Fused Deposition Modeling
An Introductory Guide
FDM/FFF technology was created in 1988 by Scott and Lisa Crump. Crump created the technology to build his daughter a toy frog using a glue gun and a mixture of polyethylene and candle wax. In 1989, Crump patented FDM technology and founded Stratasys. Stratasys created the software process that converts stereolithography (STL) files into another format to slice sections of the 3D model and determines how the layers will be printed.
In 2009 the patent on FDM technology expired allowing for a torrent of new innovation and expansion in FDM which has quickly allowed it to become one of the most common additive manufacturing systems in the world.
In FDM 3D Printing the machine extrudes thin layers of thermal plastic while following a CNC program that dictates locations and extrusion amounts. By depositing layer after layer of material the machine can build a 3D object. These objects can take on almost any shape and form as long as it can adhere to the build plate for the duration of the print and can be supported by addition structures.
Typically, when developing a program for FDM machines you will import a file into a "slicer" program such as Cura, Slic3r, Simplify3D .... which interprets the model breaking it down into layers or slices. These slicing programs take direction from the programmer which can include layer height, nozzle temperature, infill, shells, feed and so much more.
The Two Extruder Systems
There are two types of extruder systems found on most FDM machines, Direct Drive and Bowden. Each system has its own advantages and disadvantages based on the machine that they are attached to. Over the next couple of paragraphs, we will look at these systems in more detail with their pro’s and con’s and when it is a good time to use them.
Direct Drive Extruders
A direct drive extruder is assembled directly to the hot end mounting plate directly above the hot end of the printer. This short distance gives the machine better responsiveness to extrusions and retractions moves leading to a cleaner print with less stringing and artifacts. These systems require less torque then Bowden extruders and can therefore be more energy efficient. As there is not gap between the hotend and the extruder the direct drive system is very effective at printing flexible materials such as TPE/TPU and nylon. The inclusion of a direct drive system on a machine adds weight to the axis that the extruder is mounted to. This makes accelerating, stopping, and changing direction more difficult and strenuous on the machine. When used on belt driven machines at high speed this can cause belt skipping which as it sounds is when the belt stops or tries to change direction and the extruder continues to move or if the belt moves to quickly and the extruder does not catch properly. Beyond physical skipping, the added weight can lead to inaccuracies at high speeds as more torque is needed to reverse direction causing split second delays. This will leave pronounced lines on the print. In machines with ball screws on the axis with the extruder this is not an issue. Another concern with direct drive is heat creep moving through the heat sink and softening the filament at or before it enters the gears of the extruder. This will lead to a jam. Direct drives are best on rigid machines with ball screw feed and good hot end cooling.
Bowden Extruders: In a Bowden Extruder setup the extruder is usually attached to the frame feeding material forward through a PTFE tube connected by two pneumatic push in fittings. The fittings are used to secure the PTFE tube to the guide bracket and the top of the hot end. This tubes connection is essential as it guides the filament to the hot end while ensuring that minimal torque is lost. The major strength of Bowden extruders is that they are stationary and that can allow the machine to have a better rigidity, stopping, starting and turning which allows higher print speed. Due to the delay between the extruder and the hotend, Bowden systems are prone to blobbing and stringing. Bowden extruders consume more energy as more torque is required to push the material down the PTFE tube. With that being said it could be argued that this is a moot point as the axis with the extruder will require less torque to move the print head. Flexible materials such as TPE/TPU and nylon can be very difficult to print with Bowden systems as they naturally compress in the Bowden tube which can lead to under extrusion.
In recent years we have seen advancements in dual extruding. Direct Drive currently requires two full extruder and hot end setups which can multiply the issues found in direct drive. Bowden has multi in one out systems that allow with up to 4 filaments in. This is achieved by having multiple extruders and one hot end which can save a lot of time and hassle with firmware, programming, and heat issues.
We believe that each system has its place and it really comes down to what you want to 3D print and what you have to do it with. For most enthusiast with basic desktops Bowden will be the way to go. If you have a serious machine with ballscrews and want to print a single filament then Direct drive is the way to go….. especially if it is a flexible filament.
Hot End: The hot end is the mechanism that heats the filament and forces it down into a smaller size for extrusion.These mechanisms can vary in design depending on many factors so we will cover the basic parts and their function:
Pneumatic Push in Fitting: This piece is typically found on Bowden Extruders and ties the PTFE line from the extruder into the system. Direct Drive systems will have mating angles that allow them to seat together.
Heat Sink: The heat sink is a component that allows for rapid dissipation of the heat creeping through the extruder head. Heatsinks can come in many shapes and forms depending on the machines specifications but the key concept is the more surface area, the more heat dissipation. This mechanism allows heat to radiate out with the assistance of the Hot End Fan. The more efficient the heat is dealt with at this layer, the better flow your parts will get especially with materials that require high heat.
Hot End Fan: The hot end fan works in conjunction with the Heat Sink pushing/pulling hot air from the fins away from the heat sink. The efficiency of this fan and the heat sink cannot be understated. This fan tends to eat filament and needs to be cleaned on a regular basis.
Heat Break: The heat break connects the heat sink and heater block assembly together. Beyond this the heat break is designed to hold a gap between the heat sink and the heater block to allow for cooling between these two components. This is the main avenue for heat to travel to the heat sink so materials that have poor thermal conductivity are typically used with a large gap and sometime ribbing or threading to increase the exposed surface area.
Heater Block: The heater block is typically made from aluminum or brass and is used as a dock for the nozzle, heating resistor, thermocouple and heat break. In addition to holding the assembly the heater block is used to transfer heat from the heating resistor to the “meltzone”.
Thermocouple: There is a thermocouple attached to the assembly to measure temperature. The thermocouple is made of two wires made from different metals. When the temperature increases on this circuit the voltage increases and is measured in the controller determining the approximate temperature.
Nozzle: The nozzle is the piece at the end of the assembly that allows the material to flow out. Most machines have a .4mm (.0157”) nozzle by default but they can range from .05mm (.0020”) to 3mm (.118”). The size of the nozzle will determine the thickness of the extruded bead. The nozzle is heated through the heater block assembly. As this is indirect heat there can be a variance of a few degrees on an unshielded heater block. Nozzles can be made of an array of materials depending on the application and it is important to understand when to change nozzle materials;
Brass: Brass nozzles are designed for general use with nonabrasive materials like PLA, ABS and PETG. They have the best thermal conductivity and will allow you to print these materials at the fastest rate.
Stainless Steel: Stainless steel nozzles are typically used for medical and food applications. These nozzles are harder and therefore will last longer with abrasive materials. They are also less thermally resistant in comparison to brass so slower speeds are required.
Hardened Steel: Hardened Steel nozzles are recommended if you want to regularly print abrasive materials like carbon fiber, metal filled and metal filament. The thermal conductivity of hardened steel is significantly lower than brass and will require more time in the melt pool to print effectively.
Specialty Nozzles: Beyond hardened steel there is Tungsten, Titanium and Ruby tipped nozzle which are all designed to be even more wear resistant. These nozzles are costly and typically used for production runs with abrasive filaments.
Heating Resistor: Uses electrical current to create thermal energy which is then dispersed through the heater block. These resistors are typically circular and held into the heater block with a set screw from the bottom or side. The heating resistor can be horizontal or vertical depending on the application of the system.