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Journal for Technology of Plasticity, Vol. 39 (2014), Number 2 * Corresponding author E-mail address: [email protected] APPLICATION OF SINGLE POINT INCREMENTAL FORMING FOR MANUFACTURING OF DENTURE BASE Mladomir Milutinović a , Robert Lenđel a Michal Potran b , Dragiša Vilotić a , Plavka Skakun a , Miroslav Plančak a a University of Novi Sad, Faculty of technical science, Serbia b University of Novi Sad, Medical Faculty, Dentirstry Department, Serbia ABSTRACT Single point incremental forming (SPIF) is a modern, dieless sheet metal forming technique applied mostly for small batch and custom-made products of various shapes and dimensions. Compared to conventional sheet processes it offers many advantages, especially in terms of flexibility and material formability. In the field of dentistry, there is a natural need for tailored production i.e. personalized devices that are custom made for the patient, area in which SPIF is highly promising. In association with computer (CAD, CAM) and medical imaging technique (MRI, CT), this technology enables higher level of both aesthetic and functional characteristic of the product, in relation to traditional manufacturing methods. In the paper presented, SPIF technology was applied for manufacturing of a denture base (framework) of a complete denture. The research included two base materials: low carbon steel EN DC04 and stainless steel EN X6Cr17. The main objective of the research was to compare the geometry of the denture base shaped by SPIF, with an existing denture base made by the lost wax technique. Key words: SPIF, denture base, biocompatible materials 1. INTRODUCTION Production of dental components includes variety of procedures that were untill recently based on traditional techniques such as casting and machining. In the last two decades, this area have been undergoing a radical and rapid changes in terms of employing advanced manufacturing technologies such as CNC machining, advanced metal forming, precision casting, sintering, surface engineering, rapid prototyping and manufacturing) [1,2,3,4,5]. Advanced manufacturing technologies usually combine novel manufacturing procedures and machines with computer-aided

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Journal for Technology of Plasticity, Vol. 39 (2014), Number 2

* Corresponding author E-mail address: [email protected]

APPLICATION OF SINGLE POINT INCREMENTAL FORMING FOR MANUFACTURING OF DENTURE BASE

Mladomir Milutinovića, Robert Lenđela Michal Potranb, Dragiša Vilotića, Plavka Skakuna, Miroslav Plančaka

aUniversity of Novi Sad, Faculty of technical science, Serbia b University of Novi Sad, Medical Faculty, Dentirstry Department, Serbia

ABSTRACT Single point incremental forming (SPIF) is a modern, dieless sheet metal forming technique applied mostly for small batch and custom-made products of various shapes and dimensions. Compared to conventional sheet processes it offers many advantages, especially in terms of flexibility and material formability. In the field of dentistry, there is a natural need for tailored production i.e. personalized devices that are custom made for the patient, area in which SPIF is highly promising. In association with computer (CAD, CAM) and medical imaging technique (MRI, CT), this technology enables higher level of both aesthetic and functional characteristic of the product, in relation to traditional manufacturing methods. In the paper presented, SPIF technology was applied for manufacturing of a denture base (framework) of a complete denture. The research included two base materials: low carbon steel EN DC04 and stainless steel EN X6Cr17. The main objective of the research was to compare the geometry of the denture base shaped by SPIF, with an existing denture base made by the lost wax technique. Key words: SPIF, denture base, biocompatible materials 1. INTRODUCTION Production of dental components includes variety of procedures that were untill recently based on traditional techniques such as casting and machining. In the last two decades, this area have been undergoing a radical and rapid changes in terms of employing advanced manufacturing technologies such as CNC machining, advanced metal forming, precision casting, sintering, surface engineering, rapid prototyping and manufacturing) [1,2,3,4,5]. Advanced manufacturing technologies usually combine novel manufacturing procedures and machines with computer-aided

