New ComboPack Coextrusion Die Reduces Flow Instabilities and has Lowest Wetted Surface Area
Plastics are part of practically every aspect of today’s lifestyle. North Americans use over 75 billion pounds of plastic resin annually. To put this in a perspective, that’s enough plastic resin to load a freight train 20kms long- every day of the year! Packaging alone represents about 1/3 of the total use and is by far the largest single market for plastic in North America. A great deal of this packaging material is produced by the blown film co extrusion process.
Coextrusion Blown Film Dies
Co-extrusion blown film dies have been in use for many years. Traditionally, these are the concentric mandrel type incorporating a series of concentric annular passages that combine into a common one, creating a multilayer film. For producing tubular plastic film of up to three layers and die diameters in excess of 14 inches, the conventional feed block offers the shortest flow paths and fewest design limitations. However, they are not recommended for more than three layers, due to the resultant large diameter of the outer layers which dramatically increases wetted surface area.
Stackable type dies have been recently introduced into the market. These dies are well suited for structures in excess of three layers but not for less.
Macro Die Package Key Design Features
The plastics industry has become far more receptive to the concept of high quality, versatile die packages. Macro Engineering designs each die to give the customer the best return on investment (ROI). The many features incorporated into every Macro Engineering die package make this equipment rather unique in the industry. The die is the heart of the entire blown film line. In any die package, the performance will only be as good as the weakest component. For example, a clean die with low residence time can be wasted in production if the adapters are improperly sized, too long, or if the block has been set up incorrectly. The following key design features are incorporated into every Macro die package for optimum performance:
Key Design Features
Die and oscillator components are manufactured using 4340 steel and pre hardened to 30 Rockwell ‘C’. 4340 steel maintains dimensional stability and hardness levels at high temperatures. Dies are far more durable and resistant to damage. Surface finishes and nickel plating life are also improved.
Adapters are 4140 steel and pre hardened to 30 Rockwell ‘C’. Adapter bore diameter is calculated for specific resins and outputs to balance residence time and back pressure.
Oscillators and rotators are designed with high temperature, spherical roller bearings for low torque requirements, automatic self centering and minimal wear on components. To avoid oscillator cold start damage, a torque limiter is used on the output shaft of the gear box. A full 90 degrees radius elbow is used to avoid degradation in both the oscillating and the stationary block. Conventional sealing systems usually require vertical gaps on rotating surfaces to be maintained within 0.003”, in order to be effective. The sealing system on a Macro Engineering oscillator is designed on vertical, rather than horizontal surfaces. Thus, temperature differences between layers have little impact on the effectiveness of the seals. Also, the path length through the seals further reduces the chance of interlayer leakage.
Conventional sealing systems usually require vertical gaps on rotating surfaces to be maintained within 0.003”, in order to be effective. The sealing system on a Macro Engineering oscillator is designed on vertical, rather than horizontal surfaces. Thus, temperature differences between layers have little impact on the effectiveness of the seals. Also, the path length through the seals further reduces the chance of interlayer leakage.
On the multilayer oscillator and die block, each layer is configured with two side feed sections rather than one to deliver a more uniform feed to the bottom of the die.
The multilayer oscillator can be efficiently disassembled on line by removing t he die, bolts, and listing the diffusers out through the top of the block.
Ports feeding the spiral grooves are cut at a steep angle to reduce degradation areas both at the spiral diameter and at the bottom centre of the die.
A small taper is used at the bottom of the die mandrel and outer body to create larger clearances between parts during assembly. Since there is no interference, disassembly is accomplished more easily.
A heater band is provided at the bottom of each die, directly in contact with the die pin. This eliminates cold spots in the ports and reduces heat up time.
The initial cross-section of spirals, at the point where the port discharge intersects the spiral diameter, is designed to eliminate degradation spots and maintain velocity around the corner. Resin to resin or color changeovers are also improved.
