Background

Desktop 3D printing has greatly expanded the possibilities of interactive prototyping, yet large-scale interactive objects still face two major constraints. First, once an object exceeds the printer’s build volume and must be segmented, it becomes difficult to maintain both stable physical connections and electrical connections. Second, current conductive printing materials remain highly resistive, while existing selective electroplating methods are mostly limited to outer surfaces, making it difficult to support higher-load, safer, and more complex three-dimensional circuits. E-Joint was developed to unify assembly and electrical continuity within the same connection structure.

Contributions

We integrate mortise and tenon joint structures, conductive traces, and a copper plating process into one workflow, allowing segmented parts to complete mechanical locking and electrical connection at the same time. In summary, the contributions of this paper are as follows:

(1) A novel fabrication pipeline that focuses on electroplating enhanced electrifed joints to create large-scale 3D printed interactive objects.

(2) A software to separate and customize joints for large-scale 3D printed objects.

(3) The evaluation of the electrifed joints and the feasibility of the fabricated pipeline.

Mechanism

E-Joint integrates structure and conductivity into the same interface. Each electrified joint serves as both a physical connector and an electrical pathway, reducing the need for additional fasteners, wires, or post-assembly electronics.

Electrified joint

We designed four types of electrified joints for different connection scenarios. Together, they form a structural vocabulary ranging from high-strength fixed joints to replaceable modular interfaces.

• (a)The straight joint provides the most direct and stable basic connection;
• (b)The dovetail joint offers stronger axial locking and is suited for locations requiring higher tensile and shear resistance;
• (c)The expansion joint introduces additional contact pressure through deformation and is better suited for multi-line electrical connections;
• (d)The spring-clip straight joint supports detachable and repeatedly pluggable modular connections.

Internal Circuits

To support internal circuits, we create cavities around conductive PLA traces so that electroplating solution can reach the interior of the object. This converts high-resistance printed traces into low-resistance conductive paths and enables embedded electronic components inside large-scale 3D printed objects. We provided a detailed introduction to the internal circuitry in [intercircuit]. We also generate two types of component slots: surface mount slots and pin slots, so that electronic components can be integrated directly into the printed structure.

Fabrication Pipeline

The full fabrication pipeline consists of five stages. We first use Rhino3D / Grasshopper to segment the model, generate joints, place components, and design either surface or internal circuits; we then fabricate the structural and conductive parts through multi-material FDM printing; after printing, the exposed conductive PLA regions are processed with copper electroplating; the parts then go through rinsing, passivation, drying, and protective finishing; finally, electronic components are inserted and the full object is assembled by insertion. This workflow integrates large-object segmentation and highly conductive interactive circuitry into one continuous personal fabrication method.

E-Joint Design Tool

We developed the E-Joint Design Editor to support the complete design workflow. Designers can import an original model, generate selectable grid points on contact surfaces, define joint positions, and let the system automatically perform Boolean operations. They can also place electronic components from a built-in library, draw surface or internal 3D Conductive PLA Traces, and automatically generate matching internal cavities and Clamp Plates. After design, the model can be exported in separate layers for structural and conductive materials. This brings structure, connection, and circuit design for large-scale interactive objects into a single parametric interface.

Guidelines

Design Guidelines

To balance structural stability and electrical performance, we derived a set of reusable design guidelines. A single mortise-and-tenon structure is recommended to have a total thickness of at least 20 mm, and a total width and length of at least 50 mm. A single conductive trace is recommended to be at least 3 mm wide and 4 mm thick, with spacing between adjacent traces of no less than 1 mm. For assembly tolerance, the straight joint, dovetail joint, and expansion joint are recommended at around 0.1 mm, while the spring-clip straight joint uses around 0.15 mm to better support repeated insertion and removal. In addition, we provide the characteristic parameters for each of the four Electrified joints.

Fabrication Guidelines

For fabrication, we use multi-material FDM printing to produce both structural and conductive parts. To reduce resistance between conductive PLA layers, we use 100% infill, a linear filling strategy, and more than 5 wall layers, while aligning the internal fill direction with the wiring direction as much as possible. After printing, the conductive PLA acts as the cathode during copper electroplating, a process that improves conductivity by roughly five orders of magnitude. In our setup, plating begins at 0.04 A to form an initial visible copper layer and then continues at 0.02 A for several hours to achieve more uniform deposition.

Evaluation

We evaluated the system in terms of conductivity, power handling, and mechanical connection performance. For line-width testing, we fabricated test pieces with a trace length of 100 mm, a depth of 3 mm, and widths ranging from 0.4 mm to 6.0 mm. After electroplating, four-probe measurements showed that traces with a width of 2.0 mm or more achieved stable conductivity with resistance below 0.1 Ω. For power testing, we continuously increased the voltage across the traces until failure; the 5.2 mm trace reached an instantaneous load of 58.625 W and tolerated a maximum current of about 23.45 A before burning out. For mechanical testing, we examined tensile and shear behavior across all four joint types, showing that different joints excel in different scenarios such as high-strength locking, multi-line connection, and repeated plugging, confirming that E-Joint is both electrically functional and structurally reliable in practice.

Applications

We demonstrate E-Joint through three applications operating at different scales and functional requirements.