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Journal for Technology of Plasticity, Vol. 39 (2014), Number 2

techniques (CAD, CEA, CAM). In the field of medicine, these technologies are frequently associated with the progressive imaging techniques including computer tomography, magnetic resonance imaging and ultra sound imaging [2]. It enabled not only the improvements of the existing manufacturing processes, but also the process automation and visualization, design of greater variety of complex products with increased accuracy and reliability in use, introduction of innovative, state-of-the-art medical and dental services, development of the new procedures, better material utilization etc. [1,6,7] The selection of suitable technique for manufacturing of a specific product is a complex issue. It is a multicriteria decision that depends on several factors regarding part design and complexity, material processing ability, desired accuracy and quality, mechanical and physic properties of part, production quantity, and cost. In case of medical and dental devices this issue is even more emphasized due to a number of specific demands regarding materials characteristics and the fact that in most cases production is related to creation of custom-made components. Medical and dental components must be made from biocompatible materials as well as meet the general requirements of biomedical engineering [1, 8]. For example, oral cavity as a dynamic environment in which periodical fluctuations of ph, temperature and masticatory load, requires a tough material that will remain safe and consistent in the expected time of its function. According to [3] metal forming is one of the leading technologies applying to production of the medical implants. Forming technologies offer advantages in processing and improvement of mechanical properties of the material, but due to a high costs and special tools needed for shaping of a specific part, application is limited to production in large batches. Additional problem is related to low material formability, which is an issue when processing most of the biocompatible materials (Stainless steel, Titanium and its alloys) by classical forming methods. In order to meet increasing demands for individual or small-batch production of variously shaped components and low deformable materials, a group of innovative and flexible forming techniques has recently been introduced. Single Point Incremental Forming (SPIF) is a technique which unlike traditional sheet forming methods, is realized by a simple tool (spherical roller) that acts on small area of sheet plate clamped into a frame, following a tool path controlled by a CNC machine and programmed in advance [9,10,11,]. Since the workpiece is shaped gradually through a series of incremental (local) deformations, this technology enables forming of complex geometry parts with higher degree of deformation in comparison to deep drawing [12]. Other plausible features are reflected through smaller forming loads, less wear and reduction of noise [13,14]. However, the main disadvantages of SPIF technology are related to low dimensional and shape accuracy and relatively long processing time [15]. SPIF technology was primarily developed for the needs of automobile and aerospace’s industry. Over time, other branches have recognized the benefits of this technology and is currently successfully applied in many fields. When addressing its use in medicine, it is possible to produce thin walled custom-made metal components, such as cranial plate, jaw segment, face prosthesis or ankle support [16,17,18]. SPIF has also the potential of replacing existing lost wax technique for manufacturing of metal frameworks for dentures [7,8]. First results show promise, but there is a lot of room for improvement especially in dimensional accuracy and surface quality. In the paper presented, possibilities of applying SPIF technology to produce a metal denture base have been investigated. Attention was paid to the accuracy of formed components. Based on scanned shape of an existing custom-made denture base (made by casting), replicas has been made from two materials (low carbon steel EN DC04 and stainless steel EN X6Cr17) using standard SPIF tool set and CNC milling machine. The geometry of both replicas were compared with the original and results were discussed in terms of surface quality and dimensional accuracy.

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2. DENTURE CHARACTERIZATIONS Dentures are mobile dental restorations used for replacing missing teeth and surrounding tissues in patients with partial or complete edentulism. Accordingly, dentures can be classified as complete or partial dentures. Most frequent materials used for denture base construction include acrylic resins (polymethylmethacrylate) (Fig.1), with a possibility of metal framework [7, 19, 20].

Fig. 1 – Partial and complete acrylic denture [20]

Except plausible physical, biological and aesthetic properties, the main reasons for popularity of acrylic dentures are low cost and possibility of adding teeth later on. However, dentures made from acryl resin are far from an ideal solution [21]. Due to low flexural fatigue and impact resistance, they are prone to fracture. The fracture usually appears along the midline of the palate, as a result of the cyclic shifting between flexion and extension forces that arise during mastication. One of the efficient ways of reducing fracture risk is to reinforce the denture base with a metal framework. Dentures constructed in this manner, have an underlying framework that is custom fabricated from metal or metal alloys (gold, titanium, cobalt-chrome-molybdenum, cobalt-chrome…), using the lost wax technique. Acrylic resin substituting supporting tissues and artificial teeth are added later on (Fig.2).

