1Mladomir MILUTINOVIĆ, 1Dragiša VILOTIĆ, 2Tatjana PUŠKAR, 2Dubravka MARKOVIĆ,
1Aljoša INAVIŠEVIĆ, 2Michal POTRAN
1Department for production engineering, Faculty of technical sciences, University of Novi Sad, Novi Sad, Serbia
2Department of dentistry, Medical faculty, University of Novi Sad, Novi Sad, Serbia
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Abstract: In the recent years, a lot of efforts have been made with goal of improving manufacturing technologies for the production of dental components used to replace missing biological structures, support of a damaged biological structure, or to enhance an existing biological structure. Due to complex geometry and demands for high mechanical properties, accuracy and reliability in use dental devices (implants, frames, bridges, orthodontic appliances, denture base plates etc..) are predominately produced by application of advanced manufacturing technologies in combination with computer-aided techniques (CAD, CEA, CAM…).
In this paper the possibility of application of metal forming and shaping technologies in fabrication of the dental components will be presented.
Key words: Metal forming technology, dental components,
1 Name of the author, title, company, address and e-mail.
2 Name of the author, title, company, address and e-mail.
1. INTRODUCTION
In the past dental devices (implants, frames, bridges, orthodontic appliances, denture base plates etc.) were conventionally produced primarily from metal materials by using casting techniques. But over the last two decades dentistry and dental techniques have been undergoing a radical and rapid shift in terms of employing advanced manufacturing technologies such as: CNC machining, metal forming, precision casting, sintering, surface engineering, rapid prototyping, and rapid manufacturing [1]. At the same time diagnostic tools have become increasingly more sophisticated and medical imaging technology can now present patient data with high precision [2].
Advanced manufacturing technologies usually combine novel manufacturing techniques and machines with the application of computer-aided techniques (CAD, CEA, CAM, RE…). It has enabled not only the improvements of the existing manufacturing process, but also process automation, design of greater variety of complex products with increased accuracy and reliability in use, as well introduction of innovative, state-of-the-art dental services, visualization, analyze and optimization of the dental devices and dental procedures, development of the new materials and dental procedures, time and money saving of dental treatments etc.
The selection of the suitable technique for the manufacturing of a certain product is a very complex problem. It implies multicriteria decision, depending on several factors. The most important are: component design and complexity, material processing ability, desired accuracy and quality, mechanical and physic properties, production quantity, and cost. In case of 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 tailor-made components. Material used in dentistry must be biocompatible and meet the general requirements of biomedical engineering, such as: nontoxicity, good corrosion and oxidation resistance, high specific strength, hardness and toughness, the appropriate modulus of elasticity etc. As a rule, most of biocompatible materials (Stainless steel, Titanium and its alloys, etc) exhibit poor or modest processability, when traditional technologies are employed.
Modern metal forming technologies are promising way for processing of these materials and manufacturing of numerous medical and dental devices. This paper reviews metal forming and shaping methods applied in dentistry.
2. APPLICATION of METAL FORMING TECHNOLOGIES IN DENTISTRY
The use of forming technologies in medicine and dentistry is increasing permanently. According to [3] forming is one of the leading technologies applying to production of the metallic implants. Key features of forming technology are:
· enhanced mechanical properties of formed part (strength, hardness, fatigue resistance, toughness)
· high accuracy and surface quality
· close tolerances and high repeatability of part geometry
· cost saving due to a considerable reduction of process times and material waste.
· complex shapes are produced in simple way
In addition, new developed and refined forming technologies make it possible to produce parts ready-to–assembly (net shape forming-NSF) or parts which require finishing machining only on some surfaces, most of which are non-active (near net shape forming-NNSF).
Majority of metal materials used for manufacturing of dental devices can be processed by forming technology. Due to low formability some of these metals must first be heated so they can be transformed into final shape (warm and for forming), while other metals are naturally more processable and can be shaped at room temperature (cold forming). Forging, rolling, drawing, and punching are traditional forming technologies, used the most often in dentistry. However, recently introduced forming and hybrid processes such as severe plastic deformation (SPD), superplastic forming (SPF), metal injection molding (MIM), thixoforming, incremental forming etc. are becoming highly attractive option when the manufacturing way of dental devices is considered. These technologies have further increased the efficiency of the forming techniques in terms of getting dental components with more complex geometry, possibility to shape hard to deform materials, material structural improvements etc.
