Basic quick blow mold

[David Patchen]

I have to make a half-dozen bottles and want to make a simple mold for them. The bottom of the bottle is square and the top is curved, so I won't be able to turn it. I have an actual bottle to duclicate so I thought a plaster mold might be best.

I searched the board and Henry's book for info on plaster molds and found some useful stuff, but still have some really basic questions. I've never made anything more than a quick plywood number before so this is new to me.

- Is there a special type of plaster I need to use or is all plaster the same?
- I was considering coating the glass bottle I'm going to duplicate in vaseline so the plaster doesn't stick to it. Bad idea? Better ways to achieve a clean release?
- I was thinking the mold seam would go diagonally across the bottle, but how do I set the plaster around the bottle so the mold is formed in two halfs, or do I cut it later? I'd prefer not to cut the plaster+bottle to divide the mold into two halves.

Thanks in advance for your thoughts and advice.

[Greg Frankhouser]

Having had the Requisite Kent State mold training here goes;

Release: Vaseline cut with kerosene will give a better/smoother finish. Spray generic PAM is also popular.

Prep: Make sure that the bottle has no undercuts, including the bottom if the mold will have one. Most bottles have a slight indent. Most blow molds don't last, so cheap plaster will work, but I've never tested better, so...

Yes the diagonal is a good way. To set up the plaster, take clay and build a wall on your edge. You can mount the bottle on it's side in sand, or styropeanuts for that matter, but you don't want it to move. You will want to do a slop coat, A layer of thick plaster, so that you can build your edges up. If you need more meat in the mold, water down the Already set slop coat, then add more. The water is so the new plaster stick well to the old plaster. Leaving some gripping surfaces is wise as well.

Important note: You will want to have a flat something at the ready to make a flat side parellel to your mold seam. Why? So you can clamp it shut while blowing into it. NO, it's not wise to use hands, they generally arean't stong enough. Plus the steam can get a little toasty. The quick release bar clamps work well.

After the first side, remove the clay, smooth down the plaster to porvide a nice mating surface, and add some keys (divots) in the mating surface, so part 2 will fit properly. Grease the Seam and repeat the above slop technique. Gently prise the 2 parts apart.

Almost ready. Drill a few holes in the hidden corners of the mold, maybe 1/8", at least 1 in each corner. You may need more later. These are to allow any trapped air to exit the mold.

Now you have a choice, Dry Blow or Wet Blow. Dry Blow, coat the inside of the mold with Powdred graphite cut with Alcohol. As thick, well maybe 2 thin coats, maybe more depending on the amount of detail. The graphite protects the mold surface. Wet Blow, Soak the mold in water, pull out when ready to blow into. i.e. when the bubbles stop.

When I was blowing Ears of corn, The first was scrap, then I got about 10 beofre I lost enough detail to scrap the mold. (Wet blow). I'f you are doing more, I'd suggest wood, or cast iron, or graphite.

OH, if you want a big heavy mold, and have little detail, Pour your square of plaster 1/2-3/4 as deep as you need, then plunge the bottle in to your 1/2way point. Then let it set, grease, Put in keys, then pour your other half on top.

Can you tell I just got home from teaching?

Greg F

[Dave Bross]

I've never done any of this but it seems likely to work...

Henry mentioned using castable refractory for the mold. Might live a little longer if that would be a plus.

Tom Fuhrman was telling me about using wet newspaper to line a mold. He was talking about open molds so maybe no application here.

[David Patchen]

Originally posted by Greg Frankhouser
Yes the diagonal is a good way. To set up the plaster, take clay and build a wall on your edge. You can mount the bottle on it's side in sand, or styropeanuts for that matter, but you don't want it to move. You will want to do a slop coat, A layer of thick plaster, so that you can build your edges up. If you need more meat in the mold, water down the Already set slop coat, then add more. The water is so the new plaster stick well to the old plaster. Leaving some gripping surfaces is wise as well.

Thanks! Although I'm having a hard time visualizing the role of the clay and how the plaster gets applied to a bottle sitting in sand or peanuts. I'm doing one half at a time, right? I'm confused...

[David Patchen]

Originally posted by Dave Bross
Henry mentioned using castable refractory for the mold. Might live a little longer if that would be a plus.

I saw this in Henry's book but since I'm not making a turning mold I thought refractory wouldn't be a smooth enough surface. Anyone know what you'd add to it to make it smoother? Interesting option, but much longer drying time than plaster, no? I actually have half a bag of Kastolite 30 lying around.

[Rob White]

Dave, I have done plaster blow molds before, I can give you a hand.

Rob White

[Greg Frankhouser]

With the Clay you are making big wings that spring off of the corners. Thes wings provide a place to hold plaster that porvides the mating surfaces. The same way if you were making a mold off of a clay positve, the shims do.

The sand/peanuts provide a stable bed so the bottle and wings and plaster don't fall over. Also the sand will help suoort the wings, as clay stuck to glass doesn't stick extra well.

Think of the bottle, on it's side, like a boat.

Greg

[Dave Bross]

I'm guessing that vibrating the castable would get you a smooth surface by bringing the fine particles to the surface.

[David Patchen]

Ok, between these posts, Ed's book and a friend's advice, I'm getting it. Yesterday I bought 50lbs of Hydrocal and 25 lbs of clay. The bottle's small enough I might be able to just jam it into the entire block of clay as support and wings. Off to the studio later this aft to build the box to hold this stuff and begin claying up the bottle.

Thanks guys!

[Greg Vriethoff]

Just passing-on a question from a colleague.

The question:

"I'm looking for tips/suggestions on plaster blow molds if anyone has a moment.

My hope is to make one shot/one piece blow molds. Blow into them, drop them in the annealor, glass and plaster.

My questions are:

Should the molds be dry? How dry?

Plaster or Plaster/silica?

Min wall thickness of plaster?

Preheat plaster molds?"

My response:

"I would just use plaster straight-up. Let it dry for a couple days and use it at room temp. You don't want to throw it in the annealer with any moisture so use it dry. Steam would be bad for the glass and the kiln. You could use hydrocal too, but since it's a one-shot deal there wouldn't be any advantage to that. The plaster should get hot enough to not be too much of a heat sink when you throw it in the annealer (no need to preheat in my opinion)."

If anyone has anything to add/first-hand experience doing this, I would appreciate any input.

Don't know why this person wants to do it this way, just passing it on.

Thanks,

G

[Holly Wallace]

David, Talk to Spencer. I just took the casting class from him- he has lots ideas and experience.

[Pete VanderLaan]

And while you're at it, Splurge, buy the book. "Glassnotes", get it direct from Henry Halem. It's a really great book. The casting section is excellent.I find it hard to imagine doing glass without it as a reference. It even has a picture of me in it.

If you dangle money in front of him he'll be nice to you.

1-330-673-8632

[Eben Horton]

David, You didnt mention if the bottle had any detail on the sides- if its just flat sides, all you have to do is build a box out of cherry boards and burn it in..

I think you know this though..

[David Patchen]

Thanks guys, but this project is long done over a year ago! Greg (above) re-awakened this thread with his recent post above

[Jack Loranger]

I wanted to create a blow mold that was a cylinder with the outline of a local petraglyph protruding from it. I took a drinking glass about the right size and poured it full of plaster. when it hardened I wraped the plaster in a thin layer of clay about 1/8" thick then I built the outline of the petraglth with more clay and smoothed it well making sure that there were no undercuts. I built a wooden box about 3" deeper and wider that the plaster piece.
I stood the plaster/clay piece upsidedown inside the four sided box (no top or bottom) setting on a piece of plywood. I sealed the box to the plywood base with clay. I mixed up a couple of gallons of plaster and threw in about a cup of kaolin clay and a couple of cups of kiln shelf primer. I filled the box with the plaster mixture and let it set up. After it was set I pulled it off the base plywood and drilled a hole in the center of the plaster/clay piece and screwed a large eye hook into that I could tug on to pull the plaster insert from the clay wrap. After the plaster core was removed it was pretty easy to dig out the clay from the plaster mold. I drilled a 1/4" hole throught the center of the bottom of the plaster mold so the steam and water could be released. Each time, right before I blew into the mold, I would submerge the whole thing in a five gallon bucket of water for about a minute and then pull it out and set it upsidedown for a couple of minute to make sure the water was drained from it. I filled it upright and dropped the bubble into it and expanded to fill the mold. Since the petraglyth was petruding, I would have to suck just a bit to get it to come back out of the mold. I got about 15 blows into the mold over a period of about 3 years. I made drinking glasses, mugs, wine bottles and vases with it. It finally broke a few months ago.

[David Gappa]

Perhaps someone can help me out with a Plaster Mold Detail question?
I need to create five 18" diameter "perfect" spheres. We have succesfully fabricated the two part plaster mold per Halem's book and everything works fine except one detail. When we blow into the mold and try turning the pipe, the glass sticks to the side walls of the plaster. I tried mixing graphite and rubbing alcohol and painting it on the surface. I also tried submerging the plaster mold in water and blowing into soon after.


Does anyone have any other suggestions?

[Eben Horton]

did you try acetylene soot on the plaster?

personally, id go comando and free blow them. Glass wants to be round.

[David Gappa]

Thanks Eben, no I did not. How do you apply this to the plaster and where do I get enought to cover the 18" sphere?

In talking with Henry, he expressed that it is near impossible to rotate the glass in a two part sphere mold do to any slight irregularities in the plaster mold. In order to resolve our problem we kind of modified our system. We put half the mold on a table and laminated a few layers of wet newspaper inside the concavity. We then blew into it while turning kind of like a wood block. It worked like a champ for our small prototype test. Now we are going to give it a try on the large scale sphere.

[Tom Fuhrman]

That's the way Dale made his large Japanese floats for his "Float Project" I sold him a cast iron 2 piece 18" ball mold and he shipped it to Japan where they used 1/2 the mold with wet newspaper and turned it as it was blown out. The mold worked as a stationary block and the pipe was turned. just like you blow into any paste mold but done horizontal instead of vertical. it requires you to get very nice even gathers and block evenly between gathers to make sure your bubble is evenly centered and temperatures are just right. otherwise you get some thick and thin wall discrepancies. the mold has to be vented properly and needs to designed so there are no places that heat up or cool at different rates. Making proper molds for larger pieces especially, is a real art. There are not many around that really know the intricacies of designing good production glass molds.

[Dave Bross]

There's an old book on collecting Japanese glass floats from the beaches on the west coast. It's pretty hard to find now. It has pictures of a guy blowing a huge sphere in a short barrel filled with hay (yup, grass) for the mold. Lots of other cool pics of that old process. The Seattle library still has a copy.

[Justin Thompson]

Originally Posted by Eben Horton

did you try acetylene soot on the plaster?

David - "Thanks Eben, no I did not. How do you apply this to the plaster and where do I get enought to cover the 18" sphere?"

David....I know that this is about a month late, but anyway....if you haven't gotten any tips.....the acetylene soot comes from an oxy/acetylene cutting torch setup. Use just straight acetylene on very low pressure (the flame will be pretty sooty). Direct the flame to the insides of the mold and it will carbonize the walls of the mold. This process works very well for rigid and wet sand molds....never tried it on the plaster though (always used the graphite/alcohol mix)

Best of luck.

[David Gappa]

Thanks to everyone for your advice. I attached a video link of us using the 1/2 plaster mold with wet newspaper lining the interior. After it anneals, we are going to sandblast the outer layer of color to create different phases of the moon.

http://vetroartglass.com/vetro-visits-the-moon

[John Van Koningsveld]

Fire suit and flip-flops. Interesting juxtaposition.

[Pete VanderLaan]

I line old crucibles with wet newspaper.