Resin casting

[Hobbes]

I'm going to buy some rubber & resin today, to experiment with casting. I've never done this before, so I'd appreciate any tips you lot can give me.

I've got two parts I definitely want to cast:

The missile is an ALCM I scratchbuilt for use with a Tornado. The disk is a rotodome for my BAe-146 AEW, also scratchbuilt.
The 1:72 Gripen is to give a sense of scale.

Other candidates for casting include the Storm Shadows from the Airfix Tornado GR4 kit, also 1:72.

- Should I cast these items as one solid part, or is it better to make several pieces?
- If I enclose the part entirely in rubber, how can I make sure the part stays suspended in the rubber, and not float to the bottom?
- how to cast the disk? Should I set it on edge, to minimize bubbles?
- How do I remove the rubber from the part without damaging either?  

[cthulhu77]

Let me know how it goes...I would like to try my hand at that too.
                  greg

[Jeffry Fontaine]

The resident resin subject matter experts here are Wooksta, John Howling Mouse, and I believe TSRJoe.  PM them for assistance, they should be able to guide you down that awkward path of replication.  

[John Howling Mouse]

Here's the text of a msg I had e-mailed Jeffry's friend a couple of weeks ago to answer some questions he had about casting WWII bombs.
Some pointers is here (I hope).

I'll assume you're talking about 1:48 scale and that you have no molding/casting experience (this is going to suck if you already know most of the following).

Honestly, bomb-fins themselves are thin enough that they are best cast only under pressure or using centrifugal "spin" casting methods.
The thinner the subject, the less volume of resin exists to create its own catalyst effect.
While I have successfully molded and cast the halves of some 1:48 droptanks, including their fins, the results sometimes vary.

With bomb fins, it is the fact that the entire aft end of the bomb is hollow, no less.  Some companies go to the trouble of making a two-part mold to handle these.  There is a Canadian supplier who makes resin WWII bombs...but it's a pretty complicated piece of work for the end result and kind of hard to describe in an e-mail.

My 25-cent solution?  If necessary (depending on the origin part's configuration), get two of your subject bombs from whatever source kit.  Build one of your subject bomb, remove the entire aft fin section, turn it as if it was a bomb on its base pointing straight upwards and mold the boxed "X" of the fins section vertically (as a separate part that will later by CA-glued to the resin "body" of the bomb).  Use your second example of the subject to mold and cast the body of the bomb using the following technique:

For simple (solid) subjects, I have attached a pic of some halved model parts I've molded and cast in the past.  If you can keep it simple, just attach the halves to a sheet of plastic (like .030" styrene sheet you can get from Plastruct).  Some people build up a mold wall out of Lego blocks taped to the perimeter of the plastic sheet.  You can see in my photo that I've simply glued and/or used Scotch double-sided tape to hold the halved objects down to plastic sheets dropped into common desk drawer organizers you can get from Wal-Mart for a couple of bucks.

Just ensure there are no undercuts where the mold rubber can creep under anything, trapping the very object(s) you intend to mold and cast.  This usually includes removal of the locator pins on the "male" halves of the plastic kit parts (or you can drill small holes in the base plastic sheet which correspond to the locator pins).  The parts must seal effectively to the base sheet of plastic in the tray.  I use electrical tape to seal any gap b/w the outside edge of the base plastic sheet and the walls of the desk drawer organizer trays so the mold rubber doesn't slip underneath.

For molding rubber and the casting resin, you'll want to stick with something simple for a start.  Any mold product that cures at room temperature (RTV: Room Temp. Vulcanizing) and has a 1:1 ratio by volume so no fancy scales are required.  I've used all sorts of brands and types with mixed results.  You can start with fairly small kits of "Smooth-On" products.  Try "Ooo-Moo" for the mold rubber and any of Smooth-On's 1:1 resins for casting.  Make sure you buy a mold release agent that is compatible with both the mold rubber and the final resin.  I use Syn-Air's aerosol mold release agent most of the time but Smooth-On makes their own now, too.
Use Vaseline on the walls of you mold chamber/tray but not on the subject itself (better quality mold rubbers out there will pick up the texture of even a single human hair on your subject, so you can imagine it would show the smears of Vaseline, too!).

