Mold making Trutorial: Blanket Mold - pg 4
Now we need to prepare our igredients

Now we need to prepare our ingredients 

 You have diligently sanded the inside of the backup shell to remove any rough spots.  It's smoother than the hood of a Lamborghini.   You are one heck of a mold maker! 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

Don't hold that against it. 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. Don't be an enabler. Seal your boards and backup shell with lacquer and wax so urethane can't get to them.

 This is the process that I use to curb the lust that urethane has for moisture.  Apply a coat of lacquer then apply 3 layers of wax to the inside of the plaster shell. There are special mold making waxes for this purpose. You can find a hundred of them on the internet. As an alternative, you could use Butchers Wax, which is available at supermarkets and hardware stores.  As I have said before, I have used the Butchers Bowling Alley wax and their Boston Polish with equal success.

If the model is porous, seal first with acrylic sealer or lacquer. Let the coat of lacquer dry thoroughly. Start applying 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 so much that it creates pools of mold release. That will cause inhibition in the cure of the urethane.  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.  You could use bolts or whatever works for you. Here's what I came up with. I have a clamp on each corner. It's quick and easy to put on and remove. This gives me a greater and more precise amount of control on the pressure being applied. Mold making is a matter of thinking, observing and devising your own way of doing things. That's how the field advances. Just so the job comes out right. Do what works for you.

Now we need to prepare our igredients

The urethane mold elastomer I will be using is Vyta Flex 30 from Smooth-On.  I have chosen this particular mold material because the pineapple model has some very deep undercuts and I know that Vyta Flex 30 will flex enough to pull over these inch and a half deep undercuts.  This urethane 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 this photo 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, scrape 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...