Outlook and Future Directions
D Printing in the Scientific Laboratory
Overview of FDM Printers and Components
While a brief introduction to the FDM process was provided in Sect. 1.4, a more detailed overview of the components, kinematics, and figures of merit of different types of 3D printers is warranted. In general, a 3D printer must include: (i) a platform on which the object is to be built, (ii) a means of additive delivery of material in a controlled manner, and (iii) accurate and precise control of positioning in at least 3 axes. These requirements can be addressed by several approaches with trade-offs in cost and performance. For the sake of brevity, only FDM printers will be covered within this chapter.
The build-surface provides the literal base on which any 3D printed part is manufactured. Though not required for some materials, this platform is most-often temperature controlled with typical settings ranging from 45 to 120 °C depending on the material being printed. Heating of the platform improves adhesion to the surface as well as promotes even cooling of the part throughout the process, thus limiting warping of the part. Perhaps the simplest and most common method of providing heat to the platform is through resistive heating by passing a relatively high current through a series of copper traces on a printed circuit board (PCB) . Providing heat in this manner allows for the use of DC voltages and current control via a circuit employing a MOSFET as a switch. This type of heating is most effective over build areas smaller than 200 x 200 cm. Larger platforms necessitate the use of heating pads which run off of AC voltages to provide much faster heating times (even for very large build surfaces) but must be controlled by a solid state relay (SSR) and are inherently more dangerous as the voltage range in which they operate is potentially fatal if mishandled.
As previously stated, a heated build surface is not always necessary; however, there are 2 main requirements for any surface on which plastic will be deposited for
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Z. Baird, Manipulation and Characterization of Electrosprayed Ions
Under Ambient Conditions, Springer Theses, DOI 10.1007/978-3-319-49869-0_5
a 3D printed part. First, the surface must be flat to ensure that the thickness of material deposition on the first layer is consistent. Flatness is most easily realized by printing onto a sheet of glass. A typical arrangement is to stack the glass on top of the heater, often with an aluminum sheet in between to distribute heat more evenly. Second, the plastic to be deposited must adhere to the surface throughout the entire print, yet still allow for object to be removed from the bed upon completion. The use of a heated bed has an advantage in this respect, as adhesion is stronger at a correctly selected temperature and contraction of the bed upon cooling allows for the part to be removed with relative ease. Because not all plastic adheres well to glass alone, the surface is often coated with polyvinyl alcohol (PVA) based glues, acetone/ABS slurries, blue painters tape, and even hairspray. Recently there have been a number of products marketed specifically for bed treatment that are compatible with most commonly used plastics. These include BuildTak (an adhesive-backed, textured plastic sheet), 3D-EEZ, and polyetherimide (PEI) sheets, to name a few. There is no one-size-fits-all solution for bed treatment, and as such empirical testing is the normal approach for finding the correct solution for new materials. For applications of 3D printing specifically within the fields of chemistry it is important to consider the contaminants that may be introduced by different bed treatments and select an appropriate method, accordingly.
The precise delivery and deposition of material in FDM 3D printers is a concerted effort that relies on 2 components, the extruder and hot end. Extruders form the basis for volumetric material delivery by pushing a plastic filament of known diameter into the hot end. Within the hot end, the plastic is melted in a controlled zone (the melt zone) immediately prior to deposition through the nozzle orifice. The width of the orifice, the vertical positioning of the nozzle, movement speed, and the volumetric delivery of material all serve to define the layer height and extrusion width of the line of plastic that is extruded. As the extruder is responsible for controlling the rate at which material is delivered, this is perhaps the most critical aspect of achieving dimensionally accurate parts (aside from positioning error). The most common method of controlling the flow rate is to push a plastic filament of known diameter with a drive gear (or set of drive gears), the rotation of which is controlled by a stepper motor. There are a multitude of extruders and drive gear designs to select from and an ideal extruder grips the filament as well as possible while avoiding deformation, provides high torque, and allows the plastic filament to be pushed at a relatively fast linear speed.
The second component responsible for material deposition is often referred to as the hot end, due to its use of heat to transform the solid plastic feedstock into a free-flowing material that will bond to the previous layer upon exiting the nozzle. There are many different hot end assemblies available for purchase, but each functions in a nearly identical manner. In general, the plastic filament is pushed along a straight cylindrical channel capped with a metal nozzle. Heating of the filament is restricted to the region closest to the nozzle in order to limit expansion of the plastic in the constrained channel and to better control the flow of liquid from the nozzle (limit the melt zone). Some hot ends are specifically designed with a longer than average melt zone in order to achieve faster print speeds. Thermal conductivity of most plastics is generally quite low so careful control of print speeds and temperatures is necessary to achieve consistent quality within a 3D printed object.
There are 2 common ways of arranging an extruder and hot end in order to deliver plastic to the build surface: direct-drive, and the so-called Bowden arrangement. In a direct-drive setup the drive gears are located as close to the entrance of the hot end assembly as possible so as to eliminate backlash when performing retraction and prime movements. This makes for very short retraction distances and time periods and allow for the filament to be fully constrained from the time it enters the drive gears of the extruder and is melted by the hot end. The adverse effect arising from this particular arrangement is the amount of weight this adds to the print head, thus slowing the maximum effective print speed and acceleration that can be used for positioning. This is particularly troublesome for printers with large build volumes as longer belts introduce more play into the system. The Bowden arrangement allows for a much lower print head weight by locating the extruder in a stationary position, distal from the other moving components of the system. In a Bowden extruder the filament is fed into a hollow tube with an inner diameter closely matching that of the filament and the other end is coupled to the hot end. As a trade-off, Bowden extrusion systems exhibit much more backlash due to the tension of the tube, play within tube coupling components, and slight mismatch of filament and tube diameter. Because of this, Bowden extruders are more difficult to calibrate in order to maximize print quality.
Along with precise material delivery, accurate and precise positioning of the deposited plastic is critical to the production of a 3D printed object with good dimensional tolerance. There are a number of different kinematic systems employed in 3D printers, and a full discussion of this is beyond the scope of the present text as most of these systems are well-known and employed in a variety of robotic systems. For the purpose of this discussion it is sufficient to note that most, if not all, proven FDM systems rely on stepper motors, timing belts, and pulleys to realize computer numerical control (CNC) of the deposition of plastic in FDM.