Fig. 2 – Partial and complete [7] cast-metal base denture

Dentures made with metal framework offer several advantages including increased rigidity and fracture resistance, excellent strength to volume ratio, high retention, good adaptation to the supporting tissues, high thermal conductivity, minimal thickness, and good dimensional stability both during and after fabrication [19, 21]. Due to superior mechanical properties they are made thinner, thus increasing the comfort of wear for the patient. [7, 21]

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3. EXPERIMENTAL INVESTIGATION Schematic presentation of a typical single point incremental forming process is given in Fig.3. Major parameters of process are: tool radius and velocity (feed rate), vertical pitch, sheet thickness and forming strategy (tool trajectory). Proper selection of these parameters is the key factor for successful application of the procedure, especially for achieving accuracy of shape and surface quality.

Fig. 3 – The scheme of SPIF process [9]

3.1 Process modeling The modeling process for manufacturing of denture base started with creation of a CAD model. The CAD model was based on geometry of a physical denture base model which was fabricated by a lost wax technique from cobalt-chrome alloy. Average thickness of casted model was 0.8mm. In the first step, surface of the physical model was scanned (digitalized) with coordinate measuring machine-Carl Zeiss Contura G2 (Fig.4a). Acquired geometrical data or "cloud of points" was further processed and visualized in the form of virtual model (Fig4.b). Since the virtual model was not suitable for design purposes, using software packages Solid.Edge and Solid Works it was first upgraded to the 3D CAD surface model (Fig.4c) and then converted to an STL model (Fig.4d). In general, CAD model of denture base can be created by digitizing gypsum working cast model (commonly used in prosthetic practice) or eventually by processing 3D image data (MRI, CT) of appropriate jaw. In order to verify the geometry of created CAD/STL model, negative (mold) of the 3D CAD model was made by 3D powder base printing technology (Fig.4.f). After conducting visual control (Fig.4g), the mold was scanned and compared to the geometry of a physical model. As the results of analysis confirmed that geometries of models coincide to a large extent following steps were CAM modeling and selection of forming strategy. Since SPIF process is similar to milling process, classical CAM software (EdgeCAM) was applied for generating of the tool path (Fig.5) and NC code. The tool path was designed to produce two identical pieces of the denture base simultaneously. Forming strategy was programmed as “profile milling” with helical tool path, cusp height 0,02mm and 3000mm/min feed rate, respectively. The spinning speed was 100 rpm.

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a – Scanning of physical

model b – Virtual model c – CAD model е

d – STL model f – 3D printed mold g – Geometry control

Fig. 4 – CAD modeling of denture plate

a) start b) finish

Fig. 5 – CAM modeling of tool trajectory (EdgeCAM) 3.2 Machine, tool set and materials Experiments were preformed on a CNC vertical milling center (HAAS TM1) (Fig. 6), using steady frame (Fig. 7) and spherical roller with geometry as shown in Fig.8. The roller used was made from tool steel EN X210Cr12 and hardened to 60HRC. After hardening it was fine grinded and polished. In order to reduce friction and increase material formability VISKOL D S3 SAE 15W40 was applied as a lubricant. The forming tool as well as the surface of the blank was lubricated. Sheet plates of two different materials were used as blanks. First material was low carbon steel EN DC04 with initial sheet thickness 1mm, as the second was stainless steel EN X6Cr17 of 0.5mm thickness. After the forming operation, the denture bases were cut out from the blank.

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Fig. 6 - Vertical milling center HAAS TM1 Fig. 7 - Steady frame mounted on the machine

a) Geometry of spherical roller b) The spherical rollers after polishing

Fig. 8 – Spherical roller used for incremental forming

4. ANALYSIS OF RESULTS In Fig. 9 shows the original denture base and the replicas made by SPIF technology from carbon and stainless steel. After forming, visual control is performed first with goal to detect potential defects and estimate surface finish. On the both plates the only problem indentified was a slight increase in roughness of internal surface in the zone of the maximum deformation. The rest of internal surface was smooth and without tool marks. External roughness or “orange peel” effect that occurs often in sheet metal forming when the forming tool is only in intimate contact with one side of the sheet [22] is not observed. The internal roughness of SPIF shaped component is influenced primarily by the tool and step sizes, as other factors such as, sheet thickness, material properties, forming angle, feed rate and etc., may affect both the internal and external roughness. Since the surface roughness is low and localized to a small area it is not relevant and can be easily removed by polishing if necessary. Dimensional control of the obtained parts was performed by coupling with the original. For that purpose both replicas were scanned applying the same measuring procedure. The obtained clouds of points are then transformed to CAD models and compared, in GOM Inspect software environment, with the CAD model (Fig.4a). As a result of this operation so called “maps of discrepancies” were generated (Fig.10 and Fig.11) on which it is possible to validate the process precision i.e. part accuracy all over the 3D profile.

