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Molding A Molten Glass, Questions

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Plastic
Joined
Apr 18, 2013
Location
Beverly Hills
I have few questions:

1. How viscous is a molten glass? Is it similar as a condensed milk?
2. How do you release a glass from the metal mold when it already cool and hardened?

Thank you
 

johnoder

Diamond
Joined
Jul 16, 2004
Location
Houston, TX USA
As a kid about 60 years ago, I watched glass bottles being moulded /molded

Glass was NOT liquid.

It was hollow preforms - think of a glass balloon hot enough to be extremely ductile

Metal molds / moulds were split - two halves

Glass preforms went into mould / mold and it was mechanically closed.

Air was applied at open end on preform - forcing it to conform to shape of mold / mould

Mould / mold opened mechanically releasing the finished glass bottle, still red hot

It was placed gingerly on a conveyor with its identical siblings to cool.

This was large old automated equipment.

Probably better ways to do it today, but the basics are sound, quick and cheap, per unit, in a production setting
 

snowman

Diamond
Joined
Jul 31, 2004
Location
Southeast Michigan
The answer is that it depends on the temperature of the glass.

The question is a lot easier to answer if you tell us what you are trying to do.

The act of getting glass to a temperature where it will flow, and in a large enough mass to do so, without bubbles, takes some knowledge and experience in itself.
 

kapps

Cast Iron
Joined
Sep 17, 2007
Location
Orlando, FL
It also depends on the quantity and type of glass. I did some glassblowing in college and if you want consistency, you start a bubble then blow it out into a form. Glassblowing glass holds more heat which allows it to be worked over a larger temperature range. Mass production glass goes from molten to solid VERY quickly. This allows use of machines that perform the same blowing process but it takes a couple seconds instead of a minute. I don't think you want to cool the glass in the mold. Let it get cool enough to become solid then get it out of there. Some glasses need to be annealed or they will blow up randomly (sometimes weeks later).
 

atomarc

Diamond
Joined
Mar 16, 2009
Location
Eureka, CA
Some glasses need to be annealed or they will blow up randomly (sometimes weeks later).

Far from an expert on glass but I will tender a guess that ALL glass has to be annealed or it will crack or break. The glass I saw being formed into bowls and the such was dipped out of a molten vat and the appropriate size hunk was cut off the glob with scissors. The molten glass was the viscosity of very thick taffy.

Stuart
 

snowman

Diamond
Joined
Jul 31, 2004
Location
Southeast Michigan
annealed, yes, but not on the same scale. pyrex can often be annealed in open air, if the sections are thin enough. anything with any thickness, yes, absolutely.

soda lime glass, absolutely. it's all about how much it expands, and glasses with more silica in them such as borosilicate or even flint glass require a less stringent annealing process.

soda lime glass, you won't even get it cool without it breaking.
 

Trip59

Cast Iron
Joined
Mar 2, 2011
Location
Middleburg, Florida, USA
I have few questions:

1. How viscous is a molten glass? Is it similar as a condensed milk?

Depends on the glass, the temperature, and the process. Most is in the range of molasses, from room temp to colder (molasses, not glass). Lots of things you can do with it and varying temps for each.

2. How do you release a glass from the metal mold when it already cool and hardened?

Thank you

Well, if it's stuck to the mold, it's gonna be fun. Metal works well for molds, as long as it's kept cool. You get metal hot enough and the glass will stick to it. Best to cool down the mold, tools, etc. between touching the glass. I've learned a lesson or two the hard way, metal got too hot, glass stuck too much... heated the glass up again till it started to get soft and hit it with water or dipped the tool (preferred) and the glass fractured off cleanly. Not sure how to get it off the metal in that situation without ruining the glass.

Water soaked wood, soaked newspapers work for hand-working larger pieces. Graphite works well for smaller pieces, I use that for lampwork (marbles, beads, small sculpture) lots of shapes and can be machined (be nice to machine some once I get the mill up and running) to specific shapes. Much less chance of sticking, but you still want to be mindful of temperature.
 
If you ever make it to Corning, NY, there is a free monthly party in the winter months at CMoG that feature glass artists & performance called "Fahrenheit 2300.

2300° | Corning Museum of Glass

Also, several venues around the area including the museum where you can step right up and blow ornaments for a few dollars each, throughout the year.

Welcome to Hands On Glass

Many local artists or venues are supportive of experiments in glass.

Probably other areas of the country with similar access?

smt
 

9100

Diamond
Joined
Nov 1, 2004
Location
Webster Groves, MO
I needed a small cup and made a graphite mold. A glass blower at the 3rd Degree glass factory blew a bubble into it, which I cut off with a diamond blade on my surface grinder. The glass shrinks enough as it cools to come free from the mold. They use aluminum molds for a lot of starting shapes that they then work.

