Work at home Course Part I
Latex Molds
the Making of a Latex Mold
What the heck is that guy in the kilt doing behind that tree? 
Latex is a natural product. It is not the sap but rather a unique milky substance found in some plants, most notably the Hevea Brasilienesis tree. To date man has not been able to duplicate the elasticity of this marvelous creation of nature. Only latex is truly rubber. The word rubber comes to us from one Joseph Priestley (1733-1804) who used it to rub out pencil marks. Thus the English use of the word rubber for eraser. Yes. In Britain the lucky little rascals are given rubbers on the very first day of school.
Priestley, a rather unique individual, it seems not only sat around thinking up names for new products, he also discovered oxegen, founded the Unitarian church in America, was an Englishman who sympathized with the American revolution( which didn't win him a lot of friends in England) , is called the father of pneumatics (and rubbers), amongst other things. ( just FYI )
Tapping Hevea Brasiliensis
Originally native to South America,the 50 ft. tall Hevea trees are now grown on plantations in the far east. The trees are tapped in a very conscientious way so as not to injure the cambium layer under the bark. Each tree produces about one cup of "field " latex per day. This "field" latex is collected in glass or ceramic containers and brought to processing centers where the solids in the milky substance are concentrated into raw or crude rubber. Ammonia acts as a preservative for the delicate liquid and may be added to certain latex formulations before they are shipped to manufacturers for further processing.
Because latex rubber is a natural product it requires special handling and care. Liquid latex must not be allowed to freeze, threrefor it is difficult to ship during severe cold spells. Latex molds must be stored and used out of direct sunlight. Sunlight dries and cracks the rubber. Petroleum products such as motor oil will deteriorate natural rubber. Do not use them on latex molds.
Latex shrinks when applied to a model. The less solids in the formulation the more it will shrink. Also a high solids content is essential to produce a strong tear resistant mold rubber. Buy from a reputable supplier to reduce the risk of getting a low solids latex mold material. You may have to consider making your model slightly larger to compensate for the shrinkage factor.
What do I need to make a latex mold?
You will need a model, some shellac, a good quality mold making latex and some brushes.
On small items such as the hedgehog I use a small brush such as an acid brush for the first couple of coats. After that I use a larger brush. The acid brush is about 1/2 inch wide with a rolled tin handle. Acid brushes are available in the plumbing section of yoiur hardware store. They may cost 25 to 30 cents each. The reason I start with a small brush like this rather than a great big wide one is that it forces you to work very carefully. You wouldn't want to do the entire piece with a brush this small. On larger peices I start with a larger brush. The first 2 coats need to be applied meticulously and very thinly. Successive coats can be applied with a wider brush ( 1" or 1 1/2"). I will apply 12 to 14 coats to a piece this size allowing them to dry thoroughly betwqeen coats. This may be 1 to 4 hrs. between coats depending on how quickly they are drying.
Why does it have to take so long?
Temperature, humidity, and air circulation have the greatest effect on the drying time. A room temperature of about 75º would be excellent for brushing on ru
bber. I have tried brushing on latex in 85ºor 90º temperatures with fans blowing at hurricane force because a client needed a mold in a real hurry. I was putting on a coat about every 20 minutes or half hour. These attempts at breakneck speed mold making were not, let us say, entirely successful.. Without proper drying time between coats moisture is trapped under the rubber causing large air pockets to rise up. In one instance I ended up having to poke a thin cocktail straw into the air bubble and suck out the nasty tasting pocket of air. Of course I did have to go out and order a cocktail in order to get the tiny straw so it wasn't all for naught.
Shrinkage is also increased when force drying latex at high temperatrure especially if you brush on heavy coats. I havefound that the rubber pulls away from the model where there is a valley between two high points. It leaves a gap between the rubber and the model as it shrinks up. You may get away with forcing the drying process once or twice or even a few times but the one time you are really counting on having this mold in a hurry, disaster will strike. It's some kind of unwritten rule!
Let the Brushing Begin
If the model is not mounted to a board,as mentioned earlier get a board about 3 " larger all the way around than the model. Drill a 1/2" hole in the center of the board.
If the model is not mounted to a board,as mentioned earlier get a board about 3 " larger all the way around than the model. Drill a 1/2" hole in the center of the board.
I start by brushing latex on the board surrounding the model.. This cleans the brush and removes any loose hairs and deposits them in a place where they will not affect the finished casting. Do not brush all of the rubber off of the brush when applying rubber to the model. Before the brush becomes totally dry dip it back in your supply of latex so that it always remains wet while brushing. This will keep the rubber from balling up on your brush making it unusable. If you place the brush in a covered jar of latex rubber between coats it will remain soft and usable through the entire job
Some brush cleaning tips
Another method to keep your brush fresh and ready for use is to wash the brush out with amonia between coats or you may just let it soak in a jar of amonia if you wish. Make sure the bristles are totally covered or you will get a ring of latex that forms on top of theamonia infused into the bristles. Some mold makers soak thier brushes in a jar of soapy water. That helps make cleaning the brush easier. Dry the brush on a rag and be sure to start your brushing on the board if you use this method. If you wish to clean a brush that has become hopelessly clogged with hardened rubber soak it in a container of lacquer thinner. It will turn into a soft gellatinous goo that you can brush out with a fork or wire brush. This could be a lifesaver if you only have one brush left and you have to get the job done pronto.