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Journal for Technology of Plasticity, Vol. 39 (2014), Number 2

a) Cobalt-chrome alloy (cast) b) EN DC04 (SPIF) c) EN X6Cr17 (SPIF)

Fig. 9 – Original and two replicas of the denture base

As it can been seen from Fig.10 and Fig.11, there is no any point where the discrepancies overcome 1mm. For this kind of product, detected discrepancies are entirely acceptable. Furthermore, the zones of maximum difference are located near to the part edges which will be further proceeded by padding acryl resin and artificial teeth on. Rest of the 3D profiles corresponds well, with differences in range ±0.2mm. If compare the denture bases from carbon and stainless steel, maximum as well as average discrepancies are smaller in case of stainless steel. It could be attributed to thinner blank for which lesser elastic spring back is generated.

Fig. 10 – Comparison of geometry between original (cast made) and SPIF made (EN DC04 steel) denture base

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Fig. 11 – Comparison of geometry between original (cast made) and SPIF made (EN X6Cr17 stainless steel) denture base

5. CONCLUSION This paper fully demonstrates the capability and diversity of use of SPIF technology. Here, it was successfully applied in dentistry field for producing a metal base of complete denture. This is a typical custom-made product, conventionally produced by lost wax method in highly demanding procedure. A new developed procedure is flexible and based on integrate approach. By applying reverse engineering, CAD and CAM techniques, product and process design is done in an easy and effective way. The denture bases were made from two different materials (carbon and stainless steel) with initial thickness of 1mm and 0.5mm, respectively. In both cases surface quality as well as dimensional accuracy were satisfied. Dimensional discrepancies, with respect to the original were less than 1mm. New procedure (SPIF technology) enabled additional mass reduction of the dental base (sheet thickness 0.5mm against the 0.8mm thickness of the original) which is highly desirable from viewpoint of patient and comfort of wear. ACKNOWLEDGEMENT

Results of investigation presented in this paper are part of the research realized in the framework of the project “Research and development of modeling methods and approaches in manufacturing of dental recoveries with the application of modern technologies and computer aided systems“–TR 035020, financed by the Ministry of Science and Technological Development of the Republic of Serbia.

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REFERENCES [1] Gibson I. Advanced Manufacturing Technology for Medical Applications, Wiley ed., 2005. [2] Vandenbroucke B, Kruth J.P. Selective laser melting of biocompatible metals for rapid

manufacturing of medical parts, Rapid Prototyping Journal, Vol. 13, Issue 4, 2007, pp.196 – 203.

[3] Adamus J. Forming of the Titanium implants and medical tools by metal working. Archives of Materials Science and Engineering, Volume 28, Issue 5, 2007, pp. 313-316.

[4] Wall A., Heffron P, Forgings and orthopaedic implants, Bonezone, Vol 3, 2004, pp.37-39. Jiang B. G., Xianghui W., Tan Z.H. Fabricating titanium denture base plate by laser rapid forming", Rapid Prototyping Journal, Vol. 15 Iss 2, 2009, pp. 133 – 136.

[5] Bonet J. et al. Numerical simulation of the superplastic forming of dental and medical prostheses, Biomechan Model Mechanobiol 1 Springer-Verlag, 2002, pp.177 – 196

[6] Tanaka1 S. et al., Incremental sheet metal forming process for pure titanium denture plate. In: P. Bariani (Edtr.): Advanced Technology of Plasticity. Proc. 8th ICTP, Verona, Oct. 2005, pp. 135-141.

[7] Curtis R.V. et al. Dental biomaterials Imaging, testing and modeling, Wookhead Publishing ISBN 978-1-84569-424-1 (e-book), 2009.

