Mold making.

Mold making.

Postby jmass » Sun Jan 22, 2017 12:50 pm

001.JPG
Made an attempt to make a mold of a fifty foot fish belly boxcar. i cleaned all parts set them in plastic on a flat table but ended up with the mold going under the parts and encasing them. i used smooth on 121/130 followed directions on box and ended up with expensive waste. would like to try again but this stuff aint cheap. i might be able to use the bottom of the car for resin but the sides and ends are worthless. any advice?
User avatar
jmass
 
Posts: 799
Joined: Wed Jun 17, 2009 6:39 pm

Re: Mold making.

Postby areibel » Sun Jan 22, 2017 1:58 pm

It's really hard to cast a complete object like that, you have to do a two part mold. Mount the part on a base of clay (low sulphur) or paraffin and make sure it's solid so it won't "float" in the rubber. Make sure the sides are at least a half inch above the top of the part, this will be the bottom of your finished mold. Pour the rubber, let it cure completely then flip it over and remove the base material. Coat the opening well with mold release, I use a mix of mineral spirits and vaseline that is thin enough to brush on- make sure the exposed rubber on the void is well coated, but you don't want a puddle on top of the part. Pour the top plug and let it cure, you should be able to pull the plug out and remove the master. You'll probably have to either add a couple vents, either superglue a couple round toothpicks to the bottom of the master after the clay is removed or cut a small channel for air and excess resin to escape.
The ends or any other flat pieces can be a one piece mold, but you need to fasten them down to a piece of plexiglass or similar, you can use a piece of two sided tape or rubber cement. You do have to be careful with rubber cement though, some mold rubbers might react with it and not cure properly around the part.
You can check out some of the how to videos on Smooth On's website, I'm pretty sure they have some on two part molds and one piece block molds
areibel
 
Posts: 1706
Images: 7
Joined: Fri Jun 12, 2009 10:12 am
Location: Cambridge Springs PA

Re: Mold making.

Postby jmass » Sun Jan 22, 2017 2:30 pm

Thanks Al. I took the car apart and molded all parts separatly. i will try using glue to hold parts down next time.
User avatar
jmass
 
Posts: 799
Joined: Wed Jun 17, 2009 6:39 pm

Re: Mold making.

Postby ConducTTor » Sun Jan 22, 2017 11:53 pm

Only advice I can give you is, keep working at it and learning what works and what doesn't. Unfortunately there's a learning curve to everything.
What people think: "liberals/conservatives are ruining my country"
What the powerful know: divide and conquer
User avatar
ConducTTor
Site Admin
 
Posts: 8293
Images: 13
Joined: Sat May 23, 2009 4:52 pm
Location: Atlanta GA USA

Re: Mold making.

Postby railtwister » Mon Jan 23, 2017 8:53 am

I like the idea of double-stick tape to hold the part down and keep it from floating, but you should try it with a small sample first to test. If you decide to use glue, maybe white glue like Elmers would allow you to remove the parts from the base later (just soak in warm water)?

Your molds (especially the ones for the ends) look like they used a lot of excess RTR because they are so much longer than the parts you're trying to reproduce. Making the overall size of the mold just big enough to cover the part with a border that will prevent mold distortion would save $$ on the amount of silicone needed.

How necessary is the mold release agent when making the mold? A friend made resin parts using Alumilite for G scale many years ago, and he just poured the silicone directly onto the pattern. One of the properties of the RTV is that it doesn't stick to things, which is great for molds. When casting his parts, he did give the rubber mold a light coat of cheap spray paint (flat, not gloss) before pouring in the resin, which sort of acted like a release agent and also allowed him to better see any bubbles in the resin as he was pouring. It also acted like a primer when painting the part.

Keep Trying & Good Luck!
Bill in FtL
railtwister
 
Posts: 699
Images: 4
Joined: Sun Oct 04, 2009 8:39 am
Location: Oakland Park (Ft. Lauderdale), FL, USA

Re: Mold making.

Postby areibel » Mon Jan 23, 2017 11:12 am

I only use the liquid mix mold release when doing a two part mold, I've never had to use a mold release when actually casting parts. The Smooth On silicones work so well that I've never had a problem with them, I give the original a light spray before I pour the rubber (just in case) but not after that. I don't cast that much so I don't know if you might have to use it for a production type mold. I've heard that as the mold ages it loses some of it's flexibility and it may stick then, you can get some silicone oil to treat it with but I haven't gotten that far yet.

But if you're making a two part mold you need to coat the exposed edges of the rubber pretty thoroughly. If you don't the new rubber will weld to the original mold and you'll encapsulate the part and have to try to cut it out. I have a can of the spray mold release they sell, it sorta worked but you still risked ripping the mold. Someone suggested using the mineral oil and vaseline and putting it on with a small brush, that works great
areibel
 
Posts: 1706
Images: 7
Joined: Fri Jun 12, 2009 10:12 am
Location: Cambridge Springs PA

Re: Mold making.

Postby MacG » Mon Jan 23, 2017 4:38 pm

railtwister wrote:I like the idea of double-stick tape to hold the part down and keep it from floating, but you should try it with a small sample first to test. If you decide to use glue, maybe white glue like Elmers would allow you to remove the parts from the base later (just soak in warm water)?


I use a thin layer of Blu-Tack. The over stand at the sides of the master can be removed with a sharp blade.
Lok-n-Roll.de - we send worldwide :wink:
User avatar
MacG
 
Posts: 856
Images: 11
Joined: Mon Aug 15, 2011 1:19 pm
Location: Dresden, Germany

Re: Mold making.

Postby jmass » Thu Jan 26, 2017 6:28 pm

001.JPG
New molds came out much better using glue to hold down parts. i will try the resin saturday if i can find the time.
User avatar
jmass
 
Posts: 799
Joined: Wed Jun 17, 2009 6:39 pm

Re: Mold making.

Postby ctxmf74 » Thu Jan 26, 2017 7:16 pm

When I was making fiberglass sailboats we used auto wax for mold release. Rub it on to the mold then polish it with a cloth.....DaveB
ctxmf74
 
Posts: 702
Images: 32
Joined: Tue Jan 05, 2010 12:25 am

Re: Mold making.

Postby Bernd » Thu Jan 26, 2017 8:20 pm

ctxmf74 wrote:When I was making fiberglass sailboats we used auto wax for mold release. Rub it on to the mold then polish it with a cloth.....DaveB


That's tough to do on a small mold for TT scale. Been there but haven't tried it. I use a spray mold sold by Smooth On.

Bernd
Image
User avatar
Bernd
 
Posts: 854
Images: 27
Joined: Wed Sep 02, 2015 8:43 am
Location: Rochester, New York

Next

Return to General Discussion

Who is online

Users browsing this forum: No registered users and 0 guests