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  #1  
Old 07-15-2010, 10:15 PM
YanSan YanSan is offline
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Urethane vs Silicon molds

are each of these rubbers ideal for certain casting mediums? would anyone one care to point out the pros/ cons of each of these?
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  #2  
Old 07-16-2010, 12:55 PM
SPRINGFIELD SPRINGFIELD is offline
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Re: Urethane vs Silicon molds

Quote:
Originally Posted by YanSan View Post
are each of these rubbers ideal for certain casting mediums? would anyone one care to point out the pros/ cons of each of these?
In my opinion silicone rubber is superior to urethane rubber.
I have used both and I most likly will never use urethane rubber again.

1. shelf life=urethane rubber has a very short shelf life especially after the containers have been opened and reclosed.

2. Realising from model=urethane sticks like glue to most materials so many layers of mold release must be used on the model. Silicone rubber releases from most materials, no mold release required on non porus models. Porus models only need to be sealed.

3.Curing=moisture is a big enemy of urethane. The slightest bit of moisture will prevent the rubber from curing properly leaving you with a gummy mess.
Even a rainy day is enough to spoil the cure of urethane.
Silicone does not have such a moisture sensetivity and some silicone like Rebound 25
has no problem with moisture at all. However sulpher will inhibit the cure of silicone but sulpher is easy to avoid.

4. casting=Most materials cast without release in silicone molds, even plastic resins.
Urethane needs mold release for some materials especially plastic resins.

5Libruary Life= silicone rubber has a decent libruary life. Especially the Platinum cure silicones. Some urethanes turn into a gooy mess after a few years especially in hot humid weather.

6. generally silicone is more expensive than urethane rubbers but it's well worth spending the extra money.
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  #3  
Old 07-16-2010, 09:39 PM
raspero raspero is offline
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Re: Urethane vs Silicon molds

Springfield pretty much said it all.

I use polyurethane on large pieces and silicon on smaller ones. It's a matter of cost. And I also like the stiffness of the polyurethane for large molds—it's not so floppy. Silicon seems to register fine detail better. Also polyurethane vapors give me a sore throat. Silicone is pretty benign, just expensive.

R
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  #4  
Old 07-16-2010, 11:19 PM
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ahirschman ahirschman is offline
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Re: Urethane vs Silicon molds

Every thing has its place. I use polyurethane when polyurethane is called for and silicone when it is called for.

One of the main factors in selecting the appropriate mold compound is what you are using as your model, what you are casting and very important, the cost. Silicone is substantially more expensive than polyurethane.

Also, you really have to be quite careful with cure inhibition in silicones. Platinum cure silicone has a nice shelf life and low shrinkage, but it is much more sensitive to cure inhibition that tin catalyzed silicone.

Shelf life has not been a problem for me with polyurethane molds. I still use some that are over 12 years old. Like every thing in life, if you take good care of them they will last a long time.

So, mold release is a pain in the rear if you are going to be painting your casting or if you will need to add more things (Glue) to your casting. Again, when you compute your cost you have to look at the overall cost. If you are going to spend a lot of time cleaning the mold release of your sculpture to move forward then that time becomes part of your equation.

You can cast many low temperature melting metals into the proper silicone mold. Polyurethane would just burn up.

I work a lot in Florida, with high humidity, and I do not have problems with my polyurethanes. Yes, if I leave a container open for a while it absorbs moisture and then foams or becomes weaker. I have had silicone that became very thick after sitting in containers. The best thing is to buy close to what you will be using and not keep the materials around for too long. They all have shelf lives. I used to keep lots of rubber sitting on my shelves, but now I order as I need it. Works very well. So, moisture is an issue with polyurethanes, but you can take some precautions to diminish the problem. You can purge your containers to leave dry gas in them, which helps a lot.

My advice is to learn to use many polyurethanes, and many silicones and use the right one for each occasion.

Some examples:

Used silicone when I had to make a mold for very small sculptures. I was planning to do squash casting and I made the casting under 60 PSI of pressure until the silicone cured. I use lots of sulfur containing clay in my studio, and even a small amount of contamination on a tool can inhibit cure.

I use polyurethane for as many of my sculptures as I can, since it works very well and is very affordable.