From there, it's pretty much a matter of following the instructions and a little trial and error (do NOT start with something that is precious to you).  Fueltank halves might be ideal to start with.  A lot of the success of both the molding and later casting processes is due to very simple things such as thorough preparation for the task: having all the essential mixing tools & containers, latex gloves, lots of paper towels at the ready, etc. etc.

Check out this website for the best I've ever seen for learning about molding and casting:  http://castcraft.com/

Pics of the halved subjects in the mold trays can be found here on What If at:

http://www.whatifmodelers.com/forum//index...?showtopic=6743

And I recently borrowed this Ben Ridge's fundamentals how-to tape/DVD from the local library:

http://www.reproduce100s.com/get-started.html

[noxioux]

It's not that bad.  The really important thing is to make sure you mix the resins for the rubber and plastic correctly.  Incorrect mixing will lead to funny (not ha-ha) problems with your stuff.

For that one, I'd find a small box (or make one) that's just a bit bigger than the part (1/2" margin), and pack about 1/4" of modeling clay into the bottom.  Then push the part into the clay, tucking in the sides with some kind of stylus (or fingers).  Poke two small holes in the clay (for registration), and put a small piece of sprue to create a pour channel.  You might want to try to make two channels (on the same side), so the mold has a place for air to escape to.  Coat the part and clay generously in release agent, and pour your first bit of rubber.

Once that's cured, flip it over and dig out the clay, clean all the residue off everything very thoroughly, coat it all in release agent, and pour your second batch of rubber.  The voila, you've got a nice reusable two-part mold for that part.

[Hobbes]

Thanks for the tips! I managed to buy the resin yesterday, but the LHS didn't have moulding rubber in stock. He's ordered it for me (Blimey, that stuff's expensive!), should be here in a week. The resin they have is polyester, to be mixed with MEK. It's supposed to be transparent. I suspect you guys use something different (my Anigrand kits certainly aren't transparent) ?

[John Howling Mouse]

Be WERRRRRRRRRRRRRRRRRY carefuuuuuuuul with MEK, my friend.  Extremely toxic.

Does it require a 10:1 ratio or even greater delta?

The 1:1 stuff is much easier to mix, of course.

[John Howling Mouse]

Oh hell, man; just noticed that's a freakin' fantastic GRIPEN you got going on there!!!   Sorry for not noticing before: got too wrapped up in the molding/casting idea!      

Wooksta's right about the vertical pour mold being the right way to go for the cruise missile.  Don't forgot to fill any voids (i.e. that intake) with non-sulphur-containing plasticene or your mold rubber will seep into any holes and your subject will be irretrievable from the rubber once cured.

[Hobbes]

The MEK has a 1:100 mixing ratio. It's supplied in a bottle with an eyedropper nozzle, the instructions say 'use 35 drops per 100 ml' so that shouldn't be too hard.
I've got a good respirator, I'll use that when working with the stuff.

The Gripen is an Italeri 1:72 kit, done OOB. Almost finished (I just dipped the canopy in Future, so that goes on tomorrow, and then I've only got to do the tailpipe).

[Jeffry Fontaine]

I would like to know more about the cruise missile shape that you have created.  Is it supposed to represent a 1/72nd scale ALCM or something similar?  Or is it a generic cruise missile shape for 1/72nd and 1/48th scale?  Inquiring minds wish to know...

[Hobbes]

It's supposed to be a 1:72 ALCM. It's not finished yet; in fact, I just removed the intake again, and I'm sanding the nose down one more time because the thing wasn't symmetrical. It still needs fins, too.