Effective, versatile and inexpensive extruder system for direct ink writing of high-viscosity pastes

The software used for designing the extruder was Autodesk Inventor Professional 2015. A commercial FDM 3D printer, specifically the Prusa MK3S + printer (Prusa Research, Czech Republic), was used as the starting point. The first step involved printing most of the structural parts of the designed extruder using different filaments. 3D-printed structural parts included the extruder holder, gears and the nut holder and rod brackets of the linear guide system. These parts were printed using polyethylene terephthalate glycol (PETG) filament (Prusament PETG Prusa, Prusa Research, Czech Republic). The plungers of the 3 mL, 10 mL and 30 mL syringes were 3D-printed using Innovatefil® polyamide (PA) and carbon fibre (CF) filament (Smart Materials 3D, Spain). Relevant FDM printing parameters for both PETG and PA/CF parts are collected in Table 1. STP and STL files of the different extruder parts are available at the institutional open-access repository of the Spanish National Research Council, DIGITAL.CSIC [16]. The inventory of components required for the construction of the DIW-modified FDM printer, including their cost and the supplier from which they were purchased, is detailed in Table 2.

The components used in the formulation of the printing pastes included Ni particles APS 2.2–3 µm (99.9% metal basis, from Thermo Scientific Chemicals), the triblock copolymer Pluronic F-127 and hydroxyapatite powder from Merck, which was used similarly elsewhere [3, 5], and a dehydrated whey powder from Quesería La Fuente S.A.U. (Spain). Characteristics of the whey powder are detailed elsewhere [17]. The inks were prepared by physically mixing all their components using a centrifugal mixer SpeedMixer® DAC 150.1 FVZ-K (FlackTek, USA), at 2000 rpm for 90 s. For the Ni-Pluronic paste, additional cooling was required during the mixing process. Rheological characterisation was conducted in a Haake MARS II (ThermoScientific) rotational rheometer equipped with a rotary plate of 20 mm in diameter over a fixed parallel base. Small-amplitude oscillatory shear (SAOS) tests, specifically stress sweep experiments, were carried out to determine the yield stress (σ_y) and crossover point of the pastes.

The design proposed in this work is intended to extrude pastes stored on 3 to 30 mL syringes, thus making the DIW system versatile enough to cover most of the demands in fundamental materials research. The extruder comprises a trapezoidal threaded spindle connected to a stepper motor through a rigid coupler, which converts the circular motion of the motor into linear motion. The key elements of this mechanism are described in Fig. 1. Furthermore, the system has a linear guiding system, which effectively prevents any bending of the assembly, thereby significantly improving the precision of the extrusion movement. Additionally, it is integrated with a gear reduction system, carefully designed to enhance the motor torque, thereby ensuring the successful extrusion of high-viscosity pastes. The use of PETG for the manufacture of most of the extruder parts (except the syringe plungers, made of PA-CF, Section 3.4) is justified on the basis of its ease of print (no printing enclosure required; good layer adhesion), non-toxicity and superior resistance to moisture and thermal deflection temperature compared to PLA (66 °C versus 55 °C). This would allow the use of the extruder in a controlled temperature and humidity environment if the application demands it, i.e. when printing pastes with high solid contents, or when printing cell cultures. On the other hand, the use of technical thermoplastics (ASA, ABS, PA), or those reinforced with glass or carbon fibres, instead of PETG could not be justified in terms of enhanced mechanical properties of the resulting parts, and was ruled out due to operational and cost considerations (PETG is much cheaper).

While ball spindles offer superior performance than trapezoidal ones, the latter are cheaper and can be successfully used when the duty cycles do not exceed 30% of the maximum load that the spindle can bear safely and high positioning speeds are not required. Furthermore, trapezoidal spindles are self-locking, which, in this particular design, means that the nut (Fig. 1) will not move with the reaction axial forces that would push it up during the extrusion process. As a consequence, there is no need for brake systems to secure the plunger position while printing the ink.

The extrusion speed is influenced by the movement of the plunger, its diameter and the diameter of the nozzle used for printing. In turn, the diameter of the plunger will be equal to the inner diameter of the syringe used as the ink container. Finally, it should be noted that the extrusion speed is equal to the printing speed (or axial speed); so for each case, the calculation of the required speed of the plunger will obey the following equation:where \({v}_{plunger}\) and \({v}_{printing}\) are the plunger and printing speeds, respectively, and \({d}_{nozzle}\) and \({d}_{plunger}\) the nozzle and plunger diameters, respectively.

