Development of affordable 3D food printer with an exchangeable syringe-pump mechanism

Graphical abstract


Hardware description
This article describes development of a syringe pump mechanism (SPM) that converts extrusion-based FDM desktop 3D printer that is capable of extruding 1.75 mm thermo-plastic filaments, to a 3D food printer. At present, there are open source initiatives and companies offering cost-effective modifications to syringe pump extruders that can be used with inexpensive 3D printers. The PrintrBot Food & Paste Extruder is an example of such an option that is low-cost. However, it was specifically designed for the PrintrBot Simple Metal and requires rewiring during installation, which makes it difficult to adapt to other systems. The Replistruder by TJ Hinton is another syringe-based extruder that is custom-designed and open source, and has been utilized to demonstrate the printing of mechanically robust, biomimetic structures with high accuracy. However, it also has volume limitations and can only accommodate up to a 10 ml syringe as its ink reservoir. Although some open source designs, like the ''Universal Paste Extruder" by RichRap, have been made available online for retrofitting 3D thermoplastic printers, these designs are often constrained by small build volumes or the inability to retract during printing [8].
The SPM was developed to present cheaper option of food printing 3D devices in scientific researches. As it compared with commercial 3D food printers, its cost is more affordable [6]. The main mechanism costs approximately 22$. If cooling option attached, the cost increases to 72$. For this study, the authors obtained a secondhand desktop 3D printer with less than 200$ so that total cost is not more than 300$. If a brand-new 3D printer is purchased, price range is between 200 and 500$ for a suitable printer [14]. At this point cost of total device is not more than 600$ which is nearly half of the cheapest option of 3D food printers [6]. Also the cost of main mechanism without cooling apparatus is nearly half of the open source syringe mechanisms with similar function [8,10].
Researchers needing to 3D print food materials, pastes or similar fluid materials can benefit from the SPM. The mechanism is made up of two major parts: main component and an optional cooling apparatus. The main functional component of SPM consists of 23 parts in which 16 parts are 3D printable that makes it affordable and versatile. Since 3D printer materials are abundant, remaining parts are easily found items that include one trapezoidal lead screw and two brass nuts, luerlock [15] syringe and three bearings. Auxiliary items are standard hardware such as M3/M5 bolts and nuts, and luer-lock needles. SPM can also be extended with attachment of cooling mechanism that comprised of aluminum cooling block, polyurethane tubing, tubing insulator, four 3D printed parts and recycled aluminum heatsink and fan from an old pc. Hence, there is the possibility of using waste material, the proposed hardware helps to recover unused components from old devices.
The SPM relies on a guide carriage that mounted on a linear rail, which exists in some of regular 3D printers. It is a directdrive system like some of its counterparts [13,16]. The authors studied on two other different designs (unpublished work) that available in common 3D printing forums ( [17,18]) and adapted them to their desktop printer (Fig. 1). In these prelim-inary experiments, it is realized that total weight force of the mechanism, step motor and filled syringe was outside of linear rails that causes unstable pressure on the carriage and negatively effects movement which results in misplaced and uneven layers. The present study overcomes this problem by moving heavy piece, the step motor onto the carriage. Also current mechanism is smaller than these designs and occupies less space. Even difference is 1-2 cm on XY axis; big difference is on Z-axis, which is more than 7 cm that offers more volume of printing area.
A typical user can easily disassemble the mechanism (SPM), and the polymer extruder of the conventional 3D printer can be replaced back. Like as other open source 3-D printers, this gives opportunity of using same device for food materials and for thermoplastic filaments [9,13,12].
Currently SPM is compatible with 60 ml standard syringe that is widely available. However, it can be adapted to smaller or bigger volumes only by modifying syringe holder and plunger parts (Fig. 2, parts 7-11). This attribute gives SPM an advantage over its counterparts. [8,12]. Syringes with the luer-lock attribute can accommodate various standard needle sizes that offered in the range of dimensional variations between 0.1 and 1.52 mm. Furthermore, SPM can work without needles so that working range is 0.1 mm to 1.6 mm.
Despite the extrusion with SPM conducted at room temperature, the proposed cooling apparatus allows the cooled air to be carried to the tip of the syringe. It provides an advantage over equivalent syringe pump mechanisms in terms of directly reaching the tip of the syringe [8,9,11,13,12,19]. With this attribute, the layer stability of prints might be improved for some food materials whose viscosity increases under cold conditions.
There are many different types of food or bioprinters on the literature and they generally operate at low speeds [20,21]. Since the mechanism we proposed is mounted on a linear rail guide and its center of gravity is designed to be on the rail, it can operate at relatively higher speeds than other printers. In addition, many syringe mechanisms are generally developed to fit Ender or Prusa models but cannot be used with linear rail guide mechanisms [8,10,13,12]. Our system, on the other hand, is designed to achieve smoother motion with linear rail slides. Moreover, the SPM distinguishes from its counterparts thanks to its modular design. Modular design allows a heating pad can be attached to the syringe body, and by changing some parts of SPM, different syringe size could be used.
In summary, the offered syringe pump hardware has potential benefits in terms of undermentioned subjects especially for the researchers of food science; Introduced mechanism is cheap, open source, lightweight, and produced to be simple to modify. The SPM is exchanged with the extruder block of 3D printers and it uses stepper motor to drive the syringe plunger and relies on linear rails. So that, the mechanism operates similarly to conventional FDM polymer printers, making users with desktop 3D printers able to use it. It has relatively bigger size; 60 ml volume capacity that is sufficient for studies in the area of food research laboratories and can extrude in diameter range of 0.6 mm to 1.6 mm which results better layer stabilities. Syringe-pump mechanism can be used in standard 3D food printing tasks with relatively high speeds (more than 50 mm/ s).

