Post-treatment of ABS samples manufactured by FDM

. FDM is used for printing parts from thermoplastic polymers, polymer matrix composites, biocomposites or polymer-ceramic composites, nanocomposites and fiber-reinforced composites. The main disadvantage of this method is the reduced physical and mechanical characteristics due to the presence of pores and poor adhesion of layers. The post-treatment is one of the ways to improve this properties. The heat treatment has the greatest impact among all types of post-treatment processing on the surface quality and physical and mechanical properties of finished products. The paper studies the physical and mechanical properties of samples from ABS plastic (REC brand) printed by FDM and subjected to thermal post-treatment. Two methods of thermal post-treatment were considered: in NaCl powder and in closed form with pressure. The test results of the printed samples were compared with the test results of the samples obtained by injection molding. Comparison of strength and porosity showed that the properties of printed samples after post-treatment by both methods are comparable to the properties of samples obtained by injection molding.


Introduction
Fused deposition modelling (FDM) technology is the most affordable 3D printing technology, which makes it very popular all over the world [1,2].The FDM is used for printing products made from thermoplastic polymers, polymer matrix composites, biocomposites or polymer-ceramic composites, nanocomposites and fiber-reinforced composites.Recently, the field of application of FDM technology has significantly expanded due to superconstructive thermoplastics.Like any technological process, the FDM process is characterized by the presence of a number of parameters that have a great impact on the printing time, quality, physical and mechanical properties of printed parts.Choosing the right combination of technological parameters allows to ensure the characteristics of the future product and manage them [1].
It is known that parts made of thermoplastic polymers and composites printed by the FDM method have inherent disadvantages that directly affect the mechanical characteristics and appearance of the finished product.The main disadvantages are: shape distortions that occur due to residual stresses caused by temperature gradients during printing; micropores that occur in the polymer during layer-by-layer printing; micropores that occur when printing composite material due to uneven distribution of filler inside the matrix or poor adhesion between them; high surface roughness of the finished product, resulting from the staircase effect.
Most of the disadvantages can be minimized by selecting the optimal parameters of the printing process.However, only varying the process parameters often does not give the desired effect due to the nonlinear dependencies between the parameters of the FDM printing and the properties of the finished parts.Therefore, research is currently being conducted on various methods of additional treatment of products aimed at minimizing defects that occur during FDM printing.The heat treatment has the greatest impact among all types of post-treatment processing on the surface quality and physical and mechanical properties of finished products.[3 -5].