Often old glass plates and saucers or headlight lenses will have a faint circle about 1/3 of their diameter in the surface. This is the imprint of the glob of glass they started as. They were molded on a simple machine called a sidelever press, which has the bottom mold half on a table and the top on a swing arm above it. The arm is operated by a handle on the side, hence the name. The operator dips a blob of glass out of the furnace with a metal rod and drops it on the mold, then closes it quickly, squeezing it out to shape. The mold is cast iron and has no temperature control. The operator starts out with a cold mold and molds bad parts, which are thrown back in the furnace until the mold reaches the temperature that allows the glass to stay molten long enough to flow out to the mold edge. After that, the operator paces the process to hold the mold at the proper temperature. If you have cast bullets in a hand mold, you will know the drill.


Bill
 

Trboatworks

Diamond
Joined
Oct 23, 2010
Location
Maryland- USA
I have few questions:

1. How viscous is a molten glass? Is it similar as a condensed milk?
2. How do you release a glass from the metal mold when it already cool and hardened?

Thank you

The hotter you get it the faster it flows- just get some glass and a torch and start messing around with the stuff.

First job I had was in a scientific glass work company which did custom apparatus via lathe and hand work.
Interesting stuff- been too long but we had some sort of scope which the parts were placed in which clearly showed the stress in the glass before annealing- it would show up as foggy areas at the joints.
I was doing piece work and made thousands upon thousands of parts.

One tip:
Hot glass looks just like cool glass...
I still can pick up hot items which my wife has to drop & just mutter- "I used to work in a glass shop".
Seems the feeling never comes back after you singe the finger tips enough times..
 

atomarc

Diamond
Joined
Mar 16, 2009
Location
Eureka, CA
When my two boys were small I cast their initials from glass. Carved their first initial in a soft fire brick. Busted up a clear glass mayonnaise jar and filled the carving with small pieces of glass. Put the whole enchilada in my heat treating oven and let it cook. The glass melted and flowed nicely. After a annealing period and cooling...presto, a perfectly formed clear glass set of initials. It was a fun thing for the kids to be involved in as well a learning thing for me.:)

Stuart
 

9100

Diamond
Joined
Nov 1, 2004
Location
Webster Groves, MO
The stress detector is a light source with two polaroid filters in front of it. The glass is placed between the filters and stressed areas rotate the polarization which changes their brightness when viewed through the second filter.

You can make thick glass objects but you will need an annealing oven with a temperature control that ramps it down over a long period, days for thick sections. Laboratory glass blowers, especially when working Pyrex, can get away with turning the oxygen down to make a cooler smoky flame that warms the joint and deposits a layer of soot to slow cooling. That works with thin walled tubing but not with a big blob.

Bill
 

Trboatworks

Diamond
Joined
Oct 23, 2010
Location
Maryland- USA
The stress detector is a light source with two polaroid filters in front of it. The glass is placed between the filters and stressed areas rotate the polarization which changes their brightness when viewed through the second filter.

You can make thick glass objects but you will need an annealing oven with a temperature control that ramps it down over a long period, days for thick sections. Laboratory glass blowers, especially when working Pyrex, can get away with turning the oxygen down to make a cooler smoky flame that warms the joint and deposits a layer of soot to slow cooling. That works with thin walled tubing but not with a big blob.

Bill

Thanks Bill- that is what I was figuring as I thought this over. The scope brought up a effect that can been seen when one looks at flame tempered safety glass with polarized glasses.
 

surplusjohn

Diamond
Joined
Apr 11, 2002
Location
Syracuse, NY USA
Like Steven Thomas said, find a hot glass studio, I am sure there are many in your area. you will learn a great deal in a short amount of time. hot glass artists are clever people and like most artists and crafters, are happy to share knowledge.
here is one: Glass Blowing and there are industrial glass molders of course.
 

9100

Diamond
Joined
Nov 1, 2004
Location
Webster Groves, MO
Thanks Bill- that is what I was figuring as I thought this over. The scope brought up a effect that can been seen when one looks at flame tempered safety glass with polarized glasses.

Tempered glass is deliberately stressed. By heating it until there is some plastic flow, you take out all the stress. Then you cool it so the skin loses plasticity while the center is still hotter. When it cools clear through, the inner part shrinks and puts the skin in compressive stress, which is much stronger in glass than tensile or bending stress. To break it, you have to apply enough force to change a compressive area to a tensile one. If you scratch it, the balance of forces changes and things go south in a hurry.

BTW, the chief metallurgist of American Motors once related asking his equivalent in Corning glass about the tapered windows in Pompeii, which supposedly flowed downward over the centuries. The answer was that the variation was just poor quality control. If the window glass flowed, it would be impossible to make tempered glass because the stresses would just flow out.

Bill
 








 
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