OpenCAL: Low-Cost Volumetric 3D Printer

• OpenCAL is an open-sourced volumetric printing platform that uses computed axial lithography to cure entire 3D geometries without traditional layering.
• It employs commercially available components, makerspace fabrication methods, and a Raspberry Pi-based control system to ensure low cost and easy assembly.
• The platform integrates optical, mechanical, and software subsystems with custom resin formulations and dual post-processing techniques to enable rapid, reproducible prints.

OpenCAL is an open-sourced, community-driven, low-cost computed axial lithography printer and post-processing platform for volumetric additive manufacturing. It is presented not simply as a single machine, but as an ecosystem composed of a CAL-based printer built from commercial off-the-shelf components, standard makerspace fabrication methods, open software, accessible materials, and community documentation pathways intended to support replication and future modification. Its primary deployment environment is academic makerspaces, where it is designed to lower barriers to volumetric printing research while also serving as a platform for education in photopolymer science, computational imaging, mechatronics, and open-source hardware development (Waddell et al., 2 Sep 2025).

1. Conceptual basis and design objectives

OpenCAL implements computed axial lithography, a class of volumetric additive manufacturing in which an object is formed throughout a 3D volume rather than by sequentially depositing or curing layers. In the CAL approach, spatially modulated light patterns are projected from many angular directions into a rotating vial of photopolymer resin, and the sum of those exposures creates a 3D dose distribution that solidifies only the target geometry. The platform therefore inherits the characteristic VAM properties emphasized in the source description: fabrication is layer-less, support structures can be reduced or avoided, print times can be dramatically shortened, and higher-viscosity materials become more usable because the printed object does not move relative to the photopolymer volume during fabrication (Waddell et al., 2 Sep 2025).

OpenCAL was developed in response to three barriers identified for CAL adoption: hardware accessibility, material accessibility, and community adoption. Through 22 customer-discovery interviews across academic makerspaces, industry, the maker community, and research, the project derived explicit user-driven requirements. These included a total system cost below $1000 USD, assembly in less than one day, readiness to begin printing within two hours of assembly, a maximum vial diameter of 4 inches, permissible rotational runout of 100 microns, and target vial rotation speeds from 2 to 10 RPM. The emphasis on modularity, low cost, accessibility, reproducibility, and compatibility with common makerspace tools is therefore a design constraint rather than a secondary feature (Waddell et al., 2 Sep 2025).

A plausible implication is that OpenCAL should be understood as an infrastructural platform for CAL dissemination rather than only as a printer design. The source repeatedly frames the machine and its adoption strategy together.

2. Printing principle and optical subsystem

The operating principle follows the standard CAL workflow. A 3D object is converted into a sequence of 2D projection images corresponding to angular views around the rotation axis. During printing, these images are projected into the rotating vial of photocurable resin; as the vial rotates, each voxel accumulates light dose from many projection angles, so that regions intended to become the part exceed the polymerization threshold while surrounding regions remain uncured. The source explicitly links OpenCAL to the CAL reconstruction formalism of Kelly et al. and to the open-source implementation in VAMToolbox. In the standard CAL framework implemented by the platform, the accumulated dose is described as

$$D(\mathbf{r}) = \int_{0}^{\pi} I_\theta\!\big(\mathcal{P}_\theta(\mathbf{r})\big)\, d\theta,$$

with polymerization where

$D(\mathbf{r}) \ge D_c.$

The paper does not introduce a novel equation set for OpenCAL itself; rather, it physically realizes this established tomographic light-delivery model (Waddell et al., 2 Sep 2025).

The optical subsystem is centered on an AAXA P6 Ultimate projector, selected for LED-based optical output of 1100 lumens, a price of $369 USD, and expected manufacturer support. Two modifications were required. First, the projector’s 100% vertical offset meant that the bottom edge of the projected image aligned with the center of the lens, so the projector had to be physically tilted relative to the vial axis; the resulting keystone distortion was corrected using the projector’s internal computational keystoning. Second, because VAMToolbox’s projection algorithms assume a collimated light source, the native throw ratio produced excessive beam divergence. OpenCAL therefore added a 250 mm focal length lens at the end of the projector optical path, reducing divergence and shortening the effective focal length of the projected image (Waddell et al., 2 Sep 2025).

