A reliable, rapid defect-characterization process is essential to eliminate defects in coating operations. The characterization procedure should provide useable information, be easy to use by everyone involved with the process, provide rapid results, and facilitate the storage of data for easy retrieval at a later date if needed.

A common mistake is to focus on sophisticated techniques when there are simpler, basic techniques available. Sophisticated techniques require expensive equipment, a specialist to run them, and are time-consuming. A rapid analysis which gives 80 percent of the desired information in an hour is better than an answer that's 99 percent accurate, but not obtained for several days.

Avoid this pitfall with a three-step characterization procedure:

• Step 1 focuses on basic analysis of the defect, which can be made by operating personnel.
• Step 2 involves the use of sophisticated analytical tests.
• Step 3 concerns itself with storing results in a computer database for later retrieval, and obtaining reference samples to use in Steps 1 and 2.

Step 1 starts with visual examination of, and measuring the dimensions of, the defect. The size and spacing of large defects should be measured to obtain physical data about the defect. Examples are chatter and repeating spots.

For chatter, measuring the spacing of the bars can be used to calculate the chatter frequency according to the following equation:

Chatter frequency, cycles/sec = (Web speed, length/second) / Spacing, length/cycle

This frequency can then be related to a mechanical characteristic of the coater. For example, 60 cycles/sec suggests an electrical cause.

Similarly, spots found on the web are often caused by contamination on process rollers transferring to the web. These repeat at regular intervals on the sample. To determine if this is the cause of the spot, a long sample can be laid out on a floor and the average distance between spots measured. If the spot is from a roll, the measured distance between spots will the circumference of the roll, causing the defect.

A table should be available which shows the circumferences of all process rolls to help locate the one causing the defect.

The main analytical tool in Step 1 is to use a video microscope system. These microscopes use computer technology to digitize the images so that they can be easily displayed and stored. The digital images are stored in a computer database for easy retrieval and comparison with known samples.

In addition, there are software packages available to quantify the feature of the defects. Advances in computer technology have made these systems easy to use and available at low cost.

Portable systems are available which can be used in plant environments to capture reference and defect samples. Another advantage of these systems is that the image can be sent over the Internet to personnel who are not onsite but can help with analysis.

Microscopic analysis will help determine the nature of the defect. While a number of defects may appear to be similar to the unaided eye, the microscope will provide more detail. Bubbles, spots, streaks, base slivers, chatter and physical contaminants all have unique characteristics under magnification. Images of previous defects and known defect sources should be analyzed and put into a computer database. This will permit the current defect to be compared with previous defects and known sources of contamination.

Completion of Step 1 will often lead to defect elimination and can result in not doing Step 2. If more information is needed, then you will need to proceed to Step 2.

Step 2 in this procedure is to characterize the compositional differences between the good and the defective material. Scanning electron microscopy (SEM) and energy-dispersive X-ray electron spectroscopy will provide high-resolution and elemental analysis, from sodium to uranium.

Many types of data presentation are available to help understand the differences between good and bad material. Electron spectroscopy for chemical analysis and infrared microscopy are used for analysis of organic material. Commercial laboratories can perform these tests, or as an alternative, a local university might be eager to have chemistry or engineering students analyze real samples.

Don't neglect Step 3—be sure to save the data for future use and training. A standard PC database can be used to store both images and text, and is ideally suited for this purpose. They can also be accessed over the Internet or an Intranet so that anyone in the company can readily obtain the results. The database should contain the defect analysis data, an indication of the resolution of the defect and reference samples from process units.

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Based in Ft. Mill, S.C., the Assn. of Industrial Metallizers, Coaters & Laminators provides a broad range of services to the converting industry. If you have inquiries concerning products, equipment or material sourcing, converting capabilities or technical issues, the group's "Ask AIMCAL" service is available at its Web site: www.aimcal.org

For more information, contact AIMCAL at 803/802-7820, fax: 803/802-7821.