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Cast Film Dies and Auxiliary Equipment

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Application: Quality of an extrudate in a cast film system depends on the entire die system because of the synergistic effect of the components.



Reference to a cast film die typically addresses it as part of a die system because of the synergistic effect of all the system components. Proper melt delivery through capillary pipes, combining adapter for coextrusion, and spreading and thinning in the die contribute to quality of an extrudate.

After creating good quality melt in an extruder, the next step is delivery of that melt through the melt transfer pipes. These capillary pipes or adapters should not add or remove excessive heat from the melt stream. Melt transfer pipes should have substantial mass to maintain uniform temperature and be heated appropriately to the melt temperature of the polymer they are conveying. To size a melt pipe correctly requires balancing pressure drop and residence time. A longer transfer pipe gives a greater pressure drop. Simply increasing the inside diameter of the pipe will reduce pressure drop but give increased residence time.

After delivery of a quality, uniform melt to a combining adapter or die, the material undergoes spreading and thinning to the desired width and thickness. The goal is to produce a film of uniform gauge or thickness with minimal residual stress and minimal edge trim. The design of most commercial dies produced today uses control volume methods. Such design breaks areas of the die into small segments and calculates flow in the combination of these segments to provide a uniform pressure drop in each segment. One assumption is that uniform pressure drop creates uniform volumetric flow.

A die of optimum design will operate mechanically to produce the flattest possible product. Typically, cast film dies are flexible lip dies with the capability to adjust the lip gap or opening to produce the ideal product gauge. Depending on several variables, dies can have manual adjustment to achieve gauge uniformity of about ±3% to ±5%. For an even flatter film, automatic lip adjustment systems are possible.

Profile Control
While the interaction between gauging systems and an automatic die vary, typical results of automatic lip adjustment or automatic profile control (APC) are reductions of product thickness variation by half. An important consideration in APC is the speed of reaction to a signal. With cast film die gaps averaging 0.015-0.030 in., a minimum adjustment range is necessary but adjustment precision becomes critical. While range of adjustment can also be a critical parameter, proper die design will usually reduce the amount of flexible movement required by the die lip to produce an ideal product thickness. Measurement of the reaction time of an automatic die bolt is the time constant. This is the time necessary for a die bolt to elongate 62.3% of its equilibrium point. Using on/off control, power is applied to the thermal translator for some time and is off the difference between that number and 100%. APC control typically occurs in a span of 30%-70% around a mean of 50%. The measured time constant for an automatic die ranges from under 2.0 min. to greater than 20 min. Time constants should be uniform in the heat-up and cool-down modes.

A time constant as short as possible will have the most responsive control. For example, the result of having a bolt with a two-minute time constant is that the translator has executed the desired control movement two minutes after receiving a signal change from the gauging system. Rapid response means decreased start-up time. The times from start to save decrease to a matter of minutes rather than hours. Better gauge uniformity increases translate to flatter film. This gives flatter rolls, improved product performance, and increased customer satisfaction.

Another important characteristic of APC is thermal stability. Because most APC control uses a thermal bolt, this bolt must be stable and produce a uniform desired movement quickly. Recently, thermal stability has received greater emphasis with the introduction of new generation thermal translators. The new style translator is typically isolated from the die body and cooled by natural air convection rather than forced air. Natural air convection provides a more uniform cooling with reduced energy requirements while maintaining rapid response time. Because thermal translators can themselves be a source of heat, careful design considerations have been made to isolate this heat source from the die lip. Hot spots on the die lip will typically lead to film streaks or bands. Keeping the translators isolated from the die body provides cooler ambient air. This further enhances the performance of the automatic bolt.

Coextrusion
In coextrusion, a concern with individual layer uniformity and total gauge exists. People have traditionally expected the feedblock to provide uniform layer distribution. Consideration of the design of the combining adapter (feedblock), the method of combination, geometry, and the flexibility of the adapter are all important factors to the total suitability of the finished product. Physical effects imposed on the composite downstream of the combining adapter can be detrimental to uniformity of individual layers.

