Heating PLA Patterns to Facilitate Removal from Sodium Silicate Molds

Do any of you sand casting people tap the sides of the pattern to create some clearance in the sand?. It's a common thing to do with resin sand to ensure an easier release of the pattern by giving the pattern few gentle taps with something like a small block of wood until a small gap opens up between the sand and pattern. I'm using patterns mounted in a ply box so gently tapping the box sides and the inertia of the sand opens up a gap.

 

I do rap on patterns on the sides and on top to loosen them up just a little in the sand before trying to draw them out. Seems to be a pretty standard practice with sand molds. But I'm not really trying to create a visible gap, I don't want to do much more than just break any bond the sand might want to think it's made with the surface of the pattern.

Although I have heard of people rapping a little harder than that, trying to enlarge the mold cavity ever so slightly to compensate for shrinkage, when molding small parts using an existing casting for a pattern.

Jeff

 

I did cast this mold yesterday and posted results here:

http://forums.thehomefoundry.org/in...mill-gear-in-grey-iron.533/page-20#post-18380

Overall the results were very encouraging and I got casting quality using this method that I could not equal on this pattern with green sand.



Denis

 

That looks great!, excellent fine detail. I'd been wondering if it would be possible to machine the hub portion and set two of them up with adjustable meshing and lap the teeth with silicon carbide grit?.

 

Beats me, but I can see that using 3 gears could avoid developing convexity of one one gear and corresponding concavity on the other that would almost certainly occur with two gears. 3 gears would be successively lapped a:b, b:c, c:a in a way similar to the method using 3 plates to establish reference flats.

http://ericweinhoffer.com/blog/2017/7/30/the-whitworth-three-plates-method

Denis

 

I never thought about the concavity/convex issues, just emulating something like a gear honing machine. For a second there I though you meant having all three gears mesh at once in a triangle (Then I read your post carefully ). If they just all meshed at once in a line of three gears and took turns as the middle gear that would work.