3 years ago#1
Guest
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We are mass producing Plaster of Paris items using silicone moulds. However the moulds have a limited life and need to be replaced every now and then.

Making the moulds damages the Plaster of Paris masters. What is the best material out of which to make the original masters so that the making of the silicone moulds would not damage the master?

By "best" I mean a material that can be scuptured without too much difficulty.

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3 years ago#2
mickpearson
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Hi are your moulds one piece moulds that you have to pull of like a vest. a glove mould and this is how the damage will occur. if so then dragon skin silicon from smooth on will help you out to make a mould that will come of free from your plaster. without damage to your masters the other method would be if your masters are a complex shape is to make piece moulds. with your existing rubber. so that they come of the masters more easy. mould design.there is however a plaster water based resin system that i haven;t used my self i only use the hard medium ac100 jesmonite its a plaster resin. non toxic no solvents or smell. and strong like resin. but jesmonite ac200 soft composite is for carving. soft composite a combination of water based acrylic resins. and is used for soft carvable castings carving and machining characeristics are exellent displaying qualities of good edge stability. initial set ;pot life 14.17.minutes.final set 45.60 minutes.

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3 years ago#3
mickpearson
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3 years ago#4
Johan
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Thank you very much. We found the place and will look at what they can supply.

Much appreciated!

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3 years ago#5
mickpearson
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you are very welcome
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3 years ago#6
red46
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I'm having a hard time understanding how silicone is having any affect of plaster. Are you mixing the plaster right? I would suggest hydrastone but I get the impression you are sculpting the masters. Your best bet is to make a waste mold of your model, then produce a master of hard rubber. Now that would last.

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3 years ago#7
Johan
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Thank you for your suggestion. Can you give us more details with what you mean about with a waste mould?

Can one work on the hard rubber to improve the details or the sculpture?

Thank you again.

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3 years ago#8
mickpearson
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HI its me again i picked up on you saying the hard rubber. are you fighting to take the moulds of your castings. or originals

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3 years ago#9
mickpearson
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HI its me again what red46 means is use the plaster to make a mould of plaster. of say a clay model as an example.then use the plaster mould to make your originals in a hard rubber that would last a life time and you can throw out your plaster moulds because you don.t need them any more hence a waste mould .but i also picked up that you want your models made of plaster because i think you like to re sculpt them when you want to change your design am i right and im asking if you find your models are tight when you try to get them out your moulds

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3 years ago#10
Johan
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Firstly, thanks to all of you who are contributing. It is much appreciated.

I think I need to explain everything in greater detail since there seems to be confusion. This is a new area for me and I am not au fait with all the correct terms and I think this is causing the confusion. I will try to be as clear as possible, but if I use the wrong terminology try to understand what I am trying to say and teach me so that I can learn.

Firstly, our final products are images of various designs and sizes made of Plaster of Paris that are sold to arts and crafts shops for people to buy and use as the starting point of their creations. These are figures of women, faces, heads, angels, crosses etc.

We have been making these images for some time, but our quality has been very bad and our productivity very low. We now want to improve both our quality and productivity and therefore need to learn the correct and best ways of making these images.

Does anyone know what is the highest sustainable level of detail is that one can put into a Plaster of Paris image?

If I understand what you are saying correctly, we can start with any original such as clay. From this we make a Plaster of Paris mould. We then cast hard rubber into this mould to make our master image. We use this master image to make our silicone moulds which are then used for production and which are replaced as they ware out.

I'm sure there is a lot more detail involved in the actual process in order to make it work and I will need to get a proper understanding of all that as well.

Am I on the right track?

Your help is much appreciated.

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3 years ago#11
mickpearson
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Hello johan. i think you are telling me. the plaster is to soft and they are breaking to easy. there are different plasters and they all have different strengths. soft.medium.and.hard.you can look for a plaster and buy small samples of the soft and medium grades to test the quality.to see what is best .but the moulds will only be as good as the originals.some silicon rubber moulds have to be made in more than one piece so that it can come of the image easy a mould should be easy to remove from an image without breaking the original if it does not it could be because the plaster is to weak or the mould is not right and does not come of because it fits to tight and needs more than one side to come of latex is a good stretchy rubber but takes to long to make it thick enough dragon.skin. silicon can be used as it is much quicker and is also stretchy.jesmonite ac200 is a plaster you can carve with, but as i said try out different plasters plaster of paris is to soft for some applications

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3 years ago#12
mickpearson
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You can also use modeling wax that you can model in the highest detail or air drying clay but wax would be best i hope i have helped you

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3 years ago#13
red46
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I see by your IP address, you are in Cape Town, South Africa. I did a search for plaster products there and just ran into a block wall.
If you don't have the right supplies, where you are, possibly one of the building and construction supply stores will order in the plasters you need?
For sure, you need to get hold of silicone rubber to use.

