Friday, November 14, 2014
mold making and the Friday selfie
Here's an example of the reproduction molds we use. These are RTV, room temperature vulcanization Part A Part B rubber. These were made from plastic toys like Bag-O-Bugs from the toy store. I make molds of things I find in nature too like rocks and acorns and sticks (I have a butt load of stick molds) and tree bark and shells and other stuff.
You can also use a liquid silicone or latex or silicone caulk. I use all these materials depending on what I am making a mold of and how long I want it to last. The latex molds have the shortest life and take the longest to make.
After I'm done with the model and it is glued down (because it is not heavy enough to not float in the mold mix) with the piece of styrofoam (extra space to hold the crushed glass), Marc paints on isopropol alcohol which acts as a bubble release so that air won't get trapped on the surface of the model and create holes in the mold.
If I am worried about air getting trapped during casting, I will sprue the small, narrow, or deep parts. The blue wax wires will become tunnels that let the air escape to the top of the mold.
Next he builds a dam around the piece
and then pours a mix of hydro-cal plaster, silica flour, and water around the model until it is covered by the mold mix.
Then it sits until it has set, about an hour, and he removes the glass pieces that comprised the dam from the hardened plaster mold.
Now the model is encased in plaster and the next step is to remove it.
styrofoam has been removed
He places the mold wax side down on a piece of hardware cloth over a pan of boiling, steaming water. A bucket over it will help hold and direct the steam into the mold and melt the wax out.
This is why it is called a lost wax process, because we are losing the wax model after the mold is made.
We also lose the molds which are called 'waste molds', to answer your question Steve, because they can only be used one time. Most of my work has deep undercuts so the only way to get the finished casting out of the mold is to destroy the mold which is very soft after firing. If the model were perfectly drafted, it would just fall out of the mold and then the mold could be reused. In that case, we would use a different stronger mold material formula.
One last step before I start to fill the molds...measure the volume and convert it to grams of glass. I do this by weighing a container of water, then pouring water into the mold up the the 'fill' point, weigh the container of water again, find the difference and multiply by the specific gravity of the glass to get the amount of grams (or ounces depending on what unit of measure you use) needed to make the casting.
So, here is me, bundled up in the shop on the coldest day of the season so far to start filling molds. I could have done it in the warm house except I have already moved all my frit (crushed glass) over here. Actually, it's not that bad now that the sky has cleared and the sun is warming up the metal building.