6 3D Printing Process
3D printing, also called rapid prototyping or additive manufacturing, is a manufacturing technique that allows to obtain physical objects from digital models, through the creation of two-dimensional sections of the object to be manufactured, which are produced one on top of the other to form the final three-dimensional prototype.
Various printing techniques and many materials are available such as thermoplastic polymers, gypsum, paper, light-curing resins, metals, ceramics, gels and others 119, 120. Each rapid prototyping technique has special characteristics that adapt to the solution of specific problems. Therefore we will describe the main additive manufacturing techniques used in the dental field, and then we will see their clinical implementation.
FDM printing, also known as Fused Filament Fabrication (FFF), consists of the deposition of a thermoplastic polymer, which is extruded through a thermostatically controlled nozzle on a plane, layer by layer until the production of the three-dimensional model. This type of technology adapts to various low viscosity materials and thermoplastic materials, is inexpensive and relatively fast. The accuracy depends on the specifications of the printer, up to few tens of microns.
Stereolithography was the first additive manufacturing technique to be patented in 1984 by Chuck Hull 124. This technology uses a laser beam to locally polymerize resin following a computer defined 2D path, layer by layer until the designed 3D model is obtained. This type of printing is very precise and allows to print various types of resins with different properties. In the dental field there are calcinable resins, resins for prostheses, for temporary restorations, for surgical guides and other uses.
DLP printing also uses resin polymerization layer by layer, but instead of a laser such as SLA, it uses Digital Micromirror Device (DMD, the same technology as projectors) to create a polymerization mask that polymerizes the entire layer at once. This technique is very fast in objects production. The print resolution depends on the resolution of the projected light beam, but in general it is very close to that of SLA. A recent evolution of DLP printing is the CLIP 121, 122, which allows very fast production of high-resolution structures. The CLIP technology was developed by the company Carbon Inc. 123.
SLS printers use a high power laser to sinter particles of polymers, metals and ceramics. The resolution is in the order of few tens of micrometers. This technology finds various applications in the dental field, because it allows both to print polymers, for surgical guides and models, and to print ceramics and metals, with immediate possibilities to use it in the prosthetic, implantology and surgical fields.
The Material-jet printer uses a printing head that moves on the printing
surface depositing the material, which is then light-cured by a UV
light, in a process very similar to ink-jet 2D printing.
There are many materials that can be printed simultaneously; this makes
it possible to print objects with non-uniform mechanical
characteristics, where for example there are both rigid and elastic
areas in a single structure. In addition, printing multiple materials at
the same time allows to print objects with a wide variety of colors.
Printing is also very precise, because the layer of material deposited
at each passage is very thin (in the order of 20μm).
The Fused Deposition Modeling printing process on a Cartesian printer is
discussed here.
After obtaining the file in G-code from the slicing software, this file
can be sent to the printer for printing. Depending on the capabilities
offered by the printer’s electronic control board, the G-code can be
submitted to the printer in various ways.
It consists of connecting the printer motherboard to the PC via a USB
cable. It has the advantage of being a fast method and allows the
control of the printer from the graphic interface of the software
installed on the PC 2.
A big disadvantage is the fact that the PC must necessarily be turned on
throughout the printing process, and that any system crashes can stop
printing suddenly and with little chance of recovery.
Most of the motherboards for 3D printers currently on the market allow
you to insert a microSD card, from which you can print the G-code files
previously loaded on the card.
Printing from SD is very advantageous because it does not require the
use of a PC during printing. Many motherboards also allow you to load
the G-code file on the microSD without removing it from the printer,
simply by connecting the motherboard to the PC via USB to perform the
transfer of the G-code on the microSD. Printing from a memory card thus
makes it possible to have a stand-alone 3D printer, which reduces the
risk of printing failure due to problems that may occur to the PC during
that phase.
More advanced motherboards, such as open-source Duet3D 3,
have the ability to connect to the Internet and allow printer control
through the browser. This type of control greatly increases the
flexibility of the printer, which can also be monitored while you are
away from the printing place.
The web interface allows the user to remotely control even more than one
printer, provided that these are equipped with control board with web
connectivity. This is extremely useful when multiple printers must be
managed and monitored at the same time, as can happen in the educational
field or in the industrial sector. Remote monitoring can also be
performed by connecting a video camera to this card, which allows
real-time video monitoring, saving the videos for later analysis or for
the next projection for educational purposes.
