Quality Control Utilizing the Right Test Panels

Posted on Thursday, January 4, 2024

By Scott Crosley

Powder coating technologies are among the leading modern finishing techniques, offering durability, corrosion resistance, and aesthetic appeal to coated products. Central to the success of any powder coating process is a rigorous quality control regimen, and one essential component in this regimen is the use of test panels. The right test panel and process can help save a coater money, gain business, and troubleshoot concerns.

The Fundamental Role of Test Panels
Test panels function as surrogate substrates mimicking the properties of the products intended for coating. If coated alongside actual products in the powder coating line, these panels allow for a direct and controlled comparison of finish quality and performance. This comparative analysis not only highlights potential issues in the coating process, but also enables fine-tuning for optimal results.

The Panels
A quality finishing operation centers on visual and durable attributes. Substrate, surface treatment, paint application, and curing all play a role in producing quality painted parts. Multiple types of test panels should be deployed to provide the best results and makes quality control (QC) cost effective. The different panels to deploy include:

Bare utility panels: Inexpensive base metals that are clean and ready for coating.

Controlled substrate panels: Designed to remove any subjectivity out of quality testing that focuses on pre-paint and surface treatment processes. These panels require preparation before coating and must be from a consistent and traceable supply.

Controlled surface treated panels: Panels produced in a controlled environment with a prepared surface treatment for benchmark testing. They should have a similar, or slightly superior, quality to the surface treatment that is tested.

The Usages of Panels
Calibration and Benchmarking
Before initiating any coating job, it’s essential to calibrate your coating equipment and establish a baseline using test panels. This involves setting the equipment to the desired specifications: coating thickness, temperature, and cure time. The test panels serve as a reference point for ensuring that your equipment is operating correctly.

Visual Inspection
While a simple process, visual inspection of test panels is an effective method for evaluating the coating’s appearance and finish. Examine test panels for defects like orange peel, pinholes, craters, or color discrepancies. Identifying issues on test panels allows you to address them before they affect production parts.

Color Matching
Color consistency is vital in many powder coating applications, especially in industries like automotive and consumer electronics. Test panels can be used for color verification and ensuring that the coating’s color is consistent across all parts.

Monitoring Coating Thickness
Coating thickness is a critical factor in powder coating quality. Specialized gauges, such as magnetic or eddy current gauges, measure thickness. Regular measurement of coating thickness on test panels ensures consistent compliance with quality standards.

Adhesion Testing
Adhesion is another key aspect of powder coating quality. Test panels are subjected to crosscut or pull off adhesion tests. These tests assess the bond strength between the coating and the substrate. Proper adhesion ensures the coating won't chip, peel, or flake prematurely.

Salt Spray Testing
Salt spray testing simulates harsh environmental conditions to evaluate the coating’s ability to withstand corrosion. By subjecting test panels to salt spray tests, you can assess the durability of the coating.

Impact and Flexibility Testing
Depending on the application, some parts may need to withstand impacts or flexing—without damaging the coating. Test panels provide a sacrificial surface to help evaluate a coating’s resilience under those stressors.

Chemical Resistance
Proper testing on panels ensures that the coating won’t degrade or react negatively when exposed to chemicals or solvents.

Bare Utility Test Panels
These panels serve as the fundamental building blocks for powder coating quality control assessments. They act as the canvas upon which various powder coatings fundamentals are evaluated. As quick tests, they are good for color and cure and can be run with every color or job change to record batch data. Line settings should be recorded at the same time panels are sprayed. To gain the best understanding of application, the panel deployed should be of a similar substrate and size to typical production parts.

Controlled Substrate Panels
Controlled substrate panels are best used with more advanced testing, such as adhesion, salt spray, impact, and flexibility. These panels are designed to take subjectivity out of the quality checks that focus on pre-paint or surface treatment processes.

On top of job checks, a strong quality system will incorporate more advanced validations daily—or at minimum weekly. For this, the substrate variable should be removed and controlled.

It is important to keep in mind that not all metal is created equally. While it is compositionally controlled and classified, there can be great variability in how materials, even of the same grade, process. These differences can be caused by the allowable ranges of metals at the time of raw production or by how or what the material is exposed to during manufacturing and storage. The largest manufacturers pre-qualify materials for content and ability to properly accept surface treatments and coatings applications. This is not financially possible for all manufacturers and is certainly not the responsibility of a coatings service provider. Many manufacturing processes add oils, lubricants, and rust inhibitors that play a factor in a part’s ability to be optimally coated. Deploying controlled substrate panels in the QC process minimizes the impact of these factors in testing. By taking out these variables, a paint operation can understand if they are operating in control.

For coatings services, this type of panel can also be very useful for meeting manufacturers’ quality requirements and proving quality processes. It also protects against uncontrolled changes in customer purchasing or manufacturing practices; the panel should be of the same metal type as the majority of production, but if a true mixed metal system is deployed, cold rolled steel (CRS) is a great choice to create a baseline. Aluminum and zinc coated substrates can cause a lot of variability, particularly for iron phosphate.

Controlled Surface Treated Panels
Pretreatment processes play a critical role in powder coating operations: influencing adhesion, corrosion resistance, and overall coating durability. These panels provide invaluable data on the quality of a coater’s pretreatment processes. They can be painted at the same time as controlled substrate panels and allow the process to be compared to an industry standard process. This type of panel is also good for evaluating new treatments or working with new materials.

Test panels are indispensable tools for achieving and maintaining high quality powder coating finishes. The versatility of these panels, ranging from bare to surface treated, addresses testing parameters critical for quality control. The precise evaluation of adhesion, corrosion resistance, and effectiveness of pretreatment processes ensures that manufacturers can continuously improve their powder coating processes.

Using a variety of panels for different processes helps to maintain the cost, while aiding the coatings operator by providing variable elimination or comparison.

Scott Crosley is vice president, business development and sales for ACT Test Panels & Material Testing Services.