Casting resin with overhang narrow shapes?

[Fredrik Astlid]

Any idea how one can cast such details like this exhaust where the exhaust is like a girth that narrows around the inside mold part when removing the resin from the silicone mold?

 

Is the solution to devide cut the inside part slightly offset so one side will alide out first before the other side?

 

Also how to make molds where the small details like oxygen hoses hanging free can be pulled out from the mold when casting?

 

I have seen such made i 1/72 and is beyond me how the cast is pulled from mold without breaking.

 

 

Good example is thes Draken exhaust where inside is wider than the ejector cone.

 

 

Or this pilot oxygen mask hose.

 

 

I guess how the mold is planned and whre the seam is put is critical.

 

Any expertise here who know how to plan such shapes to be cast.

 

/Sincerely Fredrik.

[Rob de Bie]

4 hours ago, Fredrik Astlid said:

Any idea how one can cast such details like this exhaust where the exhaust is like a girth that narrows around the inside mold part when removing the resin from the silicone mold?

 

I have seen such made i 1/72 and is beyond me how the cast is pulled from mold without breaking.

 

 

Good example is thes Draken exhaust where inside is wider than the ejector cone.

 

Any expertise here who know how to plan such shapes to be cast.

 

/Sincerely Fredrik.

 

I'm pretty sure all of these molds are one-piece, without a seam, otherwise vacuum casting does not work. So it's not a question of a smart seam line. I would guess they use some sort of filler pin, that is pulled out before demolding. This creates a smaller cross section of the mold part inside the exhaust so it can be pulled through the nozzle opening. The pin itself needs a hole, otherwise any entrapped air will eject the pin during the vacuum casting.

 

Rob

[TheRealMrEd]

Also, some silicone products are far more flexible and forgiving, allowing stretching or deformation before they tear, allowing removal of some complex molds. Type of silicone is chosen for the product, with the more flexible stuff being softer and wearing out sooner, resulting in fewer item per mold.

 

There are also a few tricks. Say the pilot above had his left arm bent a little more, and the helmet was resting against his side. You could put a tin film of white glue across the void (something like filling in a window with a clear material, using a toothpick). The rubber mold would then be made with each half not trapping the arm, and the slight "flash" left after casting is easily shaved away. This is a trick we used to use casting wargame figures years ago.

 

Ed

 

 

[Fredrik Astlid]

Amazing answers you said the things i was wondering and having thoughts about and the plug in the mold is smart so that the silicone can be pulled out and collapse inwards when removing the cast. Also the cut and white glue in the cast form is very smart since the "flash".

 

So far i had very few successful casts but those attempts have been without vaccum for removing bubbles from resin before pouring and without pressure chamber for curing to reduce and get rid of any bubbles.

 

I will look into the pressure and vaccum chamber now.

[Rob de Bie]

14 hours ago, Fredrik Astlid said:

So far i had very few successful casts but those attempts have been without vaccum for removing bubbles from resin before pouring and without pressure chamber for curing to reduce and get rid of any bubbles.

 

I will look into the pressure and vaccum chamber now.

 

Vacuum casting equipment has become a lot more affordable, at least for a simple set. Here's what a Dutch supplier offers, for example:

 

https://www.formx.nl/tools/vacuming-and-pressure-equipment/index.php

 

I use a polycarbonate vacuum desiccator too. You can buy these at lab supply companies, mine was ~75 euros.

 

Rob