Investigation of the main mechanical characteristics of plastics for three-dimensional printing of machine parts models

Today, new methods of manufacturing complex parts using three-dimensional printing are being introduced in engineering, including in the mechanical engineering of agricultural machinery. This technique is also used in the design of agricultural machinery at the stage of physical modeling of their structures to determine certain properties of the product as a whole and its individual parts (details) on the corresponding models. Physical modeling is used when it is difficult to perform full-scale tests of the product, as well as for economic reasons. The necessary conditions for physical modeling are the geometric and physical similarity of the model and kind. The presence of such proportionality allows us to recalculate the experimental results obtained for the model in kind by multiplying each of the determined values by a constant multiplier for all values of this dimension – the similarity coefficient. However, to study the physical characteristics of a product, it is necessary to take into account the mechanical properties of the material of its model. From various sources, you can find the main mechanical characteristics of plastic threads or samples made on a 3-D printer. Their values vary greatly depending on the model manufacturing technology. The paper presents the results of a study of the physical and mechanical properties of polylactide (PLA) and polyethylene tereflatate (PET-G), which are used in the manufacture of parts by three-dimensional printing. The specific strength of polymers was respectively: for PLA 65.6…12.2 kPa·m3/kg, for PET-G 36.7…95.4 kPa·m3/kg. Specific plasticity for PLA is equal to 60.,3 %· cm3/kg, for PET-G – 468.2 %· cm3/kg. The specific plasticity for PLA is 608.3 %· cm3/kg, for PET-G – 468.2 %· cm3/kg. Mechanical properties are investigated and the obtained mechanical characteristics must be taken into account when calculating and physically modeling plastic products for three-dimensional printing.


Relevance
Today, new methods of manufacturing complex parts using three-dimensional printing are being introduced in technology (figure 1). This technique is also used when designing at the stage of physical modeling of structures of machines and mechanisms, to determine certain properties of the product as a whole and its individual parts (details) on the corresponding models. Physical modeling is a type of experimental study of an object or phenomenon based on its model, which has the same physical nature. Physical modeling is used when it is difficult to carry out fullscale tests of the product, when the size of the object of research or the values of its other characteristics (pressure, temperature, process speed, etc.) are too large (or small), as well as for economic reasons. Physical modeling is based on similarity theory and dimensional analysis. The necessary conditions for physical modeling are geometric similarity (similarity of form) and physical similarity of the model and kind: at similar points in time and at similar points in space, the values of variables that characterize phenomena for nature must be proportional to the values of the same values for the model. The presence of such proportionality makes it possible to recalculate the experimental results obtained for the model in kind by multiplying each of the determined values by a constant multiplier for all values of this dimensionthe similarity coefficient [1]. A geometrically similar model of a real part can be made using three-dimensional printing, but to study the physical characteristics of the product, it is necessary to take into account the mechanical properties of the model material. From various sources, you can find the main mechanical characteristics of plastic threads or samples made on a 3-D printer: strength limits, relative residual elongation, modulus of elasticity. Their values vary greatly depending on the model manufacturing technology.
1.2. The aim of this work is to study the physical and mechanical properties of polylactide (PLA) and polyethylene terephthalate (PET-G), which are used in the manufacture of parts by three-dimensional printing.
The following tasks are set: to determine the mechanical characteristics of PLA and PETG samples under static tension, compression, and bending.

Results and discussions
Compression tests were carried out on an upgraded breaking machine МР-0,5-1 [3,4], equipped with a loading device consisting of support plates with guides (figure 2).      The results of the research are summarized in a table that shows the specific mechanical characteristics of polymers, carbon steel, and gray cast iron to analyze the structural efficiency. Mechanical properties (strength and ductility) can be evaluated using dimensionless parameters: ) .

Conclusion
In order to prevent the appearance of residual deformation and loss of load-bearing capacity of models, the calculated stresses must be limited to the smallest value of the The specific plasticity for PLA is 608.3 %·cm 3 /kg, for PET-G -468.2 %·cm 3 /kg. Evaluation of PLA plasticity in comparison with steel γ(δ) = 0.15…0.53, with cast iron -γ(δ) = 8.3…22.5. Evaluation of PET-G plasticity in comparison with steel γ(δ) = 0.12…0.40, with cast iron -γ(δ) = 6.4…17.3. Mechanical properties are studied and the obtained mechanical characteristics should be taken into account when calculating [2-4, 6-10] and physical modeling of plastic products [11,12].