Application of additive technologies for master model creating at porcelain ware production

. The article considers the prospects of application of additive technologies in porcelain production by the example of creating a master model of a porcelain coffee pot. The results of printing a master model of the product obtained using a 3D printer with FDM printing technology are analyzed. The possible defects of 3D-printing that may occur during modelling and printing of models with complex configuration and fine plasticity are considered.


Introduction
Additive technology, or 3D modelling and 3D printing, is a young and rapidly developing manufacturing method that has found wide application in a wide range of industries, from shoe making to aircraft manufacturing. The applications of 3D printing are constantly expanding, making it available not only in large-scale production facilities, but also in the creation of single copies of fragile artwork [1][2][3][4].
Of particular interest is the possibility of using additive technologies in porcelain production, where the creation of a master model of the product is often a very timeconsuming process. The introduction of 3D modeling and 3D printing in modern enterprises can accelerate and simplify some stages of the creative process, eliminate the possibility of hidden errors and reduce the level of economic costs [5,6].
The aim of the study is to develop a 3D model of the coffee set "Dragon" with the subsequent production of a master model using additive technologies.

Materials and methods
Theoretical research methods (modelling, analysis and comparison) were applied as the main methods in the course of work, approbation of the obtained results was carried out, in particular, a 3D master model of the coffee pot and its fragments was developed and baked.
Modelling was performed using Autodesk software, using "Maya" "Mudbox" and "Netfabb" programs. The model was printed using QiDi X-MAX 3D printer using FDM technology.

Results
Approbation of existing additive technologies described above was carried out during creation of master model of porcelain coffee pot in oriental style, under the working name "Dragon". The coffee pot was intended to be an intricately shaped vessel with an intricately profiled handle and spout. The vessel's body was wrapped in rings around a dragon, a mystical symbol of Eastern culture. The curves of the body, snout made in the form of dragon's head and neck, and the handle denoting the tail are covered with scales. The style and image of the future product are the result of the authors' artistic endeavours. The complex configuration of the conceived object was of great importance for the experiment to create a master model, allowing maximum evaluation of the application of additive technology for use in porcelain production.
It should be noted, however, that in the process of modelling and printing a 3D model, a variety of defects can occur. The most common defects include: "warping", "friability", "cushioning" (protrusions and shells on the back layer of the product), "non-layering" (violation of layer geometry), "waviness" (incorrect printing speed), "fuzziness", etc. In the process of printing of the created master model, which has a complex configuration and fine plastic with embossed pattern, on 3D-printer "QiDi X-MAX" we revealed the following defects. At the initial printing attempt, voids and holes were observed forming on the model surface, contributing to the high fragility of the model (Fig. 1). This defect was the result of the wall thickness of 0.8 mm being too thin in practice. After increasing the thickness from 0.8 mm to 2 mm, the defect was no longer observed. However, after increasing the wall thickness, a print failure (misalignment of the layers) occurred which did not become apparent until several hours into the printer's operation ( fig. 1). This secondary defect was corrected by changing the export file format from "STL" format to "OBJ" format. The difference between the two formats was that the "STL" format transmitted the information about the model as a list of triangular faces which describe its surface, and their "normals". This is probably unsuitable for porcelain pieces with complex configurations and/or complicated topography, making it difficult to read and transfer information about the object to the printer. In case of "OBJ" format information about model is transferred in form of parameters of each vertex, links of texture coordinates with vertices, normals for each c b а vertex and parameters, which are set by each polygon. As a result, it was proven in practice that "OBJ" format transfers more complete information about the geometry of the model than "STL" format, which is important for the process of prototyping models in the manufacture of porcelain products with a complex shape, design, or texture. The 3D printed model can be post-processed if necessary. In our case, such processing was minimal: minor defects on the surface of the model and the removal of supports were sanded down in a wet sanding process. During post-processing it is also important to consider the softening temperature of the plastic, in our case PLA plastic (60 -65 0C), because it may cause deformation of the model. For this reason, no single area of the model surface was sanded for a long period of time.
The result of relief printing of master model's fragment from PLA plastic on 3D-printer "QiDi X-MAX", as well as the result of casting of hard porcelain on fragment of created master model are shown in Figure 2.
Illustrations clearly show, that made on a 3D-printer a trial fragment of a master model allows without distortion to carry out the further manipulations in a material, namely from the received model to make a plaster mould on which then moulding of a product from a firm porcelain is carried out (The results of the graduation projects of the Department of Materials Science and Technology of Art Products are presented in the exposition of the Mining Museum of St. Petersburg Mining University [7]. The results of scientific research of teachers and students are covered in the materials of international and all-Russian conferences, in interuniversity projects [12][13][14][15][16][17][18][19].).

