Shape Verification of Fused Deposition Modelling 3D

International Journal of Information and Computer Science (IJICS) Volume 4, 2015 doi: 10.14355/ijics.2015.04.001

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Shape Verification of Fused Deposition Modelling 3D Prints Andrej Cupar1, Vojko Pogačar1 and Zoran Stjepanovič2 University of Maribor, Faculty of Mechanical Engineering, Institute of Structures and Design, Laboratory of Design 1

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University of Maribor, Faculty of Mechanical Engineering, Department of Textile Materials and Design

Abstract In this article, the shape of fused deposition modelling (FDM) 3D printed testing objects is verified with 3D scanning technique. Leapfrog’s Creatr is used as a manufacturing device. The whole process of a materialization is explained – from preparation of a model to preparation of a printer. The study is meant to evaluate dimensional errors of printed parts and to give some advices on how to minimize them. Keywords FDM 3D Printing; Leapfrog Creatr; Comparison; Parameter Confirmation; Verification

Introduction Fused deposition modelling (FDM) or an equivalent term fused filament fabrication (FFF) is entry level additive manufacturing technology of 3D printing that has widespread over the world, since 2009 the patent for that technology expired [7], [12]. RepRap was the first promising open-source project, which idea was to build the first general-purpose selfreplicating manufacturing machine [9]. It wanted to bring 3D printing to the masses and to start new thinking of production – to download a virtual model and materialize it with a home 3D printer. To materialize means to make a real model that can be touched and held in the hand. Nowadays, low cost FDM 3D printers become even cheaper and more affordable, however, mostly enthusiasts and geeks use them. Due to the fact, that the information on how to print and what to print is scattered over the internet and is time consuming to find it. Usually it takes much of technical knowledge to understand and prepare the machine. For example, different parameters for different 3D printing devices can be found. Sometimes some parameters work for some users’ machines and for others simply do not. That makes this technology still experimental. On the other side, it is possible to buy systems that should be more reliable but they are also more expensive. These are closed systems with closed working chamber, where only manufacturer’s filament material can be used. That means same input quality, but it is usually more expensive because only one provider has monopole to deliver the filament. For an experiment in this report, Leapfrog’s Creatr 3D printer was used. It is a pre-calibrated, well build system. With the price of about 1500€, it is not the cheapest, but it offers much for that price. The deviation of the printed part is sometimes crucial for a functional prototype. Therefore, the accuracy or rather the exactness of 3D printed parts was verified and reported in this paper. The results show the tendency of deviations, and present recommendations for further modelling and research for more accurate 3D prints. Methodology Process of materialization of the object demands several preparation steps. The term preparation will be used because the whole process is observed; from the idea to the real object, where the following steps are included: 

3D modelling

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.stl file creation and potential errors revision

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G-code generation and finally

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International Journal of Information and Computer Science (IJICS) Volume 4, 2015

3D printer preparation.

Printing Preparations – 3D Modelling In order to make a 3D print, we first need a virtual model. The model can be created using different modelling software. There are several modelling techniques to get a 3D model: 

Polygonal modelling



Constructive solid geometry (CSG)

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NURBS Curve modelling

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Digital sculpting

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Subdivision modelling

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3D scanning [4], [14]

Detailed mathematical background for several techniques can be found in [3]. It is important to know that modelling techniques should produce 3D models, suitable for 3D printing. However, this suitability can be verified in the next step. Printing Preparations – stl Newly created virtual 3D model has to be saved in the proper format. Usually, computer file formats .stl, .obj or .3ds are used. That are meshes that describe 3D modelled object. Such mesh has to be watertight, without gaps or overlaps. All the triangle normal vectors should be directed outward of the object. If those requirements are not fulfilled, a .stl file has to be repaired [6]. For this purpose, freeware program Netfabb Basic [6] can be used. Open source program Meshlab [5] can repair or even rebuild the .stl file into the correct mesh as well. The main geometry of the main mesh can be changed. If errors of the input mesh are grave and at delicate places, this change is unacceptable. Then modelling preparation step has to be repeated to improve the 3D model for printing. Printing Preparations – G-Code Corrected .stl mesh has to be sliced and converted to G-code. G-code is a numerical control programming language [10], used to control XYZ robot, which 3D printer is a kind of. There are many programs to produce that code from the 3D object, but plenty of them are quite expensive. Slic3r [11] is a free available G-code generating program that works implemented in Leapfrog Repetier [8] and fits to the 3D printer. Makerbot’s Cura [13] is a freeware as well. It was also used, but some adjustments for proper printing with Creatr were needed. Repetier was used for direct 3D printer control and both Cura and Slic3r were used for G-code creation. Their user interfaces (UI) are shown in figures 1 and 2.