I use silicone to make the molds that I use to make seams of polyurethane rubbers that I use in polyurethane mold making. No release needed, so I can pour polyurethane (Very good adhesive) directly into my silicone molds.

Overall, when used properly, both will capture the most minute details. I can see fingerprints from castings done in both polyurethane and silicone. The fingerprints were very shallow, but they were captured perfectly.

If you are going to be casting harsh compounds, like epoxies (many castings) you would probably go with silicone, as your mold will probably last longer.

I would recommend that you read this catalog from Polytek http://www.polytek.com/catalog/catalog.pdf. It is very informative and the information will mostly apply to any other company that you choose for your supplies.

Oh, and one final thing... I tend to use a couple of different polyurethanes that I thicken to meet my needs. Doing that gives me tremendous flexibility. I can mix the stuff so it is very thin and runny, or very thick so that I can trowel it on to vertical surfaces with a spatula or trowel. Stick to a few products and learn how to use them well. Learn how much time you will have, how the product will change with changes in temperature (Important in Florida, since I can have my studio be any were from 60 to 100 degrees).

Hope this helps, and does not confuse you.

Good luck.

Ari.
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  #5  
Old 07-17-2010, 12:16 AM
YanSan YanSan is offline
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Re: Urethane vs Silicon molds

wow, thanks for the very informative replies! i'm still pretty clueless on alot of the process of creating a final sculpture so it helps alot. since it's going to be my first time casting i think i'm going to go with the process/ materials in this smooth on video
since i know those all work together and i'm pretty low on cash for this project, so i'm definitely going to have to go with urethane. i am planning on sculpting in WED Clay and casting in winterstone so i think i should be ok in terms of all materials being compatible.

right now my main problem is estimating how much material i am going to need. seems like the material i'll need the most of is the Brush-On 40 and plasti-paste II. i looked up on the smooth on site it says that 1 Gal of Brush-on 40 covers 7.87 ft.˛ / 0.73 m˛ at 3/8" / 0.95 cm. the problem is i'm not sure how to relate this to the piece i am planning. it's going to be 2 half life size figures and a sculpted base maybe about 2x2 ft. do i just take the dimensions of the sculpture and measure it in multiple box flat box shapes? would perhaps 2 gallons be enough to complete this mold(plus a few small tests)? also, since the plasti-paste is supposed to be used for the mother mold will i need more than the amount of gallons i need for the rubber.

i apologize if this was a bit long winded. as i said, and that is probably evident from my questions, i'm still pretty clueless about alot of this. that and i'm getting nervous and excited about getting this piece done as i only have 2 months to complete it and i've never molded or casted before(and can't afford paying someone else to). anyway, thanks for the replies and if anyone would be kind enough to answer/ help me with these new questions i would be very grateful. thank you.
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  #6  
Old 07-17-2010, 02:37 PM
Andrew Werby Andrew Werby is offline
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Re: Urethane vs Silicon molds

I don' t think that's going to work too well, Yan. The WED clay is just regular potter's clay that has some stuff in it to make it dry slower. But urethanes are very sensitive to residual moisture; they make lots of bubbles when they encounter it. In that video you referred to, it looks like the master they started with is made of plaster, not clay. All those sealants and release agents might seal in the moisture from a clay model, but I wouldn't count on it...

Andrew Werby
www.computersculpture.com
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  #7  
Old 07-17-2010, 10:09 PM
YanSan YanSan is offline
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Re: Urethane vs Silicon molds

i actually thought of the possible WED clay issue when i was writing it out but figured the sealer and mold release would cover any issues. i think i'm going to try and contact smooth on and see if they have any specific knowledge about this. i was trying to look around to see if i could find anyone who has used WED clay with urethane molds. i saw a thread on these forums of someone who did it successfully and i saw on Heidi Maiers site she said of WED

"The clay can be sealed while still wet with Krylon Crystal Clear plus an additional coat of Krylon Matte Clear to dull the shine. Once sealed, the piece can be easily molded without problems of condensation (this is best done in temperatures below 75 degrees to assure there is no sweating of the clay while molding)."

