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On the development of a robot-operated 3D-printer
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28th International Conference on Flexible Automation and Intelligent Manufacturing 28th International ConferenceJune on Flexible Automation and OH, Intelligent (FAIM2018), 11-14, 2018, Columbus, USA Manufacturing (FAIM2018), June 11-14, 2018, Columbus, OH, USA
On the development of a robot-operated 3D-printer
On the development of a robot-operated Manufacturing Engineering Society International Conference 2017, 3D-printer MESIC 2017, 28-30 June a 2017, Vigo (Pontevedra), Spain Andi Dine , George-Christopher Vosniakosa * a
Andi Dinea, George-Christopher Vosniakosa *
Costing models forofcapacity in Industry 4.0: Trade-off National Technical Univerity Athens, School of optimization Mechanical Engineering, Heroon Polytehneiou 9, 15780 Athens, Greece between used capacity and operational efficiency Abstract National Technical Univerity of Athens, School of Mechanical Engineering, Heroon Polytehneiou 9, 15780 Athens, Greece
a
Abstract a a,* on, attached to b in this case 6-axis arm with A prototype Fused DepositionA. Modelling (FDM) is reported robot, Santana , P.head Afonso , A. Zaninb, anR.industrial Wernke A prototype Fused Deposition Modelling reported on, attached to an including industrial robot, in thiscalculations case 6-axisregarding arm with high repeatability, to form a robotic 3D (FDM) printer. head The is detailed design is presented theoretical a printer. The detailed design is presented including theoretical calculations regarding high repeatability, to nozzle. form a Selection robotic 3Dof pressure drop in the the off-the-shelf electronicGuimarães, components and their inter-connection are also covered University of Minho, 4800-058 Portugal b Unochapecó, 89809-000 Chapecó, SC, BrazilandProgramming pressure drop the in the nozzle. Selection of the off-the-shelf electronic components their inter-connection are alsopaths, covered together with implementation for synchronizing the robot and the head’s extruder. of the 3D printer for together thepertains implementation for synchronizing the robot androbot the head’s extruder. Programming of theand 3Drepeating printer paths, for the time with being, to teaching the pertinent points to the via the pendant, to form a layer through the time being, pertainstotoform teaching the pertinent to theofrobot viashell the artefacts pendant, are to presented form a layer repeating through programming structures the complete object.points A number printed withand accuracy assessment, programming structures toacceptable. form the complete object. A number of printed shell artefacts are presented with accuracy assessment, which was deemed totally Abstract which was deemed totally acceptable. © 2018 The Authors. Published by Elsevier B.V. © 2018 2018 The Authors. by B.V. Under the concept of "Industry productionlicense processes will be pushed to be increasingly interconnected, © The Authors. Published by Elsevier Elsevier B.V. This is an open accessPublished article under the4.0", CC BY-NC-ND (https://creativecommons.org/licenses/by-nc-nd/4.0/) This is an open access article under the CC BY-NC-ND licensemuch (http://creativecommons.org/licenses/by-nc-nd/3.0/) information based on a real time basis and, necessarily, more efficient. In this capacity optimization This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of scientific committee of the 28th Flexible Automation andcontext, Intelligent Manufacturing Peer-review under responsibility of the scientific committee of the 28th Flexible Automation and Intelligent Manufacturing goes beyond the traditional aim of capacity maximization, contributing also for organization’s profitability and value. Peer-review under responsibility of the scientific committee of the 28th Flexible Automation and Intelligent Manufacturing (FAIM2018) Conference. (FAIM2018) Conference. (FAIM2018) Indeed, leanConference. management and continuous improvement approaches suggest capacity optimization instead of
Keywords: Fused Deposition; Robot; optimization 3D Printer; Multi-plane printingmodels is an important research topic that deserves maximization. The studyIndustrial of capacity and costing Keywords: Fusedfrom Deposition; Robot;and 3D Printer; Multi-plane printing This paper presents and discusses a mathematical contributions both Industrial the practical theoretical perspectives. model for capacity management based on different costing models (ABC and TDABC). A generic model has been 1. Introduction developed and it was used to analyze idle capacity and to design strategies towards the maximization of organization’s 1. Introduction value. The trade-off capacity maximization vs operational efficiency is highlighted and it is shown that capacity The mostmight widespread additive inefficiency. manufacturing method is Fused Deposition Modelling (FDM) due to its optimization hide operational The most widespread additive manufacturing method is Fused Deposition Modelling (FDM) due to its inexpensive platform and the open-source © 2017 The Authors. Published by Elsevier B.V. movement [1]. A filament is forced through a heated extrusion head, inexpensive platform and the open-source movement A a International heated extrusion head, including a liquefier part and a final nozzle, so that itof[1]. can be filament depositedisEngineering inforced liquidthrough form along planned trajectories. Peer-review under responsibility of the scientific committee the Manufacturing Society Conference including liquefier part andquickly a finalsolidify nozzle,tosoform thataitlayer can and be deposited in liquid form along trajectories. Deposited atracks of material this is subsequently repeated to planned form an object. 2017. Deposited tracks of material quickly solidify to form a layer and this is subsequently repeated to form an object. Keywords: Cost Models; ABC; TDABC; Capacity Management; Idle Capacity; Operational Efficiency
author. Tel.: +30 210 7721457; fax: +30 210 7724273, e-mail address: [email protected] 1.Corresponding Introduction
* *
Corresponding author. Tel.: +30 210 7721457; fax: +30 210 7724273, e-mail address: [email protected] 2351-9789 © 2018 Thecapacity Authors. Published by Elsevier information B.V. The cost of idle is a fundamental for companies and their management of extreme importance This is an open access under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) 2351-9789 © 2018 Thearticle Authors. Published by Elsevier B.V. in modern production systems. In general, it is defined as unused capacity or production potential and can be measured Peer-review under responsibility of the scientific committee of the 28th Flexible Automation and Intelligent Manufacturing (FAIM2018) This is an open access article under CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) in several ways: tons of production, available hours of manufacturing, management of (FAIM2018) the idle capacity Conference. Peer-review under responsibility of the scientific committee of the 28th Flexible Automationetc. and The Intelligent Manufacturing * Paulo Afonso. Tel.: +351 253 510 761; fax: +351 253 604 741 Conference. E-mail address: [email protected] 2351-9789 Published by Elsevier B.V. B.V. 2351-9789 ©©2017 2018The TheAuthors. Authors. Published by Elsevier Peer-review underaccess responsibility of the scientific committee oflicense the Manufacturing Engineering Society International Conference 2017. This is an open article under the CC BY-NC-ND (http://creativecommons.org/licenses/by-nc-nd/3.0/) Peer-review under responsibility of the scientific committee of the 28th Flexible Automation and Intelligent Manufacturing (FAIM2018) Conference. 10.1016/j.promfg.2018.10.004
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Conventional 3D printers utilize a gantry for continuously moving the nozzle on a horizontal plane, whilst vertical movement is intermittent a few times per plane (2-1/2 axis interpolation). There is the obvious advantage of simplicity in computer numerical control on a horizontal plane, but there are also drawbacks compared to multiplane 3D printing. These mainly concern a need of support material for overhung structures and pertinent post-processing for their removal, some reduction in strength and restriction in attainable complexity and limitations in accomplishing embedded 3D printing [2], [3]. The concept of multiplane 3D printing via a robot is undergoing intense development [4]. For instance, a machine combining multiplane additive, formative and subtractive processes in one robotic platform with an end effector interchange depending on the particular mode is presented in [5]. Major 3D printing and automation companies presented the Robotic Composite 3D Demonstrator with 8 axes, with an ability to print across fused layers in order to increase the strength of the printed part [6]. A hybrid 5-axis cnc machine involving 3D printing and subsequent milling for improved dimensional accuracy has been developed by Lee et al. [3], capable of multiplane 3D printing due to its rotational building platform. A commonly used FDM extruder has been integrated with a 6-axis robotic arm demonstrating printing in horizontal and vertical plane [2]. Another similar development utilised the screw extrusion method, whilst the motion of the robot was calculated by projecting a desired 2D image in 3D space [7]. A large-scale additive manufacturing system with a 6-axis cable-suspended robot exhibited a wide range of motion in addition to being transportable and reconfigurable [8]. This paper reports on development of a 6-axis robotic 3D printer as a proof-of-concept prototype. Its design and control are presented in Section 2 and 3, respectively, its programming is briefly outlined in Section 4, examples of 3D printed parts are given in Section 5 and conclusions are summarized in Section 6. 2. Mechanical Design The robot used was a Stäubli RX90L. It is a six axis industrial robot with payload of 4 kg at maximum speed and 6 kg at reduced speed, maximum operation speed of 12.6 m/sec, maximum reach of 1100 mm and repeatability at ± 0.025 mm. The 3D printing system mounted as an end effector on the robot should weigh less than 3.5 kg. Most of the parts of the 3D printer were designed parametrically in SolidWorks TM software and manufactured from Al alloy 2000 series on HAASTM TM-1 and TL-1 CNC milling and turning machines, respectively, according to G-code generated on SolidCAMTM software. 2.1. Extrusion system A three-tier frame was designed for accommodating electronics, extrusion system mechanism and 3D printing heads, see Fig. 1.
