Synthetic polymers are well known for their diverse applications in a wide range of fields, in the form of plastics, elastomers and fibers.

The deformation whose value increases continuously and does not recover after removing the force is called flow.

Fluids offer little resistance to deformation, and the forces of internal friction, which arise during flow, reduce the speed of deformation.

Under the action of a force, the rate of deformation of fluids increases until equilibrium with the frictional force is established, after which the rate of deformation remains constant. Flow is a key process in most operations specific to the synthesis and processing technologies of macromolecular compounds. The appeal to rheology is indispensable given its contribution to the elucidation of the flow behavior of various systems formed by macromolecular compounds.

Polylactide (PLA) has been intensively researched and used as a biomedical material, due to its high biocompatibility, good biodegradability in the human body, as well as excellent rheological and thermal properties and good molding properties. PLA has high mechanical strength.

It is also frequently used in industry. Looking at the rheological properties of PLA, the variation of the linear viscoelasticity range between 0.001 and 10 Pa was established. The crystalline melting enthalpy for PLA is considered to be 93.0 J/g. Figure 1 shows the SEM analysis of the rupture in normal atmosphere of the PLA disc manufactured by FDM and can remark the porosities between 196 - 259 µm, specific of FDM technology.

Figure 2 presents the SEM analysis of PLA after the traction test in liquid nitrogen, remarking the parallel fibrous structures, and fracture defects due to the brittleness induced by the cryogenic conditions.

The rheological behavior of real fluids and therefore of thermoplastics is presented in rheograms that reproduce the dependence between the unit shear stress and the re-slope $\stackrel{\dot{}}{\gamma }$ (velocity gradient). These rheograms are obtained from simple fluid shear experiments. The curves $\stackrel{\dot{}}{\gamma }=f\left(\tau \right)$ are called fluidity curves. Starting from these curves, the viscosity at each point of the curve can be calculated.

Rheological behavior is influenced by temperature, deformation time scale, molar mass and molar mass distribution, polymer structure. The re-slope of PLA differs depending on temperature as follows, presented in Table 1 .

The viscosity of polymer melts is influenced by molar mass, temperature, pressure, additives. The viscosity of the melts depends on the temperature, with great importance on the technological properties of the polymer, because the sensitivity of the viscosity to the temperature determines not only the choice of the processing regime but also the quality of the obtained products. With the increase in temperature, the possibility of the macromolecule passing from one equilibrium state to another occurs due to the activation energy. The dependence of viscosity on temperature can be expressed by the exponential equation depending on the activation energy of viscous flow.

$\eta =A\ast {\text{e}}^{-\frac{E}{R\ast T}}$

A—frequency term;

R—gas constant;

T—temperature;

E—activation energy.

For a constant frequency of 1 Hz, it gets η = 58.68 kJ/mol.

The flow index (melt index) is a quantity that characterizes the flow behavior of different polymers in the molten state. The determination is made on apparatus at a certain temperature and pressure, for the amount of plastic material that flows through an adjustment (capillary), in a certain determined time (generally 10 minutes). In practice, the flow index lc is used as a basis for choosing the processing method. Figure 3 shows the variation of PLA viscosity as a function of temperature, and it is noted that as the temperature increases, the viscosity also decreases. Since in the case of FDM processing, the PLA filament is melted, and the printer can be programmed according to the temperature of the melt used. Looking at the experimental part, the influence of the melting temperature on the dimensional precision of the cubes made was determined.

Nine cubes with a side of 7 mm were designed in SolidWorks software. Using the ZMorph 3D printer’s Voxelizer software, the stl file was converted to a gcode file to enable 3D printing. A nozzle with a diameter of 0.2 mm was used for processing, as in Figure 4 .

The melting temperature varied between 190˚C to 220˚C (in each 5˚C) and several batches of parts were made. The temperature was determined and controlled grace of the Voxelizer software and of LCD of the 3D printer. For control, the precision was used a digital caliper, with a precision of 0.02 mm, and resolution of 0.01 mm.

It was noted that the optimal dimensional accuracy is at the temperature of 200˚C, in the event of an increase in temperature, the viscosity of the material decreases and it flows faster through the nozzle hole, leading to oversizing of the parts.

At a temperature lower than 200˚C, the viscosity increases and in this case the flow through the nozzle hole is slower, so under-sizing of the parts will occur.

Figure 5 shows dimensional accuracy variation on the OX axis in function of PLA melting temperature, in the case of the ZMorph 3D printer.

Also, the quality and hardness of the parts’ surfaces are influenced by the melting temperature variation.

It is noted that the best dimensional accuracy along the OX axis, for PLA, is found at the melting temperature of 200˚C.

Figure 6 shows dimensional accuracy variation on the OY axis in function of PLA melting temperature, in the case of the ZMorph 3D printer.

Also, the best results regarding dimensional accuracy for PLA are at the temperature of 200˚C.

An important influence on the dimensional accuracy is the calibration of the table, respectively the working temperature of the table, respectively bringing the 3D print head to zero. In this paper, it was established the accuracy of the 3D printed parts on, all 3 axis, because can influence the assemblies modular devices, and respectively the drilling manufacturing processes of the metallic parts.

It is also recommended that during the 3D printing of the parts used in the industry, they should be made using supports, in order to present a good quality of the surfaces of the processed part, of the dimensional precision and not to reduce the part dimensionally on the OZ axis. The print head must not press into the mass, because there is a risk of missing 1 - 2 layers of deposition, in addition, mass tensions may occur.

Filament deposition takes place in a zigzag pattern to reduce auxiliary times. In Figure 5 , it is noted that the dimensional accuracy is low around 6.6 µm, because the print head presses on the mass, is not well adjusted and positioned at the zero position, and the mass is not well calibrated.

Figure 7 shows the dimensional accuracy variation on the OZ axis in function of PLA melting temperature, in the case of the ZMorph 3D printer.

To increase the mechanical strength of the PLA, reinforcements can be made with various metal or wood grains.

The prototypes for the gearbox parts were made according to the dimensions existing on the execution drawings.

The parts were designed in Inventor, SolidWorks and saved stl as in Figure 8 .

The choice of the parameters of the 3D printing regime is made using the Voxelizer software, as in Figure 9 . The accuracy of the 3D printed parts (the gearbox and the flange) is very important to realize the modular device assemblies for drilling process, to permit a precise orientation and fixation of the parts in the device, that influence the next manufacturing processes.

Figure 10 and Figure 11 show the 3D prints of the prototype of the gearbox, respectively of the flange, using PLA filaments.

Modular devices use model parts made of plastic mass to allow modular elements to be assembled to build them, being used in drilling machine building, for precise metallic prototypes manufacturing. These model parts greatly facilitate the construction of modular devices, but the dimensional accuracy of the model parts is very important for the choice of orientation and fixing elements of the devices.

The parts are designed in the 3D Inventor software, the optimal orientation and fixation scheme is established, orientation errors, fixation forces and fixation errors are calculated. Modular devices are designed using steps. Afterwards, the explode is made to visualize the component elements for assembly.

In this case, two modular devices were made, for the operation of drilling the Gearbox, respectively the Flange, as in Figure 12 and Figure 13 .