Design files summary
Complete syringe pump mechanism relies mostly on 3D printed parts. Whole mechanism and all singular 3D-printing files were designed online in Tinkercad (Autodesk, Inc, USA). Related link of design file (CAD file) has been given in the table below (Table 1). Each part name has leading number which corresponds their short/part name at build instructions section and in Fig. 2 and Fig. 3. The photo (Fig. 2) has been taken in position that each part were close to its connecting part.  [17], B was from [18] and C is the current design.  Semi-open duct part is used to arrange air tubing, to prevent entanglement. During implementation of tubing, it can be bended or stretched out. It is better that this part printed with high tensile strength filaments like PETG.
14-CableHolderDuct (Fig. 2 no.14): Cable holder duct is also used for arrangement and preventing entanglement of cables. And same like part 13, it is advised that this part printed with PETG.

Bill of materials summary
Since most of the materials are 3d printed, in the BOM list ( Table 2) the quantities of each filament used are given in a single line. The amount of filament consumed and price information for each part are also given in separate list (Table 3). Although filaments are sold in certain grams, and some parts like bolts, nuts are sold with minimum amount, the prices stated here are given for producing single SPM mechanism. Price of cooling fan and heatsink (part 35) can be deducted from total cost because it can be obtained by recycling from an old pc. Moreover, prices for shipment and required hand tools are disregarded. If cooling part is used, there should be recirculating water chiller. Since proposed mechanism will be used in research laboratories, it is assumed that this apparatus exists in regular laboratories and price of chiller is also neglected.
All materials might be found online, at any business that specializes in 3D printers, or at any hardware store. Here a widespread online store link suggested for equivalent materials.

Build instructions
A Rigid3D Zero 2 (Rigid3D Corp., Türkiye) printer [22] with Titan Extruder [23] used in this experiment. A screwdriver, M3 and M1.4 Allen wrenches, M8 wrench and tweezers are required tools for the assembly. If cooling part is used, there also should be recirculating water chiller. Part numbers associated in this section, can be seen in Figure 2, 3 and 4, which are same in design files table (Table 1) and BOM table (Table 2). Supplementary video is provided for comprehensive step-by-step building instructions as well. Furthermore, Fig. 4 is self-explanatory for orientation and position of items.
Step 2: Change filament to PLA+ and print parts 2, 4, 5 and 7-10 with same settings (except temperature of extrusion, use recommended printing temperature from the filament supplier).

