week_09 molding and casting
Tasks
design a 3D mold, machine it and cast parts from it
File
download the rhino5 file
software: Rhino, FabModules
tools: SRM-20
materials: machinable wax, silicone
location: Green FabLab Valldaura (BCN)
how to:
first i measured the material i already have (pipes from the trash) and adapted the prototype from the 3D-printing week to fit the material. within the valve there's a fissure that will hold the ring.
the first design made me unhappy when i looked at it. since the thickness of the ring is only 3mm i imagined having troubles pouring the silicone because of its viscosity. also the air hole should be on the highest part. so i redesigned it with the intention of using the Vacuum Reverse-Injection Molding Machine from ronald. the silicone will be sucked into the mold with a vacuum pump, excess material will be trapped in the glass. to be able to attach a valve to the mold there's a cylindrical hole at the top of the ring. the silicone will be poured into the cone.
i cut a piece of wax off the block using a bandsaw, which worked perfectly. nevertheless the wax needs a plane surface to later have no gaps when the two molds are put together. for greater precision and less work i modeled the positiv. then put it on the correct plane within the stock and then mirrored the model onto the stock. after that i did the same with four sphreres, which will later make sure both sides of the mold fit perfectly together. with boolean difference i substracted the unwanted geometries from the stock.
milling
i exported the rhino model as a .stl file. first i tryed to use fabmodules to create the gcode but for some reason it didn't work, mabe the file was too big? so i created the toolpath with the modela player 4. The roughing happened with a 3mm flat milling bit, for the finishing i chose to use a 1/32" bit. the settings updated automatically by choosing the milling bit in the menu.
problems
the result as such is very nice! in the drawing i had a fissure of 3mm to cut the block in two pieces while roughing. the program recognized it as too small for the milling bit so it didn't mill. when i changed for the finishing the gcode included the fissure and so the bit went into the full material. nothing broke fortunately! the modela 4 player let's you choose the area that you want to mill so i did it in two parts in the end and cut the block in half after it was finished.
casting
the silicone comes in two componetnts which need to be mixed in a certain ration and stirred. the ratio vries, so it's best to look it up in the manual, also precision is key so a scale is needed.
since there are bubbles in the mix because of the stirring, the silicone should be vacuumed. we put an adapter into the lid of a gurken glass and attached the compressor of the lasercutter to it, connecting the jar to the air inlet.
i used double sided tape to make the mold airtight and screwed an adapter into the material outlet of the mold. to test the airtightness i connected the compressor and after starting it put my finger onto the material inlet. i added some hot glue around the adapter and then it worked!
to make sure no silicone would go into the compressor another jar was installed as a trap.
to hold the two parts of the mold together i squeezed them between two pieces of board using clamps.
then, with an improvised funnel, i flipped the switch but it seemed to be not working. it looked like no material was sucked into the mold and i thought the diameter of the mold was too small for the power of the compressor. since there were more molds to be filled i gave up.
after a couple of hours curiosity took over and to my surprise it had worked!! a nice silicone ringwas in the mold.
then i cut off the excess material and tryed if the ring would fit into the valve - and it works!