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ORIGINAL ARTICLE

C. C. Chang

Direct slicing and G-code contour for rapid prototyping machineof UV resin spray using PowerSOLUTION macro commands

Received: 28 October 2002 / Accepted: 30 October 2002 / Published online: 18 December 2003 Springer-Verlag London Limited 2003

Abstract Rapid prototyping processes produce partslayer by layer directly from 3D CAD models. Animportant technique is required to slice the geometricmodel of a part into layers and to generate a motioncode of the cross-sectional contour. Several slicingmethods are available, such as slicing from sterolithg-raphy (STL) les, tolerate-error slicing, adaptive slic-ing, direct slicing, and, adaptive and direct slicing. Thispaper proposes direct slicing from 3D CAD modelsand generating a G-code contour of each layer usingPowerSOLUTION software (Delcam International,Birmingham, UK). PowerSOLUTION includes twomain modules: PowerSHAPE is used to build 3DCAD models and PowerMILL is used to produce G-Code tool paths. It provides macro language, pictureles and cutting paths for secondary developmentwork.The authors used macro commands to write aninterface generating direct slicing from 3D CADmodels and G-code contours for all layers. Most well-known controllers in the market accept the G-Code.Therefore, it is easier to apply this scheme in a CNC-machining center to produce rapid prototyping such aslaminated object manufacturing (LOM) for complexgeometries. The interface was successfully applied theinterface to the UV resin spray rapid prototyping(UVRS-RP) machine that was developed to produceRP.

Keywords Direct slicing Æ G-code contour Æ UVRS-RP Æ PowerSOLUTION Æ PowerSHAPE Æ

PowerMILL

1 Introduction

Rapid prototyping (RP) is a new forming process that

can be classied as layer-by-layer material addition inmanufacturing. It can rapidly manufacture productswithout a mould. The rst commercial RP system of sterolithography apparatus (SLA) emerged in 1988 andthere were 2234 RP systems with about 20 kinds of processes around the world by the end of 1996 [1]. Thedifferent kinds of rapid prototyping machines (RPM)were developed using different mechanisms or materials,such as SLA, laminated object manufacturing (LOM),selective laser sintering (SLS), fused deposition modeling(FDM), and multi-jet modeling(MJM) [2, 3, 4]. Systemmanufacturers sold 3289 systems around the world bythe end of 1997 [5]. In the early days of RP, the auto-

motive and aerospace industries dominated RP appli-cation. Now, RP has spread into many other industries,such as tooling, product design, medical application,architecture, and art jewelry, et al.

Most of the commercial RPMs produce partsthrough material accumulated in parallel layers. There-fore, the study of slicing methods has become animportant subject of RP techniques. The most commonslicing procedure is producing cross-sectional data fromSTL les. Approximating the surface with many smallplanar triangular faces comprises the STL format of theparts. In this way, the complex slicing algorithm issimplied and the software speed is greatly improved.

Unfortunately, mistakes and errors exist simultaneouslyin the nished part after the slicing process [6, 7]. Insteadof using an STL le, another method is to use differentformat les as data interfaces from CAD to RP. Thereare many different format les for this purpose, such as:common layer interface(CLI ) les, rapid prototypinginterface (RPI) les, initial graphics exchange specica-tion(IGES) les, Hewlett-Packard graphics lan-guage(HPGL) les, computerised tomography(CT)data, and layer exchange ASCII format (LEAF). How-ever, none of these les can eliminate mistakes and

Int J Adv Manuf Technol (2004) 23: 358–365DOI 10.1007/s00170-003-1575-4

C. C. ChangDepartment of Mechanical Engineering,Kun-Shan University of Technology,Tainan 710, TaiwanE-mail: [email protected]

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errors. There are several approaches to improving theslicing method, such as direct slicing, adaptive slicing,and direct and adaptive slicing [8, 9]. Chen et al. [9]direct slices PowerSHAPE models so that customers canuse the PowerSHAPE package to develop their prod-ucts. They use the AutoSection command to perform theautomatic slicing 3D CAD model and save the layers asa PIC (picture) le. A PIC le in layer by layer can be

used in most RP processes.This paper not only examines direct slicing from a 3DCAD model, but also the generation of a G-Code con-tour for each layer using PowerSOLUTION software.PowerSOLUTION is a powerful package for buildingmodels in PowerSHAPE and for producing the G-Codecontour in PowerMILL. It provides macro language,picture les and cutting paths for its secondary devel-opment work. The authors propose a direct slicing ap-proach based on 3D CAD models in PowerSHAPE.Then the G-Code contour of each layer is generated inPowerMILL. Macro commands are used to write aninterface that combines the direct slicing from 3D CADmodels and produces the G-code contour for all layers.The interface has been successfully applied to thedeveloped UV Resin Spray -rapid prototyping (UVRS-RP) machine to produce rapid prototyping [10].