OctoPi is an open source operative system (OS) basen on Raspbian that
can be used in small controlling board as the Raspberry Pi
148. OctoPi enable an high level of control of the printer,
with video monitoring capabilities and web interface. It is compatible
with almost every printer regardless of the control board used.
The calibration process varies depending on the printer model. The most
advanced printers perform the calibration in an automated way, and
require only a few checks by the operator. Cheaper or do-it-yourself
printers could instead need several steps where operator intervention is
required.
The calibration must certainly be done before the first use, in order to
have a reliable, precise and consistent printer. The process
[@Reference4] consists of calibrating:
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Step/mm on the X, Y, Z and E axes (E = extruder)
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Print bed leveling
To calibrate the steps/mm on the axes, you need to know some parameters of the printer components:
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Angle per step: is the rotation that makes the axis for each step, measured in degrees. Generally corresponding to 1.8 degree/step (equal to 200 step/revolution) or 0.9/step (equal to 400 step/revolution). It is dependent on the stepper motor used.
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Microstep: it depends on the interpolation capacity of the motherboard’s stepper drivers that control the stepper motor [@Reference5], 6. The microstep consists in sending low power pulses to the motor, to make it perform fractions of steps. This theoretically increases the print resolution, but as the interpolation increases, the torque decreases, so the motor may not have enough energy to perform the movement. This means that the fraction of rotation will not be performed until the number of pulses necessary to give sufficient torque to the motion is accumulated.
To note, from StackExchange answer of the user cmm 149 on ref. 6:A stepper motor torque-vs-position-error curve is like a sin curve. There is zero torque at zero displacement, and maximum torque at one full-step displacement. This doesn’t change with micro-stepping. Thus, if you are taking 1/128th of a full step, it will truly give you very little torque. OTOH (On The Other Hand) if you happen to stop, given a full step, at 1/128th of a step in error, you will also have a very small torque toward the correct position.
At the end of the story, high microstepping gives a smoother movement to the stepper given the higher movement resolution, and it need exact the same torque per angle as a low microstepping 150. At high microstepping the temperature of the stepper driver have to be assessed, especially if the electric current flowing through the stepper is high 151.
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For axes with belt drive and pulleys (generally X and Y):
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Belt pitch in mm
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Number of teeth of the pulley
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steps / mm = (step / rev * microstep) / (belt pitch in mm * number of teeth of the pulley)
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For axes with leadscrew transmission (generally Z) 51:
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Screw pitch in mm
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steps / mm = (step / rev * microstep) / screw pitch
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For the extruder:
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step / mm = (step / rev * microstep) / (diameter of the extrusion gear * pi)
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With reduction (wade extruder): step / mm = (step / rev * microstep) * (n large pulley teeth / small pulley teeth) / (extrusion gear diameter * pi)
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Adjusting the printing bed should be as horizontal as possible to have
an optimal surface on which to deposit the printing material. The
printing bed must be orthogonal to the Z-axis and parallel to the X and
Y axes. This arrangement allows to have an optimal deposition of the
first layer, which is fundamental to guarantee the correct geometry of
the printed object, as well as favoring its adhesion to the bed.
The bed level can be adjusted manually, adjusting the height through
screws, or in an automated manner. Automated leveling (Auto Bed
Leveling) is performed with the aid of one or more sensors that allow
the printer to define the printing bed position in the space. Any
inclination of the bed is then compensated by the motion of the extruder
on the Z axis 7, 8. The bed leveling must be
carried out with the extruder and printing bed at the working
temperature to take into account the deformation of the components at
high temperatures.
The auto bed leveling is performed with the G29 command. At the end of
the bed probing, the machine reports a matrix with the data relating to
the operation just carried out. This data can be used to get a better
understanding of the bed’s inclination, and help us adjust the bed and
choose the best area to print a model. Data can be plotted thanks to
scripts on websites 9. The Duet3D card also allows the
display of the printing bed heatmap directly from the web interface
10.
At the end of the printer’s heating and leveling procedures, printing
process begins. The use of brim is recommended to prime the extruder and
to increase the adhesion of the object to the bed.
To obtain a fine adjustment of the first layer there are some firmware
(Marlin, RepRap firmware) that offer the BabyStep option (in Marlin
Babystepping, 152); it gives the possibility, during printing, to send real
time axes position adjustments. The Babystep is very useful during first
layer printing, because it gives us the possibility to precisely adjust
the position of the nozzle with respect to the printing bed,
facilitating the creation of a uniform and well spread first layer.
After any adjustment with Babystep has been carried out, printing
continues until the end.