Novice at mold building, help?

[Mywhims]

Hi, I'm a new kid on this block. I am also new to creating molds for casting concrete statuary. was looking for suggestions, help on making latex molds with support shells. I am in the middle of layering latex on a concrete model. I have planned to use a bondo type resin paste material with fiberglass matt for the support shell (don't know if this will be the best choice for casting concrete statuary). (Smooth-on's products look great.....and expensive...need something more economical). I also am wondering about how to best address the severe undercuts of some statuary.....does one use a "filler" mixed in with the latex for thoes indentations, or do you layer the latex mold material on "as is" and then make more pieces to the mother mold to accomodate for the undercuts (obviously trying to avoid the machanical lock problem with the support shell). I know the support shell must fit snuggly or the latex mold will distort when pouring concrete into it. Would appreciate learning from all you seasoned artists out there. THANKX.

[Landseer]

was looking for suggestions, help on making latex molds with support shells.
Hi,
The first mistake is using latex for molds- if you want quality casts anyway, latex shrinks considerably - about 1/8- 1/4" or so per lineal foot or so regardless of brand, and since it shrinks and the supporting shell DOESN'T and you fill the thin rubber mold with heavy cement can you guess what happens?
This shrinkage in my years of experience with latex is roughly 1/2- 3/4" in something 3 or 4 feet tall/long. It starts shrinking while on the model before it's even peeled off, and very rapidly once removed. So the effect starts right with the first casts you make and they get progressively worse.

I can always tell when a cast was made in a latex mold- those warpy bumpy uneven floppy dippy sides and edges give it away every time!


(Smooth-on's products look great.....and expensive...need something more economical). I also am wondering about how to best address the severe undercuts of some statuary.....does one use a "filler" mixed in with the latex for thoes indentations, or do you layer the latex mold material on "as is" and then make more pieces to the mother mold to accomodate for the undercuts (obviously trying to avoid the machanical lock problem with the support shell).
Smooth on is probably good, I use Quantum silicone's QM140 thixotropic and YES, it runs about $100 a gallon compared to wwhat? $25 for latex now? BUT you HAVE to do a comparison and not JUST the price per gallon!

That gallon of latex contains water and ammonia- both of which evaporate and do nothing for building up thickness. I don't know how much is in the gallon, maybe someone does, but if it's 25% water and ammonia you are really only getting 3 quarts of "solids" if even that much, and I'd bet quite a bit less. It's like a can of spray paint which is mostly air and liquid propellent.

The silicone is 100% useable solids- nothing evaporates out nothing wasted.
ALso, this thixotropic formula means you apply it with a brush, it's like mayonaise, you build up a 1/4" thick layer fast and since you are doing it this way it doesn't take much rubber to MAKE the mold- you are not filling a form with it as some molds would use (block molds)


I know the support shell must fit snuggly or the latex mold will distort when pouring concrete into it. Would appreciate learning from all you seasoned artists out there. THANKX.No matter how snug the shell fits initially, if you use latex it WILL gap considerably and there is nothing you can do about this, it doesn't matter how you brush it on, how many layers, how much filler or what reinforcement you add it will shrink, and the larger the piece the more pronounced.
Latex is a PAIN to deal with undercuts in the shell, for the latex must be fully supported everywhere and if there are any gaps the concrete will cause an ugly bulge.
If you try slopping the latex in undercuts real thick to fuill them it will never cure but skin over and remain mushey inside like used chewing gum and can be distorted. You carefully design the shell in multiple pieces to deal with that, such as this mold with some of the shell pieces shown on it, and yes, I use and prefer plaster for all of my supporting shells- the five pieces are loosely laid on the silicone mold for the photo and this is bottom facing up in the image;


http://i23.photobucket.com/albums/b375/Randall2/PICT1149.jpg


The silicone can be built up to fill undercuts in any thickness and it all cures 100% but there are always opposing surfaces and this sculpture as a result needed six pieces (five shown) for the shell to be 100% sure. I could have built up the rubber thicker and made the shell is less pieces but this way is better for my use. Thicker rubber layer will cost more in materials and is stiffer, so that has to be weighed against making the shell in six pieces instead of four- the time would be about the same, the materials much less, smaller shell pieces means easier taking apart and lighter individual pieces.