Fig. 1 The 3D printing mechanism (a) as designed (b) manufactured and partly assembled second tier (c) control tier
Andi Dine et al. / Procedia Manufacturing 17 (2018) 6–13 Dine, Vosniakos / Procedia Manufacturing 00 (2018) 000–000
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3
The bottom tier is flanged for attachment to the robot. The printer is designed so as to enable use of two materials, normally the building material, e.g. PLA/ABS, and the support material. The extrusion mechanism is located on the second tier, see Fig. 1(b). Its purpose is to push the filament to overcome the pressure drop in the nozzle. It mainly consists of a driving roll supported by bearings, which is driven by a ΝΕΜΑ 17 stepper motor. There are also two pressure rolls that push the two filaments against the driving roll. These consist of a grooved wheel and knurled counter-surface to eliminate slippage. The user can choose which pressure roll will be activated by tightening the respective screws. The maximum torque of the motor is 1.765 Nm hence the maximum extrusion force is 353 N. The two 3D printer heads are located on the third tier, see Fig. 1(a). The printer head consists of the liquefier, the nozzle and temperature controlling devices, typically fins. An off-the-shelf E4D J-Head ExtruderTM with nozzle diameter 0.4 mm, appropriate for 1.75 mm filament of various materials (ABS, PLA, wood, PVC, etc.) was selected. The filament reel holder is located on the third link of the robotic arm, to provide a filament path as straight as possible and to eliminate the possibilities of filament messing in case of complicated motion. The theoretical height of the printed parts can be calculated as: Height (mm)=First layer height+(Number of Layers-1)*(Bead diameter-Overlap), where ‘overlap’ is the intentional decrease in original distance between successive layers to reduce inter-bead voids. For example, if the diameter of each bead were 0.4 mm and the overlap 0.1 mm, then the displacement from the previous layer to the next would be 0.3 mm, see Fig. 2(a).
a
b
Fig. 2 (a) Example of overlap between successive layers on a slightly inclined printing surface (b) Sections of the liquefier
2.2. Pressure drop estimation The pressure drop in the nozzle and hence the required force for extrusion is essential to know in regulating the flow rate. Key assumptions in this calculation include incompressibility of the melt, a no-slip boundary condition at the wall of the liquefier and a fully developed, steady state and laminar flow. Then, pressure drop can be predicted by momentum balance equations in conjunction with a power-law viscosity model with Arrhenius temperature dependence [9]. This model was applied for extrusion dies of cylindrical and conical shape corresponding to sections I, II and III of the liquefier, see Fig. 2(b). The power-law model reflects the assumption that feedstock (ABS, PLA) is typically shear thinning [10–13], 𝜂𝜂 = 𝛫𝛫 ∗ 𝛾𝛾̇ 𝑛𝑛−1
(1)
𝜂𝜂 = 𝛨𝛨(𝛵𝛵)𝜂𝜂 𝛵𝛵0 (𝛾𝛾̇ )
(2)
where η (Pa·s) is the viscosity, 𝛾𝛾̇ (1/s) is the shear rate. The power-law fit parameters are 𝛫𝛫 ,the consistency index (Pa·sn) and 𝑛𝑛, the flow index. This model is mathematically simple but neglects yield stress, which is characteristic of many polymer melts [14]. Viscosity dependence on temperature is significant for non-isothermal material flow through the liquefier. Thus, viscosity is separated into the temperature-dependent and shear rate-dependent terms [10]:
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The shear rate-dependent term is simply the power-law model in which the fit parameters have been calculated at some reference temperature, 𝑇𝑇0 .The temperature-dependent term is described by Arrhenius expression: 𝐻𝐻(𝑇𝑇) = exp[𝑎𝑎(1⁄𝑇𝑇 − 1⁄𝑇𝑇0 )], where α (Κ) is the activation energy [10,15]. Note that H(T) is 1 at the reference temperature, 𝑇𝑇0 . The pressure drops (Pa) in each section of the liquefier according to this model are given respectively by [14]: 1 𝑚𝑚
𝑉𝑉
𝜑𝜑
𝛥𝛥𝛥𝛥2 = (
2𝑚𝑚
𝛽𝛽 2
3 𝑡𝑡𝑡𝑡𝑡𝑡( )
)( 1
𝑉𝑉3 𝑚𝑚
(𝐷𝐷1 1
1
1
) 𝑒𝑒𝑥𝑥𝑝𝑝[𝑎𝑎(𝑇𝑇 − 𝑇𝑇 )] /2)𝑚𝑚+1
3 𝐷𝐷3 𝑚𝑚
−
1
3 𝐷𝐷1 𝑚𝑚
(3)
0
𝐷𝐷1 2
) (( 2 ) (𝑚𝑚 + 3)2 1
(𝑚𝑚+3)(𝐷𝐷1 /2)2 𝑚𝑚
𝛥𝛥𝛥𝛥3 = 2𝐿𝐿3 ( ) ( 𝜑𝜑
1 𝑚𝑚
𝑚𝑚+3
𝛥𝛥𝛥𝛥1 = 2𝐿𝐿1 ( 1 ) (
(𝐷𝐷3 /2)𝑚𝑚+3
1
𝑚𝑚+3
1 𝑚𝑚
1
1
) 𝑒𝑒𝑒𝑒𝑒𝑒[𝑎𝑎(𝑇𝑇 − 𝑇𝑇 )] 0
1
) 𝑒𝑒𝑒𝑒𝑒𝑒[𝑎𝑎(𝑇𝑇 − 𝑇𝑇 )]
(4)
(5)
0
where the dimensions 𝐿𝐿1 (m), 𝐿𝐿3 (m), 𝐷𝐷1 (m) and 𝐷𝐷3 (m) correspond to Fig. 2(b), 𝛽𝛽 is the nozzle angle of the conical section of the liquefier, and m and φ are power-law fit parameters [9,10,12]. Hence, the total pressure drop in the liquefier is the sum of the pressure drop in each section: 𝛥𝛥𝛥𝛥 = 𝛥𝛥𝛥𝛥1 + 𝛥𝛥𝛥𝛥2 + 𝛥𝛥𝛥𝛥3 According to rheological experiments of PLA at temperatures 180 ⁰C, 190 ⁰C and 200 ⁰C, it is possible to determine the power law fit parameters. The parameters of power-law can be obtained from shear stress versus shear rate plots implementing curve fitting with high correlation coefficients over 0.996 [16]. Table 1 Values of flow index (n), consistency index (K) and correlation coefficient (𝑅𝑅 2 ) [16] Temperature (oC)
180
190
200
𝑛𝑛
0.75
0.77
0.81
𝑛𝑛
𝐾𝐾[𝑃𝑃𝑃𝑃 ∗ 𝑠𝑠 ] 𝑅𝑅 2
4990
0.996
3100
0.998
2140
0.998
The activation energy (α) can be calculated by dividing the flow activation energy-𝛥𝛥𝛥𝛥𝜂𝜂 (J/mol) with the gas constant-R (8.3144 J/mol∙K). It is known that the flow activation temperature is dependent on shear rate but the latter is changing during the flow in the liquefier. More precisely, the shear rate is expected to be very high (200 𝑠𝑠 −1 ) as the melts passes through the print nozzle while low shear rate is expected near the liquefier entrance [17]. So, the flow activation energy 𝛥𝛥𝛦𝛦𝜂𝜂 of PLA was calculated for two different shear rates, 40 and 200 𝑠𝑠 −1 , based on the slopes of the plots of the true viscosity against the reciprocal of temperature (1/T) [16], as: 66.89 and 36.07 (KJ/mol) respectively. Pressure drop calculation according to equations (3-5) has been implemented in Matlab for different operation temperatures and nozzle diameters. The required force for overcoming the loss can be calculated by summing up the multiplication of the pressure drop of each section of the liquefier by the corresponding surface area, For the nozzle employed (0.4 mm diameter) the force required is 10.8 N at 180 oC dropping to 7.9 N at 190 oC and 7.5 N at 200 oC. Indicatively, for a nozzle of 0.3 mm diameter, the force required is 26.3 N at 180 oC dropping to 19.3 N at 190 oC and 20.1 N at 200 oC. 3. Control A personal computer (PC) was adopted as the supervisory controller, interfacing both with the robot controller and with the controller of the extruder board. The printing process can be started, stopped and interrupted by the supervisory controller.
