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PLASTER OF PARIS MOLD PROBLEM

Old 06-22-2009, 08:12 PM
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Default PLASTER OF PARIS MOLD PROBLEM

This is my first attempt copyingABS cowl into fiberglass,i used plaster of paris to make a mold,iused a thin layer of petroleum jelly as a release agent onthe cowl,but it still got stuck,according to the box the plaster sets in 30min so I waitedover night to be safe, it felt totally hard and dry to the touch when i tried to pull the cowl out.I am wondering was it not 100% dry or moisture free is that is why it got stuck ordid inot put enough petroleum jelly.I used petroleum jelly sparingly cause i was worried later when i will trysealing the moldtoo much petroleumJelly will create problems as well...?
Old 06-22-2009, 08:53 PM
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Default RE: PLASTER OF PARIS MOLD PROBLEM

To remove the moisture you'll either have to wait a few weeks for it to cure, or I put it in the oven at 150 degrees for about 4 to 5 hours.
Old 06-22-2009, 09:10 PM
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Default RE: PLASTER OF PARIS MOLD PROBLEM

when I did my mold from plaster I pulled the cowl front out of the plaster after it was set, about a hour, so it would release from the plaster cast, plaster tends to shrink,just a littlewhen drying so it can tighten upon a subject, it took a few days to fully cure after that.
for a release agentI used pure carnuba car wax, the one I used was a liquidtype wax, but paste wax in the can works as well, after at least 3 coats buffing each before applying the next then the molding process can be done.
that being said getting the cowl out of the mold depends on the type of mold being made, is it a split mold(2 halfs) or a one piece which is tough to get a part out of, if you have a compressor then a air nozzlehelps get the plastic part off the walls of the mold so you can get it out .
in the link of my build I posted in your Q&AI go into detail on how I made my mold from fiberglass but the same concepts can be used for plaster as well, after layingup a liquid base over thesubject plaster wrap(the kind for armcasts)can be used to make a thin strong layer so a large mold isn't needed.

photos:
my wax
subject in plaster mold
cured mold sealed with oil based Glosspolyurathane
finished cowl front






ORIGINAL: SHERDILL

This is my first attempt copyingABS cowl into fiberglass,i used plaster of paris to make a mold,iused a thin layer of petroleum jelly as a release agent onthe cowl,but it still got stuck,according to the box the plaster sets in 30min so I waitedover night to be safe, it felt totally hard and dry to the touch when i tried to pull the cowl out.I am wondering was it not 100% dry or moisture free is that is why it got stuck ordid inot put enough petroleum jelly.I used petroleum jelly sparingly cause i was worried later when i will trysealing the moldtoo much petroleumJelly will create problems as well...?
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Old 06-22-2009, 11:54 PM
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Default RE: PLASTER OF PARIS MOLD PROBLEM

You might want to lookinto using Hydrocal 30 it's better for making molds.
Old 06-23-2009, 06:17 PM
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Default RE: PLASTER OF PARIS MOLD PROBLEM

I make my molds out of 2 halves of foam and then cover each half of the foam with glass and after it cures I prime and paint it so it's nice and smooth. I use dowels to hold the two together for making the cowl. Then I start with 400 grit wet paper and finish with 1500 grit. Then I wax it with 3 coats of turtle car wax in a can. Last I use a Preval sprayer to spray on the Evercoat Mold Relase Agent. It makes a sort of rubbery layer between the mold and the glass your making the cowl out of. I alway make sure I get plenty of the spray mold release agent on the mold and let it sit overnight to dry. You will be able to see when it's dry because it acts like a glossy clear coat and makes the sanded paint glossy again like it wasn't ever sanded. After the cowl cures I seperate the two halves of the mold and the cowl just needs slight sanding at the seams. Be sure to go around the edges of the foam with glass where they go together so the mold release will be there too.

Barry

www.canopyrailsforwarbirds.com
Old 02-26-2024, 01:12 PM
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Default soft mold stuck in plaster cast

I dug out a mold my Dad made for a stepping stone. It is not silcone, but a low temperature rubber type. It is sitting in a plaster cast to hold it's shape and it is stuck in it, slightly askew. I may have to just try it the way it is and see how it comes out. When I try to carefully pry the mold off of the plaster, it is really stuck, especially around the edges. I really want to try to preserve it, as it is a momento from my Dad, who just passed. Any ideas on how to get it out? I just trying wetting it around the edges. I think this afternoon I will just try to pour it "as is" because I am afraid I am going to ruin it. Does anyone have any tips on peeling it out from the plaster? If it does not come out good, positioned as it is, should I crack the plaster from below and try to wash it away. I'd really like to preserve it. If the one I pour today, using cement and mold release, comes out OK, what material would you recommend for making a new mold?

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