A thesis for obtaining a scientific degree of a Ph.D on specialty 133 – “Industrial Machinery Engineering”. – National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute”, Kyiv, 2020.
The dissertation is devoted to the study of the spatial printing process, namely, the impact of the main technological parameters on the value of interlayer adhesion and local deformation of the applied thermoplastic polymer material during the cooling process, as well as the study of the dynamics of movement of the executive parts of FDM 3D printers, depending on the type of design scheme.
An analytical review of the current state of research on spatial printing technology is carried out. The main modern technologies of additive manufacturing are considered, the designs of 3D printing equipment with thermoplastic polymeric materials are systematized depending on the kinematic scheme of movement of the executive bodies, the types of designs of printheads based on patent searches are considered, studies of the physical and mechanical characteristics of printed products are analyzed depending on the main parameters of 3D printing and the geometry of the layers.
The review showed that the industry of modern additive equipment for the production of polymer products is developing extremely rapidly, however, approaches to the design of new types of equipment do not take into account the properties of consumables. The main design task for engineers is to optimize and modernize existing structures. Also insufficiently studied is the issue of the process of forming finished products. The conducted studies of the mechanical properties of samples do not provide accurate data on the magnitude and nature of interlayer adhesion, which is the basis of additive technologies. Even in the studies, static uniaxial tensile tests were carried out in the transverse direction of the application of the layers, a contour wall was present, and this, in turn, is incorrect for establishing the value of interlayer adhesion.
It should be noted that the question of the behavior of the melt of thermoplastic material after it leaves the print head, and the very process of its cooling and interaction with the previously applied material, has practically not been studied. A separate applied topic, also poorly studied, is the issue of the dynamics of motion of the executive bodies and the performance of equipment, depending on the design scheme.
To simulate the cooling process, a two-dimensional mathematical model of unsteady thermal conductivity and a model of the flow of a non-Newtonian non-isothermal fluid are used. To simulate the deformation of the polymer layer under its own weight with simultaneous cooling after deposition on the previous layer, a related thermomechanical problem was solved taking into account the power-law dependence of the polymer viscosity on the strain rate and the Arrhenius law of the temperature dependence of viscosity.
The results obtained showed that, under conditions of intensive blowing, the polymer in less than 1 s has time to cool to a temperature below the pour point without noticeable deformations. However, to prevent warping and delamination of products, the temperature should not go beyond certain limits, which indicates that a certain minimum period of time must be established before applying the next layer, it is different for different polymers and layer sizes. This imposes restrictions on the allowable speed of the 3D printer.
Modeling allows to obtain data on the degree of filling the cross-section of products, establish the causes of defects and select the optimal thickness, temperature and printing speed.
To simulate the process of stretching printed samples in the transverse to the direction of application of layers, a discrete mathematical model was applied using a limiting element based on the penalty method to predict the delamination of deposited polymer layers.
The results obtained showed that delamination occurs starting from the edges of the contact surface of the layers according to the brittle fracture scheme.
The presented model and method of numerical simulation make it possible to predict the strength of printed polymer products in the direction across the layers based on experimental data of tensile tests of printed samples.
Corresponding experimental studies of printed samples for rupture were carried out depending on the geometry and technological parameters of 3D printing, such as temperature and printing speed, as well as the height and length of the applied layer.