For more than a decade, feedblock profiling has been an art practiced in the coextrusion industry by shaping melt composites to compensate for the difference in polymer viscoelastic properties and the response to shear stress. Initially, only viscous encapsulation was considered in examination of nonuniform layer thickness. Choosing polymers using similar viscosities or within a given range of viscosity ratios with feedblock profiling was the method for acceptable layer distribution.

In the late 1980s, the focus moved from compensating for the deleterious affects of the die to accomplishing changes in die design. Demonstrated theory showed that a direct correlation exists between manifold geometry and layer uniformity in coextruded structures. Focusing on die manifold design as the key to achieving layer uniformity led to the introduction of unique manifold designs incorporating a large aspect ratio among other attributes. A large aspect ratio manifold minimized deformation of the combined materials. Both the planar manifold shape and the lateral manifold shape are critical to coextrusion die design.

Creating a T-slot design die with a large aspect ratio manifold provided a big improvement in coextrusion layer uniformity. Several additional attributes of the extrusion die manifold also enhanced die performance. Current designs include these attributes. Another benefit was uniform distribution force onto the flow surface to eliminate the recurring problem of “clam shelling” inherent in coathanger dies.

Cast film dies typically run with relatively small die gaps. While all dies will deflect or open under the force exerted by the polymer, clam shelling is the nonuniform deflection of a die. Because of increased force exposure, the center portion of the die body deflects more than the ends of the die. This nonuniform deflection is dynamic because die pressures are continually changing as melt pressures change due to viscosity variation and polymer throughput changes.

While providing a constant cross section, T-slot dies have historically produced a naturally heavy center flow. A pressure compensating device was necessary to achieve a balanced flow through the die. In a coathanger die, this pressure compensating zone is the preland area, i.e., the angular component resulting from the manifold placement. This angular preland was a compromise fit to the viscosity of the composite polymer. Since polymer viscosities are exponential, fitting the straight line preland to the exponential viscosity is usually impossible. The result of this type of preland is usually heavy and light flow areas in the extrudate commonly called a “W” pattern.

A second method of pressure compensation often incorporated with both T-slot and coater dies is an adjustable restrictor bar. This compensation method is also undesirable because of the disruptive effect on the composite structure and the uneven stresses induced in the extrudate. New designs enable the isolation of the unique coextrusion manifold and a similarly unique two-stage preland design. With total decoupling of the manifold and the preland, a complete freedom exists to design the ideal manifold with perfect flow balancing in the same die. The combination of this preland design and unique manifold created a new generation of die designs.

Edge Trim
Edge trim has always been an area of focus to improve productivity. Edges of product often contribute negatively to yields, line speeds, and recycle. Controlling width and composition of these edge trims can produce a significantly positive effect. Generically, encapsulation has been associated with feedblock coextrusion where it met with almost no success. Several years ago, edge encapsulation in the die became commercial.

This method of edge encapsulation enabled precise control of the edge encapsulation composition and width. Ability to control the edge encapsulation composition allows addition of a monolithic edge of a material chosen specifically for its desirable characteristics. Encapsulation materials may be chosen specifically for their price, recycleability, or melt strength. An encapsulating material of high melt strength can control edge weave in higher line speeds. A material of low cost will reduce levels of expensive materials in the recycle stream. Materials that are typically nonrecycleable can be eliminated from the recycled edge trim.

After a cast film die system has produced a uniform web that has exited the die, the web requires efficient cooling. When depositing the web on a casting roll, intimate contact must occur between the web and roll. A dual chamber vacuum box has had use to assure this contact. Another factor is to control the angle at which polymer web exits the die lip. Adjusting the vacuum on the vacuum box assures that the film exits the die perpendicular to the lip face. This prevents lip accumulation caused by the web dragging across a lip.

Conclusion
Cast film dies have unique properties. Using careful consideration to the finished product and application of proven designs, the performance of an extrusion die system can be highly profitable.


 

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