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An ArduinoTM Mega2560 microcontroller with RAMPSTM 1.4 was used for controlling the extruder. The board was flashed with the MarlinTM Firmware that manages all the real-time activities of the machine. The control language for MarlinTM is a derivative of G-code, but in this case no motion control of the axes was involved, since motion is taken care of at the robot controller level. Initially, the MarlinTM firmware was modified to operate only the stepper motor responsible for the extrusion. The motherboard, LCD, PID constants of the heater and the operation and safety conditions are defined in the program. The extruder was calibrated by adjusting the number of steps per unit length of the stepper motor. The 3D printer can be operated not only by the PC but also from the LCD interface which includes an SD card reader and displays the most important parameters enabling the user to adjust them. The robot controller, Stäubli CS7, has 12 inputs and 6 outputs for communication, three out of six inputs being high speed (1ms cycle). When the extruder nozzle has reached the proper temperature, the microcontroller of the 3D printer sends a 12 V signal to a high-speed input of the robot controller via a 24 V step up module. 4. Programming 4.1. Printing parameters The extruder system has four parameters: the extruder feed rate, the amount of extrusion, the extruder temperature and the cooling fan speed. Initially, the speed of the cooling fan was set to maximum while the extruder temperature was regulated at 190 ⁰C. The amount of extrusion was calculated as the total distance that the nozzle would travel in order to build the entire object. The flowrate of the material (𝑄𝑄̇ )through the nozzle must be synchronized with the speed of deposition (𝑉𝑉𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 ) and the cross-sectional area (𝐴𝐴) of the bead [14]: 𝐴𝐴 = 𝑄𝑄̇ ⁄𝑉𝑉𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 . The flow rate through the nozzle could be calculated by assuming a simple Hagen-Poiseuille and using the dimensions of section 2 (Fig. 2) of the nozzle [18]: 𝑄𝑄̇ = 𝜋𝜋(𝐷𝐷2 /2)4 𝛥𝛥𝛥𝛥/(8𝜂𝜂𝐿𝐿2 ). However, in this case the flow rate was calculated simply by multiplying the feed rate of the filament by the filament’s cross-sectional area. Note that the speed of deposition is identical with the linear speed of the end effector of the robotic arm. Therefore, in every printing process, the speed of the robot’s end effector (𝑉𝑉𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 ) was defined first and the feed rate of the extruder was calculated last depending on the nozzle and filament diameter as: 𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 = 𝑉𝑉𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 𝐴𝐴𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛 /𝐴𝐴𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 . At the beginning of the printing process, the robotic arm was activated by the PC and it was positioned at the starting point of the trajectory waiting for a signal from the 3D printer to start. At the same time, the 3D printer was powered and the programming code was uploaded through an SD card. As soon as the material heats up (in about 10 minutes), the 3D printer signals to the robotic arm to start the motion. At the end of the printing process, the robotic arm is guided to a safe position and the 3D printer stops after a few seconds as the length of the extrusion was calculated to stop almost immediately after the end of the process. During the printing, the extrusion can be stopped to allow for a path change. The calculation of the flow rate and the amount of the extrusion were coded in Matlab. Note that at this stage no special recalculation in flow rate or in robot speed was done at turning points of the path. 4.2. Path programming The robot path was programmed in V+ providing all the functionality of modern high-level languages, including: callable subroutines, control structures, multi-tasking environment and recursive, re-entrant program execution. The motion of the robotic arm was specified by using reference points on the printing surface, vector calculus and looping structures. Planar surfaces were programmed, but not necessarily horizontal ones. Reference point coordinates were taught to the robot by using a teach pendant and moves connecting them were added using the robot’s editor. Afterwards, using looping structures in V+, the same path was followed on parallel planes as required. A part of a code written in V+ is presented in Fig. 3. In this example of code, the reference points startp, endp, apt, bpt, cpt, dpt are defined in the cartesian space by positions and r orientation rotations (X, Y, Z, y, p, r) by using the command TRANS. After a delay of 10 seconds (DELAY), the speed of the robot’s end effector is defined at 16 mm/sec for every movement (SPEED 16 MMPS ALWAYS) and then this is moved linearly to point startp
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(MOVES). The command BREAK does not allow the controller to proceed to another command until the last one is completed. Next, the command WAIT SIG(1002) pauses the robot until input 1002 is ON, signalling readiness for printing. Following that, the FOR command executes a loop of material deposition commands 15 times to construct a layer of single-bead-walled meander-like part: this is defined iteratively by points apt and bpt, displaced according to command SHIFT by -2.5 mm and -0.1 mm in X and Z directions respectively. Movement in Z direction is due to the inclination of the printing table in this case. PROGRAM example()
FOR tt = 1 TO 15
SET startp = TRANS(955.16, 162.49, -240.97, -0.17, 179.79, -0.18)
MOVES apt
SET endp = TRANS(955.16, 162.49, -100, -0.17, 179.79, -0.18)
BREAK
SET apt = TRANS(955.16, 0, -240.54, -0.23, 179.79, -0.24)
MOVES bpt
SET bpt = TRANS(955.16, -75, -239.9, -0.77, 179.79, -0.79)
BREAK
SET cpt = TRANS(905.16, -75, -242.46, -0.8, 179.79, -0.82)
SET apt = SHIFT (apt BY -2.5, 0, -0.1)
SET dpt = TRANS(905.16, 0, -242.68, -0.87, 179.79, -0.89)
SET bpt = SHIFT (bpt BY -2.5, 0, -0.1)
DELAY 10
MOVES bpt
SPEED 16 MMPS ALWAYS
BREAK
MOVES startp
MOVES apt
BREAK
BREAK
WAIT SIG(1002)
SET apt = SHIFT (apt BY -2.5, 0, -0.1)
DELAY 5
SET bpt = SHIFT (bpt BY -2.5, 0, -0.1) END Fig. 3 Example of trajectory code in V+
5. Examples Using the integrated system, see Fig. 4(a), several shell-shaped objects were printed successfully, see Table 2 and Fig. 5. The printing surface was made of marble, see Fig. 4(b). For some of the parts the printing surface was deliberately inclined at 3 % and 1 % length and width-wise. It is pointed out that the thickness of the first layer of each printed part was assumed to be 0.5 mm, corresponding to the distance of the extrusion head over the printing surface at the beginning of the printing process.