it's funny because WED is supposed to be formulated by Disney to be able to create larger sculptures for much cheaper while still having properties of oil based clays. and as such, it's not meant to be fired. it seems like if WED is designed for larger scale work to be cast and urethane tends to be used for larger scale work, with both reasons being cost, it seems like they would have figured out a way to make it work. it would be nice to use silicon or something so i wouldn't have to worry about it, but i'm on a very tight budget, so i don't think it'd be possible. anyway, thank you very much for the input

edit: actually i just found on Heidi's site a thread where she uses WED clay and a urethane mold(although it's not smooth on brand) here

Last edited by YanSan : 07-17-2010 at 10:21 PM.
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  #8  
Old 07-18-2010, 02:30 PM
Andrew Werby Andrew Werby is offline
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Re: Urethane vs Silicon molds

I'd try this on a small scale before committing to a major effort with these materials. That was an interesting post you referred to, but it seems like Heidi had something of a disaster the first time she tried putting the urethane on the still-moist WED clay - the rubber didn't set, and she had to scrape it off and resculpt. By the time she got back to it a week later, the clay likely had a chance to dry out somewhat.

Andrew Werby
www.computersculpture.com
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  #9  
Old 07-19-2010, 12:09 AM
YanSan YanSan is offline
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Re: Urethane vs Silicon molds

reading back over the thread a little bit, the urethane was 6 months past expiration and she believed it to be a bad batch of a new formula from polytek, after talking with her supplier. but anyway, yes i was planning a doing some testing before i do the whole piece. thanks
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  #10  
Old 07-20-2010, 01:31 AM
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ahirschman ahirschman is offline
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Re: Urethane vs Silicon molds

Hi Yansan,
To estimate your volume requirements you have to use basic math. I try to break up my sculptures into nice shapes, such as cylinders, rectangles, planes, etc. That makes it easy to figure out the approximate volume that I will need to cover in my mold making material. You can often break down those simple shapes into even simpler ones. For example, a cylinder can be opened up and converted into a thin box where you just have HxWxL as your volume. It is an approximation, but close enough for molding purposes. I've been doing that for years and seldom get it closer than maybe 80% accuracy. It is not necessarily that my measurements are wrong, but since I apply every thing by hand, I may make some parts thicker and some parts thinner than I had planned.
Let me know if you are shaky with your math for calculating volumes and I'll try to provide some examples and basic formulas.

I always order a little more than I think I will need. That sure came in handy the time I did not fasten an enclosure correctly and had to scoop about 15 lbs of dripping rubber from the floor. Amazingly enough, that mold came out just fine (scooped up as much as I could, and added some from my extra pile).

The problem with not having enough at hand is that you do not have a long time between the application of coats for proper bonding. The bonds in the rubber keep forming, and after a few hours your bond will be much weaker (The time is quite variable depending on your material, so check the material info sheet).

Keep in mind that it is difficult to save opened containers of polyurethane for a long time. You can always transfer the remains to a small container and purge the moist air out of the container. That greatly extends the products life. As you work with the products you learn how the look and feel and you will usually know if something seems strange.

I know that my old polyurethane plastics (Not rubbers) will foam, so I use them accordingly, but still use them. I also take advantage of that when I need polyurethanes to foam. I add a cure accelerator, and some water and mix. The polyurethane starts to foam, and then sets. Just getting more uses out of products...

As a rule, when in doubt, make a test cure or batch. Test it against your materials for molds and against sealers and releases. Remember you have to release polyurethanes or they'll be stuck really well.

I recently tried one of smoothons self thickening polyurethane rubbers. Worked fine, but I did not like it. I would not like any of the self thickening products from any company. I am so used to thickening the polyurethane myself that I felt I had lost much control. On the other hand, if you have not gotten used to certain products, maybe the self thickening will be just fine.

I usually start with a very thin coating of the thickened polyurethane. This is gently brushed on and will ensure that all of the finest details are captured. From there I thicken the mix a bit, and as i go along, and the lower layers cure more and more I apply thicker and thicker coats.

I do the same with the mother mold. I used to use more plaster, but as my molds get bigger, and my back gets older, I trade $$ for weight.

I forgot to mention that when calculating your volumes you have to make sure you allow whatever amount extra you will need to form the seams. I like to make my seams about 1"x1" by some length, so it is easy to calculate the extra volume (30" long seam would just be 30 cubic inches of rubber).

