Advanced Multilayer Technology for Commercial Barrier Film Coextrusion
Coextruding blown PVdC based barrier film in commercial widths can be a real challenge. PVdC is corrosive and easily degradable. Special corrosion-resistant materials are required in making the die, temperature controls have to be extremely precise and production has to be interrupted frequently to take the die apart for cleaning. Degradation is not a big issue in small coextrusion dies because the residence time inside the die is relatively short. The real problem arises when trying to produce PVdC based barrier film using 14” (356 mm) or larger dies. The extended residence time inside these larger dies will accelerate the PVdC degradation. The continuous interruptions of production to take the die apart for cleaning make the process economically unfeasible.
Encapsulation
Macro Engineering has solved this problem using encapsulation in its new AMT (Advanced Multilayer Technology) MacroPack™ stackable coextrusion die. Macro took advantage of the already short residence time and lowest wetted surface area offered by its patented MacroPack™ die when it developed encapsulation for blown film coextrusion. With encapsulation, the melt stream of the corrosive and heat sensitive PVdC is coated with a layer of EVA in an encapsulation block before entering the die. The encapsulating EVA prevents the PVdC from contacting the die surfaces and protects it thermally, extending run lengths by ten or more times. The die life is extended as well since the EVA and not the PVdC makes contact with the die surfaces.
Stackable Dies
At this point, and for the purpose of clarity, it is convenient to provide some background information comparing the main features of conventional and stackable dies, and highlighting the advantages of the MacroPack™ design.
A conventional blown film coextrusion die consists of a series of cylindrical mandrels arranged concentrically. This die design is commonly referred to as the spiral-mandrel type because each mandrel has a series of helical cut spirals around its periphery. In this type of design, the innermost mandrel has a smaller diameter than the outermost one. The die requires a coextrusion die block at the bottom of the die that takes the melt flows from the extruders and distributes them to their corresponding mandrel through a series of ports.
As the number of layer on a spiral-mandrel coextrusion die increases, so does the diameter of the outermost mandrels. Increased mandrel diameter also increases the amount of metal exposed to the molten polymers. The metal area exposed to the molten polymers is technically referred to as wetted surface area. Excessive wetted surface area will increase total pressure drop and create potential for degradation due to increased residence time. This big disadvantage of conventional blown film dies brought about the introduction of the stackable die designs.
Stackable designs reorient the polymer distribution system inside the coextrusion die. Rather than distributing the polymer melt in the conventional annular flow, which is concentric with the die axis, the distribution is accomplished in a plane that is perpendicular to it. The melt is fed to each layer from the side of the mandrel.
Stackable dies are assembled from mandrels that nest on top of each other. Each mandrel is a self-contained module providing the mixing and melt distribution for a single layer. The complexity of a stackable die does not increase with the number of layers, nor does its outside diameter, which is mostly dependent on the die size.
Currently, there are three basic stackable die designs in the market:
Pancake Style. In this type of design each layer consists of a mandrel resembling a “pancake”. The polymer is fed at the level of each mandrel and is internally split multiple time as it travels through a series of channels towards the center of the die. Limitations with this design include excessive melt splitting, limitation in the number of spirals, excessive increase in the outside diameter as the die size increases, and a disproportionate number of bolts required to seal each mandrel from polymer leaks.
Upward Conical Style. In this design style each mandrel resembles a cone tapered upwards. The mandrels nest into each other providing excellent seals against polymer leaks. The polymer is side fed at the level of each mandrel and is split only once. This design is suitable for smaller size coextrusion dies, typically between .375” (10 mm) to 18” (457 mm). For larger sizes the wetted surface area would tend to increase excessively. Macro Engineering sells this type of stackable die as their TaperPack™ design.
Downward Conical Style. In this design style each mandrel resembles a cone tapered downward. The mandrels nest into each other providing excellent seals against polymer leaks. The polymer is side fed at the bottom of the die, rises to its corresponding mandrel and is split only once. This style is patented by Macro Engineering as its MacroPack™ design. It is suitable for larger size dies, typically between 14” (356 mm) and 50” (1270 mm). The MacroPack™ design offers the lowest wetted surface area of any coextrusion die within its size range.