FIG. 1: REPETIER’S USER INTERFACE (UI). Repetier Leapfrog is manufacturer-adjusted software for slicing and controlling 3D printer.

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FIG. 2: CURA’S UI.


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Printing Preparations - 3D Printer Last printing preparation happens at 3D printer’s hardware. Extruder and printing bed have to be preheated. The bed has to be cleaned, adhesion material has to be laid and filament availability checked. Changing the filament spool takes some filament leading through the filament housing and manual extruding to clean the extruder. Different colors and materials of filament can be used for 3D printing. Creatr works well with ABS and PLA materials. We did not try other materials yet. PLA is a most common choice because it is printed at 190-215°C with printing bed temperature 60°C and printed part does not warp much. ABS has to be printed at higher temperatures, at about 230°C, and bed needs to be heated to 100°C. It warps more, therefore we had problems with deficient adhesion to the bed. When the preparation steps are done, the printing process can begin. Creatr works directly connected to the computer, while some 3D printers work standalone with data being transferred over the SD card. Experimental Part For the experimental part, two testing models were created through the preparation steps described in chapter 2. First testing model was a complex assembly of different geometrical shapes. They were chosen to confirm shape exactness for inner and outer geometry, like holes and cylinders. 3D printer Leapfrog Creatr was used to confirm printing parameters. It was tested how exact 3D prints are in comparison with the 3D scan in first experiment case. This first test model has small (1mm) and big holes (10mm), cylinder and sphere with curved walls and cube with straight walls. It has also inclined thin wall of 0,5 mm. So the ability of 3D printer to create those shapes was also checked. Thin cylinder was not included in this test model, since the inability of printing was already proven. The second test model was a simple pipe, which dimensions can be measured with a caliper. All the measures were repeated and averaged to get deviations for every printing session. Equipment and materials that were used are: 

Solid Works 2012 Academic for 3D modelling

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Cura 14.01 for slicing

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Leapfrog’s Creatr for 3D printing

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Gom’s ATOS II 400 with lenses MV 135 for 3D scanning

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Gom Inspect V7.5 SR2 for inspection

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Caliper and

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PLA (Polylactic acid) plastic material for 3D printed models

Model One 3D model having several construction shapes, shown in figure 3, was created using the 3D modelling software Solid Works and printed with the 3D printer with different layer properties. Then, prints were 3D scanned and compared to the original virtual model. Color maps of deviations and parameters are shown in table 1. Our first test model is shown in figure 3. On a cylindrical plate having the diameter of 50mm and 5mm high were: 

Half of a sphere, 20mm diameter

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Hollow turned cone, 10mm high and 10mm diameter

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Cone, 10mm high and 10mm diameter

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Cylinder, 10mm high and 10mm diameter

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Cube with 10mm edge

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Half sphere hole with 8mm diameter

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Hole, 10mm diameter

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Hole, 8mm diameter

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Hole, 5mm diameter and

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Hole, 1mm diameter

Cura 14.01 was adjusted with printing parameters from Repetier Leapfrog V0.90C. Mainly, the layer height, wall thickness and number of bottom and top layers were adjusted. Manufacturer provided two types of printing with Repetier: 

Fast printing, with layer width 0,35mm and

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High quality printing, with layer width 0,2mm

In this showcase, the manufacturer’s printing parameters have been proved, and two finer layer thicknesses; 0,1 and 0,06 mm were added. However, the most important part is the shape validation. Test models were 3D scanned and compared to the CAD model. Gom ATOS II 400 3D optical scanner with measuring volume 135mm was used. It means that the package of lens to measure the objects with approximation of the virtual measuring cube with edge length 135mm was placed. One of the researchers [2] already tested its usage and accuracy. Predicted time for the same G-code differs in programs and also in versions of programs. First, Cura 14.01 was used, and printing times are shown in table 1. New version, Cura 14.03, was released, where the predicted printing times differed for the same object. However, there are almost the same predicted times in Repetier for ready to print G-code in both versions of Cura, which means mere time calculation was adjusted in different Cura versions. Measured printing times were different at all printing sessions, but they were all in the range under +-9%. The same object was sliced with both programs as shown in figure 5. Repetiers’ Slicr produces G-code with more dead moves presented with light blue lines on the left side of the figure 5. Cura’s slicer Steam Engine minimizes dead moves, which is why the printing process was finished in a shorter time. Cura also generates the G-code in a shorter time. However, it works well only on objects without small details, which is shown in figure 6, where both covers are from the same virtual object, but different programs produced the G-code.

FIG. 3: FIRST TEST MODEL EXPERIMENT.

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TAB. 1: RESULTS OF THE FIRST EXPERIMENT WITH COLOR CARD OF DEVIATIONS.