Removing filament extruder
Step 4: Shut down desktop 3D printer and unplug electricity cable. Disassemble the extruder and hotend parts of 3D desktop printer with M3 and M1.4 Allen wrenches. Only the extruder's step motor and linear guide carriage on the rail should remain in place of the extruder. Cables of the extruder fan will be used for the cooling fan. In case of an unexpected temperature rise, cover the heating cartridge with an insulator, such as teflon tape.
Part 20 is the important item of SPM that mounts the mechanism to the linear guide carriage. In this hardware, holes at the bottom of part 20 are for MGN12H carriage and connection of GT2 timing belt holder. If different carriage was used part 20 should be adapted for that carriage.

Preparing main components
Step 5: Mount part 20 to the carriage on the linear rails, with four pieces of M3 Â 6 mm screws. Attach it to the GT2 timing belt holder with two of M3 Â 12 mm screws.

Building the main body
Step 6: Insert part 2 into the 608ZZ bearing (part 25) and mount them into part 1.
Step 7: Insert 6800RS bearing (part 23) into part 1 and cover it with part 19. Slide part 19 until holes become concentric with bearing hole.
Step 8: Combine part 4 and part 22 and mount them onto the part 1 by inserting them into the part 19 hole. Orientation of part 22 should be long-side down. During joining part 4 and part 22, you may need four pieces of M3 Â 10 bolts and nuts, but they are not necessary since printed part (4) is close-fit. Step 9: Insert part 3 into the MR85-2RS bearing (part 24) and mount them into part 1 between part 2 and part 4 gears. Check for transmission, if they are connected.
Step 10: Insert two M3 nuts into the corresponding hex hole at the foot of part 1. Mount main body onto the part 20.

Preparing the step motor
Step 11: Insert two of M3 nuts into part 5. Mount part 5 onto the step motor with four pieces of M3 Â 8 mm screws.
Step motor is extruder's motor that obtained at step 4. Motor's cable position should be left-hand side, while positioning back of part 5 to yourself (see Fig. 4).

Preparing the main body bottom extension
Step 12: Part 6 has holes that matched with part 20 when placed under the foot of part 1. So that two holes at the bottom of part 6 is the place of part 9. Insert two of M3 nuts into part 9. Install part 9 onto the bottom side of part 6 with two pieces of M3 Â 12 mm screws. Orientation of part 9 is that triangle section should be downside.
Step 13: Attach part 10 to the part 9 from holes with two screws (M3 Â 8 mm and M3 Â 20 mm). No need to turn the screws, they are needed to hold parts.

Preparing the cable holder duct
Step 14: Attach part 17 and part 18 to the part 14 with two M3 Â 6 mm screws with hex socket head and nuts. See orientation and place of items in Fig. 2. Keep nuts outside. Use M1.4 Allen wrench, hold screw from the slit and insert nut.

Preparing the cooler tubing holder duct
Step 15: Attach part 15 and part 16 to the part 13 with two M3 Â 6 mm screws with hex socket head and nuts. See orientation and place of items in Figs. 2 and 4. Keep nuts outside. Use M1.4 Allen wrench, hold the screw from the slit and insert nut.

Preparing the plunger
Step 16: Insert two M3 nuts into the nut holder of part 11. Insert part 22 into the part 11 with two pieces of M3 Â 12 mm screws. See orientation of part 22 in Fig. 4.
Step 17: Remove the original plunger from syringe (part 31). Get the elastic tip of plunger and mount it on the part 11.
Step 18: Screw trapezoidal T8 rod (part 21) into the part 22 that attached to the part 11. Turn it to the end.
Step 19: Insert the newly made plunger into syringe (part 31).