a Fig. 4 The robotic 3D printer (a) general view (b) focus on printing area
b
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Note that in the case of the meander part, see Figs 4(b) and 5(c), the printing surface had to be heated to 60 0C to achieve a non-deformed shape. Uneven height of the individual ranks resulted when heating was omitted, presumably due to the large thermal gradients involved [19]. Table 2. Printed parts and their dimensions Object
1: parallelepiped
2: open box
3: meander
4: heart
Layers
17
23
30
30
Overlap (mm)
0
0.1
0.2
0.1
0
0
CAD
Dim (mm)
Err (%)
Dim (mm)
Err (%)
Dim (mm)
Err (%)
CAD
Dim (mm)
Err (%)
CAD
Dim (mm)
Err (%)
CAD
Dim (mm)
Err (%)
Length
75.4
76
0.8
76
0.8
76
0.8
75.4
75.9
0.7
78.7
79.3
0.8
-
-
-
Width
50.4
51
1.2
50.9
1.0
51.1
1.4
10.65
11.3
6.1
76.2
76.6
0.5
-
-
-
Height
6.9
6.9
0.0
5.61
-5.9
3.54
-4.3
7.2
7.0
-2.8
12.1
12.3
1.7
9.2
8.84
-3.9
Fig. 5 Example parts printed (a) Parallelepiped (b) Open box (c) Meander (d) Heart
The average percentage error in length, width and height for the four parts presented are 0.76 %, 2.04 % and 3.9% respectively, see Table 2. The main reason for the existence of errors in height is the assumption of the height of the first layer being 0.5 mm. As regards other directions, the use of joints 4 and 5 in conjunction with the weight of the 3D printer (2.5 Kg) can create oscillations affecting all dimensions. The errors in length (less than 1 %) are the smallest since length-wise motion is not affected by oscillations contrary to width-wise motion. Moreover, the constancy of flow rate neglecting acceleration / deceleration of the robot does also affect accuracy of the part near its boundary. 6. Conclusions An accurate robot, such as the one used in this work, could be perfectly used for moving a 3D printing head. If its acceleration were taken into account by the extruder motor the results would be even better in terms of achievable shape accuracy. The biggest advantage of such a system is the size of the printed part which could be as large as the workspace of the robot; in this case, its length and height could be 1100 mm and 506 mm respectively taking the
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dimensions of the printing head into account. This capability can be improved even more by establishing a movable base or a gantry-type robot, in any case safeguarding the ability of the head to deposit material on a plane tangent to the surface being created. Up to now complex trajectories can be achieved on the same plane, which is not necessarily horizontal. Indeed, several axes work simultaneously, but this is taken care of by the robot controller through its inverse kinematics routines. However, further work needs to be done, in order to exploit the capabilities of this system to the full, regarding improvement in mechanical properties and reduction in printing time. First, a slicing / toolpath planning algorithm capable of multiplane printing should be developed. Second, inner filling path patterns should be possible to specify in a standard way; smoother outer toolpaths by contour multiplane strategies could also be defined. Third, adjustment of flow rate with respect to direction or speed change is needed, typically to avoid overflow at the corners of printed objects. Acknowledgements Hellenic Petroleum group is gratefully acknowledged for their financially supporting the first author. References [1] R. Jones, P. Haufe, E. Sells, P. Iravani, V. Olliver, C. Palmer, A. Bowyer, RepRap – the replicating rapid prototyper, Robotica. 29 (2011) 177–191. [2] I. Bin Ishak, J. Fisher, P. Larochelle, Robot Arm Platform for Additive Manufacturing Using Multi-Plane Toolpaths, in: Vol. 5A 40th Mech. Robot. Conf., 2016: p. V05AT07A063. [3] W.C. Lee, C.C. Wei, S.C. Chung, Development of a hybrid rapid prototyping system using low-cost fused deposition modeling and five-axis machining, Journal of Materials Processing Technology. 214 (2014) 2366–2374. [4] G.Q. Zhang, X. Li, R. Boca, J. Newkirk, B. Zhang, T. a Fuhlbrigge, H.K. Feng, N.J. Hunt, Use of industrial robots in additive manufacturing - A survey and feasibility study, Proceedings for the Joint Conference of ISR 2014 - 45th International Symposium on Robotics and Robotik 2014 - 8th German Conference on Robotics, ISR/ROBOTIK 2014. (2014) 512–517. [5] S. Keating, N. Oxman, Compound fabrication: A multi-functional robotic platform for digital design and fabrication, Robotics and Computer-Integrated Manufacturing. 29 (2013) 439–448. [6] D. Popescu, C. Amza, Additive Manufacturing Automation for Industry 4.0, Research and Science Today. 13 (2017). [7] B.J. Brooks, K.M. Arif, S. Dirven, J. Potgieter, Robot-assisted 3D printing of biopolymer thin shells, International Journal of Advanced Manufacturing Technology. 89 (2017) 957–968. [8] E. Barnett, C. Gosselin, Large-scale 3D printing with a cable-suspended robot, Additive Manufacturing. 7 (2015) 27–44. [9] M. Hopmann, W. Michaeli, Extrusion Dies for Plastics and Rubber, 4th Editio, Carl Hanser Verlag GmbH & Co. KG, Munich, 2016. [10] A. Bellini, S. Güçeri, M. Bertoldi, Liquefier Dynamics in Fused Deposition, Journal of Manufacturing Science and Engineering. 126 (2004) 237. [11] N. Mostafa, H.M. Syed, S. Igor, G. Andrew, A Study of Melt Flow Analysis of an ABS-Iron Composite in Fused Deposition Modelling Process, Tsinghua Science and Technology. 14 (2009) 29–37. [12] H.S. Ramanath, C.K. Chua, K.F. Leong, K.D. Shah, Melt flow behaviour of poly-??-caprolactone in fused deposition modelling, in: J. Mater. Sci. Mater. Med., 2008: pp. 2541–2550. [13] M. a Yardimci, T. Hattori, S.I. Guceri, S.C. Danforth, Thermal analysis of Fused Deposition, Solid Freeform Fabrication Proceedings, September 1997. (1997) 689–698. [14] B. N. Turner, R. Strong, S. A. Gold, A review of melt extrusion additive manufacturing processes: I. Process design and modeling, Rapid Prototyping Journal. 20 (2014) 192–204. [15] T.E. Karis, D.J. Dawson, C.R. Davis, R.N. Kono, G. Kim, M.S. Jhon, S.J. Kim, Rapid prototyping materials rheology, Journal of Imaging Science and Technology. 40 (1996) 147–154. [16] S. Djellali, T. Sadoun, N. Haddaoui, A. Bergeret, Viscosity and viscoelasticity measurements of low density polyethylene/poly(lactic acid) blends, Polymer Bulletin. 72 (2015) 1177–1195. [17] A. Venkataraman, N; Rangarajan, S; Matthewson, M J; Harper, B; Safari, A; Danforth, S C; G. Wu; Langrana, N; Guceri, S; Yardimci, Feedstock material property - process relationships in fused deposition of ceramics (FDC), Rapid Prototyping Journal. 6 (2000) 244–252. [18] R.S. Crockett, P. Calvert, The liquid-to-solid transition in stereodeposition techniques, Department of Materials Science and Engineering. (1997) 221. [19] T.M. Wang, J.T. Xi, Y. Jin, A model research for prototype warp deformation in the FDM process, International Journal of Advanced Manufacturing Technology. 33 (2007) 1087–1096.