Spectral intensity measurements showed a peak intensity of $9 \text{ mW/cm}^2$ at 450 nm, and 450 nm was chosen as the operational printing wavelength. That wavelength choice is central to the material system, because the resin must exhibit suitable optical attenuation and photoinitiation behavior at the operating wavelength (Waddell et al., 2 Sep 2025).

3. Mechanical architecture, electronics, and metrology

The mechanical system is organized around a removable vial assembly, a rotation interface, a structural frame, and metrology and observation hardware. The vial includes a top lid with the rotation interface, a glass vial, and a bottom lid with a glass bottom. The transparent bottom is specifically intended to support metrology illumination in connection with Optical Scattering Tomography. Both lids are removable to ease cleaning. The standard vial is 4 inches in diameter and 6 inches tall, though the system is adaptable to other vial sizes and is explicitly demonstrated with an adapter for a 30 mm diameter vial (Waddell et al., 2 Sep 2025).

The motion subsystem is intentionally simple: OpenCAL requires only controlled vial rotation rather than multi-axis gantries or peel mechanisms. The target rotational speed range is 2–10 RPM, with rotational runout held under 100 $\mu$m. The vial top lid serves as the rotational adapter, and the build remains stationary relative to the resin during exposure except for the controlled global rotation of the container. This simplicity is presented as one of CAL’s accessibility advantages (Waddell et al., 2 Sep 2025).

The frame and enclosure are built mainly from 80/20 aluminum extrusion, supplemented with 3D-printed components and laser-cut sheet parts. Electronics and control are based primarily on a Raspberry Pi 5 microprocessor. The imaging and data-collection system can use either a USB camera or a Raspberry Pi ribbon-cable camera. The source does not enumerate the entire driver and power stack in detail, but it states that the control electronics were intentionally constrained to easily sourced commercial parts rather than custom PCBs or specialized instrumentation (Waddell et al., 2 Sep 2025).

OpenCAL also incorporates Optical Scattering Tomography as a recent VAM advancement. The paper does not report a full OST compensation workflow or quantitative scattering-correction results on the platform itself, but it makes clear that OST shaped the hardware architecture, especially the transparent-bottom vial and the integrated camera and data-collection subsystem. This suggests that scattering-aware metrology is treated as a future-facing capability built into the platform from the outset (Waddell et al., 2 Sep 2025).

4. Software workflow and computational pipeline

The software workflow begins with a 3D model and proceeds through tomographic preprocessing and synchronized machine execution. VAMToolbox is identified as the key open-source package for CAL reconstruction. The implied sequence is: import target geometry, generate tomographic projection images with VAMToolbox, calibrate projector geometry and keystone so that the digitally computed images map correctly into the physical resin volume, synchronize those images with vial rotation, expose the resin while rotating, and then remove, clean, and post-cure the printed part. The paper notes that VAMToolbox is currently a usability bottleneck and is being revised for accessibility (Waddell et al., 2 Sep 2025).

The control stack is based around the Raspberry Pi 5, which coordinates image display and rotation timing, although detailed firmware architecture and timing diagrams are not described. The software and optical calibration layers are tightly coupled: correct operation depends not only on reconstruction quality, but also on alignment of optical centerlines, geometric distortion compensation via keystone correction, and limiting beam divergence so that projected rays more closely satisfy tomographic assumptions (Waddell et al., 2 Sep 2025).

The documentation and collaboration layer is also part of the software architecture in a broad sense. CAD is intended to be shared via Onshape, while bill of materials, assembly instructions, operational instructions, and community-editable guidance are planned for Read the Docs. This organizational structure is presented as part of the platform rather than as ancillary project management (Waddell et al., 2 Sep 2025).

