Defect Reduction and Prevention
Defects are an ongoing source of extra costs and an impediment to the successful operation of an operating coating line. I know of no coater that does not have defect problems with both old and new products. Thus programs to eliminate defects when they occur and preventing their reoccurrence is an essential technical effort for a coating line.
Optimizing a current coating line to reduce defects is essential to insure economic success and remain competitive. It is also an essential process when a new coater is being designed and installed. The new coater should have improvements in design and fabrication so that it performs betters then the old designs. It should not be a duplicate of previous coating lines. If it is the new unit will have all of the deficiencies of the old unit.
Defects are defined as the lack of something necessary or desirable, a deficiency, or an imperfection that impairs worth or utility. Physical defects such as chatter, haze, streaks, repeats, ribbing, spots are typically considered as the largest class of defects in a coating line. However, there are other process and product performance defects that are detrimental, such as
• Coating weight non-uniformity and poor reproducibility
• Inadequate drying which lowers line speed and productivity
• Downtime to eliminate defects
• Drying sensitive product performance properties, adhesion, haze, are not being obtained
In developing technical programs there are several Process Success Factors that need to be considered when formulating the technical program:
• There are ~25 major process elements and each of them have several hardware components. All of them are important and need to be evaluated and deficiencies corrected. A deficiency in one element can limit the performance of the entire unit
• Defects created in any step in the coating line will not be corrected in the subsequent steps,
• The major process elements, web drive components, coating station & applicator, dryer, solution preparation, control systems, safety and Environmental must be robust.
• Online instrumentation to measure coating weight, and defects are necessary for optimum performance.
•Because of the complexity of the process detailed operating procedures are needed along with and on going training of operating personnel.
The program to reduce or to scale-up new product with minimum defects should have a defined protocol that covers all of the steps needed for success. This defines the steps needed to implement the program and insure success
These steps in the Defect Reduction Procedure are as follows:
1.Define all the product and process requirements that are needed for success in the marketplace. This is particularly important because the end product must be competitive and the overall needs should be established at the start of the effort rather then in the middle of the effort.
2.Insure availability of valid analytical techniques to measure and characterize properties and process variables. If you cannot measure an important property with the accuracy needed, then a quality product cannot be produced. Similarity, the process instrumentation needs to be evaluated to determine if it can control process variables with the precision and reproducibility needed. If a coating weight measurement precision of 1% is needed and the current method gives 5 %, the program will not be successful. Instrumentation needed should be ordered at this point in program to insure valid experimental data.
3.Insure that the web coating process can meet the required specifications. Variables such as line speed precision, drying load, drying configuration, tension control, cleanliness, web transport, substrate thickness range need to be analyzed to insure that they are adequate. It is possible that coater may not have the capability to make product at the goal specifications. Once identified, corrective action should be started.
4.Use the appropriate coating method. There are a wide range of available coating methods each of which has an operating window, which gives the best defect free product. For example, The Mayer rod gives a coating weight uniformity of 6-10 % and has a maximum line speed of 300 fpm. If you need a uniformity of 1% and need to coat at 600 fpm, this method will not be adequate. The curtain coater functions at high speed and will not coat at speeds below 400 fpm. A low volume product will not give good results. A previous Blog article, Selecting a Coating Method describes this procedure in more detail
5. Develop and implement operational procedures. The coating line is complex and has many variables that need to be controlled. There should be operating procedures for the coater and all products so that all variables are controlled and reproducible. Operating personnel should be trained to use these procedures and they should be kept current.
6. Quantify and understand defect losses. When defects occur, the type of defect and amount of product lost must be recorded. This should include product defects as well as coater defects such as coater downtime and mechanical problems. This will give an accurate depiction of losses and a method to calculate Economic impact due losses to help justify future efforts. It can help determine if any product has a superior or inferior performance
7. Use a structured troubleshooting protocol when defects occur. There will be defects with products as they are manufactured on an ongoing basis. A structured protocol will help determine the reason for the defects and to rapidly correct them and recommend improvements. Next article will discuss this.
8. Set up a computer database to store and share information. The converting process generated a lot of data, which should be analyzed to help understand the process and identify areas for improvements,
9. Implement process improvement technology. Many new technologies for the converting process are being developed each year. These should be evaluated and implemented so that coater performance is always improving.
10. Use statistical process control. The best method to control the process and insure reproducibility and good performance is to utilize one of the many quality control methods. The process is to complex to control without using one of these methods.
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