2.1 Forging
Forging is the most common metal forming process used for production of implants applied to orthopaedic surgery and traumatology [3]. It is because forging yields components with greater strength than those produced by any other metalworking process. Joint replacement implants made from stainless steel were the first forged implants in the mid of last century [4]. During the 1970s and 1980s, stainless steel has been replaced by Titanium. As stainless steel forgings, early Titanium implants required heavy post-processing machining as well hand belting and polish finishing. Typical tolerances of those implants were in the range 0,25-0,5mm[4]. Advancements in equipment, process design, and technology enabled later the development of NNSF and NSF forging implants [5]. Apart from Titanium and its alloys forged implants today may be produced from differ materials such are: Cobalt, Chrome, Molybdenum, and Zirconium [5].
Techniques for forging of Titanium are essentially the same as for low-alloy steels [6]. Forging process of mostly used Ti6Al4V Titanium alloy takes place in the temperature range 800-1000°C. Forging temperature, strain velocity, and die preheating have the crucial influence on the properties of the forged components. Because of the rapid cooling and the fairly narrow hot working range, the chilling effect of dies should be reduced to a minimum by keeping contact time as short as possible. When forging Titanium and its alloys very intensive friction occurs between die and deformed material that together with low thermal conductivity result in structure and property inhomogeneity of forged part. Diffusion of Oxygen, Nitrogen and Hydrogen causes the changes at product surface both in chemical composition and microstructure. In order to minimize these phenomena heating of workpiece should be carried out with protective atmosphere and proper lubricants applied.
As it said before forging of Titanium implants is mainly related to orthopedics field i.e. for producing of large sized implants. On the other hand, implants smaller in size as those used in dentistry (tooth implants, artificial denture, connection plates, orthodontic apparatus etc.) were exclusively produced by cutting processes. General problem of forging micro components and parts with small volume is the low heat capacity of specimen compared to the cooling caused by surrounding air and dies and therefore the traditional hot forging procedure is not applicable. Also, when considering micro forming, some size effects need to be taken into account.
In the past it was not possible to micro forge Titanium. Recent works have confirmed that it is feasible to forge micro-components instead of using traditional cutting methods [7]. The solution is found in indirect heating and design of warm tooling setup that allows permanent heating of specimen (Fig. 1)
Fig.1 The tool-set for micro warm forming [8]
1) punch 2) die insert 3) stress ring, 4) heaters
5) thermal shield 6) isolation 7) load distributor
Fig.2a shows 3D model and dimensions of a Titanium dental implant (abutment). Until now this component featuring complex geometry, high production volumes and advanced material is primarily manufactured by turning, the HEX key and flank on the side and the holes through is done by milling and drilling respectively [7]. Newly developed warm-forging (350-400°C) procedure enables complete part geometry to be obtained in only two steps (Fig 2b) [9].
a) /b) /
Fig.2 a) 3D Model of dental abutment and
b) photograph of the formed specimens after the first (left) and second (right) operation [9]
2.2 Sheet metal forming
Sheet metal technologies are also applied to produce a variety of medical implants as well dental components (cold-pressed denture base plates, dental prosthesis etc).
Sheet forming of Titanium, especially of Ti6Al4V alloy, is much more demanding and more difficult process to preform than processing steel sheets. Therefore cold-rolled titanium sheets which are used as starting material must first be annealed. Sheet forming of Titanium can be realized both as cold and warm process. Advised temperature for warm sheet-titanium forming is 350-400°C, but in some cases, such as deep drawing operation, higher temperature is applied in order to decrease the amount of operations and to obtain better dimensional accuracy of the drawn-parts through lower spring-back [3]. Usually, both titanium blanks and dies are warmed up. During the cold processes it is necessary to apply intermediate annealing. An annealing process is very often required after the processing in order to remove internal stresses of the final products.