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28th International Conference on Flexible Automation and Intelligent Manufacturing 28th International ConferenceJune on Flexible Automation and OH, Intelligent (FAIM2018), 11-14, 2018, Columbus, USA Manufacturing (FAIM2018), June 11-14, 2018, Columbus, OH, USA
On the development of a robot-operated 3D-printer
On the development of a robot-operated Manufacturing Engineering Society International Conference 2017, 3D-printer MESIC 2017, 28-30 June a 2017, Vigo (Pontevedra), Spain Andi Dine , George-Christopher Vosniakosa * a
Andi Dinea, George-Christopher Vosniakosa *
Costing models forofcapacity in Industry 4.0: Trade-off National Technical Univerity Athens, School of optimization Mechanical Engineering, Heroon Polytehneiou 9, 15780 Athens, Greece between used capacity and operational efficiency Abstract National Technical Univerity of Athens, School of Mechanical Engineering, Heroon Polytehneiou 9, 15780 Athens, Greece
a
Abstract a a,* on, attached to b in this case 6-axis arm with A prototype Fused DepositionA. Modelling (FDM) is reported robot, Santana , P.head Afonso , A. Zaninb, anR.industrial Wernke A prototype Fused Deposition Modelling reported on, attached to an including industrial robot, in thiscalculations case 6-axisregarding arm with high repeatability, to form a robotic 3D (FDM) printer. head The is detailed design is presented theoretical a printer. The detailed design is presented including theoretical calculations regarding high repeatability, to nozzle. form a Selection robotic 3Dof pressure drop in the the off-the-shelf electronicGuimarães, components and their inter-connection are also covered University of Minho, 4800-058 Portugal b Unochapecó, 89809-000 Chapecó, SC, BrazilandProgramming pressure drop the in the nozzle. Selection of the off-the-shelf electronic components their inter-connection are alsopaths, covered together with implementation for synchronizing the robot and the head’s extruder. of the 3D printer for together thepertains implementation for synchronizing the robot androbot the head’s extruder. Programming of theand 3Drepeating printer paths, for the time with being, to teaching the pertinent points to the via the pendant, to form a layer through the time being, pertainstotoform teaching the pertinent to theofrobot viashell the artefacts pendant, are to presented form a layer repeating through programming structures the complete object.points A number printed withand accuracy assessment, programming structures toacceptable. form the complete object. A number of printed shell artefacts are presented with accuracy assessment, which was deemed totally Abstract which was deemed totally acceptable. © 2018 The Authors. Published by Elsevier B.V. © 2018 2018 The Authors. by B.V. Under the concept of "Industry productionlicense processes will be pushed to be increasingly interconnected, © The Authors. Published by Elsevier Elsevier B.V. This is an open accessPublished article under the4.0", CC BY-NC-ND (https://creativecommons.org/licenses/by-nc-nd/4.0/) This is an open access article under the CC BY-NC-ND licensemuch (http://creativecommons.org/licenses/by-nc-nd/3.0/) information based on a real time basis and, necessarily, more efficient. In this capacity optimization This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of scientific committee of the 28th Flexible Automation andcontext, Intelligent Manufacturing Peer-review under responsibility of the scientific committee of the 28th Flexible Automation and Intelligent Manufacturing goes beyond the traditional aim of capacity maximization, contributing also for organization’s profitability and value. Peer-review under responsibility of the scientific committee of the 28th Flexible Automation and Intelligent Manufacturing (FAIM2018) Conference. (FAIM2018) Conference. (FAIM2018) Indeed, leanConference. management and continuous improvement approaches suggest capacity optimization instead of
Keywords: Fused Deposition; Robot; optimization 3D Printer; Multi-plane printingmodels is an important research topic that deserves maximization. The studyIndustrial of capacity and costing Keywords: Fusedfrom Deposition; Robot;and 3D Printer; Multi-plane printing This paper presents and discusses a mathematical contributions both Industrial the practical theoretical perspectives. model for capacity management based on different costing models (ABC and TDABC). A generic model has been 1. Introduction developed and it was used to analyze idle capacity and to design strategies towards the maximization of organization’s 1. Introduction value. The trade-off capacity maximization vs operational efficiency is highlighted and it is shown that capacity The mostmight widespread additive inefficiency. manufacturing method is Fused Deposition Modelling (FDM) due to its optimization hide operational The most widespread additive manufacturing method is Fused Deposition Modelling (FDM) due to its inexpensive platform and the open-source © 2017 The Authors. Published by Elsevier B.V. movement [1]. A filament is forced through a heated extrusion head, inexpensive platform and the open-source movement A a International heated extrusion head, including a liquefier part and a final nozzle, so that itof[1]. can be filament depositedisEngineering inforced liquidthrough form along planned trajectories. Peer-review under responsibility of the scientific committee the Manufacturing Society Conference including liquefier part andquickly a finalsolidify nozzle,tosoform thataitlayer can and be deposited in liquid form along trajectories. Deposited atracks of material this is subsequently repeated to planned form an object. 2017. Deposited tracks of material quickly solidify to form a layer and this is subsequently repeated to form an object. Keywords: Cost Models; ABC; TDABC; Capacity Management; Idle Capacity; Operational Efficiency
author. Tel.: +30 210 7721457; fax: +30 210 7724273, e-mail address: [email protected] 1.Corresponding Introduction
* *
Corresponding author. Tel.: +30 210 7721457; fax: +30 210 7724273, e-mail address: [email protected] 2351-9789 © 2018 Thecapacity Authors. Published by Elsevier information B.V. The cost of idle is a fundamental for companies and their management of extreme importance This is an open access under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) 2351-9789 © 2018 Thearticle Authors. Published by Elsevier B.V. in modern production systems. In general, it is defined as unused capacity or production potential and can be measured Peer-review under responsibility of the scientific committee of the 28th Flexible Automation and Intelligent Manufacturing (FAIM2018) This is an open access article under CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) in several ways: tons of production, available hours of manufacturing, management of (FAIM2018) the idle capacity Conference. Peer-review under responsibility of the scientific committee of the 28th Flexible Automationetc. and The Intelligent Manufacturing * Paulo Afonso. Tel.: +351 253 510 761; fax: +351 253 604 741 Conference. E-mail address: [email protected] 2351-9789 Published by Elsevier B.V. B.V. 2351-9789 ©©2017 2018The TheAuthors. Authors. Published by Elsevier Peer-review underaccess responsibility of the scientific committee oflicense the Manufacturing Engineering Society International Conference 2017. This is an open article under the CC BY-NC-ND (http://creativecommons.org/licenses/by-nc-nd/3.0/) Peer-review under responsibility of the scientific committee of the 28th Flexible Automation and Intelligent Manufacturing (FAIM2018) Conference. 10.1016/j.promfg.2018.10.004
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Conventional 3D printers utilize a gantry for continuously moving the nozzle on a horizontal plane, whilst vertical movement is intermittent a few times per plane (2-1/2 axis interpolation). There is the obvious advantage of simplicity in computer numerical control on a horizontal plane, but there are also drawbacks compared to multiplane 3D printing. These mainly concern a need of support material for overhung structures and pertinent post-processing for their removal, some reduction in strength and restriction in attainable complexity and limitations in accomplishing embedded 3D printing [2], [3]. The concept of multiplane 3D printing via a robot is undergoing intense development [4]. For instance, a machine combining multiplane additive, formative and subtractive processes in one robotic platform with an end effector interchange depending on the particular mode is presented in [5]. Major 3D printing and automation companies presented the Robotic Composite 3D Demonstrator with 8 axes, with an ability to print across fused layers in order to increase the strength of the printed part [6]. A hybrid 5-axis cnc machine involving 3D printing and subsequent milling for improved dimensional accuracy has been developed by Lee et al. [3], capable of multiplane 3D printing due to its rotational building platform. A commonly used FDM extruder has been integrated with a 6-axis robotic arm demonstrating printing in horizontal and vertical plane [2]. Another similar development utilised the screw extrusion method, whilst the motion of the robot was calculated by projecting a desired 2D image in 3D space [7]. A large-scale additive manufacturing system with a 6-axis cable-suspended robot exhibited a wide range of motion in addition to being transportable and reconfigurable [8]. This paper reports on development of a 6-axis robotic 3D printer as a proof-of-concept prototype. Its design and control are presented in Section 2 and 3, respectively, its programming is briefly outlined in Section 4, examples of 3D printed parts are given in Section 5 and conclusions are summarized in Section 6. 2. Mechanical Design The robot used was a Stäubli RX90L. It is a six axis industrial robot with payload of 4 kg at maximum speed and 6 kg at reduced speed, maximum operation speed of 12.6 m/sec, maximum reach of 1100 mm and repeatability at ± 0.025 mm. The 3D printing system mounted as an end effector on the robot should weigh less than 3.5 kg. Most of the parts of the 3D printer were designed parametrically in SolidWorks TM software and manufactured from Al alloy 2000 series on HAASTM TM-1 and TL-1 CNC milling and turning machines, respectively, according to G-code generated on SolidCAMTM software. 2.1. Extrusion system A three-tier frame was designed for accommodating electronics, extrusion system mechanism and 3D printing heads, see Fig. 1.
