Dewaxing

The next step in the process is called dewaxing.  This is the lost part of the lost wax process.  We really do not loose the wax, but melt it out at around 450F and reclaim it for later use.  I run my burners reducing so the wax will not oxidize.   Careful control of temperature is important so the wax will not ignite.   Dewaxing takes about 1 hour depending on the mold load. 

Before we dewax, we remove the conduit handle by putting a heated rod into it.  It loosens up and we slide it out.  Then we melt out the cup wax so the flow of wax will be unimpeded from the vents.   Finally, stainless steel wire is wrapped around the mold for extra reinforcement.  Stainless steel "U's" are clipped onto the bottom edges, again to reinforce the mold.  Dewax is where shell cracks develop and we are going to try to stop cracks from starting.  The cracks start from the expanding wax.   

The molds are placed into a basket, cup side down and are lowered into the preheated oven using the hoist.  Inside the oven is a funnel to catch the wax as it melts out.  The wax flows to a pan beneath the oven and is reclaimed.  The trick is to get the oven walls hot so there is enough thermal radiation to melt the outside wall of wax quickly.  This will allow room for the wax to expand into the melted area without much shell cracking.  Most wax expansion occurs from between 70F and 90F, depending on the wax.  We want to move through this range to the melting temperature, around 180F. 

Here are the molds following dewax. These two molds developed  tiny cracks along one face.  We patched them with a thickened mixture of slurry and stucco.  The light colored bands are the patch.  We use a heat gun to dry them.   These cracks will show up as thin fins of metal on the casting called flashing.  Easy to take care of in chasing.

A close up shot of the shell mold.  Notice how the wax is gone from the sprue and vents.  There is a brown discoloration on the mold.  this is wax that has escaped from small cracks and been absorbed in to the shell.  The next step, burnout, will remove this wax by burning it off.

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