Our goal here is to turn this PLA 3D printed plastic shape into solid glass. While the process is fairly straight forward, there are several steps that will make or break this project. After successfully doing this several times the reward at the end is incredibly well worth it!
A quick note, there are several tools necessary to complete this inscrutables, some of which are hard to get your hands on unless you know the right people or happen to have one yourself. To sucessufly complete this project you must have access to a low fire kiln (~1500 degrees fahrenheit), a 3D printer, and have proper safety equipment (gloves, masks, glasses).
Step 1: Digital Model
Begin with a 3D model of the shape you wish to cast in glass. In this particular case, using rhino, grasshopper and a plug called intralattice, found here: http://intralattice.com, we are able to produce a simple 3 dimensional grid. The file for this test object is attached.
Step 2: Print the Model With PLA
To begin this step it is important to know why PLA. PLA (Polylactic acid) has a relatively low melting point, around 500 degrees F, which is in reasonable range when dealing with a home oven or kiln. This is because once the model is cast into a mold, it needs to be melted out to create the negative for the glass.
So, using PLA filament, choose your deisred 3D model and slice it. For this example, we use Simplify3D, which is a great tool for 3D printing but is not free. Other slicing softwares that are free are just as good, but some settings that are important to use:
18% infill, support where needed, and 2 loops.
The goal is to create a water tight model so when the print is cast in liquid plaster it holds it shape. Otherwise the plaster will infiltrate and ruin the cast.
Once the shape is sliced send it to the printer! Notice the build time, for this part it says it will take just over 24 hours to print, which is a long time and a lot of filament, so make sure you can check on it throughout the print. In the first attempt one of the motors for the printer skipped and caused the shape to shear, which is not a good thing. Hard to avoid this from happened, but reprinting the file on a different machine was sucessful!
Step 3: Mold Making: Inner Shell
This is the first time safety equipment is REQUIRED! Working with silica powder and plaster powder can cause breathing and eye irritation, so wear protective glasses and a mask to protect yourself.
First step is to make a reservoir for the glass to sit in while it is melting in the kiln. To do this, cut a large piece of clay the same size as the footprint of your 3D print. To get the depth you want to be more than the volume of the 3D print, because this is where the glass will sit as its melting before it fills the mold.
This calculation can be taken from the slicing software. When infill is put to 100%, the software will tell you what the volume of the object is. With this information you can make an accurate guess as to how much glass the shape will take. Its better to be over than under, an under filled mold of glass will have holes while a overfilled mold will just remain in the reservoir. (see diagram for less words)
Lay this block of clay on to a working surface, in the example we used a piece of acrylic, and push the 3D print snuggly into the clay so it sticks into the clay. This is important because if the 3D print is not deep enough into the clay it will pop up to the surface of the plaster mix like a buoy.
The mix ratio is 2:1 plaster and silica, so for every 2 cups of plaster, add 1 cup silica. Mix these two powders throughly into one mixture. This example piece used about 8 pounds of this mixture for the first pour.
Create the outside shape of the mold. In this case, roofing paper, hot glue, duct tape and clay were used. This was to help ensure no leaks, and is quick and easy.
Add water! The clock starts once water hits the plaster so be ready to act quick. Use a 1:1 ratio, so for every 1 pound of mixture add 1 pound of water. For the pound measurement a scale is needed but another measuring tool would also work. Add this to formwork and cover the print completely with about 2 inches from the top of the print to the surface of the plaster. Check for leaks! This happened a few times so have some spare clay ready to plug any holes. Drying time in a well ventilated area should be around 30 minutes, score the bottom right before it sets completely.
Step 4: Mold Making: Outer Shell
Don't pick up the mold yet! The clay should still be secured to the acrylic or similar material. After it drys, remove the formwork and add chicken wire. This is to help strengthen the mold, like rebar in concrete. The same process of formwork with roofing paper, hot glue, duct tape, and clay can be used to construct the outer shell, which should be offset from the inner shell by at least 2 inches, this can be dependent on the size of your kiln, and your strength, as an extra inch will increase the weight of the mold.
Use the same add mixture of plaster and silica and add a few scoops of regular sand. This will help again strengthen the mold, as in aggregate in concrete. Pour into the formwork until the mold is completely covered, maker sure no chicken wire is exposed. If you are making multiple molds its a good idea to name the mold, in this case #1.
After the mold sets, remove the formwork, flip it over and extract the clay! You can also round the edges of the mold to help reduce chipping. If there are any leaks into the 3D print it will be clear at this stage. Check to see if there are any issues, if not on to the kiln!
*Side note, this mold failed as you can see the plaster in the 3D print. Luckily we made other molds, which will be seen in the rest of this instructables.
Step 5: Melt Out the PLA
During our process we decided to make several molds. This mold, using the same techniques described previously, has just had the PLA burned out. The round shape vs the square shape can make a difference, which is what we were testing, because round shapes heat in the kiln more evenly than square shapes.
To melt out the PLA, turn the mold with the holes facing down, and leave a scrap drip tray underneath. We used this firing schedule; hold at 200 F until not visible moisture, then 60 degrees per hour to 525 degrees and hold for 2 hours. In the case of this mold, we went a little bit above 525 which caused the PLA to burn and melt. If done properly this can be avoided. The off gas of PLA is non toxic, but its always a good idea to work in a well ventilated place.
Once this PLA is completely removed, you need to check the mold to make sure it is melting during this process, then the mold can be cooled back to room temperature and is ready for the glass!
Step 6: Melt Glass Into the Form
At this point you have successfully melted out a PLA print from a plaster mold and are ready for the best part, glass casting! We used glass billets, in this case clear, that are made for casting. This glass is specifically from Australia and came from a local glass dealer. They are sold by the kilo, 2.2 pounds, which is one billet. Make an accurate measurement of the volume of the glass needed using the same calculation from before to get the size of the clay. This will ensure the glass makes its way into the entire piece with some left over just in case.
Using a specific casting firing schedule the piece is brought up from room temperature, increased 100 degrees every hour to 1525 degrees and hold for 3 hours, then drop to 806 AFAP and hold for 6 hours. Then drop to 680 at a rate of 13 degrees, and then 590 at a rate of 26 degrees, and then to 70 at a rate of 78 degrees. This is to make sure the glass doesn't crack as the temperature is going up or going down, now that it is in a new shape and needs to cool properly.
Step 7: Removal and Cold Working
Finally! The glass is finished cooling, the plaster can be touched with a bare hand and the kiln is off. Depending on the cast you can have a little bit of work or a little bit more work, either way extracting the mold from the plaster takes some effort.
Beware the glass is sharp! At this stage the glass is raw and may have edges that are incredibly sharp, so please wear gloves at this stage. Break away the formwork as much as possible, remember the chicken wire which is also sharp. The plaster silica at this point will be easy to break a little force. Once the plaster is off as much as possible, or if its a complex shape, it is time to soak it in water. Use a bucket and rinse/soak the glass in the water. The plaster should fall off with a little bit of agitation.
With the plaster off, the glass is finally free! The glass needs to be cold worked, which means sanding and polishing with glass cold working tools. These tools use water to keep the glass happy as the glass is being sanded and to keep down sand dust, which again is a hazard, so wear safety equipment!
A good cold working session should get you to a finished product and viola, the process is over! In our case the final products came out well with all the flashing and sharp edges gone, leaving the shape of the original 3D print, which was our goal! Good luck with your prints and leave comments/questions below!