Spiral depths and gaps are changed upward, a minimum of 80 times from the start of the spiral section, to maintain flow in the spiral channel and accurately meter the leakage flow over the spiral land.
The spirals for each die are custom engineered with minimum of 8 port overlaps to achieve the best guage control and optical properties, over a broad range of materials and output rates.
Custom developed software accurately predicts pressure drops, shear rates, residence time, velocity and wetted surface areas. The software analyzes these critical parameters from the screen changer through to the exit of the die lip.
Residence time and backpressure are balanced to accommodate a wide range of resins and output rates. The shear rate at the walls of the flow passages is also maintained above a certain level to ensure short purge cycles.
To avoid potential problems with interlayer leakage, pressure differences between adjacent layers within a die are minimized.
Layers within co extrusion dies are combined two at a time to match flow velocities and minimize interlayer flow instability. The geometry also protects the delicate points on spiral mandrels by maintaining larger clearances between parts during assembly and disassembly.
The die lip exit is recessed into the top surface of the die to prevent accidental damage. For additional protection, flow areas plating heat treated to 58-60 Rockwell ‘C’.
On larger dies, internal heaters are provided to improve heat up times and promote thermal uniformity in the die.
Shallow recesses are provided for heater band positioning. This ensures full heater contact and simplifies installation.
Flow surfaces are polished to a mirror-like surface finish of 4-8 RMS.
For each new die project, new drawings are created and are unique to the customer.
Manufacturing is performed on Computer-Numerically Controlled (CNC) machining centers to achieve the highest precision possible.
Custom designed rheology software is linked to a three-dimensional computer generation system that produces accurate engineering drawings and CNC machining coordinates.
Macro's New ComboPack Die Design (Patent Pending)
The demands of today’s film processors have led to the development of Macro Engineering’s new multilayer blown film die. This innovative die design combines the best features of conventional and stackable type distribution systems. The newly developed ComboPack co extrusion blown film die system is composed of a lower section and an upper section. The lower section’s inner most three layers have concentric annular passages that combine into a common one. This design feature provides a lower wetted surface area and reduces the merged material residence time by nearly 50% compared with existing stackable dies. This feature will also eliminate potential inner layer flow instabilities. The upper section consists of annular outer layers that are stacked one on top of another. Again, this feature achieves a much lower wetted surface area than the conventional outer mandrel. The combination of these design features substantially reduces overall wetted surface area and eliminates the potential for interlayer flow instabilities.
In the ComboPack design, the melt is fed directly into the side feed groove from the extruder adapter to the concentric or taper spirals. The side feed groove distribution system is used to reduce flow variations prior to the polymer’s entry into the spirals.
Additionally, it reduces the wetted surface area compared to any existing binary distribution systems. It also ensures that optics and layer uniformity are consistent around the die circumference. The side feed groove approach eliminates the restriction on the number of spirals that can be used to supply each layer. For example, any of the existing stackable dies has a maximum of 16 (in combination of 2,4,8,or 16) spirals using binary distribution. Therefore to increase the quantity of spirals requires doubling t he wetted surface area of the split channel through the binary distribution section prior to entering the spirals. The ComboPack design optimizes spiral quantity without increasing wetted surface area.
It is preferable to maintain low system pressure while also keeping the residence time of a polymer melt in the system to a minimum. To produce a uniform annular flow, the design of the side feed and the spiral distribution system is created using a sophisticated computer simulation program. Combining the use of the side feeds and spirals causes each later to be mixed twice, resulting in superior distribution compared to any existing modular dies. The ComboPack die system will yield guage variations of less then +- 5% for any layer, as well as for the total film thickness.
Adding More Layers
A further advantage of the new ComboPack die system is its modular design. This gives the processor the flexibility to add supplementary layers to the basic three layer die. This is accomplished by simply adding an upper section between the lower section and the die lips. Dies with four or more layers are stationary only.