2 UV resin spray-rapid prototyping

The UVRS-RP was designed and assembled with thenancial support of the National Science Council (NSC,NSC89-2218-E-168-006, 2000) in Taiwan. It is a novelapproach to building an RP in a rather economic way.The mechanism is shown in Fig. 1. The X-Y motion

table is controlled by a linear stepping motor (lp-460 · 460, Powerly Enterprise) and moved by magneticforce. The table speed can be increased greatly. Theprecision is 1 l m and repeatability is 3 l m in X-Y table(dimensions, 460 · 460 mm) [10]. A server motor (preci-sion, 1 l m) is used to control the Z-direction motion.

Two nozzles (TS 5420, Techcon Systems) are used tospray the two different types UV resins; the acrylic typeis for body material and the PU type is for supportmaterial. The UV light (UV-Light-GY751, max. 3 kw,UV Light Enterprise) is positioned on the top of the X-Ytable as shown in Fig. 2. The UV-resin is instantly curedin a 2D cross-sectional layer by layer under UV light

exposure as shown in Fig. 3a. The nozzle diameter ischangeable, and are disposal nozzles as shown inFig. 3b. This makes it easier to arrange the variousdiameters of the nozzles for different paths such as anautomatic tool changer (ATC) mechanism of amachining center. An interface was created to translateG-codes to motion paths of the nozzle (Motion Card:DMC 1700, Galil Company).

The basic machine moves as follows:

1. First the nozzle sprays the cross-sectional area of thebody by the acrylic resin in one layer. Figure 4a

shows the direct slicing from 3D CAD that generatesall cross sections in front view.

2. Instantly, the material in body area is cured with UVlight. The planar deection can be decreased since itis 2D contour curing

3. If a support must be built, another nozzle sprays PUresin to ll up the support area. Then, the supportpart is cured with UV light. The nozzles raise onepitch in the Z-direction and continue steps 1 to 3 tillthe model is fully built.

Since the properties of materials between body andsupport are different, they can be clearly distinguishedand separated using proper force. Fig. 4b shows a owchart on determining the support area in each layer. Thesupport area is shown in Fig. 4c in a cross section.

3 STL format

Two main numerical scheme are accounted for in RPsystems:

Fig. 1a,b The mechanism of UVRS-RP machine a , the appearanceof the UV-RP machine b

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(1) How to slice a 3D model into 2D cross-sectionalcontours

(2) How to arrange the nozzle-path to ll materials upinside the 2D cross-section (or to cut the boundaryof the 2D cross-section for LOM)

The various CAD systems available today have dif-ferent CAD format les. Therefore, a common interface

between the CAD and the RP systems is required todesign a new RPM.

The STL le is the most common interface betweenCAD and RP systems. The STL format is a polyhedralrepresentation of the part with triangular facets. It isgenerated from a precise CAD model using a processknown as tessellation, which generates triangles toapproximate the CAD model. The STL le can either bein ASCII or binary format. The size of the ASCII STL

format is larger than that of the binary format, but it canbe read. In an STL le, triangular facets are described bya set of X, Y and Z coordinates for each of three verticesand a normal unit vector to indicate the side of the facet,which is outside the object shown in Fig. 5. Errors inSTL format for approaching the 3D CAD modelsometimes can not be avoided. Also, the computingprocess consumes too much time. A comparison of theSTL section and direct slicing section is provided inFig. 6. The example shows that direct slicing is moreaccurate than the STL section.

4 Producing the direct slicing and nozzle paths from PowerSOLUTION

4.1 Direct slicing from 3D CAD modelsin PowerSHAPE

PowerSHAPE is the CAD module of PowerSOLU-TION. The 3D CAD models can be built usingPowerSHAPE. There are three different slicing methodsin PowerSHAPE as indicated in the following.

Fig. 3a,b The model accumulated layer by layer through injectionnozzle a The different diameters of disposal nozzles b

Fig. 2 The mechanism of threedirection of nozzle motion

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1. Recording sequences of commands in a macro le2. Writing a macro le in a text editor3. Developing an interface by PowerSHAPE variables

with C or VB languages

The rst two methods were used to direct slice themodel [9], but the macro commands must be modied

for other models. The third method is more conve-nient than the other two methods without changingthe macro lesince there are variable parameters in theinterface. An interface that provides automatic anddirect slicing of the model in PowerSHAPE wasdeveloped.

