AIMCAL's Extrusion Coating Blog by Eldridge Mount

One feature of extrusion coating and lamination which impact on the use and design of equipment is the variable length and width of customer orders. In a perfect world the physical dimensions of a manufacturing process remain the same for many days at a time, giving potential efficiencies of scale and process stability. However, when producing products for end users, such as in commercial packaging applications, the need for special graphics and variable size and numbers of products to package, conspire to limit the size of orders in terms of both width and number of impressions.
      But the design of the line has to be fixed based on some multiple of expected widths and when this varies the width of the extrusion coating or laminating layer must be limited to conserve materials. This could be done by changing die width, by changing the die, but this is a time consuming process requiring many hours not to mention the cost of several dies, which hopefully fit the business. In stead, we use deckles for the die, which are mechanical devices to limit the width of the extruded melt curtain. Basically there are two general categories of Deckles, external and internal.
      External deckles are bolted to the die lip and simply block the ends of the die. They create dead areas for flow behind them increasing the residence time for the polymer and can lead to degraded polymer in the die (figure 1). This requires periodic stopping and purging of the die to maintain acceptable product. This is acceptable because it allows rapid change of width without too much trouble. However, as run length increases the purging requirements can impact productivity.
      Internal deckles on the other hand make use of internal plugs which fill space in the manifold and prevent the formation of dead areas in the die manifold. However, they are more expensive and can take longer to change than external deckles, but if run length is long then the cost of the change can be justified by improved productivity. Certain die geometries are not well suited to internal deckles and dies with uniform manifold cross sections are ideal for this technology. Newer die design concepts also permit easy width changes with minimal down time.
      The down side of movable internal deckles is that they can allow the formation of a degraded polymer area on the inside surface of the manifold. While running at a fixed width, this may not represent a problem, but if the width of the melt curtain is increased there can sometimes be a die line from the internal contamination which might require extensive shimming of the die or a cleaning to remove. Of course this can be minimized if the width changes can be staged to start wide and go narrower.

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Wed Aug 06 12:33:00 CDT 2008