Aluminium Casting Transmission Repair

My employer asked if I was able to assist him in fixing a transmission housing. A small piece had broken off where a bolt fastens the transmission housing to the engine. The broken piece was long gone, so we were left with no other choice but to fabricate a new one.

Step 1 : Sculpt Broken Part using Modeling Clay

To begin this repair, the original profile of the broken part had to be reconstructed. This was achieved by shaping modeling clay with an X-ACTO knife and clay tools.

Step 2 : Create A Silicone Mold

Mold Making Plan

My plan was to create a silicone mold that captured both the profile of the newly sculpted part, and the original broken part. In order to do this, I would first create a mold of the newly sculpted part. Then I would remove the clay to expose the original part so I could create a mold of the damaged features. However this mold would be a negative mold, and I required a positive mold. To create a positive mold I basically had to create a mold from a mold. This process is illustrated in the following diagram.


To create a silicone mold I used a product called MoldStar 19T. This silicone is an easy to use translucent platinum silicone that is mixed in a one to one ratio by volume. It features a relatively low viscosity and vacuum degassing is not required for most applications. Pot life is 3 minutes and cure time is 12 minutes at room temperature. For more information about this product check out the following link.


MoldStar 19T Silicone

Making the First Half of the Silicone Mold

The first step in creating the silicone mold was to construct a clay border around the sculpted clay part. This border retains the silicone allowing it to cure around the part.

Clay border constructed around part.

Silicone was prepared and poured over the part. Notice that I’m pouring from a higher elevation. Doing this reduces the amount of air bubbles that are trapped within the silicone during mixing.

Pouring silicone over prepared part

I let the silicone cure for about 15 to 20 minutes before attempting to remove it from the surrounding clay.

Silicone mold curing for about 15 minutes.

You may notice some white and gray lumps in the silicone. These lumps are pieces of clay that were placed within the silicone to take up extra space. Doing this reduces the amount of silicone needed. However, I had to be careful not to place the pieces of clay too close to the sculpted part.

Cured first half of the silicone mold

Creating the Negative Mold

I removed the modeling clay from the broken part and repaired the clay border where it was damaged during removal.


Modeling clay removed
Similar to the first mold I mixed and poured silicone allowing it to cure for about 15 minutes.


Pouring silicone to create a negative mold

Creating the Positive Mold

Once the negative mold was fully cured, it was removed from the clay border. The surface of the negative mold was prepared by applying a mold release agent and talcum powder. The talcum powder was brushed gently into all crevices. When pouring silicone over silicone, its important to have a thin coating of the talcum powder to allow mold separation. Excess talcum powder was brushed off and the negative mold was placed on the table. Another clay border was constructed around the negative mold and filled with silicone.

Positive mold silicone curing on table
Once the silicone was cured I gently worked my way around the edges, pulling them apart with my finger tips. The negative mold was no longer needed after it was separated from the positive mold.
Finished molds. (Right : Positive Mold, Middle : New Part Mold, Left : Negative Mold)

Step 3 : Lost Wax Casting

Wax placed within steel casings ready for plaster

Two wax copies of the sculpted part where cast using the previously created silicone mold. These wax copies were adhered to a piece of cardboard with melted wax. A wax sprue and riser were attached to guide molten aluminium into the part. The cardboard was hot glued to a square steel casing. Plaster of Paris was prepared and poured into these casing to act as an investment material. The casings and their contents were placed on a shelf for a few days to allow the plaster to cure. I placed one of the casing into my forge and adjusted the temperature to the lowest setting.

During the first hour, wax melted out of the plaster creating a negative cavity. Once the wax burned off, I increased the heat gradually over the next few hours to ensure the plaster would not fracture. During the last hour of heating, I placed a crucible filled with aluminum scraps into the forge. Within about 10 to 15 minutes the aluminium was melted and the plaster was up to temperature.

Heating aluminium and preparing for pour
I removed the plaster casing from the forge, placed it on a metal tray, and poured the aluminium into the sprue
Pouring molten aluminium into plaster mold
Excess aluminium was poured into a used metal muffin tray.


I removed the aluminium casting from the plaster when it was cool enough to touch. Below are a few images of the cast aluminium part. In these images you can see the sprue and the riser protruding from the part.

I gave the cast part to my employer and he was able to weld it to the transmission housing. Below are a few pictures of the final result.