Advanced Mold Making Technique: Brush-On Mold of a 3D Hollow Model
The Challenge: Filling the void on this hollow puma head so an efficient brush-on
mold can be made.
Making a mold of a hollow model presents a challenge to the mold maker. The void must be
filled as the primary step of making the mold. A "mouth core" will be used to fill the
mouth void and a "mold plug" will be employed to duplicate the hollow space which runs
through the original model.
This challenging mold of a puma head will be made using
Smooth-On's Rebound® 25 brush-on silicone rubber. There has never been an
easier to use brush-on silicone rubber than Rebound 25. Mixed 1A:1B by volume (no scale
necessary), Rebound 25 self-thickens and will hold a vertical or inverted surface, and
will cure to a soft, flexible rubber (Shore 25A) that is incredibly strong. Three to
four layers is all that is necessary to make a strong, durable production mold for
casting wax, gypsum, concrete or resins (urethane, polyester, etc.) and because Rebound 25
is a platinum silicone, it will last for many years in your mold library.
1 - Puma head cast out of concrete.
2 - Begin by cutting several
slices of water clay to use
3 - Use a piece of melamine board for a base. Center head on board and fill the cavity with clay to eliminate the void at the back of the head.
4 - Two cut off bolts are enbedded in the clay to serve as place holders for later use.
5 - SuperSeal® is applied to the model and the clay to seal the porousity.
6 - Rebound® 25 silicone rubber is mixed 1A:1B and the first coat is applied to the model.
7 - This “detail coat” should be thin so that any bubbles that form will pop and not be trapped against the model.
8 - The second coat is pigmented with Silc Pig® so its easier to see areas that are missed.
9 - Next a batch of Rebound® 25 is thickened with Thi-Vex® II.
10 - Potential problem areas are filled in to allow for easier application and removal of the support shell.
11 - A third, unpigmented coat of Rebound® 25 is applied to the model.
12 - The fourth and final coat of Rebound® is pigmented and applied to the model.
13 - The “placeholder” bolts are located and the mold rubber cut away to expose them.
14 - Needle nose pliers are used to remove the spacers from the clay and rubber mold.
15 - Ease Release® 200 is appliedto a pair of 5 inch long machine bolts.
16 - The bolts are reinserted into the clay ports that were created
by the placeholders.
17 - Parts A & B of Plasti-Paste® are mixed 1A:3B by volume and the Plasti-Paste is spread into the mold cavity and around the bolts to form the mouth core.
18 - Cavity is filled with
19 - Next, more clay is rolled out.
20 - Clay is cut into strips to make the shims that will divide the mold in half when applying the support shell.
21 - Draw the parting line for the support shell. Be careful to watch for mechanical locking.
22 - Apply the shim along the parting line across the model and down the board.
23 - Use additional clay on the reverse side of the shim to reinforce it so it doesn’t move when Plasti-Paste® is applied.
24 - A clay extension is applied to the extended bolt to act as a spacer against the Plasti-Paste®.
25 - The bolt extension and clay shim are sealed with spray shellac.
26 - Sonite® II wax is applied to the clayed areas.
27 - Universal® Mold Release is applied to the clayed areas.
28 - Plasti-Paste® is mixed together with a 1A:2B by volume mix ratio.
29 - The Plasti-Paste® is spread over the entire first half of the model, including the shim.
30 - The clay is removed from the first half of the Plasti-Paste® shell after it has cured.
31 - Sonite® II Wax is applied to the Plasti-Paste® shim and mouth plug.
32 - Universal® Mold Release is applied over the Sonite® II wax to assure good release.
33 - The second half of the Plasti-Paste® shell is applied and left to cure.
34 - Three drywall screws are drilled through the shell edges and will be used to reassemble the shell.
35 - The mold and Plasti-Paste ® shell are pried away from the board.
36 - The rest of the clay is cleaned from the cavity and Sonite® II Wax applied to make a plug.
37 - Sonite® II wax is applied to the bolts and nuts are threaded halfway onto the anchor bolts.
38 - Drywall screws are used to secure a board across the mold and will help pull the plug out.
39 - Smooth-Cast® 45D semi-rigid liquid plastic is mixed 1A:1B by volume and poured in the cavity.
40 - The anchor bolts are removed from the shell so the mold plug can be removed.
41 - The plug is removed from the underside of the mold.
42 - Remove the drywall screws that were previously installed.
Screwdrivers and a hammer are used to separate the two mold
halves from each other.
43 - Plasti’Paste® shell is removed.
44 - The mouth core is then
removed from the mouth
45 - The shape of the model and the configuration of the Rebound 25 mold allowed for the making of a “glove mold” that can be peeled off without making any cut marks that would leave a seam line that might be visible.
46 - Care is taken not to rip the mold or damage the teeth.
47 - The mouth core is placed back inside the mouth cavity and the mold seated in the shell half.
48 - Place the first of the two core holder bolts through the shell and the mouth core holes.
49 - Place the second half of the shell in place, being careful to align it properly.
50 - Place the second of the core holder bolts through the shell and the mouth core holes.
51 - Replace the drywall screws used to hold the shell firmly together.
52 - Brush the plug with Sonite® II Wax to keep material from bonding to it.
53 - Put the mold plug back into the mold, taking care to align the bolts and nut holes.
54 - Tighten the bolts to hold the mold plug firmly into place in the center of the mold.
55 - The mold is then placed into a box full of packing peanuts and leveled out.
56 - The casting will be made with Matrix® NEO® which has a 2A:1B mix ratio by volume.
57 - In this case, four cups of Part A and two cups of Part B will be used.
58 - An iron oxide powder is added to the mix to provide color to the finished casting.
59 - The Matrix® NEO® Parts A
60 - The blended mix is poured into the mold around the mold plug.
61 - Fill the mold and reaffirm that the mold structure is level.
62 - When the Matrix® NEO® mix has cured begin the demold process.
63 - Slowly remove the plug and the support shell.
64 - Gently peel back the rubber mold from the Matrix® NEO® casting.
65 - The mold and Matrix® NEO® have combined to produce a perfectly detailed reproduction.