A Custom Hopper for the Noztek XCalibur
Background
To mimic industrial melt spinning machines, we mounted our Noztek XCalibur extruder vertically. I received and then modified a vertical hopper from the OEM, and although the modifications were a much needed improvement there was still a lot left to be desired. For example, we work with small sample sizes, i.e. 100g of polymer pellets, but much more was needed to achieve enough head pressure to properly feed the extruder barrel due to its side-entry configuration. Despite my addition of vibrating motors to agitate the pellets, we had to manually push the pellets towards the barrel inlet, which resulted in inconsistent fiber throughput. Thus, I went back to the drawing board to design a fully custom hopper.
Criteria
An improved hopper would:
Have enough internal volume for 100g of input material in pellet form
Feed pellets into the extruder such that the extrusion outflow is consistent and continuous for at least 75% of the test batch
Allow for rapid integration of new hopper designs if necessary
Design
I used Solidworks Student 18-19 to model the custom hopper. The hopper is composed of two components: an FFF funnel will attach to a steel adapter. The steel adapter creates a robust attachment to the barrel that won’t break with repeated installation and removal and will withstand the heat conducted from the extruder barrel as well as convected from the heater bands below. I chose steel over aluminum because it has a lower thermal conductivity and lower coefficient of thermal expansion. Weight and strength are not concerns for this application. Fused filament fabrication allowed me the freedom to design a single piece, hollow funnel with complex, smooth surfaces. The UVa MAE rapid prototyping and machine labs group does all of the printing and support removal, so the finished parts are high quality.
By weighing a sample of pellets in a beaker, I found that 142mL is approximately the volume of 100mg of the nylon pellets that we use as input material. I designed the hopper to be thin and tall so that a tall column of pellets would sit atop the barrel inlet; The top of the funnel stands almost 5 inches above the very top of the pellet passage into the barrel, and 100mg of pellets is approximately 4.5 inches above the same point.
All of the CAD models are available on GradCAD.
How
Once again I returned to the Lacy Hall Student Experiential Center of UVA to machine the steel adapter for the new hopper. I started with a plate of steel and waterjet cut the exterior profile and holes. The waterjet has 5-axis actuation, and I took advantage of it by slanting the slotted pellet passage through the adapter. Then, finishing the part was as simple as chucking it up in the mill and programming a rounded-corner rectangular path around the outside to create the flange feature designed to fasten the funnel.
The funnel was made at the press of a button; I submitted the funnel to the department’s additive manufacturing lab to be made out Acrylonitrile-Styrene-Acrylate (ASA). The part is solid due to its thin walls, and it feels sturdy. After I received it, I added heat set inserts so it could be easily mounted and dismounted from from adapter plate.
reSults
Installing the assembled hopper was a breeze: I designed it to slip underneath a fastener installed outset in the upper hopper mounting hole and then rotate downward to line up the bottom mount hole.
Finishing the installation is as easy as tightening the lower fastener. Leaving the upper fastener a little loose gives you the ability to jostle the hopper in case the pellets get stuck, but at the right screw speed the pellets roll into the barrel without manual agitation.
I painted the steel adapter white with high temperature spray paint to prevent rust, but I didn’t mask the pellet passage. When I ran the extruder for the first time, the paint inside the pellet passage softened from the heat of the barrel, the pellets stuck to the walls, and the hopper stopped flowing pellets. After removing the paint with sandpaper, I found that the pellets no longer stuck to the adapter and the extruder outflow became more consistent than ever. Unfortunately, that is a qualitative measure, but in due time I’ll be adding elements to measure line tension and fiber diameter during production that will give me a more quantitative measure of the state of the extrusion process.
ACKNOWLEDGEMENTS
This project was supported by the U.S. Department of Energy (DOE), Vehicle Technologies Office under the contract number of DE-EE0008195.
The modified hopper was very much needed, but not everything that we needed. Thus a custom hopper was designed.
Cross section view of custom hopper modeled in Solidworks. The green component is the steel adapter and the orange component is the FFF funnel. Note the slanted passage through the steel adapter, which was achieved by 5-axis waterjet cutting.
The finished custom hopper installed on the Li Lab fiber melt spinning system