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Dan Does It ! 7
Making a Blanket Mold
Life's too short to waste it piddling around with a latex mold!
What is your time worth? Can you afford to spend four to six weeks maybe more, brushing latex rubber on a mold? No? Of course not. You have a life to live, things to do, people to see, money to make. Staying light on your feet , weaving and bobbing , keeping ahead of the crowd, those aren't just mindless catch phrases they are the words you live by. They are the commandments of one driven to reach a higher plateau. Well my success oriented friend this may be just the process your palpitating little heart of an entrepreneur has been waiting for. With this system of mold making, a mold can be made in days not weeks or months. Not only that but you can knock out duplicate molds so easily it almost seems criminal. Yes I thought that part would get the larcenous synapses in your gray matter firing. If anybody would, I knew you'd be up for a way to make money quickly and easily. No muss no fuss. Get in quick and get out! That style of thinking is right up your alley, right? Boy I hope I don't get caught in that alley!
Seein' as how you are one of those no nonsense "show me the money" kind of people and how time is money and money bein' the reason we're in business, we better git right to it or you'll start charging me directly.
Bye Baby Bunting
Let' get one thing straight right off the bat. A blanket mold is not a mold of your two year old's baby blanket. Here's the low down. What we're talking about is making a mold with a process that resembles placing a blanket over the object to be molded. No not a fuzzy warm fleece that you pull up over your head on a cold winter night either. Essentially what you do is to lay a 3/8" to 1/2" thick layer or blanket of urethane elastomer or silicone, (not latex rubber) over your model.
Daddy's gone a hunting. For to get a rabbit skin to wrap his baby bunting in.
The first thing you need to do is hunt around for an appropriately sized board on which to mount the model or you could go buy one if you are so inclined. Because are mounting the model on it we will call this piece of wood the mounting board ( how clever.) The mounting board should be 3 or 4 inches larger than the model all the way around. You can make one out of 3/4" plywood or if you're lucky you might find one ready made. I found this one on my wife's sewing machine cabinet. It's polished and lacquered up nicer than a barroom tabletop. It's mahogany, perfectly flat and by coincidence just exactly the right size. It's also free! A fella' can't pass up a deal like that. I'll just commandeer it for a spell. If you make your mounting board out of raw plywood be sure to give it several coats of acrylic sealer or lacquer. Then wax the board.
The other materials I have gathered for this project are:
Evergreen Urethane Elastomer, plastiline clay, acrylic sealer, spray release agent, paste wax, plastic tubs or buckets, spatulas for mixing and incidental items such as paper towels and disposable gloves. Supplies can be purchased from the column at the left.
Adapt and Innovate
Mold making and model making are trades where the learning never ends. What I have done is to lay out the basic principles of making a blanket mold for you. A great deal, perhaps most of what you will learn about mold making you will figure out on your own as you attempt this project. You are apt to make mistakes along the way. Do not let that deter you from your desire to grow and experience new avenues of creativity. Adding to your store of knowledge is more important than doing a perfect job the first time out. Don't be to hard on yourself. With each attempt you will become more familiar with the materials and mold making processes. One day when you least expect it, it will all come together for you and you'll say "by jiggety that's it! I've got it. I can do this stuff."
As you have noticed the example that I have chosen to make a mold of is a plaster pineapple wall plaque. Heretofore to be known as the model. The model has been previously painted and sealed with acrylic sealer so I do not have to do it again. If your model is made of a porous material such as unpainted plaster you will have to seal it with acrylic sealer. Krylon makes one such brand of sealer another is Patricia Nimocks acrylic sealer. These products are available at art and craft stores. Take the time to do a good job preparing the model for the urethane mold. Seal it well with acrylic and spray it with a release agent. Polyurethane adheres amazingly well to improperly prepared models so do this part well.
Since it is highly unlikely that you will have the exact same piece for your project, some of that figuring it out on your own stuff will have to come into play. Adapt the processes given to the best of your ability. You may come up with a better, quicker or cheaper way of doing it. Choose something simple your first time out. Once you learn how to do it in the manner described you can adapt the process to more intricate projects.
In the photo above the model has been attached to the mounting board with screws. Below shows how I am drilling through the board into the model to give the screws a starting hole.
Also drill a few extra holes to act as vent holes. Gasses build up in the mold material as they do in most of us and have to be relieved. These gasses will be released through vent holes. If you don't do this you could end up with giant pockets of gas that could ruin your mold. This is especially true when making molds over porous models such as plaster models.
Plan 9 from Outer Space
The whole is more than the sum of it's parts. Rather than just following directions blindly you need to have a holistic view of the entire process in order to absorb it effectively. That's because once you see the direction we are headed it makes more sense to your devious little reasoning mind. You can stop wasting brain energy wondering where is this going and focus your mighty powers of perception on the segment at hand. So now it's time to reveal the plan. We are going to cover the model with a blanket of clay. Then a hydrocal shell will be made over the clay blanket. The clay will be removed and a void will remain between the model and the hydrocal shell. This void we will fill with a urethane elastomer. We will then make another back up shell over the urethane mold. It sounds more complicated than it is. Trust me. On second thought maybe you better just follow your intuition.
Now that our model is mounted on the board the next thing we need to do is put plastic wrap like Saran Wrap over the model. The plastic wrap will protect the model from the water clay which we will be applying over the model in the very next step.
