Extrusion System Overview
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.
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.
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.
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.
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
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.
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
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
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.