BrushingTechnique
You must carefully brush the latex into every crivice. If tiny air bubbles form in the first few coats caref
ully brush them out with as few strokes as possible. Do not over brush. Some mold shops spray the first three coats of latex rubber on, just to be sure they get every minute spot. That's how important the first few coats are. You want to capture all of the fine detail of the model without leaving any imperfections in the rubber. Later coats may be applied less cautiously, in regards to minute air bubbles. Wait until all the whiteness is gone from the rubber before brushing on the next coat.
After the initial detail coats some mold makers apply the next coat with all the strokes going in one direction like up and down then brush the next coat from side to side so the coats have a laminated effect, much in the way plywood is made with the layers of grain crossing each other. That's fine for a large latex mold but on one this size it's just not practical that's just to much exactitude for my intestinal fortitude. My style is a less formal technique with X pattern strokes criss crossing each other the whole time. Which ever method you use try to keep large runs or drips from forming. They usually become noticeable after you've finished brushing on a coat. At which time they are difficult to correct. In the beginning you will think it is impossible to do. It does take some practice to get to the point where you make a smooth lump free latex mold, but before you know it you will be slapping rubber on like a pro. The problem with the large drips and runs is that they can keep the rubber mold from seating onto theback up shell properly, thereby distorting the casting. The world is not going to come to an end if your rubber mold is lumpy so don't get obsessed about it. Just be aware that large drips hanging off your mold is not a good thing and try to avoid it.
The first coat has dried to a nice even tan color. Now the second coat can be brushed on.
Second coat being applied. Notice how w
hite the latex is at this point. It will turn a tan color when it is dry.
The first coat has dried to a translucent tan color with no white spots remaining. It is important that you do not seal in wet rubber by brushing a fresh coat on top of it. Themold will not hold its shape where wet rubber has been sealed in. This frequently happens at the base of the figure where it meets the wooden board.
I brush out about 3" out from the base of the sculpture on the board. When the mold is done this will give me a firm piece of rubber to grab onto to pull the mold off the model. I also want the rubber to be wider than the back up shell so that when castings are being poured I can scrape any excess casting material off the mold without the back up shell interfering with the process.
Since this is a beginning mold making course this will be a simple mold with a 2 piece back up shell. I have calculated that the diameter of the mold opening at the base of the model is approximately half that of the widest point of the model. If the model were wider than that or if the mold opening (cast hole) were smaller than that, I might have to put a seam or web in the mold to allow it to open wide enough to come off the casting.We can get into more difficult molds that require webs at a later date.
I don't like to make the mold stretch to more than twice it's diameter. It is difficult to pull the rubber over the casting and may break a "green" concrete casting when it has to stretch that far. It may distort the rubber with repeated use.
Keep on a brushin'
Keep brushing on the coats. As the detail of the piece slowly becomes lost in
latex the figure takes on a rounded appearance. If you want to check on the thickness of the latex lift up the edge of the rubber on the board. You are going to brush on about an eighth of an inch, give or take a little. I brushed on 12 coats on this model. You may need anywher from 10 to 14 coats depending on how much rubber you apply with each coat.
Moving air speeds drying !
A fan blowing on the latex will reduce drying time. Commercial outfits have 6 or 8 fans blowin' up a storm to hurry things along. After all the coats have been applied you may let the mold set for a week so that it will hold it's shape. I prefer to heat cure it.
Look over the mold for places that will cause the backup shell to "hang up" or lock itself onto the mold. These nasty little problem spots are called undercuts. Mold makers hate them with a passion. The line going down the center of the mold is where the 2 piece backup shell will separate.
Fill Undercuts
For s
mall undercut areas make some thick latex paste by filling a small paper cup, like a small bathroom drinking cup, with latex. Let the latex sit overnight without covering the cup. By the next day it will have thickened into a paste. There will be a skin on top of the latex rubber. Break through the latex skin with a flat tool. Scoop out some thick latex with the tool. Spread it evenly on the undercut area. To successfully fill this undercut I had to apply two layers 1/8 " thick. Apply the first layer 1/8 "thick, let it dry overnight then apply the second layer. Do not put on more than 1/8 " in a single layer. Do not brush coats of rubber over the patch.
Let it Dry!
because it can't dry. It will wrinkle the mold on the inside ruining every casting you make. Brush al
l around the patch until it is dry. You can tell when it is dry by the golden brown color it takes on, like a Thanksgiving turkey when it's done just right
Your brand of latex rubber may not change color as much as mine.
Only experience with the product you are using will be your guide .
This is the finished patch all filled in on one side. look how it is slanted down slightly ( the mold making term is called " drafted ") so that the backup shell will slide right off.Notice it is higher at the middle of the head and lower at the end of the ear. Compare it to the other undercut ear which has not been filled in yet.
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Rubber is a natural product. It comes from a tree.
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only latex is truly rubber
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latex rubber is a delicate product and requires special handling and use. It will be damaged by freezing, direct sunlight or petroleum products such as motor oil.
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brush on even coats of rubber and let them dry to a tan color before brushing on the next coat.
Fantastico!
more pages will be forthcoming as I get time to put them up.
