Casting for Construction of Parts.

Metals and Metal Working / Machineing

Oogoo for flexible part casting or mold making.

BTY: You can use hot melt glue guns to make plastic injection molded parts.

Plastic casting

Hot plastic is injected into a mold. It must cool in the mold, then be ejected. Plastic shrinks a bit as it cools, which helps it release.

Rules:

• Smooth transitions
• No sharp corners inside 0.12" minimum radius
• "Draft" minimum of 0.5 degree. The deepest parts must be smaller than the parts closer to the outside of the mold.
• Parting line should not curve
• Uniform wall thicknesses, from 0.03 to 0.30". Thinner walls break, thicker walls will not cool.
• When a rib or wall meets a wall in a "T" shape, the rib must be thinner. About 60% as thick. Otherwise, the surface of the outside wall (top of the "T") will sink in as the plastic inside the rib cools and shrinks. You can also reduce this by avoiding 90' angle joins, or by "pinching" the walls at the point where they meet.

See also:

• https://youtu.be/iOuLRMgDzVs Avoiding bubbles in resin casting, without a pressure pot:
  • Cut fine paths in the mold away from the corners for air to escape through. These can easily be cut off from the finished part.
  • Pre-coat the inside of the mold with a thin layer of talcum powder or corn starch to lower the surface tension and allow air bubble to escape.
  • Pour from high up with a very thin stream.
  • Pour through a screen, which will break up any bubbles in the stream.
  • Inject the resin through a small opening at the end of a syringe.
  • Rub the material into the mold if you can reach into it. This works best with open face molds.
  • Tip the mold to the side while pouring to avoid any overhangs. Also, place the object in the mold when you make it so that there aren't any overhangs. If possible.
  • Bump and shake the mold after pouring. Avoid filling all the way at first, then finish off after.
• https://www.thenerdtool.com A sample part which demonstrates all the common issues with injection molding parts.
• https://www.rotometals.com/zamak-2-ingot-aluminum-4-copper-3-zinc-93-kirksite/ Kirksite is an Aluminum / Zinc alloy with a melting point of only 725'F.
• http://www.instructables.com/id/Machinable-Wax/?ALLSTEPS Making machinable wax yourself. Apparently he is just melting Plastic Shopping Bags (HDPE or LDPE, Recycle symbol #2 or #4) into paraffin wax (4:1 wax to plastic) at 300'F. Paraffin ignites at 400'F so proper temperature control is critical. Melted wax is nasty at those temperatures if it gets on your skin, so gloves and goggles are strongly recommended. Remaining lumps removed manually or by straining and then the result poured into molds. New batches can incorporate 50 to 75 percent re-melted chips from prior batches. Eventually, the plastic starts to break down from re-heating, so continual reuse is not actually possible.
• http://www.alumilite.com/ Alumilite is a nice light plastic that has excellent strength to weight characteristics.
• http://www.mcmanis.com/chuck/robotics/projects/casting/index.html Some examples of alumilite use.
• http://www.buildyouridea.com/foundry.html This guy has really nailed the casting of parts from CNC cut foam blanks