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Advances in Extrusion Coating Die System Technology
by Sam Iuliano, Extrusion Dies Industries, LLC

APPLICATION: Design features that are unique to extrusion coating address specific needs such as convenient coating width adjustment, reduced overcoat requirement, and rapid achievement of coating weight precision for a wide variety of coating materials.
The basic function of a die is to transform a molten polymer feed from a melt pipe or a co-extrusion feedblock to a significantly wider and thinner web. A typical melt pipe exit geometry is circular in cross-section with a diameter in the range of 12 to 65 mm. The exits of co-extrusion feedblocks are often rectangular or racetrack shaped with dimensions ranging from 12 by 25 mm to 20 by 100 mm in cross-section. A common die exit geometry is approximately 0.5 by 2540 mm so the spread ratio is significant. All this spreading in the cross-machine direction and thinning in the machine direction happens over a relatively short distance.

The distance from die entrance to die exit ranges from about 200 to 350 mm for single channel dies. To achieve this dramatic transformation, die flow distribution channels—often called manifolds—provide a nearly equivalent pressure drop for each polymer path along the width of the die. Many manifold designs can accomplish this uniform spreading of the melt. The simplest version is called a “T-die.” It has a constant cross section and establishes relatively uniform flow distribution.

Extrusion die tooling continues to develop and evolve to meet increasingly stringent performance requirements. Modern die designs can provide processors with convenient coating width adjustment, a reduced overcoat requirement, and rapid coating weight precision control for a wide variety of coating materials. Dual manifold dies can provide uniform layer distribution even in cases with a large difference in melt temperature. Adjustable co-extrusion feedblocks are available to compensate for changes in layer ratio, to change the number of layers, or to adjust layer distribution.

On-line Gauging As A Process Troubleshooting Tool
by Ted Schnackertz, NDC Infrared Engineering

APPLICATION: An on-line gauging system is an important source of real time diagnostic information.
An on-line gauging system goes beyond day to day control. It provides the base line data reference on current process capability, and it is a continuous monitor of that performance. Using this data to identify any performance degradation and subsequently analyzing the cyclic variations of the various displays can lead to rapid problem diagnosis. When coupled with process knowledge, it can also provide rapid resolution. Since interpretation of the data is fundamental in determining whether there is a problem to solve, understanding the different process variations is a necessary first step.

The first step in troubleshooting a process problem is determining whether the variation is common cause (process is stable) or special cause (process is unstable). Since a gauging system displays data for all three components of variation, one can build a base-line data history. This will help to determine whether the variation is common cause or special cause. A process may be in statistical control but not able to meet specifications as a result of excess common cause variation.

Short time machine direction (STMD) variation cannot be controlled by an on-line gauging system due to the short period of less than one minute. In addition, it is always present in displays. A display mode called “in-line” or “single point” can establish STMD levels under normal operating conditions to form the base for a process in statistical control. One observes STMD by positioning the gauging system at a single point on the web while collecting data for 1-2 minutes. This should occur at three positions across the web. Observing the STMD cycle again during process problems can give clues or even pinpoint a source of the variation.

An on-line gauge can usually control long term machine direction (LTMD) variation provided the process is in statistical control (common causes only). Special cause variations prevent tight control. The trending display is a good indicator of LTMD and with reports like shift and roll can form a base line reference for the process. The shift reports provide roll-to-roll average thickness and sigma variations.

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