Experimental procedure
This paper studies the physical and mechanical properties of samples made from ABS plastic (REC brand) printed by FDM and subjected to thermal post-treatment.The main characteristics of the material are given in Table 1.In order to select the printing and post-treatment parameters, the thermal and rheological characteristics of the printing material were determined before the experiments.Thermal characteristics were measured by a method based on measuring the heat flow between the test sample and the reference under controlled temperature conditions using a differential scanning calorimeter SKZ1052F.The thermal and rheological characteristics of ABS plastic used for printing samples are given in Table 2. placing in a Labtex LT-VO/20 vacuum drying chamber.Then the filament was loaded into the printer into the filament supply and heating box.The test samples in the form of blades were used in accordance with GOST 11262-2017 to conduct studies of mechanical properties.Prepress preparation consisted in designing a three-dimensional model of the sample using CAD SolidWorks, saving geometry in STL format and splitting into layers using Ultimaker Cura 5.2.1 software.The samples were printed on Total Any Form 500 PRO 3D printer.The main parameters of printing samples are given in Table 3. Fill density, % 100 The 5 samples were printed for all combinations of the raster angle and the layer thickness.The 5 samples were made by injection molding on a manual injection molding machine RLM to compare the mechanical characteristics.
The two methods of thermal post-treatment were considered based on the analysis of the literature data: -heat treatment in NaCl powder; -heat treatment in a closed mold with pressure.The using of salt in the post-treatment process allows to reduce the temperature deformations of the samples.The application of pressure allows to increase the adhesion between the layers by increasing the contact area.
The powder used in the experiment was ground NaCl.The powder particles were crushed using a LANHANG SHR-10A high-speed mixer.The resulting powder was kept at 200 °C in a Labtex LT-VO/20 vacuum drying chamber for 30 minutes to remove moisture, then cooled in air at room temperature.The printed samples were placed on a powder layer inside a metal container, and then covered with an additional layer of powder (Fig. 1).The top layer was tamped with a metal plate.The packed samples were placed in a Nabertherm N 41/H muffle furnace preheated to a temperature of 190 °C, which is higher than the glass transition temperature, but lower than the melting point of the filament material.The heating of the powder inside the container was controlled by a thermal probe.The oven timer was started when the powder temperature reached 190 °C.The samples were kept at this temperature for 30 minutes.The sample container was cooled at room temperature after heat treatment.
A special mold which is a negative imprint of the test sample shape was designed for the experiments.The mold was made of aluminum alloy grade AD000 (GOST 4784-2019).
The sample was placed into the mold (Fig. 2, a), then pressed with a plate, the shape coinciding with the shape of the test sample (Fig. 2, b).The mass of the plate was 300 g.Thus, a pressure applied to the sample was 1.4 kPa.The mold with the sample was placed in a Nabertherm N 41/H muffle furnace, preheated to a temperature of 190 °C.The heating of the mold was controlled by a thermal probe.The oven timer was started when the powder temperature reached 190 °C.The samples were kept at this temperature for 30 minutes.The mold with the sample was cooled at room temperature after heat treatment.Studies of tensile strength, porosity and deformations as a result of thermal posttreatment of printed samples were carried out.Tensile strength studies were carried out on a universal testing machine REM50A according with GOST 11262-2017.Porosity studies were carried out on the basis of determining the density of samples.Hydrostatic scales CE 224-C were used to measure the density.The density of the samples was determined by immersing them in a working fluid with a known density.The deformations of the samples as a result of thermal post-treatment were evaluated by the size change in three directions compared to the original samples.To do this, measurements of length L, neck width W and height H were made on all samples before and after thermal post-treatment.The measurements were carried out using an electronic digital caliper with a division price of 10 micrometers.

Results and discussion
The results of experimental studies are presented in Fig. 3 -5.As a result of experimental studies, it was found that after thermal post-treatment in salt, the samples withstand a load of 33.1 MPa, the porosity of the samples decreases to 3.1%.However, the samples had significant deformations (Fig. 3).The geometry changes were: -3% in length; -2% in width; 0.3% in height (Fig. 3).When post-treatment in a closed mold with pressure applied, the samples withstand a load of 36.9MPa, while the porosity decreases to 1.7%.Compared with the samples processed in salt, the samples after processing in the mold are less deformed (Fig. 3, a, middle sample).The geometry changes were: 0.6% in length; 0.3% in width; -8% in height.
As a result of comparing the strength and porosity of samples after heat treatment with the properties of samples obtained by injection molding, it was found that the properties of printed samples after post-treatment by both methods are comparable to the properties of samples obtained by injection molding (Fig. 4, 5).

Conclusions
As a result of the conducted experimental studies, it was found that thermal post-treatment in salt can increase the tensile strength by 10-12%, and post-treatment with pressure by 25%.At the same time, the porosity of the treated samples decreases by 2-4 times compared to the untreated ones.A comparison of the properties of post-treatment samples and samples obtained by injection molding indicates that thermal post-treatment makes it possible to approximate the properties of samples obtained by FDM printing to the

Fig. 1 .
Fig. 1.Thermal post-treatment in NaCl powder: (a) samples on a layer of NaCl powder; (b) a container with compacted powder.

Fig. 2 .
Fig. 2. Thermal post-treatment in a mold with pressure: (a) a sample in a mold; (b) a mold with applied pressure.

E3SFig. 3 .Fig. 4 .
Fig. 3.The effect of thermal post-treatment on the change in the linear dimensions of samples obtained by FDM printing: (a) samples after post-treatment; (b) change in length; (c) change in width; (d) change in height of samples.

Fig. 5 .
Fig. 5. Porosity of samples obtained FDM printing after thermal post-treatment (control samples were not subjected to post-treatment).

Table 1 .
The main characteristics ABS plastic (REC brand).

Table 2 .
The thermal and rheological characteristics of ABS plastic (REC brand).
The filament was pre-dried at a temperature of 80 °C for 5 hours before printing by

Table 3 .
The main parameters of sample printing