Determination of optimal 3D printing modes for composite parts for reaching the set design-engineering parameters

This paper discusses properties of various 3D printing materials, namely, plastics, including polyamides, along with composite or engineering plastics with inclusions of carbon fiber and fiberglass. We selected an optimal sample printing mode and carried out experiments, following which we determined maximal loads for test composite materials, and linear shrinkage. As the result, we determined an optimal 3D printing mode for various structures of composite material parts with the purpose to reach the set design-engineering parameters.


Introduction
Additive manufacturing (AM) is the process of creating objects by successive application of thin material layers based on the electronic 3D model.In distinction from conventional production methods, where materials are usually removed and cut from a workpiece, additive manufacturing allows adding material by one layer, which allows obtaining complex and exact geometrical shapes [1].
With the purpose to manufacture almost any product of prototype, the most reasonable and convenient additive manufacturing method will be 3D printing with plastic, namely, the FDM (Fused deposition modeling) technology.This technology is based on the principle of heated and extruded plastic [2].For object creation by FDM printing, a dedicated printed is used, which melts plastic filament and extrudes it into thin layers based on the 3D model.In other words, the FDM technology is based on successive building up and application of plastic material layers, which enables the gradual creation of a 3D object.This technology is widely applied for creation of functional prototypes, models, and finished products in various manufacturing sectors.

Material selection and sample orientation
With the purpose to provide the best properties of the manufactured product or part from plastic, optimal printing modes should be selected.Such modes are based on the certain criteria that should be met at the product printing.To define the optimal printing mode, special samples were made according to GOST 33693 -2015 (Figure 1).Further, they passed tensile test with the purpose to determine their mechanical properties; roughness and shrinkage were determined.The samples were made from three composite plastics: FORMAX is the engineering thermoplastic based on ABS with addition of carbon fiber up to 15%.It is resistant to severe loads and high temperatures.The main advantage of this plastic it its ease and stability at printing.It is also characterized by high durability and hardness.
PA ABS GF -8 consists of acrylonitrile butadiene styrene (ABS) and polyamide (PA6) mixture with addition of 8% fiberglass.This material is perfect for printing final products that require high impact and ultraviolet resistance.It is also inter to the most chemical solvents.
PA 66 GF -33 is the composite material from polyamide 66, mixed with fiberglass reinforcement by 33%.
The samples were grown from several composite material types in different directions on the 3D printer building platform: horizontally, vertically and at an angle of 45°, at various material extrusion temperatures.Table 1 with the indication of applied materials and the sample growing temperature is given below.All samples have the certain marking and the indication of the material from which the sample was printed, along with the index number, corresponding to the certain growing direction and temperature.These markings are given in Table 2.

Table 2. Sample markings of each target group
No.1 Horizontally parallel to the table No.2 Horizontally "edgewise" No.3 Vertically No.4 "Flatwise" at an angle of 45 degrees No.5 "Edgewise" at an angle of 45 degrees No. 6 Vertically at the decreased temperature No.7 Vertically at the increased temperature Sample arrangements on the printing table are illustrated below (Figure 2).Each sample set includes seven pieces, five of which were grown in five different directions, and the remaining two were grown vertically at the other temperatures, as only such samples will demonstrate exactly that the printing temperature change will rest in different interlayer adhesion, that will further have an effect on the product mechanical properties.The higher is the growing temperature, the higher is the interlayer adhesion, and therefore, samples will be "stronger" and harder to be toen by layers.However, due to other process features, the surface quality can deteriorate, and the printing mode can be affected [3].
In addition to all of the above, the growing temperature and the arrangement on the printing table will have their effect on the sample shrinkage.The higher is the material fusion temperature, the longer is the cooling time, and the larger is the material shrinkage.The closer is the sample to the building platform, the longer is the cooling time, and the larger is the material shrinkage.As the samples are placed at different angles on the building platform, and this platform is heated to the certain temperature, thermal effect on each sample group will be different.

Selection of printing parameters
Now we will provide insight of printing parameters, in which we were growing the test samples [4].
Sample growing parameters that remained constant: • nozzle diameter (0.5 mm); • layer height (0.25 mm); • blow-off: 0 %; • printing rates: are given on the Figure below, depending on the structure that has been printed, whether filling or external perimeter (Figure 3).

