Manufacturing process development for engine construction parts from composite materials by 3D printing

This paper represents the development of the general design algorithm for structures of parts made of composite materials in respect of the additive manufacturing technology. Information on the 3D printing technology by extrusion is provided. Main design stages for production tooling for the “Insert” part, with regard of possibilities and limitations of printing on 3D printers, are described. Roughness analysis of samples, manufactured on the 3D printer from various composite materials, has been carried out.


Introduction
Additive manufacturing (AM) is a technology that keeps winning admiration and interest among specialists.A look at its potential makes many experts predict that it will become a real revolution in the field of product development and manufacturing.Some experts even state that conventional manufacturing methods known to us today will become obsolete and make way to additive technologies.
It is reasonable to use 3D printing with plastic for production of shape-generating molding tools.The FDM (Fused deposition modeling) technology, in which the 3D printer extrudes, or pushes out the material from its nozzle on the printing platform, is suitable for this purpose.The nozzle moves along a trajectory set by a digital model, and prints layer by layer.
One of material types, used at 3D printing by FDM, is represented by composite plastic materials, combining several components.The advantage of using layered composite materials is the possibility to create lightweight and durable parts with double and single curvature with the use of relatively simple production processes.Besides, replacement of metal tooling and parts to those made of composite allows extending the service life without losing initial specifications, and provides a possibility of this material processing (return raw material) with its repeat production and use in other structures.
Workpiece preparation is an important design stage, at which requirements to surfaces and their mutual arrangement are set with regard of capabilities and limitations of 3D printers.The features of layer-by-layer growing at the use of FDM technology are crucial in achieving the required surface quality.During FDM printing, various physical processes occur in material, that may affect printing quality.Parameters affecting the printing results are given on Figure 1.One of important factors to be taken into account at FDM printing is the printing temperature.Different materials require he certain temperature to achieve the optimal surface quality.Incorrect temperature can cause deformation or insufficient layer adhesion.In addition, the printing rate also has its effect on surface quality.Too high rate can result in layer heterogeneity, and too low rate can result in material overheating and appearance of defects.The second important parameter is the extruder setting.Extruder pressure should be set correctly with the purpose to provide even material application.Incorrect setting can result in layer heterogeneity and incorrect adhesion.In addition, selecting suitable material is also important for achieving the high surface quality.Different materials have different properties, and some of them can provide smoother surface compared to the others.It should be noted that the surface quality also depends on printing resolution.Higher resolution allows obtaining more detailed and smooth surfaces; however, it may require more time for printing.One of the main limitations of FDM printing is the need to use auxiliary structures when printing inclinedх surfaces.It is related to the "ladder" effect that occurs due to the shift of material layers in relation to each other.A surface becomes step-shaped and has the higher roughness, which decreases the part quality and manufacturing accuracy.The minimal incline angle for manufacturing nonsupported structures is defined in the range from 20° to 56° depending on the material and processing parameters.At the surface inclination angle increase, surface waviness and roughness increase as well, and the layer incomplete fusion defect occurs.To receive the high-quality surface without support, it is recommended to limit the inclination angle in the range from 53° to 56°.
With regard of the above, models of parts with the developed spatial geometry that have the adapted structure for manufacturing capabilities and limitations of 3D printers require special attention at design.They have specific features and recommendations that will help reaching the highest quality and efficiency of printing. 1.
Define the method of the model arrangement on the printing table.Surfaces located directly on the printing table will have smoother finishing, in distinction from surfaces, located above supports.This should be taken into account when arranging the parts on the printing table. 2.
Try to minimize the use of supports.3D printers are not capable of printing in the air, so large overhangs require supporting structures.However, with the purpose to save time and material and to improve surface quality, it is reasonable to design a model on such a way that will allow minimizing the use of supports.
3. At model design, round off junctions of surfaces with a radius or a bevel.It will contribute to improving durability and aesthetic appearance of parts. 4.
At model walls modelling, take into account the nozzle diameter and the minimal printing width. 5.
Round holes, printed vertically, may be not perfectly round.To achieve the best result, it is recommended to arrange round holes horizontally.
6. Take into account the direction of load on the printed part.Printing durability in the direction, parallel to layers, can be less than in the direction, perpendicular to layers.Therefore, if a part should withstand the specific loads, this should be taken into account at design.