I can understand the money issue with molds, but this is a long term investment- the materials are NOTHING compared to the hours and hours spent making the molds, if you use cheap or crappy materials or good materials it will take no longer (likely less) to use the good stuff to begin with, than have to remake it all over again this time with the GOOD stuff after half a dozen casts.
Making molds is usally an all day affair when you add in the sealing, set up, prep work , applying rubber, making the shell etc etc.
Making the molds is not what makes you the money- making the CASTS is, so the less time spend farting around with inferior molds, and spending it instead of producing casts to sell- the better!

[Jamo]

I second everything landseer just said. Very good advice. work that you can put into the mold saves time exponetially on the casts that come out. I am lazy and don't like spending more time on something than I have to, so I put as much work into the mold as possible. Crappy molds suck. Invest in the good materials and they pay off with the time saved, headaches and number of casts that come out of them.

[Landseer]

I second everything landseer just said. Very good advice. work that you can put into the mold saves time exponetially on the casts that come out. I am lazy and don't like spending more time on something than I have to, so I put as much work into the mold as possible. Crappy molds suck. Invest in the good materials and they pay off with the time saved, headaches and number of casts that come out of them.Thanks Jamo, it's so darn true it's not funny. I have made molds for years and I've made some damn crappy ones early on in the learning curve- I mean it, you have no idea! After a while you realize an hour and a dime saved now cutting corners going cheap is going to cost ten times more real soon to fix the problem. Think it's fun to get ONE plaster cast out a mold and then have to turn around and remake the lousy mold because it was a piece of crap or the material failed like that Polytek did?

ARGGH! that's the pits!! it happens to all of us, it happens to the BEST of us.
I'm lazy too when it comes to starting a mold, I do enjoy parts of the process- especially taking the mold apart off the model and making the first cast, but generally it's a monotonus all day affair that bores the hell out of me at times when I'd RATHER be outside on a nice sunnyday, but you know that if you want to earn money and work on your art you HAVE to do the dirty work too- that includes cleaning floors, dusting, trash, shopping for materials, loading and unloading 100# sacks etc etc.

Now you're gonna love this!
That iron final I started a mold of, stupid stupid me I should know better with 30 years experience right? but in my RUSH to get it started and embedding it in the plasticene to do the first HALF, can you spot the now REAL obvious problem here that will happen down the road

http://www.sculpture.net/community/attachment.php?attachmentid=2728

once this half is (and was) covered with rubber, flipped over, the second half covered with rubber, a 2 piece shell made and then stood on end for casting hydrocal in?
The first half of the silicone rubber was gorgeous, nice thickness etc., I never got to do the second half because I suddenly realized something when I came in the room afresh and looked at it on the table planning for the second side, then went; OH S___T!!!
After I got over my disbelief I could only LAUGH!
Rules are; plan ahead, don't rush!

Sometimes the most simple dumbest little projects can just toss you on your behind when you don't pay attention.

You get two guesses and I'll tell you if you are right.

[daaub]

was looking for suggestions, help on making latex molds with support shells. I am in the middle of layering latex on a concrete model. (Smooth-on's products look great.....and expensive...need something more economical).


Hi,

It would be great to see some pics. If I am reading correctly, you a have already started the mold in latex. ?? I have used liquid latex rubber a few times and have encountered the problems Landseer mentions. The amonia, around 6%, is both unhealthy and unpleasent to breath.

You could layer in cheese cloth or some other cloth mesh every couple layers or so. This will give the latex strength and will reduce shrinkage. I have tried this and it does help. The thicker you layer on the latex, the more it will want to shrink on your model. Thin layers helps! (and takes forever) If you do need to add thick layers, do it in small sections at a time. Adding the cheese cloth will greatly reduce your ability streatch the rubber though so it may pose problems if you need to streatch it over undercuts to remove it. You probably could add in something even more rigid to further increase strength and stop shrinking. The fiberglass mesh tape stuff or even chopped fiberglass strand layered in might help.

Here is a link to an old thread of a mold I did using latex and plaster.
http://www.sculpture.net/community/showthread.php?t=2090
Jamo posted some good advice on there.