[8] Matsubara S.: A computer numerically controlled dieless incremental forming of a sheet [9] Kršulja M.: An improvement of SPIF technology, PhD thesis, 2013. [10] Oraon M., Sharma V. Sheet Metal Micro Forming: Future Research Potentials, Int. J. on

Production and Industrial Engineering, Vol. 01, No. 01, Dec 2010, pp 31-35. [11] Hussain G., Gao L.: A novel method to test the thinning limits of sheet metals in negative

incremental forming, International Journal of Machine Tools & Manufacture - Volume 47, Issues 3–4, March 2007, pp 419–435.

[12] Nakagawa T.: Advances in prototype and low volume sheet forming and tooling, Journal of Materials Processing Technology 98, 2000, pp 244-250.

[13] Lenđel et. аl. Single point incremental forming of large size components, Journal for technology of plasticity, Vol. 39, No.1, 2014, pp. 59-67.

[14] Micari F., Ambrogio G., Filice L.: Shape and dimensional accuracy in Single Point Incremental Forming State of the art and future trends, Journal of Materials Processing Technology 191, 2007, pp 390–395.

[15] http://www.mech.kuleuven.be/pp/research/spif_cranial.en.html, accessed on 10.12.2014 [16] Schaeffer L. et al. Development of customized products through the use of incremental

sheet forming for medical orthopaedic applications. 3rd International Conference on Integrity, Reliability and Failure, Porto/Portugal, 2009, pp 1-12.

[17] Silva M.B., et al. Single point incremental forming of metal sheets. Annual Winter Meeting of Danish Society for Metallurgy, Jutland, Denmark; 2009, pp. 1-14.

[18] Poštić S. Design of Complete Denture Reinforced with Metal Base, Serbian Dental Journal, vol. 60, No 1, 2013, pp 15-20.

[20] http://www.webmd.com/oral-health/dental-health-dentures#1, accessed on 05.12.2014 [21] Marcauteanu C. et al. Titanium complete denture base in a patient with heavy bruxism: a

clinical report. Journal of experimental medical & surgical research. Year XV, No.3/2008, · pp. 96-99.

[22] Ham M., et al: Roughness Evaluation of Single Point Incrementally Formed Surfaces”. Transactions of NAMRI/SME. Vol 37, 2009, pp 411 – 418.

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PRIMENA TEHNOLOGIJE INKREMENTALNOG DEFORMISANJA LIMA ZA IZRADU METALNE BAZE

DENTALNE PROTEZE

Mladomir Milutinovića, Robert Lenđela Michal Potranb, Dragiša Vilotića, Plavka Skakuna, Miroslav Plančaka

a Univerzitet u Nom Sadu, Fakultet tehničkih nauka, Srbija,

b Univerzitet u Nom Sadu, Medicinski fakultet, Odsek za stomatologiju, Srbija

REZIME

Inkrementalno deformisanje lima predstavlja modernu tehnologiju oblikovanja lima koja se realizuje putem univerzalnih alata i CNC mašina. Ova tehnologija se uglavnom primenjuje u pojedinačnoj i maloserijskoj proizvodnji, za izradu široke palete delova različitih oblika, dimenzija i stepena složenosti. U poređenju sa klasičnim tehnologijama oblikovanja lima, njena glavna prednost jeste fleksibilnost i mogućnost postizanja veće deformabilnosti materijala. U oblasti stomatologije, odnosno, protetike gde postoji prirodna potreba za individualizacijom proizvodnje zbog različitih anatomskih karakteristika pacijenata (custom-made products) tehnologija inkrementalnog deformisanja poseduje veliki potencijal. U kombinaciji sa savremenim komjuterskim (CAD, CAM) i medicinskim imiđing tehnikama (MRI, CT), ova tehnologija omogućava postizanje višeg nivoa kvaliteta protetičkih komponenti kako u estetskom tako i funkcionalnom smislu u odnosu na tradicionalne tehnike izrade. U ovom radu tehnologija inkrementalnog deformisanja je bila primenjena za izradu metalne baze totalne proteze. U istraživanjima su korišćena dva različita materijala: nisko-uglenični čelik EN DC04, odnosno nerđajući čelik EN X6Cr17. Glavi cilj istraživanja jeste provera tačnosti geometrije baze proteze dobijene tehnologijom inkrementalnog deformisanja u odnosu na polaznu (postojeću) bazu proteze izrađu livenjem. Ključne reči: inkrementalno deformisnje, baza dentalne proteze, modeliranje