Coextrusion die design doubles number of layers for packaging film and blowmolding parisons
By: Rafael J. Castillo
A new film die design improves upon the classical stackable die. Using an innovative dual spiral design to split single layers into multiple layers creates structural improvements in the final product, improves barrier properties, and saves money. Improved streamlining and the lack of sharp bends eliminates dead spots. The die gives the film producer the flexibility to meet future needs without buying another die.
Suppliers of coextruded film and blowmolded containers are under persistent pressure to improve the performance and reduce the cost of their products. They are required to extend the shelf life of packaged products and provide stronger, more flexible films at reduced costs. Coextruded product structures yield strong, economic structures.
Coextrusion enhances the polymers unique properties while reducing the unfavorable attributes of some polymers, bringing out the best properties of the individual polymers in the film structure. To do this, the packaging supplier requires versatile equipment that can produce a variety of multilayer structures at low production costs.
Enhanced Film Structures With More Layers
Incorporating more layers with dedicated extruders gives more flexibility and saves money. For instance, an ionomer skin layer can be split into two layers by substituting a less expensive commodity resin for a high-cost ionomer resin in the bulk of the skin and still achieve the excellent sealing properties of the ionomer. By adding more layers to the structure in this way, the film is made stronger and more economical. Encapsulating an EVOH layer with two thin layers of nylon produces a film with superior barrier properties while maintaining the puncture resistance and strength of the nylon. The polyamide layers provide added support for the EVOH and thus improve the flex crack resistance of the film structure. In film destined for meat packaging, combining EVOH and nylons right next to each other reduces odor and oxygen transmissions.
Splitting a single layer into two layers creates structural and physical improvements in the final product. Adding layers also improves barrier properties. For example, if we compare an individual nylon layer that is split into two layers side by side, the two layers produce a film with improved barrier properties while making the package softer with improved flex crack resistance and less curl in asymmetrical structures. Similarly, using two thin layers of EVOH instead of one thick layer reduces pinholes and improves consistency. Statistically, the two layers will have a better gauge deviation than the single layer. This is because any deviation in thickness in the film gauge is filled in by the adjacent layer of the same material. Hence, splitting barrier resin layers improves properties and reduces surface stresses, producing a film that can be more easily thermoformed.
New Coextrusion Die Concept
Ongoing research has resulted in a new patented concept in coextrusion blown film and blowmolding, known as the Dual Spiral System (DSS). The DSS has in every layer module two identical spirals flowing in opposite directions on the two opposite faces of the module. This layer splitting enables processors to take advantage of the improvements brought about from adding layers without adding additional equipment costs. Dividing each individual layer into two separate layers takes advantage of the enhanced structural and physical properties of each material layer. A five-extruder DSS blowmolding die produces a 10-layer parison.
The improvements over a classical stackable die are twofold. A layer module with two identical spirals flowing in opposite directions improves the gauge variation of the particular layer and the overall gauge uniformity of the layer. This is because any potential high spot in the gauge corresponds to a low spot in the adjacent layer in the same module. When the two layers are combined, the gauge variation in one layer is cancelled by the other. By combining two spirals in one layer module, the complexity of the die does not increase and neither does the cost of manufacturing the die.
The temperature-mixing channel is incorporated in the die in order to address material property variations during cross-sectional flow. Each spiral is fed by two ports, allowing homogenization of the melt entering the spirals by taking advantage of the thermal and velocity bias present in the flow channel. The cold, slow-moving material at the channel walls is sandwiched between layers of hotter melt. Since the hotter melt serves to lubricate the flow through the channel, there is no longer a tendency for preferential flow due to viscosity differences. The increased mixing realized from this configuration improves layer gauge control. In addition, changeover from one resin type or color masterbatch to another can be achieved rapidly.
On a conventional die a color change occurs gradually. Color decreases rapidly between the ports, but continues as a distinct band at each port for a much longer period. With the flow-mixing channel, the color dissipates uniformly around the full circumference of the bubble in a shorter period of time. The same scenario applies to resin changeovers.
The DSS uses a feature known as Taper-Lock, which for years has been a prerequisite in conventional die geometry. Taper-Lock eliminates the chance of die leakage and ensures perfect concentricity of the layer exit diameters. Each face of the DSSs modules is fitted on a taper to the adjacent plate that makes up the module, ensuring perfect concentricity of the exit diameter. This is done without an interference fit, for ease of module reassembly.
Compared to other cylindrical dies, the DSS has improved streamlining and no sharp bends to cause dead spots. Conventional cylindrical blown film coextrusion dies have stagnation regions that cause polymer degradation. In addition, because the outer layer in a cylindrical five-layer die has a polymer/metal interface five to eight times larger than the corresponding surface area of the inner layer, undesirably high residence times result, limiting the number of layers that the die can have.
The concept behind the new DSS design can be compared to currently available stack die designs through the representations in Figure 1 (above). While both designs reorient the polymer distribution system on planes perpendicular to the axis of the die, the new concept provides two independent distribution planes for every layer of polymer being extruded. Each plane of the new die incorporates a spiral system that distributes polymer along the full length of the spiral. This ensures that the polymer is evenly distributed around the complete circumference and that there are no haze lines in the exiting film caused by preferential flow inside the die spirals. In both die concepts, once the polymer has been distributed, it flows radially toward the center of the die to a point where it is redirected to flow in the axial direction. The axial flow stream begins with the inner layer and subsequent layers are added to complete the structure, which then exits the die.