Another inexpensive alternative to the two part silicone rubber may be the one part kind in the calking tubes you find at the hardware store. The link below is a thread devoted to this option. I have started a large mold using an acrylic silicone mix caulking. This kind could be thinned with water and painted on just like latex, though in much thicker layers. It seemed to bond to itself very well, kept in place and did not seem to shrink at all. I have not yet made the mother mold and removed it all yet though so I am unsure it it will work in the end.
http://www.sculpture.net/community/showthread.php?t=661

[Jamo]

I duno beats me. maybe some more hints

[Jamo]

I duno beats me. Other than little stuff, like a pour spout, keys and crisper parting lines I duno whats wrong? maybe some more hints

[Mywhims]

MY HEART FELT THANKX TO ALL OF YOU AND YOUR ADVICE/SUGGESTIONS!! You're right about the being lazy part....and this latex thing taking so much time! I was beginning to think it would be better just to purchase the molds! I'm going to sit back and recalculate my expenses and budget and try using poly or silicone for the molds....but isn't latex the most widely reccomended for pouring abrasive materials like concrete??? If I go the silicone way, I'll need to invest in a gram scale for measuring....(was trying to avoid this expese)....sigh....anyway, thank you all for taking me under your wing for a spell.

[Landseer]

I duno beats me. Other than little stuff, like a pour spout, keys and crisper parting lines I duno whats wrong? maybe some more hintsBingo you got it! A way to pour the hydrocal IN when the two halves are closed up.
I WAS going to try to salvage it by cutting a hole but decided the heck with it, it would never pour nice and level and smooth on the bottom where it needs to be flat, level and smooth to stand and be mounted on the oak base.
So the setup was left intact carefully peeling off that rubber and I added a flange at a 90º on the bottom so when I start again it will work right.

[Landseer]

.but isn't latex the most widely reccomended for pouring abrasive materials like concrete??? If I go the silicone way, I'll need to invest in a gram scale for measuring....(was trying to avoid this expese)...Latex doesn't hold up long term anyway. People use latex because it's cheap per gallon but if you do the calculations you'll find it's NOT as cheap as you thought.
You could have done that whole rubber mold in ONE afternoon with the silicone, cure it over night but 2-3 days is best for 100% cure, make the shell and start casting.

Ok, let me address the silicone tube caulk, here's a picture of one tube, I did some research and found this is 10 ounces of silicone, somewhere I saw a price of about $6 a tube and know from my local hardware store the prices vary from about $5.99 to $9.99 or so, let's go with $5 a tube.

Do the math, 10 ounces per tube for $5, that's 50 cents per ounce or $8.00 for a pound.
With me so far?

Ok, the silicone I buy is about $95 or so for 10# can (about a gallon) WITH the 1# hardener, that's 11# total, with the UPS it's $118
It's about $10.65 a pound delivered to my front door with a product warrantee.

Your hardware store caulk is $8.00 a pound and you have to pick it up, hope the store has enough of the same formula on the shelf when you need it, deal with the tubes etc etc.
You are saving $2 to $2.65 a pound AT MOST and there is NO product warrantee, plenty of acrid fumes and all the other issues.
This caulk is not meant to be used for molding it's formulated to stick to window and door frames for sealing, the silicone mold rubber is a different
product- it is designed for making molds, it is designed to stretch and last, it has ZERO fumes and almost no odor, it comes in various formulations from syrup thin to paste and everything between.

That mold I posted the photo of is about 8 pounds and that's a pretty big mold- $80 for the right rubber to be applied usually in ONE coat v/s hardware caulk you can't do that with, and which for the same 8 pounds would cost $64 worth of tubes assuming your local store has that many on the shelf when you need it.
$64 to go cheap with the wrong stuff v/s $80 to go with the right, good stuff- that's only a $16 price difference for an entire mold!

Now figure out your time of messing with window caulk, it's THICK it wants to skin over, it stinks, you cant layer that on thick and heavy and fill deep undercuts- it wont cure right, so you have to start building up coats of it to get the thickness.

