Molding is the operation that produce the mold cavity thank to a pattern.
Today, I’m asked for a spare part for a John Deere 4650. The front axle pivot was damaged by friction :
The aim is to obtain a part which will fit in the void and prevent further damage of the pin or the bore.
In order to built the mold we need a pattern. This is a part that look like the wanted one but is not made of the same material. In this case I use a two compounds silicone past that I force into the void. After it hardens, I’v got my pattern:
For economical purpose, the today’s part will share its mold with an other one. It is a replica of a casted part.Today’s patterns
A classical material for metal casting mold is silica sand (SiO2). Indeed it has many advantages, the first one : it’s refractory. But sand alone is not consistent. That’s why it is added with bounding agent. In this case it is oligo-clay and oil. This mixture becomes very constant when rammed in a flask, and can be recycled at 90%.
The two patterns are set into a flask and powdered with talc.
They are covered with sieved sand.
Ramming begins with fingers.
After the first contact layer, I add sand (no need to be sieved) and use the rammer.
This operation has to be performed in a spiral way, from sides to center. Otherwise the sand is not well rammed.
The operation is repeated until rammed sand level is higher than the flask. Then I use a flat rammer. A mallet will also do a good work.
I finish with leveling the sand with a corner iron.
The flask is turned over.
I carve the sand in order to found the parting line.
The sand is powdered with talc and the “drop” or upper flask is set up and filled with sand exactly the same way than the drag.
The the mold is opened in order to remove pattern and carve sprue, runners, gates, risers, vent holes…
The mod is blowed to remove sand grain, and closed.
After shake out, here are the parts, with ingots of exeding metal and patterns:
And finally, our John Deere is repaired.