Installing SPM
In this section, components that were prepared in ''Preparing main components" section are assembled onto the part 20.
Step 20: Mount main body (Steps 6-10) on the top of the part 20. Long-legged gear should face to back of printer (see Fig. 4).
Step 21: Attach main body bottom extension (Steps 12-13) to the part 20 with four pieces of M3 Â 10 mm screws and nuts.
Step Step 23: Take the cable duct component (Step 14) and tuck cables and cable-like pieces into the duct by stretching the slit edge of the duct. Position the duct to the left-hand side of main body (Steps 6-10), and move it up so that part 17 comes to edge of slope where holes with hex nuts (at step 10) become concentric. Use an M3 Â 15 mm screw and attach top of duct to that nut.
Step 24: Take the cooler tubing holder duct component (Step 15), position it on the right-hand side of main body (Steps 6-10) and move up like step 22, so that part 15 comes to edge of slope where holes with hex nuts (at step 10) become concentric. Use an M3 Â 15 mm screw and attach top of duct to that nut. See orientation of duct in Fig. 4.
Step 25: Add two of M3 hex nuts into part 7. Attach part 7 to main body (Steps 6-10) with two of M3 Â 15 mm screws. Insert screws from the backside by merging bottom hole of cable duct component (Step 14) and the tubing duct component (Step 15). Orientation of part 7 is that triangle section should be downside.
Step 27: Remove the plunger from the syringe (Steps [16][17][18][19] until it reaches the top of the syringe. Then unscrew T8 rod (part 21). Remove long screws from syringe holder parts (from part 7-8 and part [9][10] and open them like a gate. Install syringe (part 31) into part 7 and lock it with part 8 by reattaching long screw. Do same for the parts 9 and 10.
Step 28: Screw the T8 rod (part 21) from the top of the gear (part 4) passing through it and getting inside of plunger (Steps [16][17][18][19]. While screwing part 21 hold the plunger with your finger to let screw in. Turn the gears and check for plungers updown movement.

Building cooling apparatus
Step 29: Merge part 29 and part 30 by inserting legs of part 30 into corresponding slits on the part 29. Hang this component to the printer with M5 Â 30 mm bolt and nut from the hole of part 29.
Step 31: Attach aluminum block (part 34) to the inlet and outlet of a recirculating water chiller or similar equipment. Then put it into the part 27.
Step 32: Put part 28 onto the cooling fan and heatsink (part 35). Then mount part 27 that containing aluminum block (part 34) on it. The part 27 and part 28 should cover all sides of heatsink. Tighten with four of M4 nuts.
Step 33: Mount the part 26 with tubing (prepared at step 30) onto the cooling fan with four of M4 Â 10 mm bolts (In supplemented video we used adhesive tape and extended the way of air with some of cardboard). Put the resulted component onto the hanged part (prepared at step 29).
Step 34: Insert opening end of tubing into the part 12 then move tubing within the duct (Step 15) and attach part 12 to the tip of syringe (part 31).
Step 35: Connect cables that were removed from extruder's hotend fan (at step 4) to the pins of cooling fan.

Operation instructions
Follow these instructions to properly operate the 3D Food Printer: Prepare desired shape in a CAD program. Get.stl file and obtain g-code file by slicing it in a slicer program. Add related g-codes in the beginning of file to disable heating of bed and extruder, homing the mechanism. Plug the electricity cable, switch on the food printer. If using cooler apparatus, switch on the chiller. Remove plunger rod (Fig. 4B no.21) from syringe by turning gears (Fig. 4B no.4), take syringe and fill it with food (or other paste like) material. Set the flow rate to 8% and extrusion temperature below 20°C. Set filament diameter and layer height corresponding with size of the needle. Insert the syringe into the holder (Fig. 4B no.7) and lock both of the bottom and upper clamps (Fig. 4B no.8 and 10) with bolts. Turn gears (Fig. 4B no.4) counter-wise until plunger (Fig. 4B no.11) gets in the syringe. Hold the rod (Fig. 4B no.21) with your fingers during turning gears to lead movement of rod. Remove waste that comes out from the tip of syringe. Start printing process on the device or from the computer.