5. Materials system and post-processing

Material accessibility is treated as a central technical problem. Several tested commercial off-the-shelf resins, including Formlabs Super Clear and various craft or SLA resins, exhibited nearly 100% transmittance near 450 nm, which is problematic for CAL because some absorption is required to localize dose accumulation appropriately. To address this, the project developed a custom resin formulation based on urethane dimethacrylate with camphorquinone and EDAB:

• UDMA base resin, viscosity 9000 cP
• $17 \text{ mM}$ camphorquinone photoinitiator
• $8.5 \text{ mM}$ ethyl 4-(dimethylamino)benzoate co-initiator (Waddell et al., 2 Sep 2025)

The source states that collaboration with a local supplier is underway to distribute this formulation commercially for less than $100 per liter. It also notes that users can mix the resin themselves using only basic chemistry equipment such as a hot plate, beakers, a scale, and a fume hood.

Post-processing receives unusually detailed attention because safe and manageable handling of liquid chemicals emerged as the most prominent user concern. Two principal cleaning modes are described. The first is solvent-assisted ultrasonic cleaning. Initial tests with pure water and 30 minutes of sonication worked well for convex geometries but not for concave ones. Ethanol was then added up to 25% by volume in water, producing successful cleaning of geometries of varying complexity; this was reproduced both on a laboratory ultrasonic cleaner and on a cheap commercial jewelry cleaner. The results section states that the ethanol-based sonication method completed post-processing within 20 minutes while preserving fine features (Waddell et al., 2 Sep 2025).

The second method is a centrifugal cleaning module intended both to clean parts and recover reusable resin. In this configuration, the vial is oriented horizontally relative to gravity and rotated orthogonally about its axis. Centrifugal forces drive excess resin out through the vial openings, mesh barriers prevent the printed object from escaping, and the expelled resin is collected at the bottom of an enclosure for potential reuse. No exact RPM, g-force, or spin duration values are given, but the operating concept is explicitly presented as a strategy to reduce solvent waste and reclaim expensive photopolymer (Waddell et al., 2 Sep 2025).

6. Demonstrated performance, educational role, and limitations

Experimental validation is modest but concrete. The machine was assembled entirely from commercial off-the-shelf components and standard makerspace equipment for under $800 USD, comfortably meeting the stated target of under $1000. It successfully fabricated complex geometries in a 30 mm diameter vial, including a “Thinker” and a nut-and-bolt geometry in the custom UDMA resin. The paper reports that the complex geometry in Figure 1 was fabricated within 90 seconds, demonstrating successful optical and mechatronic synchronization. The ethanol-based sonication method preserved fine features and was completed within 20 minutes (Waddell et al., 2 Sep 2025).

The platform is intended explicitly for academic makerspaces, where it is meant to introduce advanced volumetric additive manufacturing capabilities and create opportunities for hands-on education in optics, mechatronics, photopolymer chemistry, computational imaging, and open-source hardware development. The student-led development process is itself used as evidence of feasibility: most of the team had no prior CAL experience, yet they successfully built the system and developed proficiency in its operation and improvement. Community engagement is treated as a formal subsystem of the platform. The project created a Discord server with over 150 members, reports that at least two external groups are developing independent CAL systems using COTS components, and describes a pilot deployment at UC Berkeley’s Jacobs Makerspace (Waddell et al., 2 Sep 2025).

The limitations are also explicit. Beam divergence remains too high for larger-part fabrication, so optical redesign or improved collimation is required. VAMToolbox usability remains a substantial obstacle for new users. Mechanical properties of printed parts were not characterized in the study, and safety and automated operational features such as automated print stopping and improved material handling remain future priorities. Although OST is architecturally integrated and conceptually important, the paper does not yet report a complete OST compensation workflow or quantitative scattering-correction results on the OpenCAL platform itself (Waddell et al., 2 Sep 2025).

Taken together, these characteristics define OpenCAL as an intentionally accessible implementation of computed axial lithography that combines a projector-based CAL printer, modular vial and rotation hardware, Raspberry Pi-based control, makerspace-compatible fabrication methods, a custom but accessible 450 nm resin, and two complementary post-processing strategies including a resin-recovering centrifugal cleaner. Its novelty lies not in a new CAL reconstruction theory, but in packaging CAL into a reproducible, low-cost, community-expandable platform (Waddell et al., 2 Sep 2025).