|
lost? all roads lead to plaster master Work at Home pg 5 Plaster Master guide to home business Starting Your Own Statuary Business vulcanizing the Rubber Mold and Making the Backup Shell
Contrary to popular belief Vulcanization is not the colonization of earth by Vulcans The curing of rubber is called vulcanization. Unvulcanized rubber becomes soft and sticky in hot weather (who doesn't) and brittle in cold weather. Charles Goodyear died in poverty after discovering the process that made rubber the usable manufacturing product that it is today. Others have used his process to make great fortunes. Goodyear struggled for 10 years to improve India rubber (latex rubber). He had thrown everything but the kitchen sink into his rubber mix to make it stronger and withstand extremes of temperature. Nothing worked. Nothing until one fortunate day in 1839, when quite by accident ,some latex rubber he had mixed with sulphur spilled on a hot stove. Heating the rubber was the secret! It made the rubber tough and able to withstand extremes of temperature but it still remained soft and elastic. Viola! Vulcanization!
a Tough sonovfa gun If we want our mold to be tough and extremely elastic we have two choices. We can let it rest for a week which will set the rubber somewhat. An even better idea would be to vulcanize it. We don't use the spilling rubber on the stove technique any longer. To achieve vulcanization the mold should be heated while on the model to a temperature of 120º for 4 hours. To test if the rubber is cured push your fingernail into the rubber. If the rubber springs back without leaving a dent then it is cured.
Making the Backup Shell Now that we have vulcanized our mold,the back up shell can be made. Most people vulcanize the mold and then make the backup shell. There are occasions when I have made the backup shell on the mold before vulcanizing it. I then vulcanized the mold with the backup shell on it as a unit. The reason for this is that I once made a mold over a plastiline model for a client. The client put the mold and plastiline model in their plaster drying oven at 160º overnight. In the morning the plastiline had melted. The rubber had nothing inside to give it form so no back up shell could be made. Now I take precautions against that kind of mishap. The backup shell or mother mold keeps the rubber in shape while you pour casting material into it. Backup shells can be made of resin, polyurethane, gypsum or foam. A fibrous material such as fiberglass or burlap is incorporated into the making of some of the backup shells for greater strength. Lay Out the Mother The number of mother mold sections and where the dividing lines will be is something that has to be considered right from the start. The hedgehog wll require a two piece backup shell. I knew where the dividing line would be when I started out so I made the necessary adjustments to the mold as I brushed on the rubber. The area behind the ears was built up with thick rubber to eliminate the slight undercut there. An undercut is where the back up shell will hang up and not pull off the mold. Look over the mold to see if there are any undercuts that will prevent the backup shell from coming off. Fill any undercuts that you may have missed earlier with plastiline. It is most imprtant that you fill in these undercuts. The backup shell will be locked on if you do not. Apply the Soap Draw lines on the mold where the backup shell will divide. Then apply mold soap to the mold. This will help the plaster backup shell release easily. Since the water clay we will be using for a retaining wall may try to seperate from the rubber, the mold soap will help it adhere to the rubber. the Keys that Lock it up
Start applying plaster
I start by splashing on a coat of plaster
on the clay wall. The plaster I'm using is TufCal. White HydroCal or UltraCal would also
be excellent choices. They make extremely strong backup shells. Fill in the recessed keys.
After the entire surface is coated I apply fiberglass strips or burlap. These are the size strips I will be using.
Soak the strips in plaster and press them
into the plaster coated clay wall. Cover the entire section with the plaster soaked
strips. Apply the strips around the base of the figure. Materials for this project are available at our Sculptor's Place
|
The hedgehog in the picture is not being
mugged. This is a curing compromise between the two methods. I have let
the mold dry for 4 days and now I am heating it with a heat gun for 10 or 15
minutes. Be careful not to burn the rubber you only want to heat it thoroughly all
the way through. You can build a heating box and use a small electric heater to cure the
rubber. Commercial mold making outfits have large rooms heated with gas heaters that
warm the room up to 160º.
Using water clay, build a wall along one of the
sections of the backup shell following the outline that you made.My hedgehog is divided
right down the middle and the two halves will come off from side to side.Here I am using a
tool handle to make indentations in the clay wall. These are called keys. They keep the
two halves of the backup shell locked in place. Without the two halves being perfectly
registered they would slip and cause the casting to be out of shape.
