Extrusion Dies
- Published: October 31, 2004, By Jamie Foederer, Extrusion Dies Industries (EDI)
Coating Control
Applying molten polymer to a substrate often entails drawdowns of 30:1 at line speeds of 2,000 fpm (610 mpm). To increase productivity and cut costs, converters look to run “faster and thinner,” as well as to reduce the loss of coating and substrate material in the form of edge trim. They demand advanced die systems for precise and consistent coating application. Because downtime is more costly than ever, they require deckling systems that are versatile, easy to operate, and resist leakage.
Adding to these challenges are the requirements of today's fastest-growing markets. While most extrusion coating is for conventional paperboard, paper, and flexible packaging, there is new business in high-barrier products, which include stand-up food pouches and medical packaging. Besides the roughly $0.50/lb cost of commodity resins such as low-density polyethylene (LDPE), the dominant resin for extrusion coating, these applications call for barrier, tie-layer, heat-seal, or other specialty polymers in the $1.50-$2.00 range.
The high cost of these materials makes it doubly important to employ dies that exert precise control over polymer application and minimize edge bead.
Deckles That Resist Leakage
Deckles, mounted at the ends of a die and used for varying the width of the coating, traditionally have entailed either of two problems: the interruption of streamlined melt flow or the tendency to leak.
External deckles, which apply wedge-driven compressible seals over portions of the exit gap, provide leak-free operation but create stagnant areas behind the deckle barrier. They tend to promote the formation of relatively large edge beads along either edge of the coating. Thicker than the target gauge maintained in the rest of the coating, edge beads form because the machine-direction tension applied to the moving web causes it to “neck in” or become slightly narrower, resulting in a buildup of polymer along either edge of the coating that must be trimmed away downstream.
In contrast to external systems, full-plug internal deckles completely seal off the ends of the internal flow channels of the die, eliminating areas of stagnation. In addition, because the deckle consists of multiple, independently adjustable blades, it can be used to fine-tune the edge profile of the polymer flow as it exits the die, minimizing edge bead formation. On the other hand, internal deckles require a great deal of operator experience and skill to carry out these adjustments without leakage. It is not uncommon for an internally deckled die to be shut down weekly to address leakage problems.
To combine the advantages of the two deckle types while eliminating their disadvantages, a dual deckling system has been developed incorporating both an internal and an external deckle that are linked to the same drive and move as one. The system can run leak-free for weeks at a time and by its nature is simple and easy to adjust.
Minimizing Edge Bead Formation
Internal deckle systems used for fine-tuning the edge profile of a coating are effective because of the tendency of molten polymer to exhibit transverse flow if lateral barriers to flow are removed in the lip land — the final section of the die flow channel before the exit. In a full bore internal deckle with edge profile control, there are three deckle blades, or “flags,” that seal off the internal flow channel from either end of the die — one in the preland section of the flow channel, the second in the secondary manifold section, and the third in the lip land. By positioning the lip land flags so they are somewhat farther from the centerline of the coating curtain than the other flags, sufficient lateral flow takes place as the polymer approaches the exit of the die to thin it out, reducing edge bead.
By varying the positions of the flags relative to one another, the operator has available a range of adjustments and edge bead thicknesses. However, because of the complexity of forces impinging on the polymer at this point in the process, there is a limit to the amount of lateral flow that can take place without the edges of the coating curtain becoming unstable and oscillating or wavering. Experience shows that “zero edge bead” is achieved only with a sacrifice of product quality and consistency.
Even without considering the case of costly specialty resins, edge bead reduction yields large cost savings. Consider the case of a 20-micron application of LDPE at roughly 1,000 fpm (300 mpm). When the amount of edge trim is reduced by one-third, from 0.7 in. (18 mm) per side to 0.24 in. (6 mm), the savings in resin alone amounts to 415 lbs (188 kgs) per day, representing a cost savings of $200. Added to this is the savings in terms of substrate material.
Longer Lands for Enhanced Control
Another way to save on raw material costs and to improve the quality and consistency of coated products is to increase control over coating properties as they are developed within the manifold and in the lip area.
Long land lengths — that is, greater machine-direction length of the area of the flow channel between the lips, just prior to the die exit — can provide two benefits: (1) They can yield a more uniform coating by giving the polymer more time, before exiting the die, to overcome die-swell forces that result from plastics memory, thereby reducing lip buildup that can downgrade product quality and result in downtime. (2) They can give greater scope for fine-tuning the gauge profile through adjustment of the flexible lip of the die. Long land lengths are advantageous, particularly in thin coating applications and when the die will be used to run more than one polymer.
While some die designers minimize land length as a way of minimizing leakage (since shorter lands correlate with lower pressures in the lip area), this compromises quality and productivity, since shorter lands promote more die swell and provide less control over lip tuning.
Auto-Die Innovations
Another key to controlling the properties of coating is gauge control — a critical factor in maintaining uniform coat weight so target coating properties are maintained with the least amount of polymer. While manual gauge profiling systems are used by extrusion coating processors, automated lip adjusting systems are widespread in the industry now. These systems can reduce transverse thickness variation from the ±4%-5% typical with manual systems to ±1%-2%.
Automated lip-adjusting systems center on a series of closely spaced, thermally actuated adjuster blocks arrayed along the flexible lip of the die. These blocks operate in response to feedback from a computerized downstream gauge scanner. When a thicker-than-target area is detected in the extrudate, power to the cartridge heaters at corresponding points in the lip is increased automatically; this causes the blocks to expand thermally, which tightens the lip gap in the area. Conversely, thinner-than-target areas are addressed by a reduction in power.
Options and Alternatives
The proliferation of new, specialized extrusion coating applications increases the need for converters to have access to a wide range of die-system alternatives.
A choice can be made between tool steel and stainless steel for die bodies. Using tool steel requires die components be submitted to an extra chrome-plating step — a particularly challenging process in the case of extrusion coating dies, which include deckles whose movement can be impeded unless normal irregularities in coating thickness are eliminated by subsequent machining. Yet chrome has distinct advantages: It is harder than stainless steel, providing greater resistance to deckle scratching; and for many coating polymers it provides better release characteristics, resulting in better flow and less die buildup.
James F. (Jamie) Foederer is regional sales manager for Extrusion Dies Industries (EDI), Chippewa Falls, WI. Prior to joining EDI in 2003, Foederer spent nine years in sales and design positions with Black Clawson Converting Machinery. He has special expertise in extrusion coating systems, is secretary of TAPPI's Extrusion Coating Committee, and is a member of AIMCAL's Coating & Laminating Committee. Contact Foederer at 315/529-3869; This email address is being protected from spambots. You need JavaScript enabled to view it.; extrusiondies.com
The views and opinions expressed in Technical Reports are those of the author, not those of the editors of PFFC. Please address comments to the author.
