Scaling up Three Dimensional Printing
Dini E.
1, Flosi V.
2, Rossi A.
2, Failli F.
2, and Lanzetta M.
21 Monolite Ltd.,101 Wardour Street, W1F 0UN, London, UK, enrico.dini@d-shape.com 2 University of Pisa, Dept. of Civil and Industrial Engineering, Largo Lucio Lazzarino, 56122 Pisa, (arossi, failli, lanzetta)@ing.unipi.it
Scaling up Three Dimensional Printing
Abstract
New processes and machines are proposed in academia and on the market, down to the microscopic range. On a different side, manufacturing complex parts of large size by additive processes has still unexplored potential. This paper describes the patented 3DPrinting D-Shape technology, one of the few available to build parts up to 3 tons. Three sands have been tested, calcareous dolomite, granite and pozzolana (regolith-like) with pulverized MgO, under the action of an aqueous MgCl2 binder, focusing on the optimization of the inkjet parameters.
Keywords: Additive Manufacturing; 3DPrinting; Powder agglomeration; Inkjet
INTRODUCTION
After appearing only two decades ago, Rapid Prototyping, Direct Manufacturing, Solid Freeform Fabrication, Layered or Additive Manufacturing (AM) technologies are receiving growing popular attention. Many different technology families are available including selective laser sintering, fused deposition modeling and stereolitography. New variants are proposed both in academia and on the market, also in the microscopic range, as shown in [1] for three-dimensional printing. On a different side, manufacturing complex parts of large size by additive processes has still unexplored potential, which is the target of this paper. shows that different processes have different benefits, such as material cost for the ZCorp machine, productivity for Kira, accuracy for 3DSystems, however these processes are upper limited in size, when it comes to meters. More recently material cost has been cut by one order of magnitude, however productivity (also impacting manufacturing cost) and size upper bound still remain major limitations for process substitution.
As for research, the topic of direct manufacturing of buildings or building blocks falls into the general field of Construction or Full Scale Manufacturing Methods [2]. A comparative analysis of different approaches was reviewed in [3]. These methods are now maturing. Current technologies are now being scaled up for the manufacture of full-size components and systems; these can be defined to neatly fit into three categories: Metals, Polymers and Synthetic stone. A number of construction and full scale manufacturing techniques have been developed since the mid 90's. These include Solid Freeform construction [4], Robocrane [5] [6], Additive Plaster (Bert Brink, Maxit group), Freeform Construction [Error: Reference source not found] developed at the Rapid Manufacturing Research Group in Loughborough who joined Nottingham Unversity, Contour Crafting [7] and the Monolite machine [8], which a few years later evolved into the D-shape technology. The need to build large structures was explored in an ESA project [9].The first three techniques presently are not actively pursued,
while Contour Crafting, Monolite/D-shape and Freeform Construction machines are under continuous development and are currently used to produce walls and large sculptural objects also commercially. In particular, D-shape with respect to the other techniques proved to be very effective in printing very large scale objects: for instance, it allowed to build in one single printing process a whole house.
Each of the three above cited techniques exploits slightly different deposition methods, processes and materials. The greatest difference among these techniques is that [Error: Reference source not found] [Error: Reference source not found] selectively catalyses materials within a layer of pre-deposited substrate, while the remaining ones exploit a technique, which extrudes building materials premixed with a binder or catalyst, which are thus similar to concrete. In particular, the Freeform Construction machine is in principle similar to Fused Deposition Modelling (FDM) additive manufacturing systems [10]. This system is designed to extrude and deposit material at multiple layer resolutions within the same build, above and below 6 mm and its development is presently focusing on creating complex panels (i.e., ‘building blocks’), rather than entire buildings. The particular benefit of this system when it will be fully operational should be the broad range of materials that could be used, including support material. On the other hand, the Contour Crafting machine extrudes two thin parallel layers of material, leaves the inner surfaces unfinished and the outer surfaces are robotically finished with a trowel mounted on the printing head. The space between the two layers can then be filled with sand or other materials [Error: Reference source not found].
This work describes the most recent achievements of the patented 3D-printing technology, D-shape, which was shown to be successful in various application types. D-SHAPE PROCESS AND MACHINE
The D-shape process allows building full-scale products, starting from an inorganic environmentally friendly binder to convert sand and similar granular materials into hard stone. Most local inert sands can be used, by milling to the desired mesh size, making this a sustainable technology.
It allows two printing methods: directly the entire product structure, or a set of building blocks to be assembled together, possibly using additional reinforcements.
The printing machine is a sort of gigantic plotter, with a spraying head which moves along two frames in the x–y axis space and selectively sprays on predefined areas of the sand bed a binding liquid also indicated in the remainder as ‘ink’.
The machine used in experiments has a structure of 66 m2 made of aluminum cantilevers with 300300 mm2 section.
