Laboratory 3D printing system

Present work describes the results of the development of the universal system, which capable to utilize varies 3D printing methodologies. The main goal of the study is to provide cheap, versatile and easy expandable equipment for multiple purpose research in the field of material science. 3D printing system was experimentally validated for fused deposition modeling, hydrogel, liquid dispensing and dropon-demand printing, as well as 3D photopolymerisation by UV laser and/or LED light using different types of materials.


Introduction
For 30 years of its development, three-dimensional (3D) printing has gone a long way from the method of rapid prototyping to a full-fledged production process. Various 3D printing technologies has become an indispensable field in various fields from engineering and aviation to printing live fabrics. [1] One of the significant limitations of expanding research in the field of 3D printing is the extremely high cost of installations, ranging from thousands to hundreds of thousands of dollars. Also commercial 3D printers quite space consuming, not much of them designed as on-the-table device. In addition, existing commercial samples, in most cases, do not fully correspond to the research tasks, because are intended for a single printing method and a given set of materials. On the other side cheap and simple home printers in mass focus on only one printing method -fused deposition modeling (FDM). In most of cases, such as inkjet printing [2][3][4], different kind of bioprinting [3][4][5], laser sintering [6], gel printing [7] or even FDM [8,9] research teams develop their own setup. In many respects, it is connected with the problems in adapting, modifying or expanding the functionality of 3D printers, whose design and software are proprietary of commercial companies.
Therefore, great interest is the work on the creation of open source laboratory multifunctional 3D-systems with a variable configuration. Requirements for laboratory 3D printers are fundamentally different from those for industrial or home systems. Instead of a large working volume of thousands and tens of thousands of cubic centimeters and printing speeds, compactness, versatility, and, most importantly, simplicity and openness of both the hardware and software platform come to the fore.

Requirements for the design of universal 3D printer
The main goal of this work was the development of a universal, low-cost laboratory 3D platform that can be easily reproduced and expanded in accordance with the Open Source and PEP-RAP philosophies. As a basic concept of the installation was adopted a design with a fixed table and a system of three-axis moving removable printing devices for various purposes. To cover as many different materials as possible, the basic functionality of the unit included, in addition to the traditional FDM technology, multichannel pneumatic extrusion and syringe systems for working with gels and viscous liquids, a thermoelectric drop-on-demand system for micro dosing of liquid materials in 2.5D mode and the same diode laser or fiber-optic laser for single-layer sintering or photopolymerisation.

Designing of 3D printer
The main attention during 3D printer designing was paid to the extensibility, compactness and simplicity of the design. Printer has a fixed table scheme, which is not common for compact 3D printers but allows using a liquid as a printing media. Outer dimension of 3D printer 30х30х30cm 3 (approx. 1х1х1 ft 3 ). The scheme and photos of desined 3D printer shown on Figure1.
Mainframe made of aluminum extrusion 30х30mm 2 (1.2"х1.2" 2 ), which grants a rigid and stability for 3D printer it provides the ability to mount different additional parts of the construction. Upper cover of the frame serves as base for worktable for which could be used any necessary surface (textolite, silicone heater plate, glass, Petri dishes). Work area of 3D printer 140х140mm 2 and 80mm in height. The kinematics of the 3D printer based on a 3-axis portal scheme with a universal Z-axis fastening. It is made on cylindrical guides with a diameter of 10 mm with linear bearings. All mobile parts of the printer, except for linear guides, are made of plastic on an FDM printer with an accuracy of 0.2mm. When designing the printer, special attention were paid to the design of the carriages, so that they could easily be made of light alloy on a conventional three-axis CNC-milling machine, without any modifications. To ensure precise movement, the XY axes are designed on NEMA 14 stepping motors (1.2A, 12N/cm) with integrated screw-nut shafts (4 threads, 5mm thread pitch), which allows for positioning accuracy of at least 50 μm, while significantly cheaper and more compact than ballscrew transmissions. A 20 or 28 mm stepper motor with an integrated screw-nut shaft, depending on the print head, can represent the Z-axis, depending on the design. The electronics of the printer based on the most simple, repairable and well-documented platforms Arduino MEGA + RAMPS 1.4 or Arduino DUE + RAMPS FD. In addition, these platforms are actively supported and developed by the REP-RAP community, which makes it much easier to work with them. In the basic version (FDM or single-channel printing), the 3D printer operates under the open source firmware Marlin V.1.1. For the use of multichannel printing or mixing nozzles, a modification of Marlin [10] was required, and for inkjet printing custom firmware was developed.

