Casting Fail – sorta kinda

Well … if I were a ‘Glass Half-Empty’ type I would call my first attempt at making a Two-Part RTV mold and then casting something in resin .. a FAIL! Since I’m basically a ‘Glass Half-Full’ type I will say .. *ahem* .. “It was a learning experience” …

Lessons learned –

  • *Pressure Chamber* – and say that again .. Press-urrre Chammm-berrr. It is obvious that I need this for both making the RTV mold and casting the parts. The RTV failed to fill the voids in the parts. The reason in part at least is the viscosity of the RTV I am using. This OOMOO 25 is rather thick stuff. I have been using it since it is for use WITHOUT a vacuum chamber etc.
  • Brush the RTV first: Ok. One thing I COULD have done is use a disposable brush and ‘paint’ the RTV into the recesses. This would force out the air that gets trapped simply from pouring RTV (and without using a PRESSURE CHAMBER)
  • The mold release I used wasn’t the best – which I found when separating the RTV halves. GET NEW MOLD RELEASE!
  • I printed the parts (Bridge Shoes) in WSF. MAKE THE SPRUES/RUNNERS LARGER!! reduce the size where they join to the part but make those suckers larger .. at least 2mm dia
  • Flat/Open Face molds: Try doing that using the piece of plexiglass to flatten the open side

Thee may be .. heck .. will be more items but that’s all I can come up with at this minute

Brick – FAIL!!

I wanted to see if I could make a mold so I could cast bricks in 1:48. First thing I model a standard brick in Sketchup including a 3/8″ deep frog.

That was easy enough. There are all kinds of different sizes of bricks when you start looking but the 4x8x2-1/2″ shown here is near enough a ‘standard’ or ‘common’ brick.

I arranged 11 bricks by 10 bricks in a grid and then surrounded them with a wall and a base. The idea was to create a master that I could pour RTV into and create a nice mold.

I wanted to test out WSF as a material – I haven’t done so in the past as I’ve kept myself to using FUD.

WSF – or – White Strong and Flexible is a laser sintered nylon plastic. It’s high strength is offset by it’s medium smoothness and less detail then FUD.

So. Here’s what I got. Hmmmmm. The minimum embossed/engraved detail that WSF can be printed at is 0.2mm .. about 0.008″. If you look back at the first illustration with dimensions that 4″ full-size brick width is 0.075″ in 1:48. That 0.008″ minimum detail is a full 1/10 that width. That means I got bricks with rounded edges.
A close-up of the RTV mold and a couple tries at casting bricks shows the results.

There’s a couple of things that can bee seen in this photo:

  • Pin-Holes in the RTV. That’s one of the things that I will fix at some point. The RTV needs to be de-gassed in a Vacuum chamber. I have the vacuum pump and need to purchase a couple more items to build my own vacuum chamber.
  • The bricks show well the rounded edges. Those edges would be much less if the master was printed in FUD – but that would be quite a bit more cost then the WSF print. I wanted to see how well the WSF printed and am confident that it would probably be fine to represent something like a truck sideframe.
  • The yellowish colored bricks were cast in Durham’s Water Putty and the reddish colored bricks is a plaster’ish floor leveling compound colored with Terra Cotta cement color. Neither of these made good bricks.

The bricks were a ‘FAIL’ IMO .. pretty much. If they would cast better I might could use them as old bricks. I will still use the mold to experiment with other materials to see if I can some nicely cast bricks from it. One problem is if you look close .. the backs of the bricks .. the side opposite the frog is slightly dished on many of them. That’s simply that the plaster didn’t completely fill the brick mold and surface tension pulled the edges up. I need to play around with it but I think that the answer is that I need to overfill the mold and at some point I need to scrape across the surface.

Pressure Pot and Casting

Casting Resin With a Pressure Pot
Some good basic info here. He shows the HF paint pot – shows where he runs a diverter to a second pot via a male quick-release.

Note: He says you should “never go past 80 psi. The manual states a Maximum Pressure of 80 PSI – Paint Pressure Pot. Some other applicable info there: Air consumption is 4.0 CFM @ 40 PSI – need that to select a compressor. Required Pressure: 30-60 PSI (that’s for a paint gun).

I think that 50 psi maximum pressure would be an excellent idea.

He also covers some other things such as venting the mold. Good info.

Pressure Casting Tutorial – Part 1
EXCELLENT video. He covers a little bit of everything. He makes a point in using a bolt on the inside of the outlet (no second paint pot). He also covers molds including venting.
Home Made Plastic
Ran up on this video totally by accident.I may try it just for fun .. I mean .. the cost is nothing. A Bioplastic is a form of plastic derived from biomass sources – such as the corn starch in this video (called thermo-plastic starch). I was thinking .. it might be useful for scenic items such as tires. Just a .. hmmmm .. moment.
Pressure casting resin with Cindy at
Cidy covers vacuum chamber to de-gas the resin, pressure chamber. Excellent way she sets the chamber up with shelves.