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Home > MacroLetter > 3.2 - Summer 1997
MacroLetter 3.2 - Summer 1997
MacroPack Coextrusion Die
Contents:
New MacroPack Coextrusion Die Designed for 15" to 50" Diameter
The MacroPack die system is the latest new development in annular co extrusion dies for extruding multi-layer tubular plastic film. Co extrusion dies have been around for many years. Conventional dies usually have a series of concentric annular passages that combine into a common one to create a multi-layer film. It has been proven that these dies are best suited for producing tubular plastic film of up to three layers. These dies are not recommended for more than three layers, due to the resultant large diameter of the outer layers which dramatically increases the wetted surface area.
Reducing Wetted Surface Area
The stackable, multi-layer die concept was initially brought into existence as a means of producing small diameter co extrusion dies. Existing stackable and Macro Engineering’s ComboPack (ref. MacroLetter-Spring 1996) dies are well suited for structures in excess of three layers and less than 16” and in diameter. However, the plastics industry demands dies of 16” and larger for co extruded products. Existing stackable and Macro Engineering’s ComboPack dies are fed from the outer diameter of the mandrel.
Therefore, an 18” die will have approximately 34” of outer mandrel and about 30” of spiral diameter. At this stage, the stackable dies offer very little or no advantage over conventional feed block dies which have almost the same flow paths (i.e. wetted surface area) for most of the layers.
The new MacroPack design considered the limitations and demands presented above and applied them to the development of a new multi-layer stackable blown film die for over 15” in diameter. This design is derived basically by feeding the material from the inside of the mandrel instead from the outside. It eliminates the ports and binary channels that are the greatest limitation for spiral quantity of existing stackable dies. It expands the flexibility and application of the design itself.
With the new MacroPack cp extrusion die design, an 18” diameter die will have approximately 25” of outer die body diameter and 14” of spiral diameter. This design will reduce the wetted surface area by a minimum of 50% to 60% in spiral surface compared to most existing stackable dies. Therefore, it will allow for twice as large a die for co extruded products. An 18” Macro die will have almost the same flow surface area as an existing 10” stackable die. Dies designed with a cone angle of approximately 20 degrees from the vertical axis are well suited for co extrusion dies up to 30” in diameter.
Dies designed with a cone angle of 90 degrees from the vertical axes are well suited for co extrusion dies of over 30” in diameter. Using this design, a 26” diameter die will have approximately 33” of outer die body diameter and 13” of spiral diameter. Again, this would reduce the wetted surface are by a minimum of 50% to 60% in spiral surface and allow for twice as large a die. For example, a 50” Macro die will have almost the same flow surface area as an existing 25” stackable die.
The new MacroPack design uses approximately 40% less material in manufacturing when compared to most of the existing stackable dies. In fact, most dies will have approximately 42” of outer die body diameter compared to 33” for the MacroPack design.
The stainless-steel version of the MacroPack die eliminates the need for plating and ultimately, costly stripping. This also improves the dimensional accuracy of the finished die. As well, the die is more easily repaired or resurfaced, adding to the overall life of the die.
Key Design Features
The following features are common for both the ComboPack and the new MacroPack designs:
- Each layer can be heated from the inside as well as the outside
- A large hole in the centre of the die is provided for Internal Bubble Cooling.
- All layers can be fed by using the bottom mandrel with a fixed position for the inlets, and all the extruders are at the same centerline height.
- All layers can be fed by a separate inlet ring with a variable position for the inlets. The inlet ring is located below the bottom mandrel.
- Both designs have the lowest wetted surface area compared to the existing stackable dies.
Flexibility in Processing
The new MacroPack die can be designed to process a wife variety of material in various combinations to achieve the desired film properties. These materials include: LDPE, MLLDPE, LLDPE, HDPE, PP, CPP, EVA, EAA, PVDC, PVC, Nylon, EVOH, Surlyn, Ionomers, etc
The geometry and the material construction of the die can be predesigned to process conventional barrier materials, PVDC or PVC. For example, a five-layer die, designed for PP/tie/Nylon/tie/PP, can also run PP/EVA/PVDC/EVA/PP.
For significant variations in layer percentage ratio, optimum melt temperatures, and melt shear rhea logy, additional interchangeable mandrels may be required.
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