We offered a new laboratory exercise in our Compute Aided Engineering class — Kiln Glass Casting. We have been playing with direct 3DP molds for a little over a year now. The way to know if you really understand something is to let students give it a go.
This lab allows you to experience direct mold printing (via 3DP) and kiln casting glass. Your mold will be used once(it will be
distroyed during the Kiln casting process).
You will not need draft and you can have undercuts (if you have undercuts, you will likely need vents due to trapped air).
Also, if you have significant undercuts, there may be issues of de-powdering your mold.
You will not be constrained by mold boxes BUT you may use only 12000 mm^2 (~18.6 in^2) or less of plate cross-section and the max
height is 70 mm (about 2.75 inches).
No part of your resulting glass object can be more than 1 inch (25.4 mm) thick.
Mold must be 16mm (5/8 inches) thick everywhere.
- Consistency of honey.
- Features on a surface (min 3-4mm).
- Smaller features might break off in depowdering (similar to previous labs).
- Some smaller features might not completely fill out, but they will leave an impression.
- Extruded and thinner features must be greater than 3/8″ with proper gates to allow flow and air to escape.
- Edges of smaller features will not be crisp.
- Must be able to access all parts of the mold to depowder it.
- Features on the top surface (especially open-faced) on the edge of the mold will be rounded because of the meniscus of the glass.
They designed! We printed molds. We set the firing schedule on the kiln to cast glass, loaded up the kiln, and pushed start…
The result was a 50% yield. Even after all of the reading that we had done on kiln casting (and successful runs), the thicker parts were not annealing and cooling correctly. We even had one explode with such force that it destroyed its mold! Time for more research and reading. After much more reading, we learned a new phrase — “lower strain points“. While one could employ finite element analysis to run an annealing/cooling analysis to determine a furnace curve (perhaps another time), much of art glass is explored empirically. We found an amazing book of furnace curves by Graham Stone called “Firing Schedules for Glass – The Kiln Companion”. Biblical in firing schedule nature! With several new firing schedules in hand, time to reprint molds and try again…
Yes, we know that you REALLY shouldn’t open the kiln when its glowing hot (so don’t) but we couldn’t resist taking this picture. Our resulting firing schedule is a mixture of schedules from Bullseye Glass, Spectrum Glass, and Graham Stone’s Book. We were casting regular (not premium) Spectrum Furnace Nuggets (a System 96 soda-lime glass) which is typically used in glass blowing furnaces (it is economical for class use).
Rate (F/hr) Temp (F) Hold Time ================================= 200 850 0.5 hour 500 1050 1 hour 500 1650 6 hour 9999 960 6 hour 8 800 1 hour 16 700 1 hour OFF to cool to room temperature
This schedule seems conservative and produced 100% casting success.
We believe that this curve works for objects of 2 inches (100 mm) or less in thickness. (How one determines maximum thickness is completely another issue to be discussed another time).
Our students had a great time (although if we’re not careful this could become the “shot glass lab”). We still have a few more pieces in the kiln… We can’t wait to see them.