Fig. 1 The 3D printing mechanism (a) as designed (b) manufactured and partly assembled second tier (c) control tier
Andi Dine et al. / Procedia Manufacturing 17 (2018) 6–13 Dine, Vosniakos / Procedia Manufacturing 00 (2018) 000–000
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3
The bottom tier is flanged for attachment to the robot. The printer is designed so as to enable use of two materials, normally the building material, e.g. PLA/ABS, and the support material. The extrusion mechanism is located on the second tier, see Fig. 1(b). Its purpose is to push the filament to overcome the pressure drop in the nozzle. It mainly consists of a driving roll supported by bearings, which is driven by a ΝΕΜΑ 17 stepper motor. There are also two pressure rolls that push the two filaments against the driving roll. These consist of a grooved wheel and knurled counter-surface to eliminate slippage. The user can choose which pressure roll will be activated by tightening the respective screws. The maximum torque of the motor is 1.765 Nm hence the maximum extrusion force is 353 N. The two 3D printer heads are located on the third tier, see Fig. 1(a). The printer head consists of the liquefier, the nozzle and temperature controlling devices, typically fins. An off-the-shelf E4D J-Head ExtruderTM with nozzle diameter 0.4 mm, appropriate for 1.75 mm filament of various materials (ABS, PLA, wood, PVC, etc.) was selected. The filament reel holder is located on the third link of the robotic arm, to provide a filament path as straight as possible and to eliminate the possibilities of filament messing in case of complicated motion. The theoretical height of the printed parts can be calculated as: Height (mm)=First layer height+(Number of Layers-1)*(Bead diameter-Overlap), where ‘overlap’ is the intentional decrease in original distance between successive layers to reduce inter-bead voids. For example, if the diameter of each bead were 0.4 mm and the overlap 0.1 mm, then the displacement from the previous layer to the next would be 0.3 mm, see Fig. 2(a).
a
b
Fig. 2 (a) Example of overlap between successive layers on a slightly inclined printing surface (b) Sections of the liquefier
2.2. Pressure drop estimation The pressure drop in the nozzle and hence the required force for extrusion is essential to know in regulating the flow rate. Key assumptions in this calculation include incompressibility of the melt, a no-slip boundary condition at the wall of the liquefier and a fully developed, steady state and laminar flow. Then, pressure drop can be predicted by momentum balance equations in conjunction with a power-law viscosity model with Arrhenius temperature dependence [9]. This model was applied for extrusion dies of cylindrical and conical shape corresponding to sections I, II and III of the liquefier, see Fig. 2(b). The power-law model reflects the assumption that feedstock (ABS, PLA) is typically shear thinning [10–13], 𝜂𝜂 = 𝛫𝛫 ∗ 𝛾𝛾̇ 𝑛𝑛−1
(1)
𝜂𝜂 = 𝛨𝛨(𝛵𝛵)𝜂𝜂 𝛵𝛵0 (𝛾𝛾̇ )
(2)
where η (Pa·s) is the viscosity, 𝛾𝛾̇ (1/s) is the shear rate. The power-law fit parameters are 𝛫𝛫 ,the consistency index (Pa·sn) and 𝑛𝑛, the flow index. This model is mathematically simple but neglects yield stress, which is characteristic of many polymer melts [14]. Viscosity dependence on temperature is significant for non-isothermal material flow through the liquefier. Thus, viscosity is separated into the temperature-dependent and shear rate-dependent terms [10]:
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The shear rate-dependent term is simply the power-law model in which the fit parameters have been calculated at some reference temperature, 𝑇𝑇0 .The temperature-dependent term is described by Arrhenius expression: 𝐻𝐻(𝑇𝑇) = exp[𝑎𝑎(1⁄𝑇𝑇 − 1⁄𝑇𝑇0 )], where α (Κ) is the activation energy [10,15]. Note that H(T) is 1 at the reference temperature, 𝑇𝑇0 . The pressure drops (Pa) in each section of the liquefier according to this model are given respectively by [14]: 1 𝑚𝑚
𝑉𝑉
𝜑𝜑
𝛥𝛥𝛥𝛥2 = (
2𝑚𝑚
𝛽𝛽 2
3 𝑡𝑡𝑡𝑡𝑡𝑡( )
)( 1
𝑉𝑉3 𝑚𝑚
(𝐷𝐷1 1
1
1
) 𝑒𝑒𝑥𝑥𝑝𝑝[𝑎𝑎(𝑇𝑇 − 𝑇𝑇 )] /2)𝑚𝑚+1
3 𝐷𝐷3 𝑚𝑚
−
1
3 𝐷𝐷1 𝑚𝑚
(3)
0
𝐷𝐷1 2
) (( 2 ) (𝑚𝑚 + 3)2 1
(𝑚𝑚+3)(𝐷𝐷1 /2)2 𝑚𝑚
𝛥𝛥𝛥𝛥3 = 2𝐿𝐿3 ( ) ( 𝜑𝜑
1 𝑚𝑚
𝑚𝑚+3
𝛥𝛥𝛥𝛥1 = 2𝐿𝐿1 ( 1 ) (
(𝐷𝐷3 /2)𝑚𝑚+3
1
𝑚𝑚+3
1 𝑚𝑚
1
1
) 𝑒𝑒𝑒𝑒𝑒𝑒[𝑎𝑎(𝑇𝑇 − 𝑇𝑇 )] 0
1
) 𝑒𝑒𝑒𝑒𝑒𝑒[𝑎𝑎(𝑇𝑇 − 𝑇𝑇 )]
(4)
(5)
0
where the dimensions 𝐿𝐿1 (m), 𝐿𝐿3 (m), 𝐷𝐷1 (m) and 𝐷𝐷3 (m) correspond to Fig. 2(b), 𝛽𝛽 is the nozzle angle of the conical section of the liquefier, and m and φ are power-law fit parameters [9,10,12]. Hence, the total pressure drop in the liquefier is the sum of the pressure drop in each section: 𝛥𝛥𝛥𝛥 = 𝛥𝛥𝛥𝛥1 + 𝛥𝛥𝛥𝛥2 + 𝛥𝛥𝛥𝛥3 According to rheological experiments of PLA at temperatures 180 ⁰C, 190 ⁰C and 200 ⁰C, it is possible to determine the power law fit parameters. The parameters of power-law can be obtained from shear stress versus shear rate plots implementing curve fitting with high correlation coefficients over 0.996 [16]. Table 1 Values of flow index (n), consistency index (K) and correlation coefficient (𝑅𝑅 2 ) [16] Temperature (oC)
180
190
200
𝑛𝑛
0.75
0.77
0.81
𝑛𝑛
𝐾𝐾[𝑃𝑃𝑃𝑃 ∗ 𝑠𝑠 ] 𝑅𝑅 2
4990
0.996
3100
0.998
2140
0.998
The activation energy (α) can be calculated by dividing the flow activation energy-𝛥𝛥𝛥𝛥𝜂𝜂 (J/mol) with the gas constant-R (8.3144 J/mol∙K). It is known that the flow activation temperature is dependent on shear rate but the latter is changing during the flow in the liquefier. More precisely, the shear rate is expected to be very high (200 𝑠𝑠 −1 ) as the melts passes through the print nozzle while low shear rate is expected near the liquefier entrance [17]. So, the flow activation energy 𝛥𝛥𝛦𝛦𝜂𝜂 of PLA was calculated for two different shear rates, 40 and 200 𝑠𝑠 −1 , based on the slopes of the plots of the true viscosity against the reciprocal of temperature (1/T) [16], as: 66.89 and 36.07 (KJ/mol) respectively. Pressure drop calculation according to equations (3-5) has been implemented in Matlab for different operation temperatures and nozzle diameters. The required force for overcoming the loss can be calculated by summing up the multiplication of the pressure drop of each section of the liquefier by the corresponding surface area, For the nozzle employed (0.4 mm diameter) the force required is 10.8 N at 180 oC dropping to 7.9 N at 190 oC and 7.5 N at 200 oC. Indicatively, for a nozzle of 0.3 mm diameter, the force required is 26.3 N at 180 oC dropping to 19.3 N at 190 oC and 20.1 N at 200 oC. 3. Control A personal computer (PC) was adopted as the supervisory controller, interfacing both with the robot controller and with the controller of the extruder board. The printing process can be started, stopped and interrupted by the supervisory controller.
