Defects in Coating
- Published: March 25, 2014, By Mark Miller
If you'd like to hear from Mark Miller's own lips rather than read his column titled, "Coating Matters | Defects in Coating," click on his podcast below:
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Fluid coating is only as good as the quality team says it is. Once the fluid hits the substrate many things can happen – the substrate can wrinkle, air can be trapped, and curing can do more damage than good. The key is to understand where the defect is coming from and reducing or eliminating the root cause of the defect. But before you can control the cause of the defect, you need to be able to identify the defect. Proper identification leads you down the right path, but the industry nomenclature is not the same across industries, or sometimes even within one company.
I would say the one area that coating is very different is in defect resolution (including identification, analysis, and control). Similar to how we have approached understanding coating, equipment and fluid flow, we need to be systematic in the understanding of defects. Start this process of defect analysis by listing all the possible causes of the defect-
• Substrate physical features
• Substrate chemical properties
• Coating liquid properties
• Air boundary layer properties (including static)
• Coating equipment (coating head, positioner, fluid delivery system, and hoses)
• Curing equipment
Understanding how each component contributes to defect formation is critical to defect identification and developing a path forward for analysis. So what mechanisms cause defects to occur? Some defects are continuous and some are random. Continuous defects can be streaks or ribbing shown as vertical or horizontal lines that disrupt converting operations. Random defects include air bubbles or chatter that may seem random but have a systematic response that can be identified and remedied. In order to understand the mechanisms for defect formation, you have to understand the fundamentals of the inputs. Substrates may have a surface energy that causes dewetting, rolls may be out of round and cause thickness variations downweb, or a drive may have chatter causing ribs of coating downweb.
Six common defects recognized in coating along with the common root cause solutions include-
1. DOWNWEB BANDS – this set band pattern running downweb is associated with a poor pressure gradient at the coating head. This can be reduced by improve the flow dynamics of the coating head, which is typically physical adjustments in the equipment variables.
2. UNSTABLE DOWNWEB BANDS – this shifting set of bands running downweb is associated with microvortices created at the coating head. This can be an individual streak caused by dirt or poor physical setup of the coating equipment, or interaction of the substrate to the fluid.
3. CROSSWEB CHATTER – this set crossweb band pattern is associated with a mechanical vibration. The mechanical vibration could be associated with line speed oscillation, backing roll runout variation, fluid delivery system instability, or vacuum chamber resonance. Containing this variable calls for studying the vibration frequency of the proposed variables.
4. UNSTABLE CROSSWEB BANDS – this variable crossweb band pattern is associated with oscillation or instability in the coating bead. This can be improved by helping pin the coating bead to the substrate mechanically or chemically.
5. CHEVRONS – large inverted “V”s in the coating is associated with periodic oscillation in coating. This can be improved through closer physical proximity of the coating head or reduction in acoustic waves in the vacuum chamber.
6. HASHMARKS – small “V”s in the coating is associated with gravity driven instability in the fluid attachment to the substrate. This multi-point defect can be improved with viscosity change, chemical attachment phenomenon, or curing variables.
So what can be done to prevent coating defects before they occur? There are as many defects as there are inputs, but here are some basics-
• Treat the substrate to have the surface tension and the surface energy match for proper adherence of the fluid to the substrate.
• Clean the substrate so dirt or substrate converting debris is not trapped between the fluid and the substrate.
• Monitor and control fluid flow. Small changes in delivery of the fluid to the coating station can result in coat weight variations or coating defects. Avoid air in the fluid delivery system, have proper filtration for the fluid, and use continuous flow monitors.
• Monitor incoming raw materials. Understanding if the fluid rheology has shifted or if the substrate varies in quality will lead to process shifts in the coating system.
• Avoid vibrations and fluctuations. Pumps that oscillate and drives that vibrate can create repeating defects that can match the frequency of the system creating the defect.
• Monitor line speed, tension and alignment. Mechanical issues can cause defects as often as chemical variations. Continuous monitoring of continuous systems is mandatory to understand continuous defects.
Ultimately, the best practice for defect reduction is to develop a database of common defects. Maintaining a sample library with adjoining defect description, operating conditions, incoming raw material data, and elimination procedure will reduce time and improve the knowledge base of the operators and engineers.