Layer Thickness [mm]

Predicted time (Cura 14.01)

Predicted time (Cura 14.03)

0,06

1h 49 min

1h 58 min

Predicted time (Repetier 0.90c) 1 h 47min

Measured time [min]

Time difference (Repetier/measured)

Chamber temperature [°C]

1h 51min 27s

+8%

26,2

Fig. 4a: Deviation at layer 0,06 mm, Shell thickness: 0,3mm and Bottom/Top thickness: 0,3mm 0,1

59min

1h 4min

58min

53min 7s

-9%

25,5

Fig. 4b: Deviation at layer 0,1 mm, Shell thickness: 0,4mm and Bottom/Top thickness: 0,4mm 0,2

0,35

41min

46min

41min

45min 14s

+9%

26,5

Fig. 4c: Deviation at layer 0,2 mm, Shell thickness: 0,8mm and Bottom/Top thickness: 0,8mm 29min 31min 29min 30min 6s +3% 26,5

Fig. 4d: Deviation at layer 0,35 mm, Shell thickness: 0,7mm and Bottom/Top thickness: 0,7mm

Model Two The second model was a simplified pipe, shown in figure 7, because measurements could have been repeated, and its arithmetic average calculated. Unlike the first test model, the exactness of 3D print inner geometry – the hole – and outer geometry – the cylinder – can be determined. The results are shown in table 2.

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FIG. 5: REPETIER SLIC3R’S G-CODE ON THE RIGHT AND CURA’S G-CODE ON THE LEFT.

FIG. 6: SAME OBJECT, LEFT 3D PRINT CREATED WITH CURA’S G-CODE AND RIGHT WITH REPETIER’S G-CODE.

FIG. 7: TESTING MODEL FOR EXPERIMENT 2 – THE PIPE. TAB. 2: MEASUREMENTS OF MODEL TWO.

Print No.:

I

II

III

IV

V

VI

VII

VIII

G-code software

Cura

Cura

Cura

Cura

Repetier

Repetier

Repetier

Repetier

Layer height [mm]

0,35

0,35

0,35

0,06

0,2

0,35

0,35

0,35

215

215

215

215

190

190

190

190

42min

12min 19s

12min 27s 1h 3min 38s

29min 17s

16min 51s

16min 51s

16min 51s

Printing time

46min 36s

12min 32s

14min 31s 1h 6min 23s

28min 26s

16min 42s

16min 51s

16min 36s

Outer diameter [mm]

19,96

19,44

19,83

19,91

19,71

19,79

19,84

19,74

Deviation [%]

-0,22

-2,82

-0,86

-0,44

-1,47

-1,04

-0,78

-1,28

Printing temperature [°C] Theoretical printing time (Repetier)

-1 Inner diameter [mm]

9,34

9,42

9,24

9,65

9,51

9,42

9,40

9,41

Deviation [%]

-6,62

-5,76

-7,58

-3,46

-4,92

-5,84

-5,98

-5,88

-5,9

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Height [mm]

20,65

19,75

20,16

20,16

20,07

19,90

19,90

19,92

Deviation [%]