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An ArduinoTM Mega2560 microcontroller with RAMPSTM 1.4 was used for controlling the extruder. The board was flashed with the MarlinTM Firmware that manages all the real-time activities of the machine. The control language for MarlinTM is a derivative of G-code, but in this case no motion control of the axes was involved, since motion is taken care of at the robot controller level. Initially, the MarlinTM firmware was modified to operate only the stepper motor responsible for the extrusion. The motherboard, LCD, PID constants of the heater and the operation and safety conditions are defined in the program. The extruder was calibrated by adjusting the number of steps per unit length of the stepper motor. The 3D printer can be operated not only by the PC but also from the LCD interface which includes an SD card reader and displays the most important parameters enabling the user to adjust them. The robot controller, Stäubli CS7, has 12 inputs and 6 outputs for communication, three out of six inputs being high speed (1ms cycle). When the extruder nozzle has reached the proper temperature, the microcontroller of the 3D printer sends a 12 V signal to a high-speed input of the robot controller via a 24 V step up module. 4. Programming 4.1. Printing parameters The extruder system has four parameters: the extruder feed rate, the amount of extrusion, the extruder temperature and the cooling fan speed. Initially, the speed of the cooling fan was set to maximum while the extruder temperature was regulated at 190 ⁰C. The amount of extrusion was calculated as the total distance that the nozzle would travel in order to build the entire object. The flowrate of the material (𝑄𝑄̇ )through the nozzle must be synchronized with the speed of deposition (𝑉𝑉𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 ) and the cross-sectional area (𝐴𝐴) of the bead [14]: 𝐴𝐴 = 𝑄𝑄̇ ⁄𝑉𝑉𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 . The flow rate through the nozzle could be calculated by assuming a simple Hagen-Poiseuille and using the dimensions of section 2 (Fig. 2) of the nozzle [18]: 𝑄𝑄̇ = 𝜋𝜋(𝐷𝐷2 /2)4 𝛥𝛥𝛥𝛥/(8𝜂𝜂𝐿𝐿2 ). However, in this case the flow rate was calculated simply by multiplying the feed rate of the filament by the filament’s cross-sectional area. Note that the speed of deposition is identical with the linear speed of the end effector of the robotic arm. Therefore, in every printing process, the speed of the robot’s end effector (𝑉𝑉𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 ) was defined first and the feed rate of the extruder was calculated last depending on the nozzle and filament diameter as: 𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 = 𝑉𝑉𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 𝐴𝐴𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛 /𝐴𝐴𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 . At the beginning of the printing process, the robotic arm was activated by the PC and it was positioned at the starting point of the trajectory waiting for a signal from the 3D printer to start. At the same time, the 3D printer was powered and the programming code was uploaded through an SD card. As soon as the material heats up (in about 10 minutes), the 3D printer signals to the robotic arm to start the motion. At the end of the printing process, the robotic arm is guided to a safe position and the 3D printer stops after a few seconds as the length of the extrusion was calculated to stop almost immediately after the end of the process. During the printing, the extrusion can be stopped to allow for a path change. The calculation of the flow rate and the amount of the extrusion were coded in Matlab. Note that at this stage no special recalculation in flow rate or in robot speed was done at turning points of the path. 4.2. Path programming The robot path was programmed in V+ providing all the functionality of modern high-level languages, including: callable subroutines, control structures, multi-tasking environment and recursive, re-entrant program execution. The motion of the robotic arm was specified by using reference points on the printing surface, vector calculus and looping structures. Planar surfaces were programmed, but not necessarily horizontal ones. Reference point coordinates were taught to the robot by using a teach pendant and moves connecting them were added using the robot’s editor. Afterwards, using looping structures in V+, the same path was followed on parallel planes as required. A part of a code written in V+ is presented in Fig. 3. In this example of code, the reference points startp, endp, apt, bpt, cpt, dpt are defined in the cartesian space by positions and r orientation rotations (X, Y, Z, y, p, r) by using the command TRANS. After a delay of 10 seconds (DELAY), the speed of the robot’s end effector is defined at 16 mm/sec for every movement (SPEED 16 MMPS ALWAYS) and then this is moved linearly to point startp
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(MOVES). The command BREAK does not allow the controller to proceed to another command until the last one is completed. Next, the command WAIT SIG(1002) pauses the robot until input 1002 is ON, signalling readiness for printing. Following that, the FOR command executes a loop of material deposition commands 15 times to construct a layer of single-bead-walled meander-like part: this is defined iteratively by points apt and bpt, displaced according to command SHIFT by -2.5 mm and -0.1 mm in X and Z directions respectively. Movement in Z direction is due to the inclination of the printing table in this case. PROGRAM example()
FOR tt = 1 TO 15
SET startp = TRANS(955.16, 162.49, -240.97, -0.17, 179.79, -0.18)
MOVES apt
SET endp = TRANS(955.16, 162.49, -100, -0.17, 179.79, -0.18)
BREAK
SET apt = TRANS(955.16, 0, -240.54, -0.23, 179.79, -0.24)
MOVES bpt
SET bpt = TRANS(955.16, -75, -239.9, -0.77, 179.79, -0.79)
BREAK
SET cpt = TRANS(905.16, -75, -242.46, -0.8, 179.79, -0.82)
SET apt = SHIFT (apt BY -2.5, 0, -0.1)
SET dpt = TRANS(905.16, 0, -242.68, -0.87, 179.79, -0.89)
SET bpt = SHIFT (bpt BY -2.5, 0, -0.1)
DELAY 10
MOVES bpt
SPEED 16 MMPS ALWAYS
BREAK
MOVES startp
MOVES apt
BREAK
BREAK
WAIT SIG(1002)
SET apt = SHIFT (apt BY -2.5, 0, -0.1)
DELAY 5
SET bpt = SHIFT (bpt BY -2.5, 0, -0.1) END Fig. 3 Example of trajectory code in V+
5. Examples Using the integrated system, see Fig. 4(a), several shell-shaped objects were printed successfully, see Table 2 and Fig. 5. The printing surface was made of marble, see Fig. 4(b). For some of the parts the printing surface was deliberately inclined at 3 % and 1 % length and width-wise. It is pointed out that the thickness of the first layer of each printed part was assumed to be 0.5 mm, corresponding to the distance of the extrusion head over the printing surface at the beginning of the printing process.
