Dual-Deckle System for Trouble-Free Web Width Variation
- Published: December 01, 2003, By Sam G. Iuliano, Extrusion Dies Industries LLC
Following is an expanded summary of a complete paper available on the TAPPI web site at tappi.org. On the page, click "the PLACE" in the section designated "Journals."
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Application: When incorporated into a die with an appropriate flow channel design and a modern flex lip adjusting system, a dual-deckle system will significantly improve operational efficiency by minimizing costly down time.
Recent extrusion die tooling developments provide real solutions to problems that have plagued the coating and laminating industries for years. Successful die designs for the extrusion coating and laminating industry must address a specific set of performance needs today:
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Trouble-free coating width variation for versatility and efficiency
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Ability to reduce edge bead size to minimize expensive waste
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Precise control of coating weight uniformity to achieve desired physical properties with the least amount of coating material. This precise control is necessary for a variety of coating materials with differing rheological properties.
Coating Width Variation
Devices that adjust extrudate width — commonly called deckles — have had use by the extrusion industry for decades. Figure 1 shows a system that combines internal and external deckles with a common drive unit for coordinated movement. The device in the figure represents the deckle assembly that would mount to each end of an extrusion coating die. The bronze-colored components are the internal deckle plugs that completely seal the ends of the internal channels. This full plug approach eliminates stagnation or dead areas in the channel. The silver-gray colored component at the bottom of the figure is an external back-up deckling system. External deckles can provide highly effective sealing characteristics.
By combining these two deckle systems that were previously thought mutually exclusive, a significantly better approach to deckling is possible. Just as the coextrusion of two coating materials leads to a higher performance product by combining the superior adhesive properties of one material with the superior strength properties of another, this new deckle concept leads to a dramatic performance improvement. Processors find that die systems equipped with only internal deckles need considerable experience and skill for adjustment without leakage problems. They also find that external-only deckle systems do not allow edge profile control and are not as streamlined. A combined dual-deckle system allows simple, convenient width adjustment procedures without leakage issues. The impact that a dual deckle system will have on a typical coating operation can be quantified by considering downtime due to leakage. Internal-only systems can be down on a weekly basis to address leakage problems, but dual deckle systems can run leak-free for several weeks.
Edge Profile Control
A chief goal in extrusion coating is to maintain a uniform coating thickness across the entire application width. This is not always easy. The common problem of edge bead — an increased thickness along both edges of the coating — makes trimming the edges to meet product specifications necessary. Edge bead is particularly costly in extrusion coating because the scrap includes coating and substrate. The edge beads form by an imbalance of forces that occurs when tension is applied to the coating web. As the web becomes oriented in the machine direction, it necks-in — becomes narrower — resulting in an accumulation of material on the edges. By adjusting the relative positions of the internal deckle blades, processors can minimize the width and size of the edge bead. The result is a reduced overcoat requirement of approximately 5-7 mm per side instead of 15-20 mm per side for LDPE. Waste decreases significantly.
Coating Uniformity
Three key features increase the amount of control the processor has over coating weight uniformity:
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Longer lip land lengths provide a significant response to an operator or gauge-control system adjustment
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A flow channel design is available that combines a varying geometry in the center of the die with a constant geometry on each end. This allows fully adjustable internal deckling and the varying geometry in the center helps to promote flow to the ends of the die.
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A thermally isolated automatic lip adjusting system provides extremely accurate product tolerance.
Longer Lip Lands
Many designs for internally deckled dies use a very short final lip land at the exit area of the die — only 3-5 mm long. Some die manufacturers do this to minimize the pressure generated in the lips so the die provides less challenge to seal. This represents a compromise since shorter lip lands will promote more die swell and afford less lip tuning control.
A better approach is to provide an appropriate lip land length with a good balance between the pressure drop across the lip land compared with the pressure drop across the entire die. Depending on the materials and rates of processing, lip lands can range from 10-25 mm. Longer lands can allow more accurate distribution of a broader window of materials by one die such as LLDPE and LDPE.
A new automatic lip adjusting system can provide advantages over previous systems. Since its thermal translator blocks mount outside the die body, significantly less heat transfers between the body and the blocks than with previous systems mounted within the die body. This eliminates much thermal “cross-talk” and allows achievement of accurate gage control more quickly.
A translator material such as a beryllium-copper alloy is ideal since it combines high thermal conductivity with mechanical strength to provide a rapid and repeatable response to a control action. Due to a large coefficient of thermal expansion, the translators also provide considerable stroke — ± 0.38 mm of automatic adjustment range. A lever system can provide a large amount of stroke with a smaller translator block to improve the speed of response even more.
Conclusion
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 variety of coating materials.
