How to mold a pistol? Just need a quick push in direction

[decaturvader]

My kid has an old german-style bb gun I'd like to cast.
All my experience has been thus far, are making molds and producing resin heads for 8" action figures.

I envision a flat box; and place the gun on it's side. I get my stuff from Smooth-On and the silicone molds take about 8hrs to set. Could I merely pour my silicone around the gun, filling the box until I reach half of the gun's height. Allow it to set. As it is setting up, rest some keys in strategic areas.
Once the silicone has set, pour another batch of silicone on until the gun is covered.
That would give me the whole gun, but should I run a dowel rod or something off the barrel, so I can remove the gun, clamp the two silcone molds together, and pour resin down into the hole provided by the dowel rod?
I'll have some clean-up to do...but as you can tell...I have never done something like this before and feel pretty sure I can, just need a push.
I google'd for a pictorial, and searched here...but must be using the wrong terms as I come up empty.

A picture is worth a thousand words :thumbsup
Thanks in advance!
Chad

 

[Hecubus114]

I cant see why what you described wouldnt work... although I'm no silicon expert - will it bond to itself?

 

[Sigma-LS]

You fill the box halfway with sulfur free clay. Set the gun in the clay filled up to the halfway line. You can use some plastic wrap to keep the clay from sticking to it. Make your mold keys in the clay to make sure the two halves will be aligned once there is no box holding the finished mold together. After you pour the first half, flip the box over and remove the very bottom of the box. Dig out the clay that filled the bottom half. With the now half empty box filled with half a mold and your gun, zap the first half of the mold with a lot of mold release to keep the silicone from bonding to itself. Then pour the second half of the mold.

Now you are ready to cast it. Easiest way is to pour each half separate then use a little smoothcast while the plastic is still hot before demolding to bond the two halves together. This is where the properly aligned halves of the mold come into play. With a hollow cast pretty much the same thing but slush cast the halves and then slush around the small batch in the joined mold to fuse the two together.


You can also just suspend the gun freely in the box depending on the shape of it. I did that for my bison parts recently since I needed it quick and only needed a few casts out of it.

EDIT: Watch this. Basically what you need.

http://www.youtube.com/watch?v=LSH19G_6Yeo&feature=related

 

[TMFX]

you can do a few different things, to start with silicone bonds to silicone permanently, so you can release cured silicone with a super thin film of vaseline before you pour against it.

I would recommend using water based clay (or ceramic clay) to define the mould wall, as this is the professional way. Here is a great video of making a simple, two part, box style mould.

YouTube - ‪Mold Making: 2 piece mold‬‏

You can modify a lot of the processes seen there, however. You can make retaining walls from most anything, I like styrene sheets hot glued together. Anything smooth like hard board or even cardboard, silicone only bonds to silica like sand or glass or other silicone, but it will mechanically flow into fabric or tattered paper, anything with texture, and microscopically lock onto it when it sets. So anything smooth or non porous you're safe to use silicone against without release.

You also have a gun which is taller than scissors when laid flat, so instead of pressing it into the clay as seen here, I suggest cutting strips about half as tall as your gun, and building them around to create a clay dividing wall at about the halfway point. You just want to keep your dividing wall as perpendicular to the gun or whatever you're moulding, where they intersect, so that the mould halves don't have a thin fragile edge right at the parting line.

For everyone's information, Platinum based silicone will become inhibited if it contacts any sulfur, latex is a no-no, no latex gloves, no tools or articles that have ever touched latex, and no clay that is not specifically "sulfur free". Water clay is sulfur free, it's different oil based clays you have to worry about. Tin based silicone is usually cheaper, and is only used in mould making, whereas platinum is more for silicone masks and makeups as it is approved for use against skin. Platinum does make a fine mould and holds up to the heat of casting a lot better.

Mouldmaking questions? Look on YouTube!

 

[decaturvader]

ahhhh, excellent info folks.
The videos all made it click.
Gotta get some clay and git er done.
Thanks!

 

[Sigma-LS]

Oh and to give you some glove options since the ever-important No Latex was mentioned. Vinyl or Nitrile work fine. You'll be picking bits of silicone out of your arm hair for 2 days afterward if you don't use gloves. Not from the pouring or measuring but the mixing. It's always the mixing that gets ya.:lol

I prefer Vinyl myself since I can pick up a box even at Walmart over at the pharmacy section.

 

[exoray]

Could I merely pour my silicone around the gun, filling the box until I reach half of the gun's height.

I cant see why what you described wouldnt work...

You don't want to do it as described for the simple reason that the 'top' of the silicone will be in contact with the 'bottom' of the gun and any rising air in the silicone will then rise and get caught under the gun and blemish the molds surface aka the finished surface... You want all the rising bubble to go away from the object and surface you are molding to avoid any surface blemishes... Thus the reason the object is at the bottom and silicone sits on the top in almost all cases when you pour, and the bubble rise to the surface away from the part being molded even if they don't make it all the way to the surface they almost always are away from the molded part...