Figure 3. Printing rate parameters
The extrusion multiplier, also known as the extrusion factor, is equal to 1.In our case, the layer height at sample printing was set as 0.25 mm, which is an optimal value, as its increase causes the sample surface quality deterioration.The decrease of this parameter largely increases the printing time and causes the growing risk of stratification, as interlayer adhesion will be low.We chose the standard number of perimeters, 3 (Figure 4).The printing process is largely influenced by a supporting structure, required for support of the product inclined surfaces, which is further removed.Supports can be of several types, and should be selected depending on the product geometry and the material.We used 2 support types, organic and accurate.
By its shape, organic support resembles a tree, rather solid and stable at the base, and branching as it approaches a product (Figure 5, а).It is used only in the certain points, which allows its easy removal without damaging the surrounding parts and with the minimal material consumption.While such support does not provide optimal stability for horizontal flat ledges, it demonstrated high results at support at an angle or overhangs.
When choosing the "Accurate" support style, the support shape is changed to a rectangular rectilinear structure (Figure 5, б).This support type, consisting of vertical columns that contact with the entire overhang, ledge, or bridge, functions almost for each type of hanging surfaces.However, such a support is much harder to remove, and it causes the model surface damage much more often, moreover, material consumption is considerably higher.

Study of sample properties
Mechanical tests were carried out on the TIRAtest 28300 universal testing machine (Germany) (Figure 6).The samples were placed in hydraulic clamping devices and were stretched until destruction by moving the upper crossbar, while the lower crossbar remained immovable [5].   1 demonstrated that all the three sample types have a slight relative elongation, i.e., they are considerably brittle.The sample PA / ABS GF8 No.2 has the maximal elongation and the maximal breaking strength, the sample PA66 GF30 No.1 has the maximal yield stress, and the samples with FORMAX marking are the most brittle under the relative elongation parameter.
Therefore, samples made of PA 66 GF30 material appeared to be the most durable, that were oriented at the temperature of 280 degrees parallel to the printing table plane.Following the sample printing, the sample linear shrinkage was studied [6].
Reduction in material volume and size at its forming and cooling is known as shrinkage [7].Shrinkage is an important parameter of material properties which causes internal stresses.Therefore, material shrinkage should be taken into account at the 3D printing process development.
Linear shrinkage is the difference in size between the 3D model and the obtained product on the 3D printer.It is measured in percent of the initial model size.
With the purpose of linear shrinkage definition, we carried out measurements of sample linear dimensions, using the MK-25 outside micrometer with the scale length of 0.01 mm and acceptable error of ±0.004 mm.we measure two linear dimensions of samples, а and b, corresponding to 4 and 10 mm on the 3D model (Figure 7).Having analyzed the tables, we can make a conclusion, that in each sample group, the minimal shrinkage by two sizes was demonstrated by No.5 from FORMAX (1.2% and 0.9%), No.1 from PA ABS GF -8 (0.2% and 1.1%), and No.5 from PA 66 GF -30 (5.7% and 3.6%).From this, we can conclude that the minimal shrinkage was demonstrated by the PA ABS GF -8 sample (0.2% and 1.1%).

Conclusion
In the first section of this paper, we defined the general concept of the term "additive manufacturing", and reviewed the FDM printing technology, which was subsequently used by us for test samples manufacture.
In the second section, we selected composite materials, which were subsequently used by us for test samples manufacture, selected their arrangement on the 3D printer operating table, and set the printing temperatures.In addition, we applied marking on the samples.
In the third section, we selected and provided insight of printing parameters, in which we were growing the samples.
In the fourth section of this paper, we carried out experiments using the certain equipment, obtained and analyzed the experiment results.

Figure 1 .
Figure 1.View of a sample for determination of mechanical properties at elongation according to GOST 33693 -2015

Figure 4 .
Figure 4. Setting the layer height parameters and the number of perimeters

Figure 7 .
Figure 7. Linear dimensions of samples tested for shrinkage.

Table 1 .
Sample materials and temperature modes.

Table 3 .
Mechanical test results

Table 4 .
Results of FORMAX samples and the linear shrinkage value measurement

Table 5 .
Results of PA ABS GF -8 samples and the linear shrinkage value measurement

Table 6 .
Results of PA 66 GF -30 samples and the linear shrinkage value measurement