Design, Construction, and Validation of a VAT Top-Down Printer for Ceramic Additive Manufacturing

Abstract

Additive manufacturing (AM) of ceramics by vat photopolymerization (VPP) is a process in which a slurry composed of a liquid photopolymer supercharged with ceramic particles is cured in layers in a vat by a light source located below (bottom-up) or above (top-down) the vat, resulting in a three-dimensional body, after organics removal and sintering resulting in a ceramic part. This work aims to carry out a developmental design and fabrication of a “top-down” VPP AM machine for ceramics. The criteria and boundaries of the project were: commercial DLP projector with high-pressure mercury lamp without a UV filter, layer deposition of 10 micrometers thick, vat with 75mm in diameter and 75 mm in height, spreading and leveling of layers by blades, Creation Workshop 1.0.0.75 software for control and operation of layers and movement of blades by stepper motors and also digital image projection. The result was accessible and reproducible equipment capable of depositing layers and controlling light exposure time, allowing the generation of complex bodies with their consolidated layers. This open-source project allows the community to gain experience, acquire resources, and build trust before investing in more advanced systems. Furthermore, it acts as a rich source of collaborative ideas and designs.

Keywords:
Additive Manufacturing; Open Source Machine; top-down; Ceramics

INTRODUCTION

Additive manufacturing (AM) consists of a method for the production of solid parts through computational resources by gradually incorporating layers of material, also called 3D printing ¹. Currently, this method is widely applied in several industry sectors, such as civil construction, production of mechanical elements, biomedicine, dentistry, prototyping, and product customization, among others ². AM can be performed using different techniques, such as vat light curing (VPP), powder bed melting, extrusion, and material blasting ³. Thus, AM allows the construction of polymeric, metallic, ceramic, and composite parts controlled by software that transfers a digitized 3D model of a given object to a mechanical system that prints it.

In 1984, Charles Hull patented the stereolithography method of additive manufacturing (AM), known as vat photopolymerization (VPP). This process involves selectively curing a liquid photopolymer in a vat using a UV light source. In more recent developments, the light source used to initiate the polymerization reaction can be UV light or high-energy visible light derived from various sources such as LEDs, bulbs, or lasers. The images for curing are typically projected using digital light processing (DLP) or, through a liquid crystal display (LCD), or by employing laser scanning.

The projector utilizes a set of lenses to focus the light beam onto the surface where the parts are going to be printed. Depending on the orientation of the light source, 3D printing equipment can be categorized into two types: top-down and bottom-up. In top-down systems, the light source is positioned above the vat, while in bottom-up systems, the light source is located beneath the transparent lower vat. The printing platform then moves either downward or upward as each layer is cured. In the field of ceramics, VPP stands out as one of the few additive manufacturing (AM) processes capable of producing high-performance ceramics. It achieves an impressive resolution of 10 µm and print speeds exceeding 50.000 mm³/h. This indirect process utilizes resin as a sacrificial material, which is removed before sintering. The result is parts with over 99% relative density and properties that are comparable to those obtained through conventional ceramic manufacturing methods. In bottom-up printing for ceramics, a machine requires a low-viscosity slurry to fill the space between the bottom of the vat and the build platform. This is necessary to overcome the adhesion between the formed layer and the transparent window. In contrast, top-down machines can use more viscous slurries, typically in the form of pastes. However, these machines have limitations in dragging very small parts and may have reduced printing speeds. ⁹

Having a reliable and reproducible printer is crucial for producing end-use ceramic parts. Machines represent 57.1% of the overall additive manufacturing (AM) landscape, followed by software at 19.9%, materials at 18%, post-processing at 3.7%, and others. Ceramic machines account for only 7% of total machine revenue, which is relatively low considering that ceramic 3D printing is still in the early stages of development. The technology is still quite immature, and selecting the right equipment can be a challenging decision due to the high costs involved. Additionally, these machines undergo rapid evolution, which often leads to previous models becoming obsolete.

Open-source and desktop machines provide an opportunity to experiment with prototypes and gain confidence before acquiring high-level systems. They also serve as a source of abundant collaborative ideas and designs. The current research group has previously concentrated on the design of printing equipment ^{11), (12), (13}. It is hypothesized that the use of thinner layers can mitigate issues related to insufficient adhesion between layers, thereby enhancing the overall quality of the final product. This VPP project expands upon those earlier initiatives by aiming to design and manufacture a 3D printer that can deposit thinner layers with an increased level of automation compared to previous models.

MATERIALS AND METHODS

The project involved the use of a slide projector as a light source for the photopolymerization of the upper surface layer. It included a cylindrical vat to hold the photopolymerizable suspension, a continuous elevation system for the production platform, and a mechanism for distributing the ceramic suspension layer by layer. Additionally, there was a control system (console) to manage the process. A three-dimensional sketch of this project is presented in Figure 1.

Figure 1:
Three-dimensional sketch of the prototype 3D printing machine: (a) machine and (b) control system.

