Plastruder Design

Theory

A filament based plastic extruder can be broken down into two main logical parts: the filament drive system and the hot zone / nozzle assembly. The filament drive system provides the force to pressurize the melt chamber and force the plastic out the nozzle. There are a bunch of variables that can be used to describe the system for modeling and testing.

Filament Drive System

The main purpose of the filament drive system is to provide the required push force at the required speed. There are a few important variables to keep track of here, such as FailForce, MaximumFeedrate, and DriveTorque.

FailForce is the maximum extrusion force that the drive system can produce before failing by either filament stripping, motor stalls, or filament buckling.

MaximumFeedrate is the maximum speed at which the filament can be extruded at a given force.

DriveTorque is the torque required in order to activate the mechanism and drive the filament. This is what your motor will need to provide and can be measured with a torque wrench.

When testing a filament drive system you will typically measure the FailForce that the extruder is capable of providing. You may also want to measure the MaximumFeedrate for a given force. Testing this subsystem is the first step towards designing a new extruder as many other variables are dependent on it.

Hot Zone / Nozzle Assembly

The nozzle assembly is where the filament melts and becomes molten before being force out the nozzle. There are a few important variables to keep track of here, such as MeltTemperature, ExtrusionForce, ExtrusionFeedrate.

MeltTemperature is the temperature that you are heating the filament to in order to extrude. This is a variable that you can control.

ExtrusionForce is the force applied to the incoming filament in order to cause extrusion. This is generated from the filament drive system, but can be simulated for testing.

ExtrusionFeedrate is the rate at which the extrusion leaves the nozzle and is based on the force and the temperature. When testing your nozzle assembly, this is the variable that you will typically be measuring.

Failure Modes

The most typical failure mode happens when the ExtrusionForce exceeds the FailForce. This can be avoided by keeping the filament feedrate at a speed that requires an extrusion force lower than the FailForce for the filament drive system.

The typical failure mode for this is either filament stripping, the motor stalling, or rarely the filament buckling.

Another typical failure mode is when the required extrusion force increases during operation and exceeds the FailForce of the filament drive system. This results in the same failure mode as when the extruder is running too fast.

Testing

Filament Drive System

  • FailForce
  • MaximumFeedrate
  • DriveTorque

Hot Zone / Nozzle Assembly

  • ExtrusionFeedrate
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