Week eight: molding and casting

Ele-phone holder

When it took a shape...

This week our assignment was to mold and then cast a 3D object of our choice. Lacking inspiration, I searched the web and found a cute elephant phone holder and a nice looking puzzle piece I thought I might want to cast.

The steps of molding and casting are:

  1. Design a positive model
  2. Mill that positive model
  3. Mold a negative of the positive mill
  4. Cast a positive of the negative mold

Little shopbot setup
Puzzle piece rough cut
Milled puzzle piece

This week is very straight forward. It involves using skills from previous weeks and following simple instructions on a bottle.
In choosing something to mill, I recommend something that takes advantage of the fact that you can do 3-axis milling. Neither of my designs did. They both have flat did large-format machining. I also recommend not choosing something with small pieces. The elephant was difficult to cast because the space for the trunk was so small. If you want to avoid doing a two part cast, consider designing something that is flat on its side.

Molding materials
Get out the bubbles

For molding I used Oomoo and for casting I used hydrostone. In order to not waste a ton of material, I recommend Using water to measure the volume of your object first. Fill a cup with water and pour it into your milled object. That amount of water is twice the amount of blue and twice the amount of yellow you need to mix. When it comes to casting, that is ~4 times the amount of water you need in hydrostone.
I managed to avoid getting bubbles in my mold by using the vaccuum pump after mixing and after pouring into my milled object.
Removing the mold from my milled object was difficult because there was not a lot of clearance between the shape of my object and the containing rectangle. I recommend leaving more clearance there and at the top of the object. If you break your mold then use cardboard or tape to hold it together, hydrostone is fairly viscous. Nothing really sticks to Oomoo though, so hot glue won't work.
I had a few bubbles in my cast because hydrostone is thicker than drystone. This could have been resolved by gently tapping the mold after I poured the hydrostone into it.

Completed ele-phone holder mold
Completed ele-phone cast
A random head I decided to cast since I had extra hydrostone. Has some bubbles
Spray-painted ele-phone holder