Adapted from an existing inkjet machine ¹³, the reference surfaces for the project were the frames of the aluminum table support and the linear guide supports, both of which were machined to achieve a flatness of 0.05 mm (see Figure 2a). The optimization of the design included a change in the positioning reference component of the 17 mm thick aluminum table. In the new design, the reference surfaces are the top, bottom, and back surfaces of the aluminum table, which were precision machined to a flatness of 0.01 mm. The top surface is responsible for accommodating the VAT system and leveling the slurry, while the bottom surface ensures the alignment and accuracy of the guide. This bottom surface also supports two precision rod guides, measuring ⊘12.7 mm (on the right and left), which are mounted to support a paired column. The back of the table houses a stepper motor that drives a lead screw, facilitating the linear movement of the blade systems. In this design, illustrated in Figure 2b, the frame is also bolted to the underside as a secondary component. The compactness of the project was achieved by reducing the height of the columns, narrowing the gantry opening, and improving the centering of the lead screw. These adjustments contributed to a lower Abbe error (or sine error), which refers to the magnification of angular error over distance, resulting in greater precision and reproducibility. Additionally, in this model, the LM Guides and lead screw are no longer exposed to falling debris.

Figure 2:
Frame structure, (a) preliminary as positioning reference and (b) table as reference.

The vat system features a stainless-steel sleeve containing a piston (platform) that moves up and down by converting rotary motion from a lead screw driven by a stepper motor. The stepper motor (0.5 N.m NEMA 17) has a shaft that is machined with a central hole threaded M3 with a pitch of 0.5 mm. The upper piston, which supports the part to be printed, is guided in its movement by bronze bushings and steel columns. This design ensures parallelism between the layers and prevents the piston from rotating. This system was adapted from the prototype by Garcia ¹³. Figure 3 illustrates the design of the vat.

Figure 3:
Partial section drawing of the vat design

The vat coating system involves spreading and leveling the ceramic suspension according to the guidelines in patent BR 10 2021 021544 5 A2 ¹⁴, which utilizes sequential action blades.

The pillow block supports two vertical columns (right and left) that hold two horizontal cylindrical beams with a diameter of 8 mm. This creates a translational frame designed to support the system of blades responsible for slurry spreading and leveling, as well as a servo motor for tilting the blades. On the right side, parallel to the guide, a spindle is coupled to a stepper motor (0.5 N.m), which drives the lead screw that facilitates the forward and backward movement of the gantry. The two horizontal cylindrical beams that form the gantry provide support for the spreading and leveling of the ceramic suspension. This system consists of an aluminum board that supports the servomotor (mounted on the upper and lower beams), along with a hexagonal bronze bushing installed on the lower beam. The bushing features angled blades positioned at 15° and spaced 18 mm apart on two of its faces. The aluminum plate secures the servomotor, which was selected for its high torque and accuracy in angular positioning, and it is compatible with the Arduino system used to control the motors. The servo motor is responsible for the 30° tilting movement needed to spread the ceramic slurry as the gantry moves forward and for leveling during the return travel (refer to Figures 4 and 5). Table 1 lists the components of the covering system.

Figure 4:
Composition of the covering system.

Figure 5:
Bushing/blade system and operating principle.

Table 1
Components of the covering system

Components	Characteristic	Value
Servomotor	TS8120-MG 180o (Thiankon GRC).	2.5 N.m
Stepper motor	NEMA 17	0.5 N.m
Lead Screw		Ø8 mm and 8 mm feed per revolution

An electronic control system was developed that enables a minimum downward displacement of the vat of 1.25 µm per layer, allowing for the printing of parts with layers that are 1.25 µm thick. This system, which manages the motors of the vat and the cylindrical beams of the gantry, consists of Creation Workshop 1.0.0.75 software, Marlin 2.0.x firmware, and Arduino 1.8.19, all of which are supportable with the Windows® operating system.

The electronic hardware was assembled using an Arduino MEGA 2560 R3 module, a 24V DC power supply, and two specific drivers. The power supply powers the module, which in turn powers the drivers. These drivers control both the horizontal and vertical movement of the gantry and the vertical movement of the piston vat. Figure 6 illustrates the assembly of the electronic hardware.

Figure 6:
General control system diagram.

The TB6600 was set for micro step 16 and current 1.6A. Arduino program was adjusted: configuration.h, the speeds were defined: DEFAULT_MAX_FEEDRATE {300, 300, 5, 25}. Also, the firmware Marlin program had pins set in Marlin-2.0.x\Marlin\src\pins\ramps, RAMPS.h was defined: Z_STEP_PIN 46, Z_DIR_PIN 48, Y_STEP_PIN 60, Y_DIR_PIN 61.

