The Silicone Molding Process

Step 1:

Degassing - Insert the silicone A component into the vacuum chamber to remove moisture at 30inHg VAC/ 760mmHg for 15 minutes. If the room is cold, an additional 5 minutes may be needed.

Step 2:

Create a frame for the silicone rubber pouring - Build a four-sided box
with inside dimensions equal to those of the pattern. Any rigid, smooth material can be used, including MDF, finished plywood or a Formica laminate.

Step 3:

Add vents and gating to the pattern -
Venting allows air to exit the tool. Without venting, air pockets will prevent a
complete filling of the tool cavity. Vents are made from 1/16 inch (1.6 mm) metal or plastic dowels. Cut the dowels long enough to extend from the pattern surface out through the top of the tool. Attach the vents with cyanoacrylate (super glue) to all high points of the pattern and any areas that are likely to trap air when casting the silicone rubber.

Attach a gate to the part using cyanoacrylate. Make the gate from a 1/4 inch
(6.4 mm) rod stock cut to a length which extends from the part surface through the top of the tool. If casting a viscous urethane, use a larger diameter rod.

After the tool has cured, remove the gate rod and attach a small paper cup over the gate channel. The cup will be the reservoir for the cast urethane.

Once assembled, coat all surfaces with mold release. Since mold release selection depends on the type of silicone rubber, refer to the manufacturer's
recommendations.

Step 4:

Use a black permanent marker to mark the parting line of the master
model. This will make it easier to see where to cut the mold after the silcone has hardened.

Step 5:

Position the master model in the frame. The master model should be in
the air to allow the silicone rubber to flow under it.

Step 6:

Mixing the silicone ALPINA Additive Silicone Rubber with the
hardener -

•  Silicone A component Köraform A42 – 90%

•  Silicone B components: Köraform A41/42 BL and A 42BW together –10%

Step 7:

Second degassing – Place the mixed silicone rubber in the vacuum
chamber until all of the bubbles created by the chemical reaction have collapsed, approximately 5-10 minutes.

Step 8:

Pour the mixed silicone rubber into the prepared frame. Please note, the
liquid silicone rubber is poured in the corner of the frame to allow a smooth flow around the master model.

Step 9:

A third degassing process is possible after the silicone rubber has been
poured into the frame. (This is not mandatory as all the bubbles created during the pouring action will float to the surface during curing.)

Step 10:

After curing the silicone rubber, remove the frame and cut the mold
using a dissector knife along the parting line. In the picture, special tweezers are being used to aid in the cutting process.

Step 11:

Separate the master pattern from the silicone mold. The mold is ready
for casting.

Choosing the Casting Material and Method

Urethanes may be gravity casted, vacuum casted, pressure casted or injected. The determining factors are: available equipment, the material's pot life and the difficulty in filling the tool.

Gravity casting is simply pouring urethane into the tool cavity without vacuum or pressure assistance. In vacuum casting, the tool is filled and an ample supply of urethane is poured into a reservoir created by the gate cup. The tool is then placed in a vacuum chamber. The evacuation of air draws the urethane into the tool. A similar process is used for pressure casting with the main difference being that the chamber is pressurized to force the urethane into the tool cavity.

•  When the cavity is difficult to fill, try injecting the urethane using a large syringe or a caulking gun.

Fast Cast Resins