3.1 Design Considerations
In this paper, the proposed 3D printer employs a blending extrusion method, which differs from traditional filament-based 3D printers. Therefore, it is necessary to consider not only the structural sagging due to weight but also the extrusion characteristics of the extruder. Additionally, the design of the extruder must take into account factors such as the screw shape of the extruder, temperature control for material melting, control of bottlenecks occurring during material extrusion, and cooling devices to ensure material supply stability, all of which are crucial for output stability.[7]
The sagging of the extrusion unit is a primary consideration in the gantry structure, which is applied to most 3D printing methods. This sagging refers to the deformation amount of the frame to which the extrusion unit is attached. When both ends of the frame are fixed, the maximum deformation occurs at the middle of the frame, interacting with factors such as the nozzle size of the extrusion unit and the bed's levelness, leading to extrusion errors. Theoretically, the thickness of the extruded plastic resin should be uniform and adhere to the bed regardless of the direction in which the bed moves. However, if extrusion errors occur, variations in thickness and adhesion degree affect the overall quality. Therefore, it is necessary to verify the maximum sagging amount at each position of the extrusion unit through structural analysis in the design of 3D printers.[8]
3.2 System Appearance and Extruder Design
As shown in Fig. 1, the design was advanced to enable output with a maximum size of 1000mm x 1000mm x 700mm, and the frame was made of aluminum square tubes with a size of 50mm x 50mm. The movement of each axis was controlled by a step motor, and for the Z axis, four motors were installed at each corner to distribute the load. The bed was designed to be 1200mm x 1200mm, 200mm larger than the maximum size that can be output, to prevent deviation in output. For the outer frame, it was designed to be 1600mm x 1600mm with 200mm intervals on the left and right.[9]
For the extrusion frame, a 40mm x 80mm aluminum profile was used to ensure torsion and part installation space, and two LM guides were installed to ensure drive straightness.
The extrusion section can be roughly divided into a rotating section, extrusion screw, nozzle, etc., and a 35W DC motor with a 150:1 gearbox was used for extrusion to ensure a maximum torque of 450Nm, and the maximum rotation speed was 20rpm. The screw and motor are connected using a coupler, and a nozzle with a diameter of 5 mm is attached to enable the extrusion of a large amount of resin. The weight of the extrusion section is about 4.4 kgf, and a hopper is installed in the material input section so that the raw material can be supplied regularly.[10]
In the case of an extruder extrusion screw, shape design is important to ensure the material extrusion force. A single-axis screw extruder has low energy consumption, good quality, and stable operation. It also has low noise and a long life, making it suitable as an output screw for 3D printing work and small-lot production of a wide variety of products. In addition, as shown in Fig. 2, the distance between the blades (lead) is constant, which is a characteristic of the plastic injection screw, so the volume decreases as you go to the metering section, pressure is generated in the material, the depth becomes narrower, and a gradient is formed, which is advantageous for material extrusion.
The melting temperatures of different materials vary. If the temperature is set too high, the material may burn or undergo thermal decomposition, resulting in deformation and making it difficult to achieve the desired properties. If the temperature is too low, the material will not melt properly, leading to incomplete mixing, material clumping, or nozzle blockages, which can cause printer malfunction. To control this, individual temperature control devices were installed at both the barrel and nozzle sections. If heating devices are only installed at the nozzle tip, the material will be extruded without proper mixing. Conversely, if the system is only installed at the barrel section, the material will cool and block the nozzle during extrusion, so both sections were equipped with heating systems. This ensures that the material melts first in the barrel section and, through the rotation of the extruder, is mixed and then transported to the nozzle section for extrusion.[11]
In 3D printing, a bottleneck refers to the condition where the excessive supply of molten material prevents proper extrusion, causing the material to cool down due to the cooling fan and block the extruder, hindering proper extrusion. To prevent the molten material at the upper part of the nozzle from cooling and solidifying, a screw was additionally installed to rotate the molten material. This prevents the material from solidifying due to the cooling fan before extrusion, thus avoiding nozzle blockage and maintaining an appropriate temperature for stable extrusion onto the bed. The cooling fan at the front of the extruder is used to prevent layers from becoming messy when printing models with sharp ends, small products, or when the next layer is built while the previous layer hasn't properly solidified. Typically, the cooling fan of an FDM filament printer prevents heat from being transferred to the nozzle neck, but the cooling fan of the developed extruder lowers the temperature of the molten material at the top of the nozzle to a similar level as the nozzle temperature control, aiding in stable extrusion.
