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Web coating defects: Steps to improve coating-weight uniformity By Edward D. Cohen, Ph.D., president, Edward D. Cohen Consulting, Inc.
Editor’s Note: This article is the second in a four-part series that discusses the major web-coating defects, their various causes and the technology and methods needed to eliminate them and to prevent their recurrence. Part One appeared in the 2011 Quarter 1 issue.
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oating-weight non-uniformities are one of the major loss categories in the web-coating process. Because the performance of the coating is a function of the actual coating weight, a uniform coating at the desired coating weight, over the entire substrate, is essential to producing a saleable quality product. The coating weight must be uniform within a roll, down-web and cross-web and give an average coating weight that is within specification. If the coating weight, uniformity and roll average vary outside of limits, the product will not function as intended and will be scrapped, resulting in a yield loss and increasing costs. Another source of losses is a higher coatingweight lane in the machine direction. This heavy lane does not completely dry and causes roll laps to stick together in storage, and the roll is not useable. If too much coating is applied, it may not affect product performance but can raise costs by wasting material. This can be a major concern when expensive raw materials are used or may be in limited supply. Another need for
TABLE 1: Coater variables that affect coating weight •
Coater line-speed uniformity, web tracking
•
Coating-solution viscosity, percent-solids, surface tension, temperature control
•
Coating applicator type, fabrication precision, variables level and variation in run
•
Coating-station design, fabrication, rigidity, settingsprecision sustainability in production run
•
Pump uniformity
•
Substrate flatness
•
Preventative maintenance
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www.convertingquarterly.com • 2011 Quarter 2
improved uniformity is that many of the newer products have thinner coating weights and tighter specifications. Technical approach To obtain a uniform coating weight over the entire substrate, the coating applicator must deposit a uniform, wet coating thickness. The applied coating will then maintain its thickness throughout the coater unless it is physically damaged and is distorted. This damage can be done by contact with a roll that contacts the coating or by high air velocity in the dryer. Difficulty in obtaining high uniformity depends on several coater variables (see Table 1). All of these must be optimized and reproduced for improved uniformity. Because of the high number of variables, a structured process is the best approach to optimize and upgrade uniformity. A random procedure will be time-consuming, expensive and may not achieve the best operating conditions. The suggested major steps in this process are: • Determine product-uniformity requirements • Select coating-weight measurement technique • Select appropriate coating method • Optimize and maintain coating solution and coater-operating conditions • Upgrade coater to meet desired requirements, if needed • Institute operating procedures, quality-control systems and data-management systems. Determine product-uniformity requirements The first step in the improvement process is to determine the uniformity specifications needed. These specifications should be based on the performance required by the customer and should not be either over- or under-specified. To characterize and control uniformity, there are several key measurements: • The coating-weight values in the Transverse Direction (TD), across the substrate from edge to edge • The coating-weight values in the Machine Direction (MD), from the start to end of the substrate roll • The roll average that is calculated from the previous values • Average of all rolls in production campaign.
