Glass Knife

[flisk]

I've been telling myself I would try to do this for a long time. This past week I made my first attempt at it.

Although the one in the picture below is not made of glass, instead it's a 2 part epoxy/resin, the priciple should still be the same.

I was in one of those stores that ends in Mart and I found in the hobby section a neat little kit that you can put your child's fist in the fill it up with resin and have a keepsake. Well, my mind ignored the fist on the cover and instead saw the beginnings of a knife.

To make a long story short I made a mold of the blade I wanted to use and tried it out.

This is what came out after 24 hours:



Already I see some things I want to change and I envision how I will do the glass knife. There was alot of dirt mixed in with this mold at the base of the blade, I'm still trying to figure out how that happened, maybe some dirt fell on it after I set it on the workbench; who knows.

Basically I intend to make a hard mold and melt down some cola bottles and try to pour them into the mold.

[TexasJack]

I assume you're using sand for a mold? Have you thought about using the 'lost wax' process? Make your knife model out of wax, close it in the mold, heat the mold - wax runs out and leaves the exact impression behind.

[flisk]

I'm not familiar with that process.
I'm also not too sure how the wax would hold up against liquid molten glass being poured into a mold of it.
I'm not sure that it would work with the two part epoxy/resin either.
The mold I used is a latex type rubber, very soft and "fluidic", then i poured the resin into the mold and let it sit for 24 hours.

For a glass version I'm looking at different mold materials that can withstand the molten glass A) without melting the mold B) without burning the mold and leaving behind non-clear burnt areas on the blade itself.

I was thinking perhaps Plaster of Paris, but I've not tested it yet.

Heck, I might even consider making a lead mold...

[TexasJack]

Plaster of Paris is hygroscopic and would probably explode. Lead would probably melt.

There is a fine sand that is used for casting parts and I would assume that glass (being sand) could also be cast in it.

A friend of mine works (worked?) at Ford making engine parts. (Jobs are a bit iffy now.) You can take a part that is broken - say a gear - and make an impression of it in the mold sand. Then do the same with the other side. Fill the cavities with wax and join them when it hardens to make a duplicate of your part. Add a sprue so the new material can get into the mold. Put the wax part into an enclosed mold and heat it up. The wax melts and runs out, leaving a perfect impression of the part. Fill with steel (in his case) and the new part is formed.

This process is centuries old. I think Ruger uses it to make pistols.

[R. Lemmen]

Using sand as a mold medium will give you a very gritty surface and it will fuse with the molten glass unless you are using some form of refractory sand. In the glass molding industry, tools are typically machined out of Heat-Shock resistant alloys like H-13 or Tungsten Alloys.

[justice]

i think clay or ceramic molds should work. no?


....justin

[AUBE]

check with a glass supply company..they carry different products for casting glass.
you can also lampwork it instead of casting...basically take a lump of glass and heat it in the forge(or with a torch) untill it is molten...then u can shape it by pressing it against different tools (graphite plates, wetted wood, etc) kind of like forging but you dont have to hit the material...just press it.

[Go4it]

I have used the lost wax technique for a number of years with bronze and aluminum (temps from 1600-1900 degrees) and have used a two layer investment. the inner layer next to the original wax was 75/25% plater/fine silica sand mixture with an outer layer of 50/50 plaster/medium sand mixture. We did this because it was a lot cheaper than the ceramic investments and didn't require as high a kiln temp. It is a lot messier, but more forgiving as well

The key is to kiln dry the investment at 1200 degrees for a couple of days. This makes the investment absolutly dry and prevents any blowouts. Moisture in the investment is what causes almost all the hazards of this type of casting.

The shape you are using is relatively simple so vents and sprues probably aren't necessary as long as what you are pouring is hot enough to pour smoothly. But that is a whole different topic of discussion.

this is my first contribution to the forum, but i finally came across a topic for which I can speak with some authority.

Thanks

[TexasJack]

this is my first contribution to the forum, but i finally came across a topic for which I can speak with some authority

And we're glad you did!