A scheme for producing 2D contour from models isshown in Fig. 7. First, the X-Y planers for all layersare produced to slice the model. Then, the boundariesof the intersection between the X-Y planers and themodel is located in all layers. There are three steps for

developing an interface of automatic slicing algorithmwith VB in PowerSHAPE:

1. Load the link le between PowerSOLUTION andVB. The name of link-le is SolutionOLE.ocx. Thismust be put in drv:\WINNT\system32 and REG-SVR32.exe must be executed to connect the VB and

PowerSOLUTION2. The interface of the automatic slicing is written withVB language. The ow chart is shown in Fig. 8 andthe dialog box for the slicing interface is shown inFig. 9. There are two main parameters, slicingthickness and slicing number, given in Fig. 9. Whenthe slicing pitch is entered, it automatically produces2D contours in the model, as shown in Fig. 10

3. All self-developed macro les must be put intothe PowerSHAPE environment. Firstly, enter thepulldown menu to nd the function \Application\

Fig. 4a–c Direct slicing from3D CAD to generate all crosssections in front view a, the owchart of determining thesupport area sprayed b, therepresentation of support area,As j c

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Add-Ins\Manager, and load in the name and pathof macro les for automatic slicing in PowerSHAPE

After nishing the above procedures, the interface forautomatic slicing can be tested as follows:

1. Load 3D CAD model have into PowerSHAPE asshown in Fig. 6a

2. Execute the macro le for auto-slicing. The dialogbox, which includes slicing thickness and slicingnumber, is shown in Fig. 9

Fig. 6a–c 2D contour by direct slicing from 3D CAD model a,2D contour by slicing from STL format b, the errors between twocases c

Fig. 7 The scheme of producing 2D contours from the model inPowerSHAPE

Fig. 5 The triangular formatfor STL les

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3. 2D contours can be generated in the macro commandas shown in Fig. 10. The green lines represent the 2Dcontour of all layers

4.2 Motion path of the nozzle from PowerMILL

The motion path of nozzles is generated in PowerMILL,which is the CAM module of PowerSOLUTION. Themain purpose of PowerMILL is to produce the G-codeof the tool path. The main difference between the cuttingtool and the nozzle is the motion in the Z-direction. Thenozzle is used to spray materials on each layer frombottom to top while the tool is upside down. Nozzle

paths of each layer are generated and then the motioncode is transmitted to the controller(DMC 1700, GalilCompany) to drive the nozzle. Another interface fortranslating G-codes into special codes that can be ac-cepted by DMC 1700 must also be developed.

All application interfaces for UVRS-RP machinehave been successfully developed. In Fig. 11 the yellowlines show the nozzle path of all layers on the right handside, the various nozzle diameters can be chosen on theleft hand side, and the red lines show nozzle path in theZ-direction form bottom to top.

5 Results and Discussion

In Fig. 12a, it is clear that the boundaries of bothbody and support material are quite smooth whenusing direct slicing scheme. Also, the support material(PU resin) is easy to separate from the body material(acrylic resin) as shown in Fig. 12b. PU resin is softand bendable at room temperature while acrylic resin

is hard and stretchable. It is shown in Fig. 12c. Anexample of RP that was completely built using theUVRS-RP machine is given in Fig. 13. Advantagesresulting from applying the VB to develop directslicing and nozzle path interfaces in PowerSOLU-TION are:

1. When direct slicing from a 3D CAD model, errorsand mistakes can be greatly reduced, and the speed of calculation can be accelerated

2. Nozzles can be arranged like machining tools, so itis possible to assign different nozzle diameters to

Fig. 8 The ow chart of automatic slicing algorithm by VBlanguage

Fig. 9 The dialog box of the interface of automatic slicing includedslicing thickness and slicing number

Fig. 10 An example for the direct slicing from a model (the greenlines are 2D slicing contours)

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different paths. The smaller nozzle, for example, canbe used to spray the boundary of each layer, whilethe larger nozzle can be used to ll up the inside of boundary. This can greatly increase the accuracyand speed of RP. This method will also be used infuture studies

3. For most of printing types from RP manufacturerssuch as Sanders, Actua, 3D Printing, etc., the life of nozzle is limited and expensive. The disposal nozzle iseasy to maintain and is very inexpensive

4. Generally, it is difficult to write control systems andinterfaces for developing a RP system within a shorttime period. This paper presented an easy way of

developing the interface for the needs of differentRPMs using macro commands in PowerSOLUTION

The scheme and RP machines have a number of limitations as well, such as:

1. The control system and transfer interfaces must bemounted in PowerSOLUTION environments

2. The amount of material sprayed is difficult to predictin the starting and ending periods of all paths. This

affects the accuracy of the RP model. Therefore, eachnozzle path must be extended to avoid these twoperiods when the RP body is being built

3. The support material in the inner structure is hard toremove cleanly

4. The concentration in the material depends on manyparameters and is hard to control. The TaguchiMethod was used here to nd optimal conditions forthe present

6 Conclusion

Using macro commands in a commercial CAD/CAMsystem is a good way to develop control system andtransfer interfaces through VB or C languages in an RPmachine. This paper demonstrates that it performs wellin direct slicing scheme and in arranging nozzle pathsthrough PowerSOLUTION environments. There arestill some limitations in both software and hardware, butthese should be improved in the future.

Fig. 11 An example forproducing Nozzle pathsgenerated from 2D contour forall slicing layers (The differentdiameter of nozzles can bechosen in the left side)

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Fig. 12a–c The body (acrylic resin) and support (PU resin)material cured by UV lamp in one layer

Fig. 13 Rapid prototyping made by UVRS-RP machine (slicingthickness: 1 mm, nozzle diameter: 0.5 mm)

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