a Fig. 4 The robotic 3D printer (a) general view (b) focus on printing area
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Note that in the case of the meander part, see Figs 4(b) and 5(c), the printing surface had to be heated to 60 0C to achieve a non-deformed shape. Uneven height of the individual ranks resulted when heating was omitted, presumably due to the large thermal gradients involved [19]. Table 2. Printed parts and their dimensions Object
1: parallelepiped
2: open box
3: meander
4: heart
Layers
17
23
30
30
Overlap (mm)
0
0.1
0.2
0.1
0
0
CAD
Dim (mm)
Err (%)
Dim (mm)
Err (%)
Dim (mm)
Err (%)
CAD
Dim (mm)
Err (%)
CAD
Dim (mm)
Err (%)
CAD
Dim (mm)
Err (%)
Length
75.4
76
0.8
76
0.8
76
0.8
75.4
75.9
0.7
78.7
79.3
0.8
-
-
-
Width
50.4
51
1.2
50.9
1.0
51.1
1.4
10.65
11.3
6.1
76.2
76.6
0.5
-
-
-
Height
6.9
6.9
0.0
5.61
-5.9
3.54
-4.3
7.2
7.0
-2.8
12.1
12.3
1.7
9.2
8.84
-3.9
Fig. 5 Example parts printed (a) Parallelepiped (b) Open box (c) Meander (d) Heart
The average percentage error in length, width and height for the four parts presented are 0.76 %, 2.04 % and 3.9% respectively, see Table 2. The main reason for the existence of errors in height is the assumption of the height of the first layer being 0.5 mm. As regards other directions, the use of joints 4 and 5 in conjunction with the weight of the 3D printer (2.5 Kg) can create oscillations affecting all dimensions. The errors in length (less than 1 %) are the smallest since length-wise motion is not affected by oscillations contrary to width-wise motion. Moreover, the constancy of flow rate neglecting acceleration / deceleration of the robot does also affect accuracy of the part near its boundary. 6. Conclusions An accurate robot, such as the one used in this work, could be perfectly used for moving a 3D printing head. If its acceleration were taken into account by the extruder motor the results would be even better in terms of achievable shape accuracy. The biggest advantage of such a system is the size of the printed part which could be as large as the workspace of the robot; in this case, its length and height could be 1100 mm and 506 mm respectively taking the
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dimensions of the printing head into account. This capability can be improved even more by establishing a movable base or a gantry-type robot, in any case safeguarding the ability of the head to deposit material on a plane tangent to the surface being created. Up to now complex trajectories can be achieved on the same plane, which is not necessarily horizontal. Indeed, several axes work simultaneously, but this is taken care of by the robot controller through its inverse kinematics routines. However, further work needs to be done, in order to exploit the capabilities of this system to the full, regarding improvement in mechanical properties and reduction in printing time. First, a slicing / toolpath planning algorithm capable of multiplane printing should be developed. Second, inner filling path patterns should be possible to specify in a standard way; smoother outer toolpaths by contour multiplane strategies could also be defined. Third, adjustment of flow rate with respect to direction or speed change is needed, typically to avoid overflow at the corners of printed objects. Acknowledgements Hellenic Petroleum group is gratefully acknowledged for their financially supporting the first author. References [1] R. Jones, P. Haufe, E. Sells, P. Iravani, V. Olliver, C. Palmer, A. Bowyer, RepRap – the replicating rapid prototyper, Robotica. 29 (2011) 177–191. [2] I. Bin Ishak, J. Fisher, P. Larochelle, Robot Arm Platform for Additive Manufacturing Using Multi-Plane Toolpaths, in: Vol. 5A 40th Mech. Robot. Conf., 2016: p. V05AT07A063. [3] W.C. Lee, C.C. Wei, S.C. Chung, Development of a hybrid rapid prototyping system using low-cost fused deposition modeling and five-axis machining, Journal of Materials Processing Technology. 214 (2014) 2366–2374. [4] G.Q. Zhang, X. Li, R. Boca, J. Newkirk, B. Zhang, T. a Fuhlbrigge, H.K. Feng, N.J. Hunt, Use of industrial robots in additive manufacturing - A survey and feasibility study, Proceedings for the Joint Conference of ISR 2014 - 45th International Symposium on Robotics and Robotik 2014 - 8th German Conference on Robotics, ISR/ROBOTIK 2014. (2014) 512–517. [5] S. Keating, N. Oxman, Compound fabrication: A multi-functional robotic platform for digital design and fabrication, Robotics and Computer-Integrated Manufacturing. 29 (2013) 439–448. [6] D. Popescu, C. Amza, Additive Manufacturing Automation for Industry 4.0, Research and Science Today. 13 (2017). [7] B.J. Brooks, K.M. Arif, S. Dirven, J. Potgieter, Robot-assisted 3D printing of biopolymer thin shells, International Journal of Advanced Manufacturing Technology. 89 (2017) 957–968. [8] E. Barnett, C. Gosselin, Large-scale 3D printing with a cable-suspended robot, Additive Manufacturing. 7 (2015) 27–44. [9] M. Hopmann, W. Michaeli, Extrusion Dies for Plastics and Rubber, 4th Editio, Carl Hanser Verlag GmbH & Co. KG, Munich, 2016. [10] A. Bellini, S. Güçeri, M. Bertoldi, Liquefier Dynamics in Fused Deposition, Journal of Manufacturing Science and Engineering. 126 (2004) 237. [11] N. Mostafa, H.M. Syed, S. Igor, G. Andrew, A Study of Melt Flow Analysis of an ABS-Iron Composite in Fused Deposition Modelling Process, Tsinghua Science and Technology. 14 (2009) 29–37. [12] H.S. Ramanath, C.K. Chua, K.F. Leong, K.D. Shah, Melt flow behaviour of poly-??-caprolactone in fused deposition modelling, in: J. Mater. Sci. Mater. Med., 2008: pp. 2541–2550. [13] M. a Yardimci, T. Hattori, S.I. Guceri, S.C. Danforth, Thermal analysis of Fused Deposition, Solid Freeform Fabrication Proceedings, September 1997. (1997) 689–698. [14] B. N. Turner, R. Strong, S. A. Gold, A review of melt extrusion additive manufacturing processes: I. Process design and modeling, Rapid Prototyping Journal. 20 (2014) 192–204. [15] T.E. Karis, D.J. Dawson, C.R. Davis, R.N. Kono, G. Kim, M.S. Jhon, S.J. Kim, Rapid prototyping materials rheology, Journal of Imaging Science and Technology. 40 (1996) 147–154. [16] S. Djellali, T. Sadoun, N. Haddaoui, A. Bergeret, Viscosity and viscoelasticity measurements of low density polyethylene/poly(lactic acid) blends, Polymer Bulletin. 72 (2015) 1177–1195. [17] A. Venkataraman, N; Rangarajan, S; Matthewson, M J; Harper, B; Safari, A; Danforth, S C; G. Wu; Langrana, N; Guceri, S; Yardimci, Feedstock material property - process relationships in fused deposition of ceramics (FDC), Rapid Prototyping Journal. 6 (2000) 244–252. [18] R.S. Crockett, P. Calvert, The liquid-to-solid transition in stereodeposition techniques, Department of Materials Science and Engineering. (1997) 221. [19] T.M. Wang, J.T. Xi, Y. Jin, A model research for prototype warp deformation in the FDM process, International Journal of Advanced Manufacturing Technology. 33 (2007) 1087–1096.