Forgot to put index features in the first half of a mold

[aulrich]

As the title says I forgot to put index features in the first half of a silicone mold what are my options to fix this.  

Or how best do I cut the index feature into the existing half.

 

 

[Travis]

What is the mold made of?

POP, Durhams, etc... you can carefully drill rounded indentions into the perimeter face.  Nothing vertical just create 1/2 circles and the mold halves will easily mate without issues.  Often a mold will register with no alignment pens depending on how you make them.

If RTV I would make the equivalent of a cork borer.  Take a piece of hollow metal tubing and place in your drill press.  Take a file and ease the edge to sharpen it.   Then you can carefully remove a circular plug from the mold and insert pins.  Can also just cut wedges from the perimeter of the finished half and then mold the second half.

[mark poulson]

1 hour ago, Travis said:

What is the mold made of?

POP, Durhams, etc... you can carefully drill rounded indentions into the perimeter face.  Nothing vertical just create 1/2 circles and the mold halves will easily mate without issues.  Often a mold will register with no alignment pens depending on how you make them.

If RTV I would make the equivalent of a cork borer.  Take a piece of hollow metal tubing and place in your drill press.  Take a file and ease the edge to sharpen it.   Then you can carefully remove a circular plug from the mold and insert pins.  Can also just cut wedges from the perimeter of the finished half and then mold the second half.

Be sure to use a really sharp knife.

[aulrich]

Yes it is RTV, I think for this one I will just take a scalpel and cut wedges out of the edges.  Its also not perfectly flat so that should help in this case.  I really need to get some Lego.

[mark poulson]

Do forms made with Legos leak?  I had that happen with a form wooden form I glued down to some plexiglass.  The UV resin I used snuck between the wooden form and the plexiglass.

Would the same thing happen if I used Legos to make the form?

[Hillbilly voodoo]

Yup I have made this mistake once or twice lol

It’s no big deal if you have only poured one half of a two piece mold. I have either cut out cones with a scalp or taken a large drill bit and drill some shallow holes. Make sure you use lots of mold release and pour the other side

Now if you poured both sides you can create locator pins with small doweling. Use your master to make sure your mold is aligned and close it up. Drill through one side and part way into the next side of the mold. Push the doweling through the one side of the mold part way into the opposite side. Cut the doweling flush to the outside of the mold 

And there you go you have wooden locator pins

I have made this mistake a few times rushing well making molds before I go to work or running out the door. The wooden dowel trick is how I fixed molds I forgot to use mold release and had to cut them in half

hope the above makes sense 

[Hillbilly voodoo]

2 hours ago, mark poulson said:

Do forms made with Legos leak?  I had that happen with a form wooden form I glued down to some plexiglass.  The UV resin I used snuck between the wooden form and the plexiglass.

Would the same thing happen if I used Legos to make the form?

I use the Lego method and the clay does a good job of preventing the RTV from leaking. I have never used glue of any kind. I have been reusing the same clay and Lego for years 

[Travis]

Never really had issues with lego method either... except for different application.

You can take an aluminum mold for soft plastic bait and build your lego wall around the mold and leave around 1/4 to 1/2 inch gap.  Heat hot plastic and pour over to make a soft plastic mold of the aluminum mold.  You can then pour plaster of paris or Durhams into the soft plastic mold and make a copy of the aluminum mold.  I have had hot plastic leak through the lego mold box twice over the years.  

[wchilton]

I have used clay to create mold box for open pour.  Have also used it to create a quick/dirty master.  Clay will leave a matte finish where the silicone contacts it.  I kind of like that look for top and sides of a mold.  Do not use clay with any sulfur content.  Have heard that it can mess with silicone curing.  I just use Plastillina #1 modeling clay from the art supply store.

[aulrich]

Yea regular kids modeling clay does interfere with curing, my resin supplier had the right stuff.

You got to love spray mold release, the mold popped apart with no stuck bits.