made a rtv mold one piece mold of a railroad box car door.detail on one side only. the detail comes out super but the back side is not smooth and the door seems to be thicker.tried to cover it as it sets but not much better
Detailer
Thats pretty much standard from what I’ve seen. You’ll need to sand it flat. Resin dust is a major problem and you’ll need a mask as well as doing it outside if you can. I’ve not done much casting but if you don’t do a two sided mould you will always get issues.
To sand I use sanding blocks from a hardware store, ones with foam backings, they are cheap so you can use them without worrying about distroying a good file.
Phil
One piece molds open to air are not a good idea for two reasons. One is the reason the OP mentioned- it is hard to pour the mold with exactly the right amount so that neither surface tension nor capillary tension do not make a curved edge.
The other reason is that the two-part urethane resin relies on heat generated by the chemical reaction to mixing in the hardener. If part of the actual part is in contact with the air, this area does not get heated as well as those surfaces not in contact with the air, and hence will not fully harden. I overpour just slightly when I make a one-piece mold, then place a block of particle board with a smooth surface on top of mold, with a weight on it to hold it in close contact with surface of mold. The mold must have been poured with the pattern on a smooth surface to begin with, so it represents a smooth surface around the cavity. This will give you a flat and fully cured back surface.
thanks guys for the info will give it all a try
Problems with resin dust are easily avoided by wet sanding. I use my laundry room deep sink but a bowl or bucket will work just as well. For moderately rapid material removal I use 400 Grit Wet or Dry sandpaper. For a true flat surface, I tape my sandpaper to a piece of glass from a 5" x 7" picture frame. Tempered glass would be safer but, I’m not putting much pressure on the glass and, I did tape the back and edges with duct tape…
sounds good will give it a try thanks