With the bottom rubber slice in place, press a sprue base former against the bottom wall and halfway down into the rubber. Insert the end of the model into the base former, lower it down, and gently press it halfway into the rubber. Using sharp-tipped tweezers, begin placing keys at the corners, around the base former
and along the sides about 1⁄8" from the edge and base former. If the keys are too close to the side, they may shift during vulcanization and
break through the outer edges of the mold. The keys, standing on their bondable, powder-free cut edges, are pressed halfway into the rubber. Try to use keys that will cause even and symmetrical displacement. Don’t use skinny keys on one side and fat ones on the opposite side, because the displacement will be uneven, and the model might shift out of place.
After establishing the perimeter and the high stress area around the sprue base with medium keys, add fill-in keys in staggered positions. Think staggering as in bricklaying, because it minimizes leakage alleys and improves registration. Add the smaller keys between the eyescrews to further lock in tight registration. It is important to have
the keys stand straight up and down, to use appropriately sized keys, and to not have the keys resting against the model. Sometimes, it helps to use a clean tool like a watchmaker’s screwdriver to make a hole in which to shove a tiny, thin key. The trick is to avoid making the hole too big, because you don’t want gaps between the keys and the rubber. Gaps compromise bonding.
Once the keys are in place and the depths and positioning are reviewed, the whole surface is powdered. Use a soft bristle brush to spread the powder over every surface, especially the sides of the keys and where the rubber meets the model and the frame. Use compressed air
to gently remove all the extra powder because just a very thin coating is needed. Blowing with
your breath could result in accidental moisture deposits. Next, peel the safety paper off the top slice, flex it inward, and lower it into the frame. Gently press the rubber downward until it is
firmly in place with no air pockets between the slices. The motion is downward, not side to side or sliding, because you do not want to disturb key/model placement, or cause distortion.