Titanium and its alloys are characterized by poor drawability. Limit drawing coefficient, m=d/D, as one of the criteria for evaluating sheet ability to deep drawing operations, in case of cold process of Ti6Al4V titanium alloy is m=0,83-0,76 while during the warm processing is m=0,71-0,63. Similar to forging, strain velocity significantly affects the sheet forming process of Titanium, and therefore it is advised to form titanium sheets on the hydraulic presses with the velocity lower than 0,25m/s [3]. Corner radiuses and clearance between a punch and die are also very important. Since Titanium is very prone to galling process and formation of Titanium ”build-ups” on the tool surfaces it is essential to ensure the effective separation of the contact surfaces.
2.3. Superplasticity forming
Superplastic forming (SPF) is a net-shape forming process used with superplastic materials, a unique class of materials that has the ability to undergo extraordinarily large tensile deformation. Ductilities of 1000–2000% are commonly observed in metallic superplastic materials although, commercially, elongations of 300% is sufficient to form even the most complex component [10]. High ductility is also encountered in superplastic alloys during torsion, compression, and indentation hardness testing. Phenomenon of superplasticity appears in many materials including ceramics and composites [10]. There are two essential requirements for the occurrence of superplasticity: 1) high temperature, usually greater than half of the melting point, 2) stable microstructure, with fine grain size, high-angle grain boundaries and grain boundary diffusion [11]. The major advantages of SPF process are: excellent accuracy and fine surface quality of final part, absence of springback phenomena and residual stresses, large and complex workpieces can be shaped in only one operation, less tooling costs, and cost and weight saving.
Computer aided simulation is an important part of the SPF process that enables design and analyze of the various aspects issues of the process. The most critical issues of SPF are related to material behavior during forming (limited predictive capabilities of material deformation and failure) and low production rate.
The pioneer work and driving force for future developing of superplastic applications in dentistry and maxillofacial surgery was the manufacture of dental implants using high-strength, mill-annealed titanium alloy [12]. Since then, the range of applications has expanded and now includes: lower and upper complete and partial denture frameworks, implant-retained over dentures, cleft palate plates and hollowed-bulb obturators in dentistry with further studies on maxillofacial applications [11]. Unlike the application of SPF technology in the industry which usually means the production of standardized elements in large quantities, dental components (like dental prosthesis) are tailor product made to fit a patient's oral shape. Therefore, one of the basic conditions for economical application of this technology in dentistry is to find the way for cost-effective die production. A possible solution is the application of ceramic dies.
In Fig.3 the scheme of superplastic dental prosthesis forming process of Ti6Al4V alloy by Argon gas pressure developed by Curtis et al. [12] is given. The ceramic die insert (7) used to produce a dental prosthesis is made first based on a mould of the patient's oral shape taken by the dentist. In the next step the die assembly (steel furnace chamber and ceramic die insert) is mounted on hydraulic press and induction heated to 900°C.
Fig.3 Scheme of die assembly for SPF [13]
Once a Titanium alloy plate (140 mm diameter and 3 mm thickness) has been inserted into the die set, it takes approximately 90 min to reach the sheet insertion temperature, somewhere between 800 and 900 °C. A low clamping pressure is applied at this stage to maintain Argon gas (3) flows above and below the forming sheet to protect against oxygen contamination. During forming process clamping force of about 60KN is applied, as the workpiece is shaped by Argon gas at pressures of up to 4,2MPa. It needs up to 3 hours to complete forming process. Fig.4 shows a dental prosthesis made by SPF.
Fig.4 Partial upper denture-Ti6Al4V shaped by SPF [11]
2.4 Severe plastic deformation
Severe plastic deformation (SPD) is a term describing a group of metal forming techniques in which an ultra-large plastic strain is introduced into a bulk metals and alloys without any significant changes in the overall dimensions of the specimen [14]. The components obtained by SPD exhibit high strength ductility and fatigue resistance. This process causes the formation of ultrafine grained microstructure (submicron or nano-size) in the initial material. In order to gain such microstructure, starting (grained) material must have predominantly high angle boundaries, the structure must be uniform over the billet volume and the large plastic strains may not have generated internal damage or cracks [15]. A few procedures have been developed within SDP technology: Equal channel angular pressing (ECAP), High pressure torsion (HPT), Accumulative roll bonding (ARB), Reciprocating extrusion-compression (REC), Cyclic close die forging (CCDF), cyclic extrusion compression (CEC), Repetitive corrugation and straightening (RCS), severe torsion straining (STS), super short multi-pass rolling (SSMR). The schemes of the principal SPD methods are presented in Fig 5.