Optimized extrusion coating die systems for flexible packaging
Flexible packaging is likely the largest market for extrusion coating and is certainly the most demanding. Production volumes are great, calling for coating systems that provide the highest levels of efficiency and raw material savings while entailing the least amount of downtime. The importance of ensuring the integrity of food packaging, the precision required for printing and sealing operations, and the complexity of certain coating structures—these call for stringent standards of quality and consistency. Finally, the increasing demand for thin multilayer structures heightens the importance of advanced techniques for coextrusion.
Thickness Profile Adjustment
for Coat Weight Uniformity
Many components in the extrusion coating line play a role in meeting these requirements, but the most critical element is the die. Whether the process be coating, lamination, or coextrusion coating, the die performs the key function of distributing the coating material uniformly across the width of the web in order to produce a uniform coat weight. Because polymers used in extrusion coating have flow properties that are more complex than those of ordinary fluids, the manifold or flow channel inside the die must be engineered in accordance with the flow properties specific to a particular polymer. The goal of the die designer is to create a nearly equivalent pressure drop for each particle path along the width of the die.
Fine-tuning of the transverse thickness profile takes place at the point where the coating material is about to exit the die. To provide this fine-tuning, one of the lips of the die is designed to be “flexible” in that it consists of identical adjustable segments arrayed along the width of the die exit. These can be activated to make the die gap smaller at points where initial coat weight is greater than the target, or larger where it is below target. The adjustment can be carried out manually by rotating lip-adjusting screws on each segment. Most new extrusion coating dies, however, come with an automated system that continuously measures coat weight by means of product thickness scanners and corrects for variations. Adjustment is performed by thermally actuated bolts. At lip segments where thickness needs to be reduced, for example, the bolt temperature is increased so that the bolt expands, restricting flow.
Two graphs show the difference between the thickness profile at startup of the die and the profile upon fine-tuning to achieve target thickness. While a cursory glance suggests that the graphs are the same, a closer look shows a significant improvement after profile adjustment, with a more uniform coat weight distribution. The example in this case is that of a 1,095-mm wide LDPE web with target thickness of 20 microns. Before profile adjustment, the web showed a 2 Sigma standard deviation of 0.5 micron, with an average thickness of 22 microns. Profile adjustment yielded a 2 Sigma standard deviation of 0.4 micron, with average thickness at the target level of 20 microns.
The value of this thickness control for economy and product consistency can be illustrated by considering a round-number example: a 1,000 mm web with target thickness of 20 microns, coat weight of 17 g/sq.m, and line speed of 100 m/min. Every minute of production yields 100 sq.m, which translates into 1,700 g or 1.7 kg per minute. In an eight-hour shift that amounts to 816 kg. In contrast, working at 22 micron average thickness yields an eight-hour total of 1,056 kg. In other words, better thickness control across the entire width makes possible savings of 240 kg per shift.
Edge Profile Control:
Key to Waste Reduction
Another key to significant material savings is control of edge bead, the heavier, thicker edges of a coating produced by neck-in. Ultimately a manifestation of the elastic nature of molten polymer, neck-in is caused by the imbalance of forces when draw-down tension is applied to the molten web, causing a reduction in the width of the web as it leaves the die. An important factor in the degree of neck-in that takes place is the air gap, which is the distance between the die lips and the nip roll / chill roll contact point. The greater the air gap, the more pronounced the neck-in and the larger the edge bead. (Other factors affecting the degree of neck-in include the melt strength of the polymer, the line speed, the coat weight, and the die lip gap.)
Polyethylene and some other polymers used in extrusion coating require an air gap large enough to provide sufficient time for the material to oxidize. This is necessary for the formation of an adequate bond between coating and substrate. On the other hand, the heavy edge bead that results means considerable waste of material. Since both the coating and the substrate in the edge bead must be trimmed away, the material cannot be reused.
With a die incorporating edge profile control (EPC), it is possible to achieve significant reduction in edge bead. Essentially the EPC system is a special type of internal deckle. Like all deckles, it is designed to vary the width of the web by adjusting the components at each end of the die. In addition, the EPC system makes it possible to fine-tune the edge profile of a coating as well.
In the EPC die, the deckle components for adjusting the edge bead profile are located within the flow channel. They include the manifold quill, which is the primary manifold plug; and the secondary blade, which fine-tunes the edge profile and is in the secondary manifold. Immediately following the secondary blade are the deckle rod and final lip flag, which set the width of the web.
Edge profile control is achieved by adjusting the relative positions of these components. By driving the manifold quill inward relative to the final lip flag and rod, flow within the die towards the edges is dramatically reduced. The secondary blade is positioned so that the width of the on-target material is maximized.
In a schematic showing the EPC components, the offset positions of the deckle components are shown, with dimension “A” describing the offset position of the manifold quill and dimension “B” describing the offset position of the secondary blade relative to the final lip flag and rod. The internal deckle components can be adjusted simultaneously to vary the overall coating width. Individual components are adjusted independently to profile the edges.
