Enhancing mechanical properties of 3D printed thermoplastic polymers by annealing in moulds

Abstract

Five series of specimens with two different print orientations (−45/45 and 0/90) and two print layer thicknesses (0.1 and 0.2 mm) were made. In total 60 specimens with 100% filament infill were made. One specimen series (20 pieces) was isolated as a reference or thermally untreated. Before the thermal treatment (annealing), two specimen moulding methods were used: NaCl powder (granulation 63 mm: 20 pieces) and Calcium Sulphate (Gypsum: 20 pieces). During the annealing, specimens immersed in NaCl powder were heated in a drying oven to the filament melting point (for PLA: 200°C, with a duration interval of 30 min), while the treatment of the heated specimens in gypsum was performed at a temperature of around 190°C, with duration interval of 3 h with the observed temperature inside the mould of about 100°C. An ultrasonic bath and a drying oven were used in the gypsum treatment. Temperature measurement and control during both annealing treatments were performed using a thermal imaging camera, while the temperature control inside the drying oven was performed using a digital thermometer. After treatment, the specimens in the moulds were cooled at room temperature, and the dimensions of annealing and untreated specimens were controlled. Surface morphology was characterised using scanning electron microscopy (SEM). The SEM analysis reveals improved internal structure after heat treatment of the PLA specimens. These results show that the investigated specimens after heat treatments had better structural properties than the referent specimens. Tensile testing on a universal testing machine in compliance with the ASTM D638 standard was also performed. The referent PLA specimen with −45/45 and layer thickness of 0.1 mm had the highest tensile stress value (64.08 MPa) while the specimen with minimal tensile stress value before fracture was 0/90, 0.2 mm (54.81 MPa). Heat treatment in gypsum showed the most significant increase in strength with −45/45 (0.1 mm) being the strongest (71.66 MPa) while the strongest specimen treated in sodium chloride was −45/45 (0.1 mm) with maximum tensile stress of 70.08 MPa. The mechanical characteristics of the PLA were characterised using the Vickers microhardness tester. The PLA microhardness value was calculated according to standards ASTM E384 and ISO 6507. The referent PLA specimen with −45/45 (0.2 mm) orientation shows the maximal microhardness value (125 MPa), and the minimal microhardness value was observed for the 0/90 (0.1 mm) orientation specimens (108 MPa). The heat treatment specimens in gypsum have a better hardness (185 MPa) than those treated in gypsum (165 MPa), with microhardness increasing by about 12%. The essence of the work is reflected in the additional filament processing to achieve a better structural and mechanical performance of the materials and reduce the anisotropy that is characteristic of 3D printing.

Optimisation of tensile stress of poly(lactic acid) 3D printed materials using response surface methodology

“…Heat treatment also appears to decrease the maximum elongation of test specimens before breaking with the average maximum displacement being around 6 -7 % smaller than the untreated specimens [23]. Hence, though heat treatment in moulds led to increases of strength and material became more brittle.…”

“…The specimens were made in a laboratory with a temperature of 25 °C and relative humidity of 55 % [23].…”

“…The drying time of the specimen mould was six hours at 100 °C. Annealing heating treatment of the specimens immersed in plaster was performed at a temperature of 190 °C for 3 hours to achieve a temperature inside the mould of about 100 °C [23,24].…”

“…After that, they were cleaned and prepared for testing the material's mechanical properties. The air gap between the layers is narrowed, and the surface's roughness is reduced [23]. That is, the layers and threads stuck together better in the material.…”

“…The following Table 6 shows the average value for the maximum tensile stress for each specimen's combination. In the stress-strain analysis, annealing in plaster moulds increases the strength of the specimens on average by 12.7 %, while annealing in NaCl moulds increases the strength on average by 9.7 % compared to the reference specimens [23]. Interestingly, while annealing in plaster showed the most significant increase in maximum stress, annealing in NaCl moulds showed the most significant increase in modulus of elasticity due to good dispersion and uniform distribution of NaCl within the PLA matrix [23].…”