9. Molding and Casting

design a 3D mold, machine it, and cast parts from it

Making "Hugging Teddy bear" : softrobotics

For this assignment, I have two weeks. I try to make hugging Teddy bear with PneuNets Bending Actuators. This mechanism is inspired by softrobotics the video below.

Idea sketch

It’s very simple mechanism. there are two parts: body, arms. Body looks like ball and contains air. If I hug Teddy bear, the body will get pressure. And then Teddy bear also hug me because the arms are bended when the air goes inside.

Making Mold

Design Mold : body

I want to make a ball which has empty space inside. I split the ball to half and going to stick it together. I made two mold parts; bottom part is outside of the ball, top part is inside of the ball

Download (ipt, stl file)

Making Wax block

We have a suitable wax for Machining, but It was only 4mm thickness. I melted it with a heat gun. this list is the needed things for making thicker wax block.

wax blocks to melt (I also the classmate’s used wax chips after being machined.)
2 Molds for a Wax block (Acrylic box: Laser cutting)
heat gun (oven would be much better)
pot
refrigerator (much faster to cool it)

Download (Acrylic mold for wax block pdf, dxf file : 170mmX130mmX70mm)

Failure

first time I tried to melt each surfaces of the two blocks and glue it together, but it doesn’t work.
Masking tape let wax leak. I used duck tape after

Flattening wax

for machining, I made the wax flatten using shopbot and Vcarve Pro. make a pocket toolpath 3mm depth using upcut endmill 1 flute.

pocket depth = 3mm
Diameter = 6.0mm
Pass Depth = 4mm
Stepover = 3.0mm (50%)
Spindle Speed = 14000rpm
Feed Rate = 200inches/min
Plunge Rate = 60inches/min

download the flatten files (crv, gcode file)

Machining Mold

Make a tool path using Modela Player 4.

import stl File
change the origin to right way, make sure the scale of the material
choose Z-top surface to cut
Cutting Area : Check ‘Partial’, lower left X:0, Y:0, Upper right X:170, Y130

Cutting parameters

-Roughing
Tool : 3mm square
XY speed : 30mm/s
Z speed : 10mm/s
Spindle : 7000rpm
Cutting-in Amount : 1mm
Path Interval : 1.8mm
Finish Margin : 0.2mm

-Finishing
Tool : R3 ball (6mm diameter)
XY speed : 30mm/s
Z speed : 24mm/s
Spindle : 7000rpm
Cutting-in Amount : 0.2mm
Path Interval : 0.2mm
Finish Margin : 0mm

Create tool path : Roughing takes about 3hours, Finishing takes about 2 hours

bottom mold

top mold

Making arms (using 3Dprinter and open-source design file)

I 3d-printed mold files and Casted it (I used ke1402. Next time, I want to modify my design and make with CNC. I followed instructions and downloaded mold stl files from the link below.
http://softroboticstoolkit.com/book/pneunets-bending-actuator

3D print the downloaded stl files
Casting : Mold Star® 20T Silicone
Mold Star® 20T is an easy to use translucent platinum silicone that is mixed 1A:1B by volume (no weighing scale necessary). It features a relatively low viscosity and vacuum degassing is not required for most applications. Pot life is 6 minutes and cure time is 30 minutes at room temperature.

Assemble

I made two same body parts and glued it with silicone. then glued it with arm parts.

Body part casting (2EA)	glue it with silicone
glue two casting parts	body part Assembly
It was very effective to clean the surface with acetone before gluing it

Failures

it was really hard to take silicon cast out from the mold which is wax. I used WD7 which is penetrating oil. It was much better to take it out


	I broke a mold to took it out

The arm parts what I modified were too thick to bend it by hand. It is too hard to bend it. so I had to redesign the design files. I read design rules for softrobotics on the website.