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How Fused Deposition Modeling (FDM) Printers Work

Working of FDM 3D Printers

Fused Deposition Modeling (FDM) is a 3D printing technique pioneered in the 1990s by Stratasys. In fact, the term ‘FDM’ is the trademark of Stratasys. The company continues to be a leader in manufacturing 3D printers all over the world, including India. 
Alternatively, the 3D printers that are based on this technology are also called as Fused Filament Fabrication (FFF), Plastic Jet Printing (PJP) or material extruding printers, which is the generic name for these 3D printers.

The 3D printers that work on FDM technology consist of the printer platform, a nozzle (also called as printer head) and the raw material in the form of a filament.

The Printer Platform
The printer platform or the bed is typically made of some metal, ceramic or hard plastic, and each successive layer is deposited on this platform. 

The Nozzle / Printer Head
The nozzle of FDM printers is attached to a mechanical chassis which uses belt and / or lead screw systems to move it. The entire extrusion assembly is allowed to move in X, Y and Z dimensions by a motorized system. A fourth motor called as the stepper motor is used to advance the thermoplastic material into the nozzle. All the movements of the head and the raw material are controlled by a computer.

The Raw Material
The raw material is typically production grade thermoplastics, though sometimes metal is used as well. The thermoplastic material is capable of being repeatedly melted when exposed to heat and re-solidified when the heat is withdrawn. The thermoplastic filament or metal wire is wound as a coil on a mounted spool. It is then fed through the printer nozzle. The better class of 3D FDM printers allows the temperature of the nozzle to be maintained just close to the glass transition temperature of the material being extruded. This allows the material to be extruded in a semi-liquid state, but return to solid state immediately. This results in a better dimensional accuracy.

In principle, any thermoplastic can be used as raw material for FDM printers. Commercially, a few of the popular choices of raw material include nylon, Acrylonitrile Butadiene Styrene (ABS) and its variations, polycarbonates, ply-lactic acid, polystyrene and thermoplastic urethane. MED610, a raw material that Stratasys provides is bio-compatible. Their ULTEM material too is certified by the aerospace industry. 

The FDM 3D Printing Process
When the FDM printer begins printing, the raw material is extruded as a thin filament through the heated nozzle. It is deposited at the bottom of the printer platform, where it solidifies. The next layer that is extruded fuses with the layer below, building the object from the bottom up layer by layer. 
Most FDM printers first print the outer edges, the interior edges next and lastly the interior of the layer as either a solid layer or as a fill in matrix.

In some objects / models, there are fragile ‘overhangs’ that will droop unless they are given some support. FDM printers incorporate a mechanism whereby these support structures (called struts) are printed along with the object. They are later removed once the build is complete. These struts are usually of the same material as the object. Some printers have a second extruder to specifically deposit soluble thermoplastic struts when there is a need to prevent the overhangs from drooping. These struts may be of a different composition than the thermoplastic used for the 3D model. They are later dissolved by an appropriate solvent.

How accurate are 3D FDM Printers?

Remember that a 3D printer works by depositing raw material layer by layer along the X, Y and Z axis. The accuracy of the 3D printer therefore depends upon the minimum distance the nozzle can travel vertically (the Z axis). Minimum the distance it can move, more the points along the sinusoid that it can capture, and better the accuracy.For Stratasys 3D printers, which are the pioneers of the FDM printers, the current best possible dimensional accuracy is about 0.127 mm. Of course, the choice of raw material too plays an important part in achieving dimensional stability. It should also be remembered that the accuracy comes at the cost of printing time required.

A few advantages of FDM 3D printers include:

  • a wide range of FDM printers are available in the market today
  • the raw material is inexpensive, durable and maintains dimensional integrity
  • there is a wide choice of raw material
  • they are affordable
  • low turnaround time

One disadvantage is that if the desired level of accuracy is extremely high, then the FDM printers may be found wanting.

FDM 3D Printers find application in:

  • creating prototypes for Fit, Form and Function testing
  • rapid tooling patterns and mould inserts
  • creating and testing any parts that work under thermal loads
  • production of precise and complex end-use parts e.g. jigs & fixtures

Sectors that use FDM 3D Printers include:

Overall, FDM 3D printers give a very high value for money and are very popular in India and other countries.