The mold making silicone I use has a LONG working time, well over 2 hours in my experience that you have from mixing it to apply it to the model, while that time is going by the rubber sits exactly the same as it was mixed, it doesnt want to skin over, thicken or change, you have loads of time and don't have to panic and rush.
Yes, you DO need a gram scale, but I bought mine for $39 I think at a kitchen supply place on line, it weighs grams and ounces/pounds and runs on a battery, it's a one time purchase it's your equipment and tool for work.

You do NOT need to vacuum de-air the stuff, it is only suggested, you mix it carefully and well and apply it right and avoid almost all air bubbles. Typically the minor occasional bubbles I get are from trapped air in details in the MODEL as the rubber is brushed on, not the rubber itself.

Last issues is, this silicone caulk is NOT "hand lotion" and it concerns me when I read of people kneeding this stuff with xylene, bare hands etc BAD idea! you don't know what all is in this stuff and what long term health risks from skin contact might be.

[Lunarburn]

I agree that silicon mold rubber is a beautiful mold material, but expensive.

Here is a good alternative. For about half the price. Mixes 1 to 1 and will hold up to concrete.
http://www.industrialpolymers.com/brush-on_molding_gels.html

[Landseer]

I agree that silicon mold rubber is a beautiful mold material, but expensive.

Here is a good alternative. For about half the price. Mixes 1 to 1 and will hold up to concrete.
http://www.industrialpolymers.com/brush-on_molding_gels.htmlThat appears to be urethane rubber, you might have missed the thread I posted about my experiance with THAT stuff from Polytek, the stuff tears very easy- I lost about 4 very expensive molds made of that because Polytek's rubber didn't cure right and then they said it was because one of the parts was inconsistent and sent me a replacement kit (forgetting I had wasted four $100 kits plus all my time and hundreds of pounds of plaster)

That was when someone here suggested Quantum and I obtained a sample and have been with it ever since.

The problem with urethanes is the stuff WILL tear pretty easy and once you get a nick or tear the tear goes like glass. It may be half the price of silicone but again, very often you get just what you paid for!
The urethane I used 74-40 worked very different from the silicone, you have to be fast with the urethane because it starts gelling in minutes to where you can't work it with a brush any more, the silicone will stay like wallboard paste for a good two hours or more and sets more slowly- plenty of working time.

[Landseer]

Latex:

Disadvantages -- Low-cost latex products generally shrink – on the order of 10 to 20% depending on product. Making molds with latex rubber is slow and time consuming. Brush-on molds made with latex require as many as 20 brush coats, with 4 hours of drying time between each coat. Time factor for making a brush-on latex mold is ten days or more. Many latex products have an ammonia odor (however, there are new latex products on the market with lower shrinkage and no odor). Latex molds are generally not suitable for casting resins.
Polyurethane:

Disadvantages –
urethane rubber has the worst release properties and will adhere to just about anything. Thorough model preparation (we’ll cover this topic later) is essential to successful mold making with urethane rubber. Urethanes are moisture sensitive and may bubble if exposed to too much moisture (making molds outside on a very humid day, for example). Limited shelf life after opening – remaining product may be affected by ambient moisture in the air. (Smooth-On makes a product called “Xtend-Ittm” that greatly extends the shelf life of unused urethanes).
=========

I did a little research;

Cementx #80 is a vulcanizeable Natural Latex compounded from specially processed Latex with a solid content of between 68 and 70
percent.

That means out of the gallon which costs $33.60 plus shipping, 30-32% is just water and ammonia that evaporates out- you lose 1/3 of it.
That would mean to get a full gallon it would cost about $41.00
This stuff is even more expensive in two ways- cost and labor- up to 20 coats with at least 4 hours dry time each, taking maybe 10 DAYS to make a mold, costs than I thought this morning.
So as you can see, latex v/s silicone or caulk v/s silicone the silicone wins no matter how you dice it.

[Jamo]

heres a universal rule that goes everything not only mold rubber.You get what you pay for. Silicone may cost you some bling but in the end do you really wana cheap it out on something you spent months making.Your time is more valuable than money any day.

[Landseer]

You get what you pay for.
Your time is more valuable than money any day.Jamo is right there, this applies to just about everything.