Mechanical System
Extrusion System Overview
- 1) Funnel
- The funnel was added to the system to ensure that PLA pellets would be slowly loaded into the system such that the system wouldn’t jam and would stay hot enough to ensure constant extrusion. To do this, the funnel was designed to hold pellets in it while heat from inside the barrel melted them enough to drop the pellets down into the barrel and begin the extrusion process.
- 2) Barrel
- The barrel is manufactured out of a 1’ length, 1” OD, ½” ID, .25” wall thickness multipurpose 6061 aluminum tube. The barrel is threaded at one end with ⅝ - 18 UNF internal threads for 1” in length to interface with the nozzle. It also has a small 8-32 UNC threaded hole that prevents the barrel from rotating through the use of a set screw. Finally, the barrel has a ½” hole drilled into the top to hold the funnel.
- 3) Nozzle
- The nozzle is is crafted out of ¾” aluminum stock. The extrusion hole is measured at 2.6mm. Although we desire 1.75mm diameter filament, we will pull filament out of the extrusion hole with a spooling system decreasing its overall diameter. The nozzle is also threaded with ⅝ - 18 UNF external threads for ¾” in length. These threads allow us to attach or remove the nozzle from the barrel at will. The choice of UNF threads maximizes nozzle to barrel contact for increased thermal conductivity.
- 4) Housing
- We opted to build our system housing out of ¼” laser cut mdf and t-slot joints. Building with just laser cut mdf and t-slot joints allows us to iterate rapidly and meet all the structural and thermal properties required by our system. Mdf provides sufficient structural rigidity allowing us to hold the barrel in place through the use of a set screw, and withstand the torsion between the motor, screw, and barrel. Finally, it is heat resistant and can be in contact with the high temperature barrel.
- 5) Screw
- We chose to use a ½” auger as our plastic extrusion screw. Augers have a rotating helical blade which allows it to be used as a screw conveyor. This conveyor effect allows us to push PLA pellets towards the top of the barrel. It also provides enough forward pressure to squeeze melted PLA through the nozzle. To integrate the 3rd party auger into our system, we had to make a few modifications. First, we removed the gimlet and cut the auger to length with an abrasive saw. Second, we milled a .120” coupling slot into the the base of the auger to interface with our motor.
Post-Processing System Overview
- 6) Rollers
- We printed the rollers out of ABS to ensure that they would be durable and grip the PLA filament adequately. The bottom roller is driven by a small motor, and supported on the other side by a motor that has been converted into a bearing with the removal of a gear. A rubber band wraps around the bottom roller, twists, and then wraps around the top roller to drive it. The motor speed is determined by the digital output from the calipers, because faster motor speeds will result in thinner filament and slower motor speeds will result in thicker filament.
- 7) Calipers
- We are using digital calipers to measure filament diameter. A custom 3D printed plug allows the reading on the calipers to go into the arduino as digital input and control the speed of the spooling motor (read more about that here). The calipers don’t naturally want to close, so a rubber band interacts with the tip and the housing in order to make the tips hug the filament. The only other modification on the calipers was a minor surgery to remove a few small internal parts and allow the calipers to slide more easily.
- 8) Water Bath
- The water bath serves to make sure that filament is hardened by the time it reaches the calipers. This way, the calipers will get an accurate measurement on the thickness of the filament, and the rollers will be able to grip the filament without deforming it. The water bath could be upgraded to something much higher tech, but the current version is perfectly functional.
The final transferring of the filament onto a spool will be done manually, as the system cannot be safely run without someone there at all times and this necessary element of interaction will prevent someone from walking away during operation.
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