3,25

-1,26

0,8

0,79

0,35

-0,51

-0,48

-0,38


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Results with Discussion As the results in table 1 show, deviations for the model one are in a range from -0,5 to +0,5 mm, which is also shown on the color range histogram at the top of table 1. Color card of deviations shows best fitting at 0,1 mm and 0,2 mm layer thickness. The highest deviation is at 0,35 mm layer thickness – there is the most of red and blue color, which are highest deviations from -0,5 to +0,5 mm. However, all 3D prints are smaller than virtual source model. Diameter of all base cylinders is about 0,6 to 0,8 mm smaller than the virtual model. Table 2 shows deviations of pipe’s printed parts in comparison to its virtual parts. As the results show, the objects have 5,9% smaller holes, 1% smaller outer walls and are 0,5% lower. This is not merely a simple shrinkage of an object, which could be simply eliminated with .stl part scaling. There are two options for correcting this state. The first one is to adjust the G-code program, to take in account this dimensional errors, and the second one is to repair the model and recalculate values, but this is possible just for very simple parametric objects. Prints’ height also depends on the first layer height. It can be assumed that the height is the most accurate dimension. This also depends on the thickness of the adhesion material that can vary. Height and radius error is also reported by Calderon et al. (Calderon et al., 2014). Replicator, which is similar to Creatr, registers about 0,55 mm error in outer diameter. On the other hand, the measurement deviations are in average about 0,2 mm, 0,6 mm and 0,1 mm for several observed geometry. Therefore, the question is if it makes sense to consider these deviations. First, it depends on the purpose of use of the part. If an exact functional and durable mechanical part is needed, probably some other manufacturing technique will be used, for example SLS. However, if just a general shape or general assembly is needed, the measured deviations are acceptable. Moreover, this is usually the case and purpose of FDM. It also depends on the fact if just one object or more objects are printed at same time. Visual check of the print surface at both experimental parts confirms that the best results are with parameters that the manufacturer proposed, with 0,2 mm and 0,35 mm thick layers. With 0,1 mm layer thickness and 15% longer printing time, the shape exactness is similar to 0,2 mm thick layer. Layer thickness of 0,06 mm builds very fine products, but the printing time is more than twice as long as for 0,2 mm thickness. It makes sense only if a detailed product is needed but then the infill should be set at least to 40%. Otherwise, there appear holes in top layers because there is not enough infill support structure to hold the melted filament up, as 3D print comparison in table 1 at 0,06 mm thick layer shows. Rectangular patterns in figure 4a show holes in a virtual model in the top layer that were created in mesh post processing. Furthermore, in figure 6 the 3D prints of the same object are shown in which G-code was created with different slicer programs. Cura was used without support structures and created good surface finish, but details were lost – typewriting in figure 6 was almost unreadable. On the other hand, Repetier creates great and readable typewriting. Repetier’s G-code had one-hour longer printing time, but there were many support structures so the time cannot be directly compared. Conclusions And Future Work Leapfrog’s Creatr is used as a rapid prototyping system that brings ideas from the virtual to the real world. If being used as a very precise prototyping method, there are two ways for adjustment. First is to adjust the G-code as G code G41 and G42 does for cutter radius compensation [1]. But this solution requires complex programming that is not necessary suitable for all machines. More simple is to manually adjust the model parts in the start, where this geometry deviation is considered. But there is a human factor included, where errors can be made or something missed out. There are already tools in commercial programs such as Rhinoceros, which can resize the holes in existing NURBS geometry. It has to be operated manually. Simple plug-inn or script can manage this automatically. If the mesh model is already created it can be analyzed on curvature and proper curved areas can be inflated or

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deflated. Several 3D programs are capable to sculpt a 3D mesh. With a plug-inn or script this reparation can be first detected and then adjusted automatically. FDM or FFF technology is still a kind of experimental way of product materialization. Usually, the enthusiasts are those who build and use these machines. Anyway, people who do not know much about the stepper motors or Arduino boards but they want a result – a 3D print. Moreover, they are talking about 3D printing, which makes this technology popular. In the future, this technology will be more and more accessible. Accessibility does not merely mean availability at stores and low prices, but also easy to use, quality products and reliability of the whole machine. Ready to print desktop 3D printers can be already bought, but still many unexpected things can happen during the printing process. In the future, some improvements are provided. Firstly, constant conditions in a building volume are needed, which can be achieved with a closed chamber that allows precise temperature control. Second are structural improvements of extruders that have to be separated from the chamber with flexible barrier, so they do not overheat. Third is the use of a new G-code generating program and controller called Simplify 3D, but this program is not a freeware. Entry-level additive manufacturing is very useful in the field of education and industrial design, where price and time are important factors. It is also important for individuals who want to build something on their own. They can start to build the 3D printer first, or they can buy assembled 3D printers and then make different things. If that things are accurate enough depends on the purpose of the use of the parts. However, precise work always requires some experimental prints to improve sufficient import-export equality. REFERENCES

[1]

Automation, C., “Cnc automation.” [Online]. Available: http://www.cncauto.com/standard-g-m-code-supported.aspx.

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Brajlih, T., “Testing the accuracy of ATOS 3D optical scanner measuring volumes,” Annals of DAAAM & Proceedings, 2007.

[3]

Farin G., Curves and Surfaces for CAGD: A Practical Guide. Morgan Kaufmann, 2002.

[4]

Farin, G., J. Hoschek, and M.-S. Kim, Handbook of computer aided geometric design. Amsterdam [etc.]: Elsevier, 2002.

[5]

Meshlab, “Meshlab.” [Online]. Available: http://meshlab.sourceforge.net/.

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Netfabb, “Netfabb.” [Online]. Available: http://www.netfabb.com/.

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Paul, B., “Law in the Making.” [Online]. Available: http://lawitm.com/post-one-part-one-patents-and-3d-printing.

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Repetier, “Repetier.” [Online]. Available: http://www.repetier.com/.

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Reprap, “RepRap.” [Online]. Available: http://reprap.org/wiki/RepRap.

[10] Reprap, “Reprap Gcode.” [Online]. Available: http://reprap.org/wiki/G-code. [11] Slic3r, “Slic3r.” [Online]. Available: http://slic3r.org/. [12] Stratasys, “FDM_Technology.” [Online]. Available: http://www.stratasys.com/3d-printers/technologies/fdm-technology. [13] Ultimaker, “Cura.” [Online]. Available: http://software.ultimaker.com/. [14] Vaughan, W., “[digital] modeling.” New Riders, [Berkeley, Calif.], 2011.

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