Yoshimoto Boxes

The Yoshi Box is a hinged toy reminiscent of the folding patterns of a Yoshimoto Cube, but it employs mechanical hinges to allow the pieces to be folded into a endless cycle of shapes.  The pictured model is available for purchase from Shapeways or Sculpteo.

This toy is made up of eight bisected boxes, each attached to two of its neighbors with mechanical hinges.  This sample was built in nylon SLS with the hinges already fused together.

The small tabs enable two models to be snapped together so they'll move in harmony with each other.  I suppose you could hide small treasures inside the boxes, too.

The inspiration for the Yoshi Box came from Yoshi Prime, a previous Yoshimoto-like toy I built on my SD300 using living hinges.  It's built in one piece with thin, flexible plastic connections that permit it to be folded in each axis.

Here's a closer view of a living hinge.  To build a vertically-oriented living hinge on the SD300 I bridged the connection between adjacent cubes with a wall 0.3mm thick, with a graceful curve to spread the stresses as much as possible.  I suspect it would have a limited life because of the amount of bending required, although it hasn't broken.  (Yet.)

This model combines the box-like style of the Yoshi Box with the living hinges of Yoshi Prime, forming a group of 8 small treasure-boxes connected by living hinges.

Comparing with SLS and PolyJet

I uploaded my spherical 2x2x1 puzzle (the 'practical' one) to Shapeways and tested some samples built in other materials. (It's available for anyone to purchase.)


The parts built in nylon SLS (left) were functionally identical to the PVC parts built on my SD300 (right). In fact, they were interchangeable. Both materials tolerate the flexing that's required to install the last piece.


In contrast, a part built in Objet "PolyJet" (left) arrived already broken. Although the PolyJet Fullcure material claims to withstand bending, it's apparently brittle at this 1.6mm thickness. The SD300 part (right) is sufficiently strong because the posts were built in a horizontal orientation.


But the PolyJet puzzle is sufficiently intact that it works okay, despite the broken piece.

Process comparison: SLS

Here's a part that includes an edge that tapers down to 0.10mm in thickness, far thinner than most 3D fabrication processes are rated for. I had copies built by Invision HR, SLS, and my SD 300 and compared how they handled the thin taper.

At left, the blue part was built by an Invision HR. It truncated the taper before the end, and the portion it built was brittle so the leftmost taper broke off. But this isn't a fair comparison because the part was built in a vertical orientation, so the build didn't take advantage of the Invision HR's excellent Z-axis resolution.


The SLS part was built in bright white polyamide. It was difficult to get a picture of it, but you can see the thin tapers were built to their full length. My caliper measures the thickness as 0.19~0.23 mm, about twice the thickness specified in the STL file, but the tapers are sufficiently tough that they can survive handling. But the nylon exhibits signs of fatigue after flexing, suggesting it would eventually break.

At right, the SD300 built the thinnest part of the taper to full length with a measured thickness of 0.16~0.17 mm. The taper is thinner and more-flexible than the SLS part, but it's tougher too. It showed no fatigue after flexing, which isn't a surprise: it's basically just a single thickness of PVC sheet. But elsewhere, on the top of the curved 'foot', the top sheet of PVC would probably peel off if it was exposed to much use.


That weak spot presented a good opportunity to test the effectiveness of solvent-finishing, which I discussed in my last post. I dipped the part in Weld-On 2007 and tested it against an untreated part. I put both parts into the cavity for which they'd been designed to fit and put them through mechanical tests. Afterward the untreated part showed some wear, at left in the picture below. The weak layer is still attached, but it's peeling. The solvent-dipped part at right is in good condition, exhibiting no signs of wear or damage.