3.3 Structural Analysis
Table 3
Properties of Structural steel and Aluminum
| Elastic Modulus (GPa) | Density (kg/㎥) | Poisson’s Ratio |
Structural Steel | 210 | 7850 | 0.29 |
Aluminum | 70 | 2700 | 0.3 |
Structural analysis was conducted based on the designed extrusion frame and extruder shape. The structural analysis conditions, as shown in Fig. 4, involved fixing both ends of the assembled extrusion frame and extruder. The deflection amount due to self-weight was checked with the extruder positioned on the left, center, and right. The materials were specified as structural steel and aluminum according to the design conditions, and their properties are shown in Table 3. The analysis focused on the maximum deflection amount, considering the extruder positioned in the center of the frame. The analysis results, as shown in Fig. 5, indicated that the maximum stress and deflection amounts when the extruder was positioned on the left, center, and right were (101 MPa, 0.3 mm), (116 MPa, 0.489 mm), and (102 MPa, 0.299 mm), respectively. The maximum stress occurred at the extrusion frame joint and LM guide, but since the yield stress of structural steel and aluminum are 275 MPa and 170 MPa, respectively, it was deemed safe. Additionally, the deflection difference between the center and the left and right was found to be 0.18 mm, and since the height of one layer when using a 5 mm nozzle is 0.8 mm, it was considered safe as it does not exceed the layer height.
The pellet extruder system consists of a stepper motor, hopper, screw and barrel, and nozzle. To verify the design feasibility of the injection screw, FSI (Fluid-Structure Interaction) analysis was conducted. This analysis involves using a finite element approach to model the stress and deformation responses of the components to the pressure, temperature gradients, and specified boundary conditions of the surrounding fluid, indicating a fully coupled interaction between the fluid and the solid.
The analysis conditions are shown in Fig. 6, and the comparison was made between using the existing auger bit and the developed injection screw. The analysis results, as shown in Fig. 7, indicate that in the case of the auger bit, pressure does not occur in the transport section, resulting in only about 1.1 MPa of pressure at the outlet due to the nozzle shape. In contrast, with the injection screw, the pressure increases linearly towards the nozzle due to the gradient effect, and the pressure formed in the transport section is maintained to the nozzle chamber, resulting in a stable pressure of about 1.7 MPa at the nozzle tip.
3.4 System Fabrication
The fabrication process is divided into the following stages: manufacturing the external frame and printing bed, attaching motors and components for operation, installing limit switches, and configuring the circuitry. For the external frame, aluminum square frames and joints were cut and machined to the required specifications and assembled to complete the external frame. Subsequently, power supplies connected to the MCU, stepper motors, motor drivers, and limit switches were installed.
The proposed 3D printer system configuration, as shown in Fig. 8, utilizes a total of three axes: X, Y, and Z. Therefore, motor drivers and motors are needed to control each axis. The motor drivers controlling the motors are independently controlled for each axis. The external power supply uses a 24V power supply unit, but to supply power to seven motors simultaneously, individual power supplies were applied for each axis. For the Z-axis, the movement of four motors is synchronized, so two motors were connected to one motor driver.
For the pellet extruder, as shown in Fig. 9, the initial 24V power supply is used to set the temperatures of Control 1 and Control 2 (barrel and nozzle sections) to the melting temperature of the mixed material. Then, to prevent the stepper motor of the extruder from stalling, the stepper motor of the mixing screw is activated, causing the entire system to operate. Each operating system can be turned on or off offline, and the system is configured to immediately stop in case of an error. Each temperature control unit can be set individually, and the extruder and the left screw also have separate speed controls, allowing for suitable screw speed adjustment depending on the situation, making it an ideal system for controlling various products.
The fabricated extruder consists of a controller and an extruder. The controller can control the motor power, temperature, and rotation speed. When power is supplied to the controller and the target temperature is set, the extruder starts heating. After adding pellets to the hopper and setting the rotation speed, the extrusion drill rotates and extrudes the pellets. As shown in Fig. 10, a driving test confirmed that the pellets were extruded as the extrusion drill rotated.
Through final assembly, as shown in Fig. 11, a large 3D printer with dimensions of 1.6 × 1.6 × 1 m was produced. The assembly precision of each axis was checked using electronic measuring instruments, and it was confirmed that the maximum error was 0.3° for the X and Y axes, and ± 1 mm for the Z axis. This ensures a manufacturing precision with a maximum error of 5 mm for a print size of 1 m.