Deckles typically are positioned by iterative trials until the optimum result is achieved. This setting then becomes the recipe for a particular processing scenario comprised of resin type, coat weight, line speed, and air gap.
Without edge bead profiling, edge bead can be up to six times the target web thickness and trim widths can be 15 to 20 mm per side, but with the EPC system it is possible to shrink bead size and reduce overcoat requirements by a third, to the 5 to 7 mm range.
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 300 m/min. When the amount of edge trim is reduced to one-third, from 18 mm per side to 6 mm, the savings in resin alone amount to 188 kg per day. Added to this is the savings in terms of substrate material.
In a pilot line trial, the white paper substrate was under-coated so that the edge build-up on the wound roll is visible. In the photo, there is a heavy build-up on the right (with no profiling) and minimal on the left (after EPC profiling), where the coating edge is barely visible.
Internal deckles provide streamlined flow at all width settings, sealing the entire flow passage so that “dead” areas are not created when width is reduced. Position settings are repeatable, and both manual and motorized width adjustment systems are available.
Used in combination with an internal deckle, an external deckle adds effective sealing capability. “Dual deckle” systems are available in which internal and external deckles are adjusted in concert. While external deckles do not allow edge profile control and do not provide streamlining when used to adjust web width, die systems equipped with only internal deckles require a great deal of experience and skill to adjust without leakage problems. Internal-only systems can be down on a weekly basis to address leakage problems, whereas dual deckle systems can run leak-free for several weeks. Typically, downtime frequency is reduced by a factor of 4 to 6.
Special Challenges in
Coextrusion introduces a whole new dimension of challenge for die designers. Chief among their goals is to maximize layer uniformity in order to eliminate distortions that degrade product quality and consistency and can compromise key functions such as barrier.
A common coextrusion problem is called viscous encapsulation, in which a lower-viscosity layer becomes thicker towards the ends of the coating and often completely surrounds the other layers.
The problem results from the tendency of lower-viscosity layers to migrate toward areas of high shear stress, such as the walls of the flow channel. In doing so, the lower-viscosity layer acts as a lubricant for the higher-viscosity layers, and the individual layer thickness profiles become non-uniform. Viscous encapsulation is more pronounced when there is high shear stress at the layer interfaces, when the viscosity difference between layers is large, and when the die is very wide, since there is more time for the encapsulation to develop.
To resist viscous encapsulation, the primary manifold—the largest chamber in the flow channel inside the die—must be engineered to promote lower shear stress levels. The red, orange, and yellow areas in the illustration show high shear rate fields. Shear stress equals viscosity times shear rate. The shear stress level at the upstream manifold wall is relatively high, and lower-viscosity materials will tend to migrate toward this wall as they progress down the flow channel, resulting in interface distortion. To reduce the shear rate and thus shear stress at the upstream wall, the primary manifold is engineered with an elongated teardrop shape, as shown at right in the illustration, rather than the classic teardrop shape.
3-D flow calculations confirm that the elongated teardrop manifold shape generates less shear on the manifold walls and at the layer interface locations (as indicated by the dotted lines in the cross section views).
Ease of Maintenance
to Maximize Uptime
Deckling that minimizes die leakage is only one of the measures available to increase coating-line productivity by reducing downtime.
To minimize the frequency of shutdowns for cleaning, flow channel surfaces are highly polished, resulting in low surface energy. Industrial hard chrome plating is often recommended, for example, since it has nearly half of the surface energy of nickel plating, so that deposits of surface build-up take longer to accumulate. Finish levels also affect surface lubricity and the length of runs between cleanings. Chrome-plated surfaces are much harder, around 65 Rockwell C, than alternative platings and resist wear from cleaning and handling. Since chrome-plated surfaces wear the least, the surface finish remains highly polished for longer periods for durable, build-up resistant performance.
Because of the high operating temperatures in extrusion coating, carbonized residue can become trapped within the die, causing lines or streaks in the coating. Often this degraded material builds up at the approach to the final lip land. One device for removing this build-up is a tool has the shape of the lip flag and deckle rod built into it, so it can be used immediately. To gain access to the lips, the external deckle can be retracted from the die lips while remaining engaged for alignment when it is returned. There is no need to disassemble the end seal or packing plate for routine lip cleaning.
When it is time to split and clean the die for preventative maintenance, the entire deckle assembly on each end of the die can be removed by detaching four mounting bolts. There is no need to disassemble the deckle and then recalibrate all of its components.
A custom-engineered deckle cart with a jack stand is available for handling the deckle components and safely wheeling them away from the die. This protects the maintenance crew while also preventing wear to the die or the deckle.
Modern extrusion coating and laminating die systems incorporate a range of technologies for controlling coat weight, minimizing edge bead, optimizing coextrusion, and reducing downtime. As a result, flexible packaging manufacturers can obtain greater economy through reduced raw material consumption and increased productivity. At the same time, they can achieve the high levels of quality and consistency so essential for today’s sophisticated packaging products.