7.
At modelling if parts that should be interconnected, take into account the tolerance or the gap between them.Gaps are required to provide the possibility of parts connection.The optimal gap value depends on the model size, orientation, material used, and other factors.The gap within 0.15-0.2mm is usually recommended. 8.
If required, a model can be divided into several parts, which will be placed on the printing table.This will allow overcoming the printing table dimensional limitations and to increase the printing quality.9.
At modelling of parts with developed spatial geometry, take into account manufacturing capabilities and limitations of 3D printers.Recommendations and limitations, illustrated on Figures 2-7, may serve as useful guidelines when designing and printing such models.

Design and manufacturing
As an example, with the purpose to optimize the technology of manufacturing shape-generating molding tools from composite materials, we will choose the "Insert" part.The "Insert" part is a component of the NK-32-2 engine combustion chamber structure, and is designed for creating reverse currents with the purpose to decrease the flame rate for more efficient engine operation.
Design of shape-generating molding tools of the "Insert" part starts from building the part and die 3D models (Figure 8) in Siemens NX software.With the purpose to create production tooling (internal tool), we performed the part and die deduction from the parallelepiped.Therefore, we obtained a 3D model of the "Insert" part production tooling (Figure 9).We chose the 3D printer PICASO Designer X S2 as the equipment for manufacturing the "Insert" part production tooling.
To reproduce the "Insert" part production tooling model on the 3D printer, it should be translated to the g code (instruction, understandable for the 3D printer).Therefore, the 3D model, created in Siemens NX, should be saved in STL format for the subsequent file preparation to printing.The model slicing was performed in PrusaSlicer software, according to the above recommendations.Arranged models of the "Insert" part production tooling in the building platform, and the "cut" 3D model layers are illustrated on Figure 10.With the purpose to manufacture the certain product or part from composite material with the best properties, we should select the optimal material and printing modes.To define the most suitable material and the optimal printing mode, we carried out experiments, in the course of which we grew samples from different types of composite materials, at different extrusion temperature values.The surface roughness was defined under the printed samples.
The following parameters were constant when growing samples: ➢ material filling density (30%); ➢ nozzle size (0.5 mm); ➢ blow-off (0%); ➢ printing rate (45 mm/s).Surface roughness is an important aspect in modelling, and it can be controlled through growing methods and layer thickness.However, sometimes the surface of grown models of insufficient quality, which is expressed in high roughness.This may create the need in lasting manual finishing, especially in the case of complex-profile surfaces.Figure 11 illustrates the roughness measurement process, where each sample is measured in two directions.This allows obtaining the comprehensive view of surface characteristics and to determine defects.Analysis of Table 1 data demonstrated that various materials have various roughness.The least roughness by the Ra parameter was demonstrated by the sample from ForMAX composite material at the nozzle temperature of 270°С.

Conclusion
In the first section of this paper, we developed the general design algorithm for structures of parts made of composite materials in respect of the additive manufacturing technology.
In the second section, we designed the production tooling for the "Insert" part, with regard of possibilities and limitations of printing in 3D printers.In addition, we grew and measured tests samples from three composite materials for the purpose of roughness analysis.

Figure 1 .
Figure 1.Physical processes occurring in material in the process of FDM printing.

Figure 7 .
Figure 7. Building the model with holes

Figure 9 .
Figure 9. 3D model of the "Insert" part production tooling

Figure 10 .
Figure 10.Arrangement and cutting layers of the "Insert" part production tooling3D model

Table 1 .
Data given in Table1demonstrates the results of sample roughness measurements under the Ra parameter.Sample roughness values, measured in longitudinal and cross directions.