My 5-year-old daughter and I have been having fun designing and 3D printing her creations from Tinkerplay, formerly known as Modio. She's able to design something and feel ownership over it. This gives the printed toy more of a story for her, and thus a much longer half-life in her attention.
The Tinkerplay parts are really well designed and snap together with ball and socket joints. In the app, the default size is a ball with a diameter of approximately 10 mm, with the ability to scale up or down to 125% or 75%. I was curious if the ball and sockets would work at much smaller sizes, so printed a selection of Tinkerplay parts using the Autodesk Ember 3D printer, a digital light processing 3D printer.
Here, I've printed and assembled Tinkerplay parts as small as 20% of their normal size -- ball joints of 2 mm diameter. Smaller parts are probably possible. I was actually having trouble losing parts during the cleaning step where the parts are immersed in a liquid!
Step 1: Create in Modio, Scale and Orient in Meshmixer, 3D Print
All the creatures were created in Tinkerplay. Tinkerplay outputs an STL of the parts, which I scaled to the smaller sizes in Meshmixer. Each part is designed to print on an FDM 3D printer without supports or rafts. I rolled the dice and assumed they would print similarly well on Ember, so did not generate supports. All the parts I tried were able to print. All three of the prints shown were printed in Ember's standard prototyping resin at 25 um layers.
Note that while my daughter plays with the creatures she creates and we together print with FDM, these smaller Tinkerplay creatures printed with Ember were my own experiments. They're too small to really be fun, and don't make suitable toys.
Step 2: 50% - 5 Mm Diameter Balls
Here's a bulbous crab printed at 50% scale. It snaps together easily, all the joints move, and it's easy to pose and make it stand.
Step 3: 25% - 2.5 Mm Balls
Here's a propellor-headed 4-legged critter printed at 25% scale. Similar to 50%, it snaps together and poses easily.
Step 4: 20% - 2 Mm Diameter Balls
This is the half-snake half-scorpion from the included creatures in Tinkerplay. Printed at 20%, the ball joints are approximately 2 mm. There's a quarter in the images to show the scale. Assembling them, particularly this many, was tricky and required tweezers. A few of the sockets broke during assembly. Once assembled, they move and pose, and for their size, are remarkably robust -- I can carry the creature around without it coming apart. I haven't tried printing anything smaller because of the few broken joints, and the likelihood of losing the parts during the cleaning step after they're printed.
Step 5: Close-up Images of the 2 Mm Balls and Sockets
Here are some close-up images of the 2 mm ball and socket joints and the geometry as seen from within Tinkerplay. In one of the images, a graduated scale with 100 um lines is visible behind the ball.