As other 3DPrinting technologies, this process relies on the agglomeration of inert powder materials, which can also be found in situ and milled to achieve the desired granulometry (0.1-2 mm), through a special binding liquid. The STL file of the 3D part model from commercial CAD software is processed by a self made CAM to determine
the x-y contours at each z coordinate. The machine gantry architecture and operation is summarized in Figure 1.
Table 1 Commercial Additive Manufacturing machines based on different processes (last updated 2012).
Process SLA FDM POLY-JET SLS 3DP LOM
Maker 3DSystems (USA) Cmet(JPN)
Strata-sys (USA) Objet (IL) 3DSy s-tems (USA) Eos (GER) ZCorp (USA) Kira (JPN) Model SLA5000 VIPER-PRO RM6000II 1200ES EDEN500V PRO SINTS750 Z510 PLT-A3 Max part size X [mm] 500 1500 610 250 500 550 720 350 410 Y [mm] 500 750 610 250 400 550 380 250 300 Z [mm] 580 500 500 300 200 750 380 200 300 Layer tickness [mm] 0.05/0.15 0.05/0.15 0.1 0.245/0.33 0.016 0.1/0.15 0.2 0.089/0.102 0.15 Material liquid photo-polyme r liquid photo-polyme r liquid photo-polyme r thermo-plastic resin liquid photo-polyme r sinter -able powd er sinter-able powder powder adesive paper Accuracy [mm] ±0.2 ±0.2 ±0.2 ±0.3 ±0.2 ±0.25 ±0.2 ±0.3 ±0.25 Roughness 30° (Ra) [µm] 18 17 * 43 20 16 18 17 21 Material cost [$/kg] 510 870 * 1710 1290 165 540 Processing % time (active vs total) 51.67 78.57 * 77.45 * 50 42.86 75 83.87 Productivity [mm3/h.] 145.45 396.69 * 85.56 * 189.7 2 155.84 436.36 140.76
BINDER SUPPLY Z-AXIS NOZZLES ARRAY COMPACTING ROLLS 1 M X-Y LAYER Y-AXIS X-AXIS POWDER SUPPLY CPU GEAR MOTORS
Figure 1 The three dimensional printing machine setup. The powder deposition follows three steps:
1. deposition of given amount of powder, controlled by pneumatic pistons; 2. planing of the powder bed, by a linear blade;
3. compaction by rolling to achieve a constant density.
The x-y transverse axis is a 6 m aluminum blade with a hopper that deposits uniform horizontal layers of granular material with constant thickness (default 5 mm) by compressed air. The powder is then compressed by rolling cylinders to achieve a homogeneous sand pressure prior to start printing on the newly deposited layer with a target apparent density of 1.2-1.4 Kg/dm3 on the final part. As for the final part porosity, 70-75% of the volume is powder, with 10% binder and the rest is for voids. The transverse is lifted at each layer by hydraulic rams.
Pulverized metal oxide (MgO 16% wt.) dispersed in the mixture reacts with the binding liquid (MgCl2(H2O)n 64% wt. and other additives under 3%.
The binder is selectively deposited by an array with 300 spraying nozzles at 20 mm axis spacing (Figure 2). The maximum binder viscosity is 3° E (22 cStokes or mm2/s), between -10° and 60° C. According to the droplet formation principle, this printhead can be classified as 'continuous jet' [11].
The selected aqueous solution represents a compromise between maximum surface tension, which prevents the droplet ejection through the nozzle, and minimum viscosity, which allows a sufficient flow.
Four passes along the x axis are required to cover the whole 6 m length by transversely shifting in the y direction the nozzle array by 5 mm at each pass.
In the binder/powder interaction the following three major phases can be identified [12]: 1. impact between droplet and powder bed, with risk of grooving and powder
rearrangement;
2. diffusion of the liquid binder among powder grains. Speed depends on the binder viscosity;
3. powder engulfing and binding, by a chemical reaction according to the material system under use.
Figure 2 Detail of nozzles and droplet ejection, printed layer, tested samples in bed and after drying, calcareous dolomite powder mix, nominal length 100, 50, 20 and 5
mm, width 5 mm, printhead travel speed 10 mm/s, excess pressure 0.2 kPa. The printhead z and x axes are controlled by gear motors with encoder with 1 mm position accuracy and travel at 20 m/s (the optimum in the tested 10-30 m/s range), with a machine productivity of 11.85 layers per hour (involving 2 powder spreading and 2 powder rolling passes and 4 printhead passes) or 2.133 m3/h. 2-ways hydraulic on/off microvalves (A and F class) control the inkjet binder by a 24 Volt DC on/off servo driven solenoid in pulsed mode to regularly jet drops of preset volume at 150 Hz. Valve on/off response time is in the order of 10–15 ms. A gear motor and an encoder ensure a 1 mm positioning accuracy. All the machine actuators are PLC operated.
The finest features to be designed should consider that the compression (UNE-EN-1926) and the bending resistance (UNE-EN-12372) of the cured parts are respectively 175–200 and 15–35 Kg/cm2. Some other machine features are listed in Table 2. At the end of the process, the unbound powder is removed from the final part by vacuum and can be directly recycled.
