Adaptable Bed for Curved-Layered Fused Deposition Modeling of Nonplanar Structures: A Proof of Concept

Adaptable bed setup

The adaptable bed uses two elements: a pin-bed (PEICEES^® 6″8″) and a foam (OSASIS^® fine-celled thermoset phenolic plastic foam; Fig. 1a). The printer used for this study is Rostock V3.2 Max 3D. The working principle consists of all the pins starting at their maximum height (distance from the top of the pins and the top section of the foam), then they are pushed (nailed) downward, while the foam keeps them at the height required to form the mandrel. The pin-bed is placed on top of the foam (Fig. 1b). The pins have an initial height of 18 mm and a diameter of 1 mm. To keep the pins at the right depth, the foam was selected neither too soft to avoid looseness nor too stiff to require high magnitudes of force to push the pins.

OPEN IN VIEWER

FIG. 1.

Fabrication setup, (a) OASIS foam and pin-bed, (b) pin-bed on top of the foam, (c) the complete fabrication setup, including the pin-pusher (Supplementary Fig. S1), and (d) diagram of the working principle (Supplementary Video S1). (e, f) Sphere lattice results for a separation of the meridians rasters of 45°. (g, h) Surface lattice results for a separation of the rasters in the xz-plane of 5 mm. (i, j) Microscopic comparison between (i) 3D printed and (j) pin-base bottom surfaces. 3D, three-dimensional.

A tool to push the pins was designed with a spherical tip (Fig. 1c). This tool fits on the extruder and increases the allowance angle to conform surfaces with sharp changes on slopes, while pushing one pin at a time, avoiding the collision between the fans and the pins. Considering the distance from the center of the tool to the contact point with nearby pins as ρ, a reduction on the slopes achievable results whenever ρ is greater than the diameter of the tool tip. The complete setup is shown in Figure 1c, where a section of a cylinder is presented as an example of a mandrel.

Forming mandrel on the pinned-base: G-code generation

The extruder trajectories to form the mandrels on the pin-bed were generated through MATLAB^® scripts where the intended form was parametrized and discretized in segments of 2.5 mm (pin head diameter). The trajectory of the pin pusher involves moving in the z- direction, nailing one pin at the time into the foam. Then the extruder moves in z+ direction, next in the xy-plane to the following location (Fig. 1d). The process is repeated until all the pins are nailed at the right depth required to conform the shape needed. The fineness of the mandrel depends on the dimensions and location in xy-plane of the pins. The Rostock machine has a x–y and z resolution of 100 and 12.5 μ, respectively. This positional accuracy keeps each pin at the correct height to attain the predefined shape on the pin-base.

The concept presented in this study was tested with two different mandrels: a fraction of a sphere with radius of 50 mm and a surface formed by z = 0 . 0 0 4 − x + y − x y + o f f s e t − p i n p u s h , where “offset” is the pin's height, and “pinpush” the length of the pin pusher.

G-code generation for nonplanar lattices

Another script was developed to generate the trajectories to print the nonplanar lattice samples. The semispherical lattice had a radius 50 mm with five circular rasters and meridians rasters separated at 45°. The complex surface lattices consisted of rasters in the yz-plane and xz-plane separated by 2 and 5 mm, respectively.

The samples were fabricated with white polylactic acid (PLA). Nozzle trajectories were discretized in linear segments of 1 mm length. The spherical lattice was parametrized using polar coordinates, discretized by 1°, which was roughly 0.87 mm. These lattice structures were printed on both the adaptable bed and FDM mandrels. Tape was added to enhance adhesion while facilitating the removal of the sample. FDM mandrels were modeled in SolidWorks^® and manufactured with green PLA, 10% infill, and layer thickness of 0.2 mm.

Mandrels formed on the pin-bed

The two mandrels formed in the pin-bed are presented in Figure 1e and g. If a geometrical change is demanded and a new mandrel must be formed, the foam is removed manually from the bottom of the pin-bed. Then, placing the foam block again pushes all pins back to the original height. The geometric accuracy was quantitatively validated by pushing only one pin to a total distance of 10 mm. Taking the total height of the pin to be 18 mm (prior pushing), the resulting height was measured using a digital T&O^® Vernier. The average of 11 heights measured was 8.08 ± 0.36 mm when compared with the expected height, results an error of ∼1%.

3D printed mandrel versus pin-bed mandrel

Considering the pushing tool traverse speeds, 1500 mm/min upward and 1000 mm/min downward, the total setup times to form the pin-bed mandrels and the amount of printing material demanded to produce one sample are presented in Table 1. For both FDM-printed and pin-bed mandrels the material used for the samples was obtained by weighing them in an Analytical Balance ME54E by Mettler Toledo^®. Weights showed a significant saving in material employed. Fabricating the samples on an FDM mandrel demands >1000% of material. The pin-bed setup requires additional components, that is, the foam, the pin-bed; however, the amount of saving in materials justifies this. The pin-bed costs 20 USD and one piece of foam costs 0.4 USD (split into eight sections in this study); contrarily, every printed mandrel costs ∼1 USD.

The nonplanar lattices fabricated on the FDM mandrel were placed on the pin-bed and vice versa (Fig. 1e–h). Results were compared with an inverted optical microscope with a magnification 50 × by ZeissV^® (Fig. 1i–j). Regardless of the mandrel used, similar results were observed: (1) a rough bottom surface due to the tape and (2) defects on the top layers attributed to imperfections of the nozzle. From these, rasters diameters from both mandrels were measured. A minimal difference of ∼30 μm measured can be attributed to the pins being at lower heights than the intended ones.

The approach presented worked properly for lattice samples. Several opportunities were identified for improvements. Important reduction in formation time of the mandrel on the pin-bed can be obtained by automating the pins displacement, however, including linear actuators for such a number of pins could result in an expensive setup. Increasing the number of pins with smaller diameters will allow a finer discretization of the surface to be formed benefiting the fabrication of quasi-solid structures. When printing structures where the rasters are placed next to each other, a poor number of pins could lead to defects on the surface finish.

The proposal presented could be used in setups where the platform has mobility if the pin-bed is properly mounted on the platform. Once the pins are nailed, the foam could keep them in position regardless of the movement of the platform.