Tired of sales people knocking on your door? Hang this on your door and hope that solicitors in your neighborhood can read (most, but apparently not all, can read).
I needed a "No Soliciting" sign for our front door and I was not pleased with the stickers I could get at the local hardware store....they are functional, but not so great looking. So I turned to my Makerbot to produce exactly what I wanted. I thought plastic would not be too durable in the sunlight, even if painted and I wanted to try my hand at metal casting since I had never done that before. I designed the sign to be about 8" long so I split it into two pieces to fit on my TOM. I added some sprue/vent structures into the design to allow casting in pewter. I experimented with several different casting mold and sprue structures and found that this seemed to work best. Since this is a one-sided piece (back side is flat), I used a piece of porcelain tile as the back half of the mold, and used plaster as the front half of the mold. I glued the plastic master to the tile with a little acetone, built up a box around it, poured in plaster, baked the plaster, removed the plastic master, then poured the molten metal. That is the basic process. To finish it, I sanded/polished the metal then painted the background red.
I think it came out really nice and expands the capabiltiy of the Makerbot beyond just plastic. Next time I want to try some "lost-ABS" or "lost-PLA" type casting so I can create more complex geometries.
This was my first experiments with molding metal, so I made lots of mistakes and there are probably better ways to do things. Please be careful with molten metal if you try this yourself....it is hot and dangerous.
It is fun learning a new skill.
- Print the sign in two pieces
- Sand the junction between the two pieces so they are perfectly flat and mate together (the bottom few layers of plastic tend to stick out a little)
- Use a little acetone to bond the pieces together and bond them to a flat porcelain tile....just a little acetone.
- Flood the whole surface with acetone and quickly dump it out. Then let it air dry a few minutes. This will smooth out the surface a little and make it easier to release from the plaster mold.
- Build up a box around the plastic (I used wood and hot melt glue to stick it to the tile.
- Mix up some thin plaster of Paris (about 1:1 ratio of powder to water) to fill the bottom of the mold. Slosh it around good to make sure it fills all the details and the bubbles come to the surface.
- Mix up some thicker plaster (about 2:1 ratio of powder to water) to fill the rest of the mold.
- Let the plaster harden for about 30 minutes then remove the wood mold and remove from the tile.
- I found it best to leave the plastic master in the plaster at this time. If I try to remove it, it tends to damage the plaster. Instead....
- Heat the plaster in an oven at about 200F for about 1 hour. The plastic will soften and bubble a little and be very easy to remove from the mold. It destroys the plastic, but the mold will be perfect. Clean up any stay plaster. Note: I tried various release agents on the plastic to get it to release from the plaster (WD-40, petroleum jelly) but none seemed to work as well as the method above.
- Continue to heat the plaster at about 450 for a couple hours to drive off ALL the moisture and pre-heat the mold.
- Pre-heat the porcelain tile as well in the oven.
- As the mold/tile are heating, melt your pewter. I used some lead free pewter from http://www.metalshipper.com/pewter/pewter-bars.html. It came in small bars that were easy to deal with and the price seamed reasonable. I metaled the metal on a little camp stove in a cheap stainless steel measuring cup.
- Remove the mold/tile from the oven, clamp them together and hold them vertically. Pour in the metal into the main funnel until it starts to emerge from the 4 vents.
- The metal will cool in about 10 minutes then extract the metal piece. It seems to damage the plaster mold so I could only get one piece per mold.
- Clean off any extra plaster and pewter flashing. Sand the surfaces with progressively finer grits til it is polished. I used 120, 180, 300, 400, 600, 1000, and 1500, grit paper by hand. It only took about 15 minutes to polish it nicely.
- I spray painted the inside with 4 coats of red paint. Then polished again with 1000 and 1500 to remove the paint on the top surfaces only.
- Hang on your door and hope that solicitors in your neighborhood can read (most but not all can read apparently).