A one piece cut block mold is a popular mold making method that has some definite advantages. This time saving technique yields an accurate production mold which also minimizes seam lines. Reduction of seam lines cuts down post finishing of castings and has less impact on detail. Smooth-On silicone rubber compounds cut very evenly and smoothly and are thereby an excellent choice for making a mold of this type. The following shows how to make a one piece cut block mold using silicone rubber and demonstrates how it can be used for casting plastic or a variety of other materials, and how this same mold can be used for a rotocasting application.
Original model of 'David' to be molded.
Back view - the undercut areas below the shoulders requires a mold rubber that is flexible and strong
Secure model to platform so that It does not float in liquid rubber. Use glue gun, modeling clay, caulk or screw gun.
Build containment field around model (size / type depends on model) Measurements are taken to build a wooden box.
The wooden box is positioned over the model .
Top view- the model is centered inside the walls of the box.
SuperSeal® is brushed on the surface of the porous plaster model to seal the surface. This prevents the silicone mold rubber from locking into the fine pores.
Ease Release® 200 is sprayed on the inside surface of the box walls.
Seams are sealed with a hot glue gun.
Part A of the silicone rubber is measured out by weight using an accurate gram scale.
The proper amount of Part B is also added.
Parts A and B are mixed thoroughly.
For best results, the liquid rubber is vacuum degassed in a vacuum chamber. Allow For volume expansion in mixing container (5X or more).
Degassing removes air introduced into the rubber during the mixing process.
A reading of 29 inches of mercury is necessary to achieve an air-free mixture.
The mold rubber is poured in a single spot at the lowest point of mold.
The rubber seeks its level up and over model, this displaces air and helps minimize bubbles.
After material cures, the front of the mold is marked to avoid cutting on the wrong side.
Containment walls are removed.
Demolding Model - Use a razor knife to cut seam. Start at top of head and work down.
An extra set of hands helps pull the rubber slowly away from the model as it is cut.
A zig zag pattern is cut to aid in registering the seam back together.
The model is extracted from the mold.
The mold exhibits perfect detail captured from the original model.
The mold is carefully put back together, making sure that the seam is aligned correctly.
Rubber bands are used to secure the seam.
Smooth Cast® 300 resin is mixed and poured into the mold and allowed to cure.
Plastic casting is demolded - A perfect replica of the original!
Crystal Clear® resin is cast into the the mold and cured in a pressure vessel at 60 psi.
This versatile mold is also used for roto casting. The seam is once again carefully registered back together.
This time, strapping tape is used to secure the seam.
The mold is secured to the platform of the rotocast machine using ratchet-style straps.
SmoothCast Roto® resin is poured into the mold, the hole is 'plugged' and the rotocasting begins.
The machine rotates on a horizontal and vertical plane at the same time.This evenly disperses the uncured resin over the surface of the mold.