I was a little worried about the curing problems mentioned. I have used Polytek without any issues for about 15 years and this is the second time I have heard of problems, so I did a little research about quality control. It turns out that Polytek (And, I would imagine any other company making these products) mixes and cures a sample from every batch that they make. They also take and store uncured samples. If they get a complaint they can check both, the cured, and they can make another cure at the time of the complaint. They told me that they have a very low problem rate. I know that "Lost in NY" had issues a while back. I once received a container that had leaked about 1/2 to 1 gallon of one of the two parts (2 5 gallon buckets) and they shipped me another 5 gallon bucket within a few days.
But, you need to follow the mix ratios and you need to keep your products covered while you use them, between uses, and you need to purge moist air out of all containers if you expect polyurethanes to last.

Hope some of this is helpful.

Ari.
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Old 08-25-2010, 02:57 PM
YanSan YanSan is offline
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Re: Urethane vs Silicon molds

Thanks Ari, that was indeed quite helpful.



this is my sculpture at the moment, i'm not liking it all that much right now, but i'm not done yet, so. i figured i should try and figure out how much rubber and plasti-paste i'm going to need to i was trying to measure it out, except i am having issues figuring out how to calculate it.

as you can see i split it up into 3 main boxes to measure, but i'm not sure how to calculate it verses the material. the materials are given in cubic inches, but since the mold/ mother mold are only supposed to be about 3/8in thick i'm not sure how to compare them. i know the BrushOn 40 rubber is about 424.8 cubic inches per gallon and the plasti-paste is 360.5 cubic inches per gallon, but i'm not sure how to calculate that in reference to the sculpture.

sorry if this seems simple, i'm not too sure what i'm doing, plus now i've had to cope with a crazy eye surgery that left me unable to work on this for a few weeks.
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Old 08-25-2010, 07:17 PM
raspero raspero is offline
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Re: Urethane vs Silicon molds

I calculate my sculptures as a few boxes: one for the body, a smaller one for the head, a few long ones for the arms and legs, and then I figure the total surface area of the boxes X 3/8 inch thickness which gives me a volume of rubber. That's still not enough though. I add some more to that. For instance, if I calculate I need 3 quarts, I buy at least a gallon. It's amazing how much of it gets wasted: drips, spills, hardening in the pot. The seams take a lot more rubber too. It adds up. Nothing is worse than running low on rubber in a mold making process. I've learned that the hard way.

R
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Old 08-25-2010, 10:40 PM
YanSan YanSan is offline
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Re: Urethane vs Silicon molds

thanks raspero! i started to calculate the surface area of the boxes i generalized, but then i didn't understand how to calculate in the 3/8ths thickness and started to become confused and couldn't figure it out. i think i calculated about 2670 square inches, but didn't know where to go from there. is it just 2670x.375? to get the cubic inches?
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Old 08-26-2010, 07:59 AM
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ahirschman ahirschman is offline
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Re: Urethane vs Silicon molds

Hi YanSan, The trick to calculating the needed volume (and subsequently, weight, etc) is to simplify your sculpture.

The way I do that is by mainly using boxes and cylinders to compute a "surface area".

Once you have a surface area, then you can add height to arrive at a volume.

For example (You know this, but just a refresher), if you want to cover a very basic shape that is flat and is 10x10 (inches, cm, etc) then you would have a Surface area (how many 1x1 squares you can fit in your shape) of 10 x 10, which is equal to 100.

If you now want to cover something that is flat and is 10 x 20 its surface area would be 10*20 = 200 (You can fit 200 1x1 squares into that shape).

Now, for sculpting we do not need to be exact, so many times I break things down into the closest flat rectangular shape.

For example, If an arm in your sculpture would just fit inside a cylinder that has a diameter of 4", you can calculate the area of that cylinder or you can pretend that the cylinder is composed of a rolled up sheet of square paper. If the diameter is 4" then the Circumference of the circle is C= Pi * Diameter which is equal to 3.14 * 4 or roughly 12.5.
If the Cylinder that we are talking about (encompassing the arm or body part of your sculpture) is 20" long then the "Sheet of paper" that would encompass the body part (You take your cylinder, cut it the long way, and open it up flat) would be 20" x 12.5" with an area of 20*12.5 = 250 square inches.