Press the plastic
wrap down snugly. It can extend out several inches from the edge of the
model on the wood since we will be trimming back the excess later.
Now we need to make the blanket of clay.
The clay I am using is a gray water based clay. Not that it matters but I have found that the gray colored clays do not stain the plaster as much as the red and orange clays do. Start forming the blanket by placing a roll or lump of clay between two 3/8 " thick boards. The consistency of your clay is critical in order to be able to roll it out. If it's too wet you'll have a big sticky mess on your hands. It's like sticking you rolling pin in a pot of tar. If it's too dry it will crack and crumble when you try to roll it. So here's what you do. Start with a rolling pin that you borrow from the kitchen and roll out slabs of clay about a foot long and 4 to 6 inches wide. Because you are using 3/8" thick spacer boards every slab will be exactly 3/8" thick. The boards take all the fight and guesswork out of what could be a bear of a job. For me the hardest part of the job is getting the rolling pin back into the pantry without getting caught.
Press the slabs down tightly to conform to the shape of the model . Be careful not to press the clay to hard on the high spots of the model. This will cause the clay to thin out and your mold will have a place you can see light through.
Add consecutive slabs of clay joining them together where they meet until the entire model is covered.
At this point some engineering of a pour hole and vent must be considered. Air must be allowed to escape and the liquid urethane must be allowed to flow in smoothly
Lets take a close look at the design of our model.
At the highest parts of the model we need to install a means of letting the air rise up and escape. We also will need a place to pour in the urethane. At the highest elevation points we will place cone shaped risers made of clay. The yellow area represents the clay. You will notice in the diagram above that some parts have been made thicker to let the air flow up and out. If the clay followed the contour of the model in those places it would trap the air in pockets ruining the mold. This is the quick and easy way it uses more material but sometimes materials are cheaper than labor. This is the method I will be using.
Another method to let the air out of the pockets would be to place straws in the clay at the high points. In the diagram above yellow represents the clay and the straws, white represents the plaster shell. Notice only one clay cone is necessary to act as a pourhole. With this method the clay can conform more closely to the model. It takes a little more time and effort but you use less urethane for the mold.
Now the smoothing out process begins. A natural sponge is the traditional item used to do this work. One called an elephant ear sponge is the perfect choice.
Should it come to pass that you are unable to obtain one such elephant ear sponge; a small oval synthetic sponge cut in half, as shown below will suffice. Wet the sponge and rub the surface of the water clay to remove any rough texture. You will soon develop the technique to create a fine even surface. The object of is to get the surface glassy smooth. Holes and rough ridges on the surface will keep the backup shell from registering properly on your finished mold. The smoother you make the clay now the less work you will have to do later in the plaster stage. After wiping with the sponge the surface might look lumpy though they are smooth lumps.
Lumps and bumps in life are inevitable.
Up and down and in and out curves are fine on your mold but you do want to reduce a bumpy surface. Fortunately for your blanket mold we have a way to smooth them. Wooden tools that are specially made for smoothing and flattening clay are available. They are ergonomically designed and have the proper shaped ends for gliding over undulating clay surfaces. Would that we could all have ergonomically designed ends and glide over undulating surfaces. Life would be a dream. Oh yeah!
However back to our saga. A satisfactory tool for smoothing down lumps can be made from a broomstick which you have flattened on one side. It's not quite as glamorous as the store bought'n ones but it will work.
There are thin metal scrapers and rubber squeegee's for smoothing purposes also. A combination of all the tools and methods above can be used to get the desired finish. There is a knack to working and smoothing water clay in this fashion but fortunately the learning curve is a short one. There is a sense of pleasure in working with these natural clays and getting them to come out smooth and blemish free. Achieving mastery of these techniques will give you a sense of accomplishment .
Test the thickness of the clay blanket at this time. Take a paper clip and straighten it. Poke the straightened paper clip into the clay at various locations. If you run into any thin spots add the proper amount of clay there. Pay special attention to high points and sharp protrusions. The clay becomes thin over those areas. Smooth the tiny holes left by the paper clip testing.
Ok your clay blanket is now all neat and pretty. If you have to leave it for any length of time cover it with plastic wrap so the clay doesn't dry out. If the clay starts to dry it will shrink and crack.
Next you need to cut and wax 4 boards to construct a form around the model. I am using Butchers wax which is available at grocery and hardware stores to seal these boards or from http://www.butcherwax.com. Apply three coats, polishing each to a sheen between coats. I try to avoid using shellac on boards where urethane will be cast. Urethane has a pure unadulterated lust for moisture. It craves the stuff like some people crave the nectar of the vine. If the urethane draws out the moisture in the shellac it will cure with a tacky surface or not at all.
Also urethane will bond to shellac like all get out if you don't wax it well or spray mold release on it. Lacquer or acrylic sealers are better choices for use with urethane since they are solvent based not alcohol.
Once they have been waxed the side form boards should be installed 2" or 3" from the edge of the clay. Use screws to attach the forms. You will need to remove one of the boards and replace it in exactly the same position as part of the process. The screws will allow you to do that.
Soap 'er up
Give the clay and the boards a coat of mold soap.
Materials for this project are available by clicking the links on the left or visiting the Dan Does It !7 supplies page.
I'll make your mold for you E-mail me about your project.
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