 

[exoray]

I also want to point you in the direction of my little secret to getting a super clean parting line...

http://www.therpf.com/f9/getting-even-seams-two-part-mold-98557/

Note, it's like the no-no way of casting I just talked about above, but it's only a temp pour of silicone, it's disposed of and proper pours are made that pull the bubbles away form the part... It's also slightly wasteful but IMO the $ lose of silicone for a super clean part line is worth the cost...

 

[tictoc]

I also want to point you in the direction of my little secret to getting a super clean parting line...

http://www.therpf.com/f9/getting-ev...the results of your BB gun pour when you can!

 

[exoray]

But, it was hard to pour. It was cool in the garage. Would a warmer day make it easier to pour I wonder?

I prefer cooler days, as it give the silicone more time to degas...

May I suggest you pick up some silicone thinner (aka silicone oil) and cut your silicone just a bit... You will need to experiment with your choice of products... Too much will weaken the silicone and make it soft, but a little bit in my experience helps degassing and pouring and has nil effect overall on the cured silicone... I usually spike between 5-10% silicone oil when I mix, but that amount varies with the silicone I'm using as they are not all created equal... If I'm already using a soft silicone say a shore 20 I spike it with substantially less as I don't want it to turn into rubber worm consistency... But with a firm silicone like say a shore 40 you can hit it with about 10% thinner and bring it down to say a shore 30-35 level that pours and degasses better...

I personally degas it all anyway under vacuum as it simply makes for better molds in the end, especially since I mix it with a paint mixer on a drill that fluffs a lot of air into it...

 

[TMFX]

This is all very good info regarding thinning silicone, you can also thicken it chemically, and speed up the cure chemically, but that may be a bit too much info for a beginner. For any keys or registration, use a loop tool to cut channels into the clay, or push in shapes like an end of a sharpie seen in the BITY video I linked. I've also used big steel balls, like 1/2" in diameter, push them 1/3 of the way into the clay, pour the first silicone half around them, then flip over and remove the clay and steel balls. Release the silicone and pour up the second half, this recreates the steel balls on this part of the mould, and since they were 2/3 exposed, they act as snaps to hold the two pieces of the mould together. Don't use marbles as they are glass and will bond permanently to the silicone.

You can see my mould and use of steel balls as key place holders here:
http://www.techmagicdesign.com/blasterPistol.html

You can just use Mold Max 30 from SmoothOn, it's a great tin cure silicone, no worries about sulfur, and it's ready for any pouring situation without additives. I'd get a couple trial sizes for ~$30 each and then all you need is a drill gun and mixing bit, some cups, and a kitchen scale.

And the trick to pouring up silicone without degassing is to stand on a stool or counter with the mould on the floor. You pour slowly so the stream is thin, and the air bubbles tear out of the stream as it cannot keep surface tension under stretching like that. Pour onto the dividing wall and allow the level to rise up and over the form, this will naturally evacuate air instead of pouring onto the item and trapping air under the initial blop of silicone.

When you're casting resin, brush a film of talc or baby powder into the silicone mould, and blow out the access. This leaves a film of microscopically rough texture which will wick resin into all the tiny details by breaking the surface tension of the resin and capillary action. This is a great trick to be sure not to trap bubbles in details.

 

[exoray]

and speed up the cure chemically, but that may be a bit too much info for a beginner.

From my experience accelerators are even a bit much for the experienced sometimes... On a small mold an extra drop or two of cure accelerator can be disastrous, cutting the kick time from hours to a few minutes... You have to really watch yourself with the accelerators... Also if you don't mix the accelerators 110% your mold will not cure evenly, and you could pull it off too soon and find gooey parts that have not cured yet because they didn't get their even dose of accelerator... Also too much accelerator and the silicone loses it's rubberiness and will rip and crumble and chunk off a lot easier...

The one benefit I have found to the accelerator is that it improves adhesion to other silicones, especially helpful when doing repairs... The silicone I primarily use will not bind to itself like many other silicones do, but if I add a few drops of accelerator it sticks like glue to itself...

 

[Sigma-LS]

Accelerator is a godsend for brush on molds. Once you get the main detail coat on without accelerator and a little backup, you can slap on the thickness without dealing with tons more layers and wasted dripping silicone. But yeah, tiny amounts. I have an eyedropper of Thivex (sp.?) 2 and it's still almost full after 2 years and being used half a dozen times.

 

[TMFX]

yea thivex is the way to go for brush up moulds, I usually don't need to accelerate a cure, just change the viscosity. I can usually wait 24 hours which is about the longest curing silicone time I know of.

 

[tictoc]

Exoray -
Now that I have experimented with the silicone, the thinner would have made things a ton easier. Even just a little thinner would have made it easier to pour and kept me from getting spiderwebs of silicone on the garage floor.

Tech-
what is the wormlike channel around the pistol in your mold? Is it another "key" to help the mold fit together snug and minimize flashing?

...sorry to hijack your thread decaturvader....I'm done

 

[Jedifather]

What can be done if the silcone oozes into the inner workings of the gun and you can't get it apart(please don't ask why:confused ). Someone had suggested Acetone, but wasn't sure if it would harm the finish