Now, a torso fits poorly into a cylinder because it is fairly flat, but you can use this "wrapping a sheet of paper" scheme for just about any thing. Take a tape measure and measure around the torso, and then the height. The area is the length around the torso times the height.

If you have a funny shape you can usually break it down into simple ones.

If your math is not great, and you cant do integrals then just look the formulas up (I did a quick search and found http://www.math.com/tables/geometry/surfareas.htm, for example.

So, now you have a list of surface areas.

The volume, as raspero pointed out is simple to compute. Again, for ease of understanding, lets say we have a 10 x 10 flat sculpture we want to cover with 1 inch of rubber. Then the area is 10x10 = 100.
How many 1x1x1 cubes of rubber can we put on top of our 10x10 sculpture? Well, 100 cubes would fit nicely. So, the volume is the Area X the height.

I always keep this in mind, because it makes life easier. If you have a sculpture with many parts, and the sum of the surface area turns out to be 1250 square inches then I tell myself that covering those 1250 square inches with 1" of rubber would require 1250 cubic inches of rubber. Now, if I only want to use 1/2" of rubber I just divide my 1250 by 1/2, and so forth.

Now, many vendors sell their rubbers by weight, so I can look at the technical bulletin for the product to calculate my needed weight. I just went to Polytek.com and looked at the technical bulletin for their 74-30 pour-able Shore D30 polyurethane rubber. That particular rubber has a density of 27.5 cubic inches per pound.

So, for my sculpture that has a total surface area of 1250 square inches with a 1/2" thick cover of rubber I will need 1250 x 1/2 = 625 cubic inches of rubber. If my rubber weighs 27.5 cubic inches per pound I can figure out the needed weight
625 cubic inches / 27.5 cubic inches per pound = 22.7 lbs of rubber.

But, you will always need more rubber, because you usually have more complex shapes and need a seam. Also, even for flat shapes, you usually form some sort of lip to extend the rubber past the sculpture area and minimize the chance of damaging your "Data" (The data is the area of the mold that has sampled your original sculpture...)

In my case, I have a mold that allows me to "cast" 1x1x12" strips (Now I have one that allows me to go 1x1x48"). I pour some rubber into these molds and when they are partially cured I pull them out (The square strips) and use them to form a seam over my rubber coated sculpture. I have to pre-think of where the seams are going to go, etc. I will add some thickened rubber to this seam to make sure it is 100% adhered to the rest of the mold. So, I always have my initial surface area for the sculpture, and then I have the length of seams (since the mold is 1x1, the volume is just the length of the seam in cubic inches). Now, it may not seem like much, but for a rubber that weighs 27.5 cubic inches per pound you will get about 2' of seam per pound!

Yes, you can always get away with less material, but I don't like to be penny wise / pound foolish with molds that I will keep for 15 years. On the other hand, if it is a one time mold...

What else do you need for your mold? Certain shapes will need some sort of keys to keep them secured to the mother mold. Again, figure out all your extras and add them up.

You will have some leftover in the bucket (brush, etc.) and you do not want to run out of rubber before you are done.

My personal preference is to use low viscosity rubbers and thicken them as needed. This allows me to start with a very thin coat that will catch all the details (Down to the faintest fingerprint on the clay) and as thick as peanut butter towards the end of the cycle. Versatility!
Cabosil is a great thickener, but it absorbs humidity over time (Polyurethanes will foam). There are many other thickeners that work great (get them from the same place you get your rubber).

Well, sorry for the lengthy reply. Hope this helps.

Ari.
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Old 08-26-2010, 07:30 PM
raspero raspero is offline
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Re: Urethane vs Silicon molds

Yes, that's 1000 cubic inches.

The polyurethane I use is Smooth-On 121, which is 27 cubic inches per pound. 1000 cubic inches is 37 pounds. 3 one gallon units at 16 pounds each would give you plenty of rubber and some extra to practice with on a smaller piece before you commit yourself to the main thing.

R
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Old 08-27-2010, 09:24 AM
YanSan YanSan is offline
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Re: Urethane vs Silicon molds

thanks again Ari for the comprehensive reply, much appreciated! thanks raspero!

i guess i had the right idea originally but then somehow confused myself into thinking it'd be more complicated. thanks again!
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