Block Mold - Multi Part | Page 2
When the clay has been smoothed out to 90-degrees of the model it is time to add the mold keys. The mold keys allow the two halves of the mold to register precisely when the mold halves are put together. For this purpose, we use "acorn" nuts embedded in the clay parting line about a half inch from each (Figure 8) other filling the entire clay surface.
Figure 9 shows all of the acorn nuts placed along the clay parting line in two rows. You can't be too generous adding mold keys in this step.
Now it is time to create the mold box, or matrix as it is often called by mold makers. This is done using foam board. A piece is cut to create a wall at least double the height of the highest part of the model. Score lines are the made on one side of the foam board about one to two inches apart. Then the board is flexed so that the scored lines open to allow the foam board to easily bend around the clay base. Use a hot glue gun, glue the foam board along the bottom of the base board. Sometimes an extra hand helps to keep the foam board in place while it is being glued.
After the foam board wall is completed, small pieces of clay are placed inside where the foam board meets the clay parting line to seal the gap. If the gap isn't sealed, silicone will leak out. Clay the entire gap and then up the sides where the foam boards meet each of its sections.
When you finish adding clay to the inside, you should add clay the outside where the foam board meets the baseboard. Complete this around the entire perimeter, Note the red duct tape. The tape was added to reinforce the mold as the weight of the silicone to be poured is heavy and would push out the wall of the foam board without this reinforcement. Duct tape is also be added across the top to further hold the walls see Figure 13.
In this step you must visualize where the "air traps" might occur when the silicone fills the mold. Where there is trapped air, silicone will not fill the area, and a gap or mold deformation will occur. It was determined that the hand of the model which sticks out might be an issue, so a small metal wire was cut and glued with the hot glue gun (left arrow) from the base to the hand. Next a straw was cut (right arrow) and glued from the hand upwards to the main part of the statue. Later the wire and straw will be cut from the cured mold, leaving an open tube to allow air to flow to the top.
Once the clay has filled the inside gap and the base of the foam board, the mold has been reinforced with duct tape and the air traps have been resolved, it is time to add mold release in anticipation of the silicone pour. Using a good silicone release such as ArtLube-100, give all surfaces three light coats of the release allowing each coat to dry between applications before the next one is applied. This is a critical step to prevent the mold material from adhering to the model and mold box. With silicone this is not a big issue since silicone often needs no release. However, it is always a good habit to practice as it allows for a cleaner de-molding.