OpenVCAD: A Volumetric Multi-Material Geometry Compiler

Abstract

Modern additive manufacturing has made significant advancements in multi-material fabrication techniques that allow for site-specific control of material deposition. With these advancements, design tools have fallen behind machine capabilities in specifying volumetric information. Traditionally, design and fabrication workflows have expressed multi-material objects as several single-material bodies. By storing only the information about the surfaces of the geometries, information about the volumetric composition of the solids is unrepresented. The intense interest in compliant mechanisms and meta-materials demands a new design workflow that can support architecting material distribution throughout an object. To address these needs, we present OpenVCAD, an open-source volumetric design compiler with multi-material capabilities. OpenVCAD provides a scriptable suite of geometric and material design methods that enable efficient representation of complex objects with hundreds of materials. OpenVCAD allows functional specification of multi-material volumes that are parameterized on spatial locations, yielding complex multi-material distributions that would be impossible to describe in alternative workflows.. This paper will present the OpenVCAD pipeline, and demonstrate its use though the design and manufacturing of functionally graded multi-material components.

How it Works

A compiler for multi-material designs

The OpenVCAD project introduces a parametric modeling language alongside a compiler, to articulate and generate volumetric designs. The workflow for volumetric design within OpenVCAD is displayed in the accompanying diagram. Designers utilize the OpenVCAD modeling language to define their objects. Subsequently, these designs undergo compilation to form an OpenVCAD tree, facilitating exportation into diverse volumetric formats, including PNG stacks suitable for inkjet 3D-printing.
A central facet of OpenVCAD's functionality resides in its material nodes. These nodes enable the application of intricate material transitions, digital alloying, and image processing techniques to child geometries. This contrasts with the conventional 3D-printing procedure, where multiple materials are superimposed through a combination of single-material surface meshes. OpenVCAD's approach obviates the need for such complexity. Moreover, the platform enables designers to express objects featuring internal compositional variations, such as lattice structures and meta-materials. This innovative capacity extends the scope of design possibilities beyond conventional limits.

OpenVCAD workflow diagram

The OpenVCAD modeling language

Using the modeling language, geometric forms and material configurations can be efficiently articulated within a singular file structure. The subsequent example illustrates the application of OpenVCAD in designing and compiling a multi-material entity via functional grading and constructive solid geometry principles. OpenVCAD employs a node-centric modeling language, akin to the OpenSCAD language with a pivotal distinction – OpenVCAD is tailored to accommodate multi-material designs. OpenVCAD files, designated as .VCAD files, adopt a straightforward text-based format. These files encapsulate a hierarchical assembly of nodes, reminiscent of abstract syntax trees (ASTs) or constructive solid geometry (CSG). Categorically, the OpenVCAD modeling language incorporates three node classifications: geometric primitives, compositional directives, and material specifications. Referencing the figure below, the orchestration of OpenVCAD nodes is demonstrated, exemplifying their utility in generating designs characterized by multiple materials.
OpenVCAD modeling language example

Object Showcase

Multi-material functionally graded logo.

Multi-material functionally graded screwdriver constructed using a Stratasys J750 Polyjet printer. Multiple materials are used to add a soft-touch handle and a multi-color shaft.

Soft actuator printed in three materials. This example was created using a scalar field equation in polar coordinates to generate the geometry, and was then functionally graded in threshold mode using three distinct scalar fields over the entire bounding box of the model.The model was printed using the Stratasys J750 InkJet printer.

Multi-material functionally graded airplane wing. The object’s quality was enhanced by decreasing the cell size of the gyroid pattern along the length of the wing. This preserves geometric detail in thinner sections of the wing. Similarly, a heavier, stiffer material (blue) gives way to a lighter, more flexible material (red) along the direction of the wing. In all, only 8-lines of the modeling language are needed to generate this object.

Multi-material functionally 3D-benchy.

Multi-material functionally graded lattice structure

Multi-material organic lattice structure where each member is functionally graded. Each strut is functionally graded to form a meta-material where the center (shown as a red material) exhibits better compression resistance when compared with the blue material at the ends. The blue material exhibits better adhesion between connected strut endpoints. The struts are transformed and combined via a summing operation to yield a composite lattice.

Pre-surgical Planning

A notable application of OpenVCAD lies within medical image processing, particularly in the realm of pre-surgical planning. The subsequent illustration underscores this application, showcasing OpenVCAD's utilization in creating a 3D-printable representation derived from a patient's MRI scan. OpenVCAD seamlessly processes DICOM image stacks, assigning materials and executing image manipulation. These designs are subsequently exportable as bitmap stacks, apt for Inkjet 3D-printing. However, it's noteworthy that the OpenVCAD medical image processing workflow may necessitate supplementary licensing. For further insights into this aspect, inquiries can be directed to charles.wade@colorado.edu.

BibTeX

@article{OpenVCAD2023, title = {OpenVCAD: An open source volumetric multi-material geometry compiler}, journal = {Additive Manufacturing}, pages = {103912}, year = {2023}, issn = {2214-8604}, doi = {https://doi.org/10.1016/j.addma.2023.103912}, url = {https://www.sciencedirect.com/science/article/pii/S2214860423005250}, author = {Charles Wade and Graham Williams and Sean Connelly and Braden Kopec and Robert MacCurdy}, keywords = {Volumetric design, Multi-material additive manufacturing, Meta-materials, Lattice-structures, InkJet 3D printing, Functional grading}, abstract = {Modern additive manufacturing has made significant advancements in multi-material fabrication techniques that allow for position-specific control of material deposition. With these advancements, design tools have fallen behind machine capabilities in specifying volumetric information. Traditionally, design and fabrication workflows have expressed multi-material objects as several single-material bodies. By storing only the information about the surfaces of the geometries, information about the volumetric composition of the solids is unrepresented. The intense interest in compliant mechanisms and meta-materials demands a new design method that can support architecting material distribution throughout an object. To address these needs, we present OpenVCAD, an open-source volumetric design compiler with multi-material capabilities. OpenVCAD provides a scriptable suite of geometric and material design methods that enable efficient representation of object with complex geometry and material distributions. OpenVCAD allows functional specification of multi-material volumes that are parameterized on spatial locations, yielding complex multi-material distributions that would be impossible to describe using alternative methods. This paper will present the OpenVCAD pipeline, compare it to related work, and demonstrate its use through the design and manufacturing of functionally graded multi-material components.} }