Safety precautions:
Be careful not to pinch your fingers between gears during operation. Since extrusion temperature set to below the room temperature, be careful about heating cartridge of previous printer as it exposed to air.

Possible drawbacks and solutions:
3D printers have a safety feature that prevents cold extrusion to avoid damaging extruder when there are issues with the hot end. In this case, add M302 S0 code to the beginning of g-code file, which permits cold extrusion. The extruder motor could be inverted in the default configuration, thus during operation it can be seen that the T8 rod moving on the opposite direction. Add M92 Z-400 command by providing negative step/mm values. Change step value (4 0 0) according to your printers setting. There may be issues with air flow rate of cooling apparatus. In this case, replace the fans with higher-powered ones.

Limitations:
The SPM is connected to a linear guide carriage in order to minimize vibrations during movement. In this case, the proposed system is not suitable for use in devices that do not have a linear rail carriage such as the common sling bed printers (Prusa [24] or Ender [25]). However, thanks to the modular structure of the proposed system, it can be adapted to this type of devices by changing/modifying only one part (Fig. 4A no.20) without changing the whole system. Either part of SPM could be modified or a linear rail upgrade could be done for the printers ( [26,27]).

Validation and characterization
The printing assessment of SPM was conducted by a factorial design. A hollow cylindrical shape was printed in size of (40 mm Â 40 mm Â 20 mm) and the factors including nozzle height (NH, 0 mm-3.30 mm), printing speed (PS, 20-80 mm/s) and flow compensation (FC, 90-100%) were inspected using full factorial design in our another study [28]. As a result of experimental design, optimum parameters were selected for NH 0 mm, PS 50 mm/s and FC 100%. As the rheological behavior influences the extrusion, printability evaluation conducted with three different food paste differing in their viscosity [29]. To validate the working of SPM and observe possible printing capabilities of different nozzles, an oleogel mixture (OG) prepared to assess 0.6 mm diameter nozzle, a mushroom fortified semi-solid food mixture (MF) for dysphagia patients prepared to assess 1.2 mm nozzle and a meat analogue (MA) prepared to assess 1.6 mm nozzle [30][31][32]. A flower structure and hollow circle were drawn in Tinkercad (Autodesk, Inc.) and exported as a.stl file and sliced with Ultimaker Cura 5.0 [33,34]. Printing of edible inks were conducted with settings in Table 4.
Resulting prints were analyzed with image processing programme, ImageJ, in order to accurately measure the perimeters and the gaps (Fig. 5) [35]. Each formulation was printed at least in triplicate and shape fidelity calculated with the equation below [36].
Shape fidelity ¼ Measured dimension Theoretical dimension Â 100 Hollow cylinders and flower shapes were successfully 3D-printed with three different food inks (Fig. 5). Shape fidelity values of diameters ranged between 99.3 and 100.7 %, heights ranged between 100.2 and 100.8% and accuracy of gap size ranged between 99.0 and 102.3 % (Table 5). Results showed that at high layer numbers, deviation of height accuracy increased, this could be related with rheological properties of food materials. However deviation of all dimensions were not higher than 3% which demonstrates that the performance of the SPM was satisfactory for the food materials. Flower shape were 3D-printed without retraction, on the other hand during printing of cylinders different retraction speeds were used. A filamentous structure could be observed between the skirt and the print body when no retraction was used however with relatively lower retraction speed OG sample accumulated and represented a ridge on the retraction point (Fig. 5.B).
In conclusion, an extrusion-based FDM desktop 3D-printer was modified by replacement of extruder part with our proposed mechanism, SPM. With the attachment of SPM, 3D-printer was converted to food printing capable laboratory scale 3D food printer and various shapes with different nozzle sizes and variable layers, were successfully printed. This research showed that the visual acceptance of paste-like foods can be increased by SPM-mounted 3D food printers, so that they can be a possible solution for malnutrition in patients. The SPM exposed that it could be reliable and inexpensive option for the laboratory size 3D food printers.