I am a beginner and thought I could post to help others avoid some mistakes. I’ll just pour them out at random for now. Maybe I can save someone some cash worth of casting resin?
Use baby powder in you molds, make sure you dump out the mold to prevent any extra from messing up the cast. One tiny clump, messed up my pour!
When using a color dye, don’t think because it spreads out while you’re mixing, that you can stop. It spreads out quicker than the parts A and B blend together. Just a drop goes a long way with the dye.
Give your parts a sprue that can be cut off later. This gives somewhere for the trapped air to go. I have just cast a bunch of O scale pilots and the steps are ruined by trapped air. Urrggg.
It’s nice to have a large hole to pour into. The mix goes in better and you can rock the mold a bit, and hopefully get the air out.
More to come from me, when I get it right!!
Maybe someone else can save me some more frustration.

What was I thinking? This ain’t easy! Gotta be something better to do than this???
Urrggg.
(http://www.largescalecentral.com/public/album_photo/0b/bb/01/1b851_09c6.jpg?c=04cc)

Joe. Use a commercial mold release agent, instead of the baby powder.

If you are using a colorant, Mix really well equal parts of it in both the “A” part and the “B” part before combining the two parts together.

For really fine detail parts, use the thinnest resin you can buy. Water thin.

The longer pot life ( time between mixing the two agents and the time that it starts to thicken ) the more time that you will have to “DE-AIR” your molds.

If you get air trapped in the same spot all the time, cut an extra spur to that spot, or even all the way to the outside of the mold. Start to fill, and when resin starts to leak out of the air pocket, plug it up. No more air! I have one mold with two holes drilled into trapped air spots,that I plug after starting to fill and 5 vent spurs.

WD-40, LPS, or even Pam, in a pinch works well as a mold release too. I pour the resin into one end of the mold and let it run towards the other end. That helps in getting rid of any air pockets. Since most of my casting just involves switch frogs, the molds are all open at the top. I use the CR 900 High Strength resin for these.

The mold on the left still needs to be cleaned up a bit before I use it.

Thank you!
I bought some Alumilite 80D because it said great strength and longer pot life than the one in their starter kit. I think it’s coming out brittle. It also needs about eight hours to cure instead of the 1-2 they claim. They also say use baby powder.
I’m learning I guess?

Keep learnin’ Joe. Then I dont have to experience the pitfalls after you!!

Baby powder (Unscented talc) should be a very thin coating. I used to dust wax molds for lost wax casting. Sacrifice a nylon stocking, I’ll not venture into your method of obtaining said … cut a circle, pooch the center, fill with powder, gather and tie off. It helps to have a compressed air gun handy. You just want enough powder remaining after dusting (tap with bag) and blowing to coat the surface, remove too much, lower the pressure. The powder will help pull the