This is pg 3 of the Dan Does It! 7 Article Making a Blanket Mold
Blanket Mold
You have diligently sanded the inside of the backup shell to remove any rough spots. It's smoother than the hood of a Lamborghini. The next step is to apply a barrier coating to keep the urethane from soaking into the plaster shell and becoming permanently adhered to it.
Urethane has a drinking problem
It's not a big deal. We all have weaknesses, secret cravings that if taken to excess would ruin our lives. It's just that urethane is looking to score some moisture from wherever it can, whenever it can. The mold boards are likely candidates, since wood contains moisture. It will also try to hit up the plaster backup shell for a dose of moisture. Don't let a good mold material go bad. Seal your boards and backup shell with wax or lacquer before pouring urethane against them.
This is the process that I use to curb the lust that urethane has for moisture. I apply 3 layers of wax to the inside of the plaster shell. There are special mold making waxes for this purpose. As an alternative you could use Butchers Wax which is available at supermarkets and hardware stores. I have used the Butchers Bowling Alley wax and their Boston Polish with equal success.
Apply the wax with a soft cloth.
Rub it in well, polishing
it
to a high gloss in between each coat. Then spray over the
waxed shell with mold release. Also spray the model with mold release.
Spray on two light coats and do not miss any spots. Do not spray on
pools of mold release. That will cause inhibition in the cure of the
urethane. If the model is
porous seal first with acrylic sealer. Then spray with
release.
Repair the plastiline fillet at this time if it has pulled loose. Smooth it out and press it back into the groove. Clean the mounting board of any mold making debris.
Putting it all back together
Then replace the shell in it's berth over the model, snuggly nestled in between the frame boards. If you look down into the pour hole of the HydroCal shell at this time you will see a void all around the model where the clay used to be. This is the space that we will fill with urethane mold elastomer. Replace the one board you removed to get your pry tool under the shell.
The next step is to devise an arrangement to hold the shell held down while the urethane is being poured in. You may devise your own method. Here's whut I come up wit.
I like to use C-clamps to hold the shell down. The clamps are quick and easy. Another method would be to drill holes through the HydroCal shell and the mounting board and bolt the shell down.
Now we need to prepare our ingredients
The urethane mold elastomer I will be using is Evergreen 20. I have chosen this particular mold material because the pineapple model has some very deep undercuts and I know that evergreen 20 will flex enough to pull over these inch and a half deep undercuts. Another quality in it's favor is that it has an oil in it that develops a slick surface that will release plaster and gypsum cement castings extraordinarily cleanly. Evergreen 20 is a 1 to 1 ratio mix. That makes preparation of the materials simple. Just use equal amounts of A and B components.
The object in the middle of the photo Above is the clay removed from the HydroCal shell. I have estimated that I will need slightly more than that amount so I have added extra material to each bucket. I would like to mix exactly the correct amount. I certainly do not want to mix an excess. If I err I want to do it on the short side. I can always mix up a small batch to make up the difference. I hate to waste anything so if I mix too much I end up running around the shop looking for something to make a mold of in a hurry. This has resulted in some pretty interesting molds. Time and practice are the best teachers in estimating the amount of material you will need. You can do it mathematically by figuring the volume of the area to be poured.
Mixing the evergreen 20 urethane First use a metal spatula to mix the side B component. It has solids that may have settled during storage. Do not use wooden mixing tools in preparing urethane. They will introduce moisture into the mix. Measure equal amounts of A and B components and add to your mixing bucket. Mix for 90 seconds with a metal spatula or an electric mixer. Empty the contents into a second bucket scrap the material from the sides and bottom into the second bucket also. Mix for another 90 seconds or longer if need be scraping the sides and bottom at least once during this operation. If the urethane is not well mixed soft spots will occur in the mold or the mold may cure with a tacky surface.
Pouring
Now pour the mixed urethane into the pour hole. Pour it slowly in a thin steady stream. By pouring in a thin stream air bubbles are broken as they travel down the long thin line.
The liquid entering the cavity through the pour hole will force the air in the cavity to rise up and out the vent hole. You will see the urethane rising up in the vent. The level in the two holes will eventually equalize. Fill the holes to within 1/4" from the top. If you come up short, immediately mix enough to finish topping off the mold. Do not wait until the rubber has set to add more. Add it while it is still liquid. Now the hard part. Wait at least 24 hours.
Waiting, waiting, waiting ...I can't take it...
Hmmm, everything seems to be OK
Congratulations! Unlike yours truly, you have extraordinary will power and have managed to not bother your mold for a full day! OK you've earned your reward. Remove one board. Pry up the shell. Get your first look at how your mold came out. Do not remove the urethane mold from the model just yet. Now here's the interesting part. You can use the HydroCal shell to keep on producing more molds. Here's how it's done.
Make another mother
Put the first shell aside for the moment. This hydrocal shell will for now and ever more be known as the master shell. We won't be needing it for a while. We are going to make another HydroCal shell and call this one the production shell. This new shell is going to be slightly trimmer than the master shell. Spray the outside of the urethane mold with mold release.
I have cut the vent and pouring sprues off the urethane and plastered right over top of the mold. No vent or pour hole will be needed in the production shell. Notice how the shell does not come all the way out to the frame boards. The production shell was made with the same fiber glass and HydroCal mixture as the master shell. Now remove the production shell and remove the urethane mold from the model. Put the new mold in the brand spanking new production shell.
This is a view of the of the urethane mold in the backup shell. Now that you have a master shell and the model still mounted on the board it might only take you a day and a half to make duplicate molds! Simply repeat the process to make as many molds as you want. A person of your cunning could take this idea, build a business around it and make themselves a tidy little nest egg.
Materials for this project are available by clicking the links on the left or visiting the Dan Does It !7 supplies page.
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