The enhanced structural and physical properties of each layer material can be taken advantage of by dividing each individual layer into two separate layers. A multilayer film is stronger than a monolayer film of the same thickness. Correspondingly, a coextruded film has higher melt strength and can be produced at a higher output rate with better bubble stability. A layer module with two identical spirals flowing in opposite directions improves the gauge variation of the particular layer and the overall gauge uniformity of the layer. Multilayer conventional dies typically have ports starting at staggered positions in relationship to an adjacent layer. However, in the case of dual layer division distribution spirals, the layer gauge deviation is reduced because the output in each of the two spirals in the module is the same.
Gauge Deviation Experiments
An experimental study on overall and layer-to-layer gauge distribution was conducted on a three-layer 120-mm DSS blown film die. The die was designed to run a nylon/tie/LDPE structure of 25 percent/10 percent/65 percent at a nominal 70 kg/hr. The extruders are three 50-mm general purpose single-flighted screws. The idea behind the analysis was to determine if the theory behind the layer splitting provided an improvement over the traditional stackable die without layer splitting. The hypothesis was that overall gauge distribution would greatly improve because adding layers has been proven to decrease gauge distribution. However, for coextrusion dies the layer-to-layer gauge distribution is a more important element than the overall gauge distribution. It is the layer thickness that controls, for instance, the uniformity of the barrier properties of the film.
The barrier property of the film is directly proportional to the thickness of the barrier layer. If the barrier layer distribution has high and low spots, the barrier property is only as good as the lowest spot on the barrier layer. This causes processors to compensate for the low spots by making the entire barrier layer thicker than it has to be, causing a dramatic increase in material costs and a subsequent reduction in profitability. Furthermore, a film with good overall gauge distribution does not necessarily have good layer-to-layer distribution.
The layer-to-layer film thickness was measured by running the die without its tie layer resins. In this way, the individual barrier layers can be removed from the remaining film to be individually measured. Table 1 lists the process conditions and setup of each of the trials performed on the die.
Figure 2 (right) consists of printouts that were supplied by Octagon Process Technology GmbH of Germany, which used its automatic offline film thickness profiler to determine the gauge deviation of each of the samples in the trials.
The gauge deviation measuring from sample 1 was +/- 6.1 percent (2 sigma). The nylon layer distribution by itself, sample 2, was determined by running the DSS without the adhesive layer and this gauge distribution did not exceed +/- 8.4 percent (2 sigma). Finally, the DSS was run with all three layers processing fractional melt LDPE. In this case, the gauge distribution was +/- 4.6 percent (2 sigma).
The layer-to-layer gauge variance for thin barrier layers is typically much higher, on the order of +/- 13 percent, for stackable dies without layer splitting.
Increased Flexibility In Design, Processing
The ability to extrude various resins through each layer increases the processors ability to design and produce various film structures on one line. The processor with versatile equipment can add layers without increasing equipment costs. The DSSs split-flow layer concept enhances film properties without additional cost to the die.
Adding more layers to a blown film die increases the versatility of the line by allowing it to run a greater number of polymers and film structures. It allows entry into new markets by running new structures without modifying the die geometry. Furthermore, using a greater number of layers allows cost savings, since expensive layers can be divided and less expensive materials used in the structure.
The experimental analysis performed on a three-layer DSS system showed that the overall film gauge distribution was in the order of less than +/- 5 percent for polyolefin resins and +/- 6 percent for barrier materials. The layer-to-layer gauge distribution was slightly higher at +/- 8 percent. Layer-to-layer gauge distribution is typically much higher for stackable dies.
The DSS doubles the number of layers in a die without a corresponding doubling in cost. In traditional modular dies each layer is essentially a single-layer die. In the DSS, each module is essentially a two-layer die. The DSS coextrusion die provides a doubling of the number of layers in a die, improved streamlining, mixing of melt flow, and temperature isolation.
Consumer demand for packaging is constantly evolving. Having the ability to produce a large number of layers gives the film producer the capability to adapt to future demands and eliminates the need to order new capital equipment to meet changing market requirements.
|Dual Spiral Systems has placed coextrusion dies in the United States, Asia, South America, and Europe. The company recently shipped two 350-mm five-layer DSS coextrusion dies to a processor with plants in Europe and North America. Both plants specialize in films for the food industry. The dies are mainly used to produce barrier films containing EVOH and nylons but are also able to process polyolefins without die modifications. |
The die in the United States is running the structure shown in Table A.
The process data collected from extensive trials on the die include pressure, gauge deviation, residence times required to switch from one color to another, and compatibility of different resin grades. The processor has reported data readings for an output of 360 kg/hr as shown in Table B.
The layer-to-layer gauge deviation was measured by running LDPE adjacent to nylon 6 without a tie layer and then separating the nylon from the LDPE since they do not adhere to one another. The separated layer was then measured using a gauge profiler.
The compatibility test consisted of assessing different resin grades at different layer percentages to see where the limits of the die were in terms of interfacial instability. The most extreme case was to run a 4.5 melt index tie resin at 3 percent of the total output in layer D while maintaining the remaining layers at their maximum outputs running a .2 melt index LLDPE. Because of the wide viscosity difference between the tie resin and the LLDPE and the fact that the tie layer was very thin, interfacial instability would be expected. However, the DSS die did not exhibit the V-shaped flow phenomenon commonly associated with interfacial instability.
Editors note: Rafael J. Castillo is with Dual Spiral Systems Inc., Hamilton, ON. This article is adapted by permission from a presentation given by Castillo at the 2001 Annual Technical Conference of the Society of Plastics Engineers.
Plastics Auxiliaries & Machinery, March/April 2002