Discussion
The technology for making a master model using additive manufacturing technology boils down to two main steps: 1) 3D modelling and preparation of the finished 3D model for printing; 2) 3D printing and post-processing of the resulting master model, if necessary.
The 3D-modelling stage is carried out using 3D-modelling software such as "3ds Max", "Maya", "ZBrush" and others that allow designers to design jewellery and household items, as well as to visualize design projects [5]. When moving to the second stage (3D printing) to create a master model for porcelain production, the choice of printer is of great importance. Modern 3D printing printers differ according to the printing method used: extrusion (FDM or FFF), wire (EBF3) or powder (DMLS, EBM, SLM, SHS, SLS). In addition, the choice of printing technology is important, as is the curing process (SLA, DLP). In general, the principle of all additive technologies is to create three-dimensional objects by layering one layer of polymer material on top of another with a certain pitch.
This article discusses how to create a master model of porcelain coffee (Approbation of additive technologies for creating a master model was carried out as part of a diploma project by a student of the Department of Material Science and Technology of Art Products at St. Petersburg Mining University, Grudinina A.M. Employees and students of the university carry out various studies in the field of material science, as evidenced by a number of publications, patents, educational and methodological materials. [20][21][22][23][24][25][26][27][28]) pot using extrusion (FDM) printing method. The choice of FDM (Fused Deposition Modeling) printing technology was driven by the fact that the FDM method supports printing in a wide range of materials and thermoplastics (standard, structural and high performance): PLA, ABS, PET, TPU, etc. [8][9][10] The advantage of this technology over others is also that it enables the creation of strong, wear-resistant models with extensive post-processing options (sanding, polishing, painting) and with lower costs. However, like any other technology, FDM-printing has several disadvantages, the main of which is the relatively low printing speed and the need to create and then remove special supports for product elements that become overhanging in the printing process, which requires subsequent technical processing of the product [11].    Maya and Mudbox were chosen to design the product and a computer-based solid model of the future product was generated (Fig. 3-9). The model was created considering the peculiarities of the material from which the coffee pot is to be made (in our case porcelain) and the technology of manufacturing products from it. In porcelain production in modelling the master model avoided sharp elements, therefore all details had to be smoothed or given a rounded, smooth outline. In the process of modelling, shrinkage inherent in porcelain and ceramic products during drying and firing was also considered [29]. For this reason, the virtual model and, consequently, the subsequently printed 3D model had to be larger than the finished product by the percentage of shrinkage of the ceramic mass. Thus, the main requirements for the 3D model used for printing were closed and integral surface, the presence of properly oriented surface ("normals" (A normal is a geometric perpendicular to a tangent (line or plane) passing through a tangent point; the main normal is that normal to the spatial curve which lies in the plane most closely contiguous with the curve) of the surface must be directed outward, because they define the boundaries of the object and allow the 3D printer software to distinguish where the inner, and where the outer surface of the model).
The wall of the model must have had a certain thickness, which depends on the material used and the printer's resolution. For example, the QiDi X-MAX 3D printer has a resolution of 0.4 mm (extruder nozzle diameter), while the minimum wall thickness for model rigidity must correspond to twice the printer's resolution, i.e. 0.8 mm. If these requirements are not met, it is likely that an error will occur in the printing process that will deform and distort the final model. To prepare and optimize models, use specialized programs like "Materialise Magics", "Netfabb", etc., which allow you to analyze the final 3D model and correct errors automatically if detected. However, this method does not fix problems in all configurations. The more complex a model, the more likely it is that manual editing is required. At creation of the master model the analysis on presence of errors and their correction was made with use of the program "Netfabb", at the expense of use of the command "Repair". How this looks in the computer programme is shown in Figure 10. When preparing the model for printing, it was important to consider the size of the printer's desktop. In standard printing, the model is printed as a whole. In the case of threedimensional ceramic products such as cups, teapots, coffee pots, etc., it is advisable to divide each product into as many parts as possible and place them directly on the plane of the 3D printer's working area. Such a methodical approach allows reducing printing time, avoid deformation of the product under its own weight in the process of printing a full product and minimize the possibility of reducing the quality of the surface, i.e., the formation of printing debris. At creation of master model of "Dragon" coffee pot, we used "QiDi X-MAX" 3D printer, which has a working area of 25x25x30 cm. Final preparation of the 3D model was performed using "QIDI Print" slice program, which, as well as "Simplify3D" and "Cura" programs, allows setting necessary product printing parameters (layer thickness, degree of model filling with material, temperature of table and plastic, printing speed, printing supports, printing material, etc.).
An important step in the process is the choice of printing material. PLA plastic was chosen to create a master model of a coffee pot for porcelain production. This type of plastic has a low softening point (600C), which, from a technological point of view, is sufficient to remove the plaster mould from the master model. Secondly, the melting point of PLA plastic is lower in comparison with other laminates which may provide a more accurate and higher quality print of complex embossed surfaces. Third, PLA plastics are the most environmentally friendly type of polymers used in 3D printing, which is important in porcelain production, where health hazards are quite high (Assessment of environmental risks and ways of overcoming them have been presented in a number of works by scientists from various organisations, including the St. Petersburg Mining University [32][33][34][35][36]) [30,31]. Practically, we have found that the most suitable layer height and printing speed parameters for PLA plastic, respectively, are 0, 16 mm and 30 mm/sec for printing complex-profile and complexrelief models. The parameters obtained during 3D model creation and printing are translated into a so-called G-Code (program code with a list of commands for the 3D printer), which was downloaded into the 3D printer for 3D printing.

Conclusion
It was not the task of this article to analyse the further stages of the creation of the service, and therefore it is not necessary to talk about how the further production processes, up to the glaze coating of the product, are carried out. The testing of additive technologies for the creation of a master model for porcelain production has, in our opinion, taken place. Some of the problems that may be encountered by those who work with these technologies were analysed and ways of solving them were shown. Based on the above, it can be stated that additive technologies, compared to traditional methods of creating master models for porcelain and ceramic production, have obvious advantages, as they contribute to the intensification and diversification of the model creation process. Practical implementation of 3D printing will allow achieving high accuracy and detailing of ready-made models, eliminate hidden mistakes before the mass production, assess functionality and ergonomics of future products, reduce harmfulness of production. Additive technology is a promising way to produce master models, which should be studied and actively implemented in porcelain and ceramic production.