There are two components in the time required to start a production run. The first is the time required to install the coating applicator. The second is the start-up time for a coating process is defined as the time from the initial application of the coating solution to achieving steady state operating conditions and the production of commercial quality product. It can vary from 5 to 20 minutes or more, depending on the coating process, solution delivery equipment and the design and response time for the drying process. During this time the product can be out of specifications, AT 400 fpm this can result in between 10,000 and 50,00 sq. ft of unacceptable product being produced. If the coating weight, physical quality or drying sensitive performance properties are out of specifications and corrective action is needed, considerably more product can be lost until quality product is being manufactured.
For a high-volume manufacturing run, this loss is a small percent of the total sq ft being produced and may not be significant. However, this loss can be a significant portion of a low volume production run. Therefore, reducing the start-up time is becoming more important as the number of low volume products that are being produced on a coating line increases. If expensive and limited availability raw materials are being used the losses can have even a high cost.
Since there are many new low volume high value in use products that are being developed reducing the start-up losses is essential.
The factors that can lead to a long start-up time are:
• Coating weight not on target and out of limits and poor Md & Td profile uniformity
• Coating quality defects such as, chatter, bubbles and streaks
• Dryer control slow to achieve conditions and stabilize
• Either over drying or under drying
• Web transport line speed and tension control non-uniformities
There are technical activities that can be taken to minimize these times are.
To minimize time to attain coating weight:
• Use on-line coating weight gauge to rapidly determine coating weight so that corrections can rapidly be made
• insure all solution % solids , viscosity and temperatures are in accurately measured and in specifications. This will insure correct settings are used for coating applicator.
• For premetered coating methods calibrate all pumps and flow meters
• Insure all coating applicator settings are accurate and are at specifications
To minimize time to obtaining good coating quality:
• All of the factors that affect coating weight are also important for coating quality.
• Insure coating delivery system and solution are free of contamination and bubbles.
• Use on-line surface inspection to rapidly detect defects so that corrective action can be immediately taken.
Because of energy savings considerations the dryer set points may be set well below desired operating conditions, when the coater is not in operation. To rapidly obtain desired conditions with the large volumes of air in the impingement and floater dryers, a rapid response computer control system is needed. In addition, supplemental heaters can be used to initially supply the extra heat need during the initial coating start. They would turn off when temperatures are close to desired conditions.
With small-volume products, different coating methods may be needed for specific products. The time to change a coating applicator needs to be minimized because during that time the coating line is not producing product.
To reduce changing applicator time:
• Install cartridge coating station. This can result in change time of 5-20 mins versus hours for standard coating system.