Fused deposition modelling
Base FDM module based on robust and simple E3D V5 hot end [11] for 1.75mm filament, combined with Bowden extruder. As extended version we prepare an open source screw extruder [12], slightly modified for best fitting to our design ( Figure 2).

Syringe dispensing system and Pneumatic dispensing system
Compact syringe dispensing system has completely new design and exists in four different versions: 1. main single channel syringe module with 5ml capacity and 4 kgf/cm2 max pressure (Figure 3a), 2. dual syringe module with 2х5ml capacity and 1,5 kgf/cm2 max pressure (Figure 3b), 3. precision single channel syringe module with 1ml capacity and 0,5 kgf/cm2 max pressure (Figure 3c) 4. 4-channel revolver print head combined with two external dual syringe module with 2х10ml capacity and about 8 kgf/cm2 max pressure (Figure 3d).
Pneumatic dispensing system (Figure 3e) consist of medical device microcompressor (max pressure 0.1MPa), electromagnetic 3-ways gas valve and 5mm PVC hosing. As a reservoir, any syringe with volume between 1 and 20 ml could be used.

Drop-on-demand system and laser module
Drop-on-demand system (inkjet print head) based on well known, easy to control and reliable C6602 HP (Hewlett-Packard) thermoelectric nozzles cartridge (Figure 4a), which is well described and documented on PWDR open source project [13]. Print head driven by regulated voltage booster (output 12-32V) and two Darlington arrays controlled by main Arduino controller. As a coherent radiation source commercially available 5V 405-450nm laser diode module with measured power 320mW was used (Figure 4b).
Module set in passive radiator and controlled by Arduino through MOSFET.

FDM module validation
FDM module was tested on printing a complex model of human skull with 400mk brass nozzle and 200mkm layer height without support and infill. Commercially available bronze PLA (polylactic acid) filament was used as material as it provides good visualization. Screw extruder tested on printing disk with internal grids with 600mk brass nozzle and 250mk layer height. Specially synthesized poly-ε-caprolactone copolymer powder with low melting temperature (68 O C) was used as a material for printing experiments [11]. As it shown on picture, 3D printer showed good print quality in both cases ( Figure 5).

Syringe dispensing module testing and Pneumatic dispensing system
All dispensing module were tested in single channel mode. As a test, material PLA solution on tetraglycol, which has ability hardened in water, was used [16]. . The printing result clearly shows that designed 3D printing system are capable to effectively work with viscosity liquid and provide printing in liquid media.

Drop-on-demand system
Drop-on-demand system was tested on two common tasks: bind a powder layer and ink plotting on the copper surface. As a testing material for binding experiments reaction hardening system includes tricalciumphosphate (powder) -phosphoric acid salt solution (binder) was used [17]. During printing process, a drop (~40pl) was shot every 100mk in tricalciumphosphate powder to form a single layer a square shape (Figure 7a). For inkjet surface plotting standard HP in was used without any additional copper surface treatment (Figure 7b). Both result, the structure and the surface plotting, shows that the quality of inkjet printing enough to gain a 100mkm resolution which is comparable with commercial 3D systems.

Laser module
Photo curing system was tested on thin layer (~300mk) of photoresistive material (Fun To Do Standard Blend RED, Fun To Do, Netherlands) on glass substrate ( Figure 8). The obtained samples resolution are around 500mk, which a not as good as on modern stereolithographic equipment but it mostly depends on coherent beam source. So it can be greatly improved if needed with installing high quality laser and optics.

Conclusion
As validation experiments show, all tested 3D printing technique including FDM with filament and powder material, viscous liquid and gel dispensing printing, thermoelectric Drop-on-demand and laser photocuring could be provided on developed universal 3D printer. With a cost less than 400$, open architecture and a very short conversion time (not much then a few hours) to change printing method this machine could become a cost effective and versatile researcher instrument. Following of the Open source and REP RAP philosophy will allow scientists to take a full advantage of the potential of 3D technologies, adding a new experience to their development at the same time.