as: orange peel 52.35%, frame effect 13.42%, blister 12.08% and crater 6.71%. The root causes of encountered error types may be based on the same reason or may stem from different reasons. For this reason, it is essential to correctly define the error type and make an evaluation based on the location of the error on the part. [caption id="attachment_123740" align="aligncenter"] Figure 1. Current State Error Pareto Analysis[/caption]  

3.2. Solution Plan

Following the fishbone analysis performed for the current situation, field work was conducted taking into account all possibilities. It was observed that during powder coating, the problems in the problematic area arose from gun adjustments not being consistent, such as gun nozzle size, gun angle, bench grounding, and air pressure.

3.3. Evaluation of Results

Orange peel mentioned in the Pareto analysis (inadequate grounding in the problematic area, not making air and gun adjustments) produces inevitable results. As a result of the root cause analysis performed, the root cause of the error was identified as inadequate gun positions, apparatus cleaning, and air pressure in the problematic area. For this reason, applications in the problematic area were increased. In the current state error type Pareto analysis (Figure 1), the orange peel error was seen as 52.35%, whereas, as seen in the post-improvement error rates chart (Figure 2), no orange peel error was encountered. The labor, melting, recycling, etc. costs gained after the improvement made are shown in the total gains table (Table 2). [caption id="attachment_123741" align="aligncenter"] Figure 2. Current State – Post-Improvement Error Rates Chart[/caption]

4. General Conclusions

Powder coating use worldwide is increasing day by day. The reason it is used more in areas such as lighting, automotive and white goods is that it exhibits low cost with high corrosion resistance. Surface defects damage both appearance and the coating's ability to provide protection. Some of the most common defects are cratering and other surface tension-induced defects, dirt and blistering. Preventing or improving such problems is very important. The logic that quality products are shown by quality coating enables us to compete with European countries. When possible error causes in the sample lighting part are examined, the Pareto analysis identified orange peel error at 52%. In solving the identified error, with improvements made to gun positions, apparatus cleaning, and insufficient air pressure, no orange peel error was detected. Thus, by eliminating this error, the operation gained TRY 18,337/annually.

5. References [1] B. Fitzsimons and T. Parry, Fitz's Atlas 2 of Coating Defects, MPI Group, Surrey, U.K., 2011 [2] Kenneth B. Tator, ASM Handbook Volume 5B: Protective Organic Coatings, ASM International, Ohio, USA, 2015 [3]RochesterInstituteofTechnology,PowderCoatingProcess, https://www.rit.edu/affiliate/nysp2i/sites/rit.edu.affiliate.nysp2i/files/pdfs/powder_coating_process_final.pdf [4] Pulver, Electrostatic Powder Coating, http://www.pulver.com.tr/electrostatic-powder-coating