Table 2 D-Shape machine features (more in paper text).
Powder size 0.1-20 mm
Printable area / Machine size 25 / 710 m2 Layer thickness 3-20 mm (±1 mm)
Printing time 5-15 min./layer (depending on thickness)
Resolution 7.5 mm
Peak absorbed power
(printing/sand vacuuming) 2 / 20 kW
EXPERIMENTS
Three sands have been tested, namely calcareous dolomite, granite and pozzolana (regolith-like), at 83% wt., with the MgO at 16% wt. and the addition of high strength 20 mm polymer fibers (1% wt.) for the first two mixes. The three powders have been characterized by optical microscope granulometry respectively at 1-1.5 mm, 1-2 mm, and 0.5-2 mm.
The primitive balls (process voxel) is about 6 mm.
The printhead travel speed has been tested in the 10-30 m/s range, with 20 being the optimum. At the lowest speed the part size where larger than designed. At the maximum speed, corresponding to the maximum productivity, errors have been observed caused by insufficient flow, so the binder pressure was raised by 40 kPa; however at high speed other errors are induced by vibrations caused by the large masses and accelerations.
The excess pressure in the hydraulic nozzle circuit was tested in the 20-40 kPa range, 30 being the optimum. 20 kPa are sufficient for a single nozzle and the flow is not sufficient with more nozzles engaged. Over 30 kPa the powder bed grooving becomes excessive unless the printhead travel speed is increased.
The servo driven solenoid of microvalves showed no appreciable effect in the tested 75-150-300 Hz frequency range and 150 being the default.
2,268 samples have been produced with the following parameter combinations. For each of the three powder mixes, three travel speeds and three pressures have been tested in order to explore a wider range about current default. The number of replication is 7 lines to cope for process variability. The length of lines is 5 (also corresponding to the layer thickness), 20, 50 and 100 mm to take into account the edge effect at the start/end of lines for excessive/insufficient binder input and because of the inertia in the binder pumping system. Individual lines, adjacent lines and overlapped lines have been printed in the mentioned 27 experimental conditions to understand possible interactions between line configurations. The nominal line width is 5 mm, just below the actual process resolution and process voxel.
Figure 3 shows an example of graph for conditions A. single line
B. two adjacent lines and C. overlapped lines/layers.
Figure 3 Example of length (el), depth (esm-esM) and width (ebm-ebM) errors on printed primitive lines [cm], in sizes 0.5, 2, 5 and 10 cm, for single (A), two adjacent (B) and two overlapped lines/layers (C), for the calcareous dolomite mix, for 20 m/s printhead travel speed, excess pressure 30 kPa, and microvalve opening 150 ms. On the abscissa the sample lengths in cm are shown. The ordinate represents the (positive/negative) length error el. The minimum and maximum line thickness error esm and esM, and the minimum and maximum line width error ebm and ebM for 7 lines are also shown.
DISCUSSION
Experiments on the tested excess pressure, doplet frequency and travel speed have shown no benefit over previous default parameters, which represent the optimal compromise between productivity and quality: higher parameters would allow higher productivity but cause higher part errors.
The maximum observed error is in the order of 10 mm for shorter lines (5 and 20 mm), maybe due to the large nozzle size ( 5 mm) and inertia of the hydraulic system. The
error falls down to few mm on longer lines (50 and 100 mm) resulting in errors under 10 mm on 6 m parts.
It can be observed that errors are conservatively mostly positive, allowing manual post-processing to achieve the required dimensional accuracy. Increasing the process parameters in order to increase productivity is most probably not compensated by the higher time and cost required for part post-processing by skilled artisans.
As a conclusion the suggested parameter combination indicated probably represents the optimum for the three investigated material systems, which have their own time constant, being dominated by the relative surface energy and other process variables, governing the binder/powder interaction peculiar phenomenology.
The similar size feature of conditions B. and C. show that a good merge is achieved both in adjacent lines and overlapped layers, producing good mechanical properties to final parts.
In addition to parameter optimization, these experiments also serve for calibration purposes by compensation of errors by the machine control software.
CONCLUSION
Demonstration parts up to 3 tons have been built in one piece. As with other AM technologies, large parts, in turn, can also be assembled as building blocks.
Experiments to improve the inkjet aspects of the D-shape technology have been outlined, showing that it is good a candidate to build very large parts, filling a void of current competitive technologies.
This technology may be suitable for rapid tooling (e.g. for large casts).
Research is ongoing on part measurement, also for (in-)process monitoring and control of the printing process, to allow an early detection of malfunctioning and anomalous events that may affect irreversibly the final result. A standard vision system with a suspended camera centered above the powder bed seems suitable according to preliminary tests based on comparing each layer with the corresponding section of the desired 3D model. Further research is required for the extension of the proposed machine architecture to the 3DPrinting of metal powders and other material systems and to build a robotized version of the machine consisting of a smaller moving printer. REFERENCES
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