The Creation Workshop program is designed to activate stepper motors and configure the machine’s X and Y axes, as well as the Z axis. The program includes commands for forward and backward blade movement in G-code pre-slice format. The commands are as follows: <Delay> 10000; G1 Y500 F2000; <Delay> 15000; G1 Y-500 F2000; <Delay> 15000. This setup allows for a 20-second exposure time for the image and provides a displacement of 100 mm with a blade travel speed of 1 mm/s.

The tilting movement of the spreading blades is controlled offboard by an Arduino UNO R3 module, which is activated by two mechanical limit switches located on the upper left beam of the metallic frame. The 2.5 N·m servo motor, when supplied with 7.2 V, draws approximately 120 mA of current. However, since the Arduino output is limited to 40 mA, the servo motor requires an independent power supply through the red (+) and brown (GND) cables. It is essential to ground the GND cable to the GND of the Arduino board. The yellow terminal receives pulse signals (for clockwise and counterclockwise motion) from the Arduino module via pinouts 3 and 4. Figures 6 and 7 provide a simplified diagram of this setup.

Figure 7:
Electronic model incorporated into the main hardware

The complete console must include a power switch, a cooling fan, an emergency stop switch, and operation indicator lights: energized (yellow), running (green), and emergency (red). Figure 7 illustrates the electronic model that was integrated to operate the blades.

To promote the photopolymerization of ceramic suspensions, the 3D printing machine currently under construction employs a Hitachi CP-S318 projector, which is equipped with high-pressure mercury light. This projector was specifically chosen because it lacks a UV filter, a critical feature for enabling photopolymerization. To support the projector and a set of converging lenses, a height-adjustable bracket has been installed at the rear of the structure. The focus of the light projection is precisely set on the surface of the suspension placed on the moving platform (or piston) of the vat, aligning exactly with the level determined by the leveling blade. Although this equipment is typically used for commercial slide projectors, it is advisable to use a UV protection window, along with safety glasses that have UV filters and gloves for protection.

RESULTS

The assembly of the 3D printing machine has been completed, incorporating all the proposed features (Figure 8). The machine is capable of producing layers with a thickness precision and repeatability of 10 micrometers, as certified by a micrometric dial indicator. During the printing process, a slurry of Zirconia 3Y-TZP was used, and it was crucial for the first layer to adhere securely to the platform. This is important because the initial layers can drag during the application of subsequent layers. To ensure this adhesion, an Acrylonitrile Butadiene Styrene (ABS) or aluminum disc with a diameter that matches that of the mobile platform was affixed to it using double-sided tape before the additive manufacturing (AM) process began. This method effectively prevented the dragging of the print layers. Once the production of each part was completed, the ABS or aluminum platform was easily removed for cleaning.

Figure 8
Complete assembly of the 3D printing machine

A set of blades for spreading and leveling is essential to the coating process. Figure 9 illustrates the experimental design of these blades. Initially, a pair of hugging blades (Figure 9a) and a stylus (Figure 9b) were secured to a bronze bushing. However, after printing a test piece, it became evident that this method of attachment hindered access to the space between the blades for cleaning and made it challenging to replace them. To address this issue, the fixation was modified to incorporate neodymium magnetic bars, representing a design innovation in this study. This change facilitates both the leveling of the blades and their maintenance. Additionally, the design of the blade tip was revised; the approved configuration includes a thick stylus for spreading (Figure 9c) and a flat-angle blade for leveling (Figure 9b).

Figure 9:
Blade designs: (a) hugging tip, (b) stylus tip, (c) flexible rubber tip, (d) flat-angle tip, and (e) flat tip.

Currently, the slurry feed is done manually using a funnel, spatula, syringe, and pastry bag. However, work on automatic feeding is ongoing.

The printing of Zirconia Tosoh 3Y-TZP was conducted to evaluate the machine prototype. A photopolymer slurry containing 40 vol% powder was prepared, and bar models were printed using a layer thickness of 25 µm and an exposure time of 20000 ms. After sintering, the relative density of the parts reached 98%. Figure 10 illustrates the green bars after cleaning, both post-printing and post-sintering.

Figure 10:
Bars printed: (a) green and (b) after sintering.

CONCLUSIONS

The additive manufacturing machine we designed and built successfully met the prototyping objectives by producing layers thicker than 10 microns. This achievement was made possible through the integration of an electronic system combined with the VAT platform movement motor. We resolved recurring issues such as the dragging of newly photopolymerized layers, layer surface finish, and the cleaning and replacement of blades through innovative solutions like magnetic fixation and adjustments to the blade’s advancement and return speed. As a result, it created accessible and reproducible open source equipment capable of depositing layers and controlling light exposure time, which allows for the generation of complex structures with consolidated layers.

ACKNOWLEDGMENTS

The authors thank the support provided by Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), grant number 2021/12612-7.

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Edited by

• (AE: Rafael Salomão)

Publication Dates

• Publication in this collection
  17 Mar 2025
• Date of issue
  2025

History

• Received
  01 Oct 2024
• Reviewed
  04 Dec 2024
• Accepted
  23 Dec 2024