The Re-making of Aragorn (Step 9)

Whew! Welcome back to my "Re-making of Aragorn" series. I apologize for taking so long to post again, but I have been busy finishing my Aragorn model and photographing the entire process! Now let's catch up.

In this step, I will be explaining the things to keep in mind when preparing your model for the molding process. The biggest thing to remember when casting the resin in your mold is that air is your enemy. Trapped air in a mold will prevent resin from reaching into that area and will ruin your casting by causing those areas not to form. This could result in major repair work to fix the casting, or the lost of material if you choose to scrap the piece because not enough of it formed. So you must consider the final negative shape in your mold and learn to anticipate trapped air and ways to prepare before you pour the liquid rubber mold.

A scientific fact (and a neat experiment):
When liquid is poured into a vessel, the pull of gravity will cause the liquid to level. This can be observed by taking two containers (like 2 liter bottles) and hooking them together. Leave the tops off of both and pour liquid only into one container. Gravity pulls the liquid down and allows it to flow into the other side of the container. The liquid replaces the air that was in the container as the air is pushed up and out of the second container and the liquid levels to the same height in both containers.

Now, close the top of the second container. Now pour your liquid... Gravity does the job again, but this time, the air has no place to go. When air is compressed, it builds pressure and can be very strong. the trapped air is stronger than gravity's pull and the pressure of the liquid. this causes the liquid to stop rising in the second container while it continues to rise in the first. this creates a situation where the liquid does not flow all of the way into the shapes and is un-level.


Now apply the above fact to a mold. If liquid is being asked to flow into an area that traps air and does not allow it to leave the mold, then the liquid resin will not flow into that area (see the illustration). Notice that the hand shape has several fingers that extend upward into the mold rubber and are cut-off from the pour spout. Air in the fingers does not have anywhere to go, so resin will not flow into them and will result in a cast hand that will not have any more than the finger used as a pour spout.



To allow air to escape the mold and fully cast the part, we only need to create air shafts from the trapped areas that flow up to the top of the mold. These are called "chase holes" as the resin will chase the air out of the mold. Remember that liquid will level, so DO NOT make the chase holes out of the side of the mold. Doing so will cause the resin to drain off at a certain level and not fully fill the mold (causing a mess and not forming the part). Chase holes always rise to the top of the mold allowing the liquid to fully rise to the top of the mold also. In this verison (see illustration), the chase holes allow air out of the fingers and the resin can flow fully into the mold and you get a full casting. You then only need to remove the extra resin, formed in the chase holes, from the part with a knife and a little sanding. It is vastly better to remove small extra bits, than to have to repair un-formed areas.



Another way to go would have been to cast the part with the protruding areas formed down into the mold. That way, gravity works for you and resin can flow down into those areas and air can flow out of the fingers. If you still have small areas that turn up into the mold (see forefinger in illustration), you only need to make a chase hole all the way up to the top of the mold or connect that area to another area that doesn't trap air (pictured). You can think of the way that plastic model kits are made with "runners". This extra piece can be easily removed and sanded and will ensure that the forefinger will form in the mold as air will be pushed out to the main area of the mold.



So, having the shape of my Aragorn body in mind. I know that I will be pouring the mold through the feet and need to identify areas that will trap air going up into the mold. For those areas, I need to plan and prepare chase holes before I pour the rubber. I know that the bottom of his traveling coat and the ends of the blanket will trap air and cause trouble.




So, I use dowel rods (cut to length) and bond them to those areas and connect them to the model's base (which will be "up" in the final mold). These rods and the Super Sculpey model will be removed from the mold and create the chase holes I need.

.

Now lets's create the mold casings!