Applying fluid dynamic principles to extrusion tooling design benefits any medical device company involved with continuous extrusion. Benefits include improved product tolerances, reduced waste, and stable, predictable performance.
It is well known that different polymers have unique properties making them desirable for certain applications. As an example, some products may require a stiff, tough cover with high electrical insulating properties, while others might need to be soft and flexible for medical applications. It is important to recognize that the flow characteristics of different polymers and polymer blends are unique and can be measured.
Polymers are non-Newtonian fluids — the viscosity changes as the force applied to the material changes. To measure and compare how different polymers behave when they flow, a device called a rheometer is used, and shear viscosity data is obtained. As polymers are exposed to higher shearing forces, their viscosity tends to be lower, and at a shear stress that is too high can damage some polymers. Melt fracture is a common example of this. Shear viscosity data can be used to compare how materials flow relative to each other, and the tests can be run at different temperatures to show how heat can affect the flow.
Many blends of polymers are extruded, including some mixed with color additives, some that are reground or use recycled materials, some that contain cross-linking additives, and blends of different materials along with fillers. These blends affect the flow characteristics and should be considered as well. Ideally, whatever is going to be extruded should be measured.
In practice, as a polymer is pushed through the extruder, adapters, and tooling, the flow channel geometry changes, but the flow rate should remain constant. The shear forces the polymer is exposed to vary during the process. The forces are lower when the channel is large and higher when the channel becomes small. Therefore, the shear viscosity data are important: they help to understand how the polymer changes as the force it is exposed to changes. Of course, it is possible to break down the steps in the flow path and understand what is happening along the way, but this becomes tedious. The modeling techniques available today make this easier to manage.
To begin any tooling evaluation or design, it is important to first understand a few basics about the process, including the type of application, the rheology of the materials being extruded, the size range of the product, flow rates, tolerances required, and requirements unique to the specific application. Once the first phase of the tool design is complete the computational fluid dynamics (CFD) work can begin. It is common to design the tooling using solid modeling software, allowing everyone involved in the design process the ability to visualize what the components of the tool will look like, and how they all fit together.
The CFD steps are iterative beginning with the initial design of the tool and the results of the calculations. As a simplified example, perhaps in the initial design the calculated pressure is too high. The flow channel size is increased to lower the pressure, and the calculation is rerun. Once the calculated pressure is within the desired range, the distribution can be adjusted to ensure that the final product is uniform. If multiple sizes of products and line speeds are to be run using the same head, CFD indicates how this will affect the process, and the tool can be adjusted accordingly.
CFD can be used to identify problems in existing tooling and to optimize new designs. It helps to visualize the flow of the material through the tooling to see areas that may be stagnant or places where the shear that the polymer is exposed to is too high.
In some applications, a tool must produce a narrow range of products consistently. A good tool design allows the process to run without the pressure being too high, while minimizing the volume of polymer within the tool. The polymer is exposed to less shear, and less energy is required to run the process. A good tool design requires less operator intervention as well. Other customers might want to run a wide variety of products and polymers through the same extrusion line. CFD can help determine where limitations might occur, or it can be used to design tooling that can increase the processing window.
To get the best insight into an extrusion process, it is essential to understand what the flow path looks like from the extruder through to the die exit. In many cases problems start because of low flow or stagnant areas caused by transitions from one area to another, and these are not always intuitive.
Using CFD provides more information about the extrusion process. It is possible to predict what happens if a different resin is used, or help explain why changing from one lot of the same polymer to another lot affects the way a process reacts. Gaining this additional insight leads to a better overall understanding of what is happening within the extrusion process and ultimately help processors reduce cost and increase quality.
This article was written by: John Ulcej, Director of Technology for B&H Tool Company, San Marcos, CA. For more information, Click Here.