The designed mechanism (Fig. 1) must guarantee that the motor delivers sufficient force towards the plunger to overcome the resistance of the paste to flow through the nozzle. The aim of our design was to achieve a printer head to extrude pastes with a wide range of storage modulus (G′) 10³–10⁷ Pa. In our previous work [18], an axial load of 0.14 kN was determined experimentally (in a mechanical testing equipment) for extruding Ni-Pluronic pastes (solids content of 77 wt%), stored in 3 mL syringes (\({d}_{plunger}\) = 8.89 mm), through 0.61 mm tapered tips (\({d}_{nozzle}\) = 0.61 mm). The axial force measured, when converted to stress values, corresponds to 2.26 MPa, which will be taken as a reference for the extruder dimensioning for different syringe diameters.

The dimensioning procedure of the spindle-nut system consisted of the following steps; all values, calculations and equations used are summarised in Tables 3 and 4:

Based on the previous calculations, the chosen motor must generate a torque of 15.2 N·cm at a maximum angular velocity (i.e. spindle turning speed, Table 3, Eq. 2) of 3.5 rpm. In addition to these requirements, there are other factors to consider. Before selecting the motor, it is important to determine the amperage and voltage provided by the controller that is soldered onto the board of the 3D printer. The PRUSA MK3S + printer has a board with Trinamic 2130 drivers, which operate within a voltage range of 5–46 V and can deliver up to 2 A if they are equipped with an appropriate heat sink [19]. Furthermore, it is necessary to establish the maximum dimensions of the motor. The outer dimensions of stepper motors adhere to the standards set by the National Electrical Manufacturers Association (NEMA) in the USA, with various sizes available, such as NEMA 14, 17 and 23. Since the extruder system needs to be lightweight and compact, a NEMA 17 motor was chosen, specifically the model 42SH47-4A. Table 5 summarises its key technical characteristics [20].

The selected stepper motor is capable of delivering a torque significantly higher than the required torque for extruding the tested Ni-Pluronic pastes stored in a 3 mL syringe (44 N·cm >  > 15.2 N·cm). In other words, it is able to apply extrusion forces of up to 400 N. Although the motor is capable of operating with a current of up to 1.68 A, the current limiter of the controller is set by default to restrict the current flow to 0.8 A. This value will be used to calculate the maximum motor speed (Table 6), which comfortably exceeds the application needs (93.8 rpm >  > 3.53 rpm).

One of the objectives of this work was to design an extruder capable of working with different syringe volumes. To maintain the extruder within suitable dimensions, syringe sizes of 10 mL and 30 mL were selected in addition to the previously mentioned 3 mL syringe. Changing the syringe diameter (hence \({d}_{plunger}\)) affects the values of \({v}_{plunger}\), F_a, R and T_a (Table 7). The values of T_a for 10 mL and 30 mL syringe volumes exceed the maximum torque provided by the NEMA 17 stepper motor selected (Table 6), thus requiring a reduction system to increase the output torque.

Among the available alternatives of reduction systems, the use of Chevron-type double helical gears was selected. This type of gear eliminates axial loads, thereby obviating the need for bearings. Moreover, it yields lower noise levels than straight-toothed gears. A gear system with a transmission ratio of 1:6.25 was designed, meaning that the output shaft will rotate 6.25 times slower than the input shaft. Since the transmitted power must remain constant, the gear system will deliver 6.25 times the input torque provided by the motor (i.e. ca. 280 N·cm). This way, the reduction system would allow the extrusion of ink contained in any of the 3 syringe volumes used, as the total torque would be significantly higher than the required torque T_a (Table 7). It was necessary to use a gear train consisting of two identical stages vertically arranged instead of a single pair of gears, in order to reduce the diameter of the reduction system and make it more compact. Otherwise, a significant reduction of the available printing area would occur. Also, the requirement of the compactness of the reduction system conditioned that the gear coupled to the motor (gear 1, Fig. 2) had only 16 teeth. Based on the number of teeth of this gear and the required rotational speed of the output shaft, the number of teeth for the remaining gears and their respective rotational speeds were calculated, and the results are shown in Fig. 2.

The gear attachment to the motor shaft was achieved using an insert square nut within the gear body and a fixing stud that applies pressure against it (Fig. 3a). For gears 2 and 4, two nuts and their corresponding set screws were utilised. Since the insert square nut needs to fit inside the gear body, the flank width (b) should be greater than 5.7 mm. Additionally, the tooth gap (h) should be larger than the hole required to accommodate the fixing stud with a diameter of 3 mm. As detailed in the experimental part, the gears were 3D-printed with the FDM head. Figure 3b shows some details of the printing conditions.

A linear guide system was designed (Fig. 4a) to ensure that the plunger of the syringe remains concentric with the syringe throughout its travel. If the plunger and syringe are not entirely parallel and concentric, this would generate higher bending stresses in the extruder holder (Section 3.3) and increase friction losses during the plunger movement. The linear guide system (Fig. 4a) consists of a pair of 8 mm diameter precision-ground steel rods, and two LM8UU linear ball bearings. The piece (nut holder, Fig. 4b) that serves as a coupling between the nut and the plunger is securely attached to these rods using two cable ties. The nut holder also features a groove where the plunger of the syringe is inserted. The nut is ultimately fastened to the nut holder using three M3 × 16 ISO 7380 screws and three threaded inserts. The two steel rods are connected to the front frame (see Section 3.3, Fig. 5b) via two small-assembled pieces secured with screws (rod brackets, Fig. 4c). The same steel rods are inserted into two holes present in the lower end of the front frame (Fig. 5b). Both nut holder and rod brackets were 3D-printed as detailed in 2.1 section.

The as-designed extrusion system needs a lightweight holder structure to integrate all the components into a printer head that will be vertically mounted on the X-axis carriage of the printer. The structure was fabricated in multiple pieces using 3D-printed PTEG parts (see materials and methods), assembled together using threaded inserts (Fig. 5). PTEG was chosen for its low density (1.27 g/cm³), less than half that of aluminium (2.7 g/cm³). In addition, working with PTEG allowed us the on-site production of the different parts through 3D printing, facilitating successive redesign stages to improve the performance of the extrusion system until reaching the design presented here (Fig. 5). Furthermore, another significant advantage of 3D printing as a manufacturing method is the possibility of selecting the internal infill of the printed parts, using only the material required to support the loads acting upon them. As a reminder, it should be pointed out that 3D-printed materials are highly anisotropic, similar to layered materials. Thus, the design of structural parts must always consider the orientation of the printed stacked layers with respect to the applied load, as loads parallel to the layer stacking plane may cause delamination.

Description of the different components that, when assembled, make up the extrusion holder system is as follows:

As mentioned before, the dispensing system should be able to accommodate syringes of three different volumes: 3 mL, 10 mL and 30 mL. To achieve this, a series of interchangeable parts were designed to be used depending on the selected syringe:

Throughout the design phase, a balance was sought between the compactness of the assembly and the component accessibility. Three objectives were prioritised:

To achieve these objectives, all the parts were assembled using small-sized ISO 7380 and DIN 912 screws, threaded inserts and M3 square nuts. This enables the detachable connection of the entire assembly. The elements used in the assembly are also included in Table 2.

Figure 7 illustrates a 3D model of the fully assembled extruder system (Fig. 7a), along with photographs of the actual parts and the assembly process: unassembled PTEG parts (Fig. 7b), assembled extruder without the syringe (Fig. 7c) and fully assembled extruder coupled to the 3D printer (Fig. 7d).

Figure 8a depicts a screenshot of the Prusa MK3S + electronics with the modified elements for connecting the new DIW printer head (highlighted in red). Initially, the connection of the extruder to the Prusa MK3S + was accomplished by simply replacing the connection of the E-axis filament extruder motor of the FDM printer with the connection of the stepper motor of the new DIW extruder. For this latter connection, a conventional 4-terminal female header was used, equivalent to the one utilised by the Prusa MK3S + printer (Fig. 8b).

Furthermore, since the new extruder operates at room temperature, it was necessary to disable the filament extruder thermistor in the original Prusa FDM printer. This was achieved by installing a 5.6 kΩ resistor (Fig. 8c).

Due to the larger dimensions of the DIW extruder system compared to the original FDM extruder of the printer, there is a reduction of the maximum print volume from 250 × 210 × 210 to 220 × 175 × 70 mm. To update this information in the printer, the dimensions of the printing area in the slicer software need to be modified only by adding a Y-axis offset. This could be achieved easily by using code through the serial port of the printer rather than modifying the printer firmware. Specifically, we will input the command “M206 Y-35,” followed by “M500” to save this information in the EEPROM, ensuring that this configuration is permanently retained even when the printer is switched off. To verify if the entered value is active, the command “M503” can be used. By following these steps, the printer will be correctly configured to accommodate the new DIW extruder, and the modified print volume will be used effectively for future printing tasks. Once the FDM printer is configured to work with the extruder, additional commands “M302,” “M83” and “G91” will be required to “set cold filament extrusion,” “use of relative coordinates for the extruder” and “use of relative coordinates in the XYZ positioning.” Command G1 Ex Fy should be used to accommodate the plunger in the syringe, with x and y being the extrusion length (in mm) and motor speed, respectively. Finally, the nozzle and sensor positioning are set by using the “G28W” command, to avoid crushing the nozzle on the printing surface.

According to economic studies reported by other authors [11], commercial DIW extrusion devices with similar functionalities to the one presented in this work would have a price range between 800€ and 2000€. Excluding the FDM printer value, the manufacturing cost of the new printer head is ca. 100€ (Table 2), reducing substantially the budget compared to commercial DIW extruder systems.

Three inks were chosen to evaluate the performance of the new DIW extruder. Their main characteristics are summarised in Table 8.

The chosen inks are representative of the printing range capabilities of the new DIW extruder, based on their rheological characteristics. The whey ink corresponds to an example of a viscous paste. The yield stress of the paste is 1110 Pa, which is relatively high but comparable to other inks found in the literature, such as ceramic (1500 Pa) [21, 22], graphene oxide (2300 Pa) [8] or skimmed milk powder (2000 Pa) inks [23]. The crossover point gives an idea of the difficulty of extruding the paste through small nozzles, with the whey ink being at the upper limit of the ink printability range (< 2500 Pa) [24]. On the other hand, the crossover point of the Ni-Pluronic ink falls within average values and corresponds to an example of an ink optimised for printing in DIW devices. Finally, Pluronic is an example of a very-low-viscosity ink that is commonly used as a binder in multiple materials and biomaterial formulations.

In all cases, the performance of the extruder was excellent independently of the syringe size used (Figs. 9 and 10). Extrusion of whey was used to assess the material retraction capability of the extruder, since the whey ink is a highly elastic paste. Results were incredibly good although some retraction issues were found when operating with the 30 mL syringe. It was observed that at certain points where the DIW extruder movement stopped, the applied retraction was not sufficient to contain the dripping inertia of the material. This can be seen in Fig. 9b in the form of small droplets scattered on the external surface of the printed structure. However, this issue was easily corrected by increasing the retraction value in the slicer. When comparing the performance with a commercial ceramic printer using the same ink [17], the present DIW extruder demonstrated superior control of flow and retraction. In the case of printing with Ni-Pluronic or Pluronic inks (Fig. 10), no defects related to either retraction or flow control were observed.

The current extruder has been used to print 3D structures for chemical process intensification such as stirrers (Fig. 9a and c), catalysts/catalyst supports (Fig. 9b and d) or 3D electrodes for energy storage (Fig. 10a) and bone scaffolds (HAP, Table 9). These are only examples of the versatility this novel extruder offers in terms of the range of materials that can be printed for different purposes.

Finally, to highlight the quality of the printed parts and operational advantages of our current extruder, a comparison with similar literature has been provided in Table 9. To do so, the main features of similar design and capabilities approaches as highlighted in the paper, namely ink viscosity, extruding volumes, retraction control and cost, are considered together with examples of their printed objects. By the nature of the materials shown in the first half of Table 9, it is easily appreciated that the extruders developed in [11‐14] were only intended for working with low-viscosity inks. El Mesbahi et al. [15] proposed a new extruder for 3D printing of large ceramic parts (high-viscosity inks), but the publication does not provide details of any of the mentioned features, apart from allowing the use of multiple syringes with different volumes; hence, it has not been included in the comparison. Table 9 also includes examples of extrusion systems developed for printing high-viscosity inks [23‐25]. Comparison with the work of Roopavath et al. [27] is especially relevant as in terms of piece quality, considering that the authors used a commercial (5000 €) 3D printer from Cellink AB.