Mold blown glass has been an essential strategy for production glasswork especially in the Czech Republic and other Scandinavian countries. Although the Czechs have become especially good at this style of glassworking and wooden mold-making using a lathe, it requires special skills, tooling, and large solid material. Unless you are a Czech mold-making master, it is also very hard to get accurate forms by hand carving on a lathe.
The fin-mold strategy I've been developing has allowed me to prototype mold-blown glass forms with accuracy relatively quickly, with a band saw instead of a lathe, and with wooden boards as opposed to solid hardwood.
I arrived at this approach out of necessity. Without regular access to a CNC mill, I found it very expensive and slow to go to professional mold-makers. The price made it difficult to refine forms over multiple versions of a glass and too slow to go through many versions quickly even with a large budget. As many industrial designers know, shortening the window between concept and prototyping, and lowering the time and monetary cost of every version allows you to arrive at a much more thoughtful and functional outcome quickly.
This Instructable assumes prior glassblowing and woodshop knowledge to execute, but anyone who is interested in how glass is made may find it interesting.
Step 1: Choose Your Profile
-Cherry boards about 1" thick. Cherry is a good wood choice because it isn't noxious when it burns, and it's dense so it will withstand burning longer than most other woods. DO NOT USE PLYWOOD. The glue is poisonous when it burns! (The amount you need will depend on the size of your mold)
-Wooden Dowels (around 16)
Step 1: Sketch out a glass that you want to make. That profile should be radially symmetric and have no undercuts in the horizontal direction. If you think about opening a mold in two halves, you don't want any glass getting in the way and keeping the mold from opening once it's cooled down and solid.
Draw that profile in illustrator, autocad, or somewhere that you can make precise adjustments to your line. Add an enclosing rectangle that represents the size of the wooden fins you'll be cutting the profiles out of (mine is drawn in cyan). One you start blowing glass into the mold, it will start to burn away so you need some extra material back there for strength. I recommend at least 1.5"
Step 2: Cut and Attach Profiles
Print out copies of the enclosing rectangle and profile at 1:1 scale. Cut out wooden rectangles at the dimensions of the cyan bounding box. Then, tape the prints to the pieces of wood, using the bounding box as registration lines. When you go to cut your profiles on the band saw, make sure you have some extra tape handy. As you cut the profile, you'll also be cutting through the paper and you'll probably need to do some re-taping as you go.
Once you have all your profiles cut, you can prepare to attach them to the top and bottom faces of the mold. I used the same registration strategy for marking out the hole placement as cutting the profile. Print out the radial hole pattern you will need to attach the fins to the top and bottom boards with dowels. Tape them to the boards that will sandwich the fins. You should have 2 boards for the top, and 2 for the bottom. The two bottom boards should be full rectangles and the top two should have a semi-circular profile cut out of it that will allow the glass to drop into the top of the mold when it's closed.
Next, drill out the tops and bottoms of the fins. Make sure the drill holes are centered along the width of the fins, and the appropriate distance in from the edges to make them register with the holes on the top and bottom boards. If your profile is small, you'll also have to belt sand a bit of the corners off the ends of each of the fins so they don't interfere with each other. Those don't have to be precise since the glass doesn't actually touch that part of the fins.
Then knock those fins into the corresponding holes and finish by knocking the top boards onto the fin dowels.
Step 3: Finishing the Mold
The last thing to do is to get all the fins and faces squared up and add some faces on the sides for the latches and hinges. Those pieces can be plywood because they won't be getting touched by the glass.
Once your done, you can soak your mold in water overnight and it'll be ready for glassblowing. You'll have to "burn it in" before it starts to work nicely. That basically means blowing a few rounds of glass into the mold. This burns the faces of the wood which touch the glass and builds up a layer of carbon on them. That layer of carbon protects the wood from burning even more and gives the glass a smooth surface to ride along as it spins and expands. Make sure you soak your mold between each time you blow into it.
Step 4: Refinement and Results
You can repeat the mold-making process as many time as you need to refine your shape. As you can see in the pictures, the final shape I actually approached a mold-maker with was very different than the profiles in the first fin molds. I probably went through the entire process about 4-5 times before I reached a shape that I was happy with and that could be mold-blown nicely.
The wooden mold was used to make the first few batches of glasses. The final shape was tweaked just slightly before modeling it to be CNC'd out of aluminum. The mold actually ended up being made as 4 parts that were then fastened into two halves. It had to be done that way to create the sharp bottom corner and small ring on the very bottom of the glass. Both haves also have vent holes to allow steam to escape, and the inside faces of the mold get lined with cork. I also added an extra "overblow" profile to the top of the shape. That part actually gets cut off when the glass is cold and helps keep the lip of the final glass thin.
To finish the glasswork, the overblown material is "hot-popped" (skip to 2:00 min) off which is a fancy style of scoring and splitting the glass with controlled heat. The lip is then fire polished to round out the edges. This method is not only faster than shaping the glass by hand, but it also allows for much thinner and even walls. Even for a very good glassblower, making a simple cup takes somewhere between 7-10 minutes in the hot shop where the mold-blown method takes about half that time. There is some cold-working to be done to finish the cup, but it doesn't require all a furnace so it's much more energy efficient.
Each mold is also much cheaper. A very simple wooden mold can cost a couple hundred dollars to have made and take a few weeks to get back, while the cherry I used to make the fin mold cost me $4 a board foot. I only ended up having to use about 3 board feet per mold, so it only cost me $12 plus the cost of hinges and a few hours of work per version.
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