Home > MacroLetter > 10.2 -Autumn 2006 > Fundamentals of Cast Film Extrusion II

Fundamentals of Cast Film Extrusion Technology
(Part 2 of 3)
 

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Flat Die System

It can be said that the die system is the heart of any coextrusion line.  The die system is formed by the coextrusion feedblock, the flat die and the melt transfer adapters that transport the different molten polymers from the extruders to the feedblock inlet ports.  The quality of the coextruded film and the productivity of the process are greatly dependent on the design and performance qualities of the die system.

The primary function of the die system is to form a multi-layered film that is uniformly distributed across the width of the die with thickness variations on the film and thickness variations on each individual layer within industry accepted tolerances (not to exceed ±2.5% for the total thickness and within ±15 to ±20% for each layer).

Upstream from the feedblock are the melt transfer adapters.  The design criteria of this capillary system must consider parameters such as material residence time, pressure drop and temperature control.  For instance, an excessive pressure drop could be addressed by increasing the pipe diameter; however, this in turn would increase the residence time of the material and increase the possibility of material degradation.  Also, accurate wall thickness sizing and proper heater specifications are necessary to prevent the pipes from heating or cooling the melts that they transport.  It is the task of the designer to find the proper balance between all these variables.

The coextrusion feedblock arranges the different melt streams in a predetermined layer sequence and generates as many melt streams as layers are to be in the final coextrudate. Once this is done, each stream adopts a planar geometry, meets its neighbouring layers and the final planar coextrudate is formed.

Coextrusion feedblocks are grouped into two categories:  Fixed and variable geometry blocks.  In the upstream section of these blocks the so-called selector plug or selector spool is found.  This cylindrical shaped removable part is responsible for routing each melt stream into its final position in the coextrudate.  The plug, if required, also splits those streams with a material that feeds more than one layer in the structure.  If a different layer sequence is required, it can be achieved by simply changing the plug.

Fixed geometry blocks are most effective when the production line is devoted to only a few different products that are similar in their rheological behavior.  However, it is worth noting that these blocks have removable flow inserts that could be machined or replaced if required to process a wider spectrum of materials.    

Variable geometry feedblocks are ideal for the coextrusion of high added value materials or when the scope of the production line is more diversified.  In general, these blocks feature movable internal components that can adjust the width distribution of an individual layer prior to meeting with neighboring layers and/or its velocity, which in turn affects its shear rate and viscosity.  Thus, problems inherent to coextrusion such as that of layer distortion and interfacial instability can be overcome with adjustments of the feedblock.  

In spite of all the capabilities of coextrusion feedblock technology to address flow anomalies inherent to coextrusion flows, the production of an optimal coextrudate is only possible if the feedblock operates in conjunction with a die conceived and properly designed to process a coextrusion flow.  The perfect synergy between the die and the feedblock is what will guarantee a high quality product.

A well designed die must guarantee that in the process of spreading the coextrudate coming from the feedblock the flatness of each individual layer is maintained within a tolerance of ±15 to ±20%.  It must also be designed so that the residence time is not excessive in order to prevent degradation problems or in some cases to prevent undesired heat transfer between layers.  The die must also be designed so that the pressure drop is kept at a level that is normal within the extrusion process.

It is also critical that the die has the appropriate size, sufficient mass of steel and proper mechanical design to guarantee thermal stability and to minimize the so-called clam-shelling problem that manifests itself as an excessive deformation of the die lips when the die is subjected to the high pressures inherent to the extrusion of thin films.   
     
Recent advances in die technology have boosted the productivity of cast film production lines.  Special reference can be made to the so-called internal deckles.  Inserted on both ends of the die, the deckles allow changes to the film width and the consequent reduction of trim.  They can be fixed or adjustable and their length can exceed 20 inches.

Edge encapsulation technology has been introduced in recent times to reduce the negative financial impact of material waste caused when the trim of the coextrudate is not recyclable.  The previous figure shows a band of a single material being coextruded side by side with a coextrudate.  The encapsulation material is of low cost, recyclable and has high mechanical properties. The encapsulation material mainly forms the trim, which allows for its reinsertion into the production process and great savings in material cost.  In addition, edge encapsulation technology is fully compatible with the internal deckle technology.

Cooling Unit  

The cooling unit is comprised of a primary quenching roll, a secondary roll, a motorized roll positioning system for proper vertical and cross machine direction alignment of the rolls, and in many cases a vacuum box and/or air knife.

The rolls are typically chrome plated to achieve a better surface finish and to enhance the heat transfer process during film cooling.  The cooling agent is commonly water that circulates inside the rolls.  The primary quenching roll cools one side of the film while the secondary roll cools the opposite side of the film.

The die is positioned above the primary quenching roll at an angle that varies from 45° to 90°.  The distance between the die lips exit and the roll ranges from 0.8 to 2 inches.

The cooling system allows the line to operate at high speeds.  As the line speed requirement increases, so do the diameters specified for the rolls.

The rolls must be perfectly aligned with the web to guarantee a uniform tension and to minimize thickness variations across the width of the film.  In addition, the angular velocity of the rolls must be well controlled to prevent film thickness fluctuations in the machine direction.

The use of a vacuum box, connected to the die fixed body, is necessary in certain applications, like that of Cast PP, that require a more efficient cooling.  PP materials, if not cooled aggressively, tend to form crystals that ultimately give rise to hazy films.

The vacuum box removes entrained air between the primary quenching roll surface and the film to minimize the air barrier between the hot web and the roll.  This air barrier, if not reduced, acts as a thermal insulation cushion that impedes the film cooling process. The box also reduces the amount of necking in the film and the air gap and allows higher line speeds to be utilized.

The vacuum box can be combined with an air knife or an air chamber to further enhance the web cooling.

(To be continued)