Most definitely, for your finished products, you need to stick to plaster. I don't know what plaster products, are available in your area. If you can get your hands on #1 pottery plaster made by USG, it is the best general purpose plaster and usually more available than some of the others. It is also less expensive than some.
I found on the USG site, a mention of distribution to Africa, although I couldn't find a link to a distributor there. You might spend some time investigating. If you can get hold of USG products, they have an amazing variety of plasters for different applications. http://www.usg.com/worldwide/locations.html

On this next page, you can see the different plaster products and their uses. http://www.usg.com/industrial/art-statuary.html

See if you can latch onto a supplier where you can get what you need. Also, I noted a link on that same page, to smooth-on for those rubber products.

You need a good plaster with strength but not too much weight. You need silicone rubber compound for making your rubber molds. You also need to spend some time viewing the videos at smooth-on.

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3 years ago#14
red46
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Also, I might mention, regardless of the plaster you use, you need to do what is called "slaking", which means at least a 3 minute rest before mixing. This give every particle time to become wet. This is a must. If you haven't been doing this, it just might be the reason for weakness in your plaster. Also, mixing time is important, as well as the water to plaster ratio. A full hard fast 3 minutes for plasters and 5 minutes for hydrastone. It takes speed and energy to fully activate the plaster so you will also need to use a drill and paint mixing blade, at the very least.

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2 years ago#15
MouldMaster
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your moulds a very good idea mate

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1 year ago#16
Smee 2
Guest

Make very hard and dense plaster castings, and once set after 2 or 3 days, keep the surfaces very smooth and polished, and keep your blocks wet - as in in water all the time.

When molding, blow off the excess water - the puddling and drips - do not dry the plaster at all, fill the cavity with silicon, and then close the mold and then stick the plaster back under water.

Silicon will NOT stick to smooth wet plaster.

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1 year ago#17
Wolff
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red46 wrote:
Also, I might mention, regardless of the plaster you use, you need to do what is called "slaking", which means at least a 3 minute rest before mixing. This give every particle time to become wet. This is a must. If you haven't been doing this, it just might be the reason for weakness in your plaster.


I will mention the slaking is mainly to get rid of lumps of dry material, and release some of the air, in some 30 years of using Hydrocal and the other USG plasters I've rarely bothered with doing that because all of the USG gypsum cements (Hydrocal, ultracl, hydrostone duracal etc) REQUIRE mixing them with a mechanical mixer in order to set properly and gain the stated strength according to their published spec sheets, but honestly, I think half of that stuff is a load of bs since I've never followed their timed mixing directions, slaking time, nor weighed the plaster, I go by how the mix feels by hand to know it's ratio is good, and I just mix it well for 2-3 minutes with a jiffy mixer on a drill and never once had a problem of any kind.

If you are making plaster molds for SLIP casting, that's a whole other thing, you Do want to be sure the ratio is correct 100% of the time for each section or you can get variances in how fast parts of the mold absorb water from the slip.

They specify a weight, mixing time and temperature mainly because if you are a large commercial firm producing 1000 casts a day and want all of them identical, you want to do the mixing etc exactly the same every time according to specs, for the other 99% of USG's customers- that's us- those who hobby cast half a dozen pieces in a day or a week, it hardly matters whether one cast has the spec'd 12,000 PSI compressive strength or another one has 10,800- in the real world (that's here!) compressive strength of plaster is useless, it doesn't matter how high the compressive strength is when all it takes to break off a fragile corner is one small bump on a hard surface.

Compressive strength for statues is not the same as breaking strength from SHOCK, it is sudden shock force which snaps off corners and breaks small parts off like noses, toes etc.

This applies to concrete too, most people don't realize how weak and fragile concrete really is to resisting shock force, it has great compressive stength but a minor bump with a hard object can break sharp corners and projections off, that is why most concrete garden things have rounded features and very little details and undercuts.

As an aside, I no longer use USG products or their Hydrocal, I found a much better plaster that is slightly harder and denser than Hydrocal, and it does NOT need to be mixed to the extent USG seems to have an obsession over, you just mix it well and you are done with it.
I'm using Densite plaster, and the great thing is it's packaged in 50# bags instead of the damnable 100# back killers USG's Hydrocal comes in, I find 100# of it runs me just about what the Hydrocal cost me but I don't have to buy a 40 bag pallet of it like I did with Hydrocal!

The more of the plaster you add to the water the stronger it gets no matter how much or little you mix it (within reason) I've had to get Hydrocal to set as FAST as possible, or when using for making a mold, and I've mixed it to almost wallboard paste thickness to where you can trowel it on a wall and it could stick there it was so thick, and it hardens up like stone.
I also use very warm to hot water for the mix if I want it to set faster, 100 deg F is the ideal temperature for the water.

I believe the biggest difference between some of their formulas is how much water they tell you to add, more water = weaker plaster, less water = stronger, and I've made Hydrocal harder than their Hydrostone even.
The big key is make sure it's mixed well, no lumps.

Here is a comparison of both Hydrocal and Densite together, pretty clear that the Densite's specifications for compressive strength is almost twice what Hydrocal shows, the weight per cubic foot is slightly lighter for Densite, you also use slightly less water:

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1 year ago#18
elize
Guest

Hi
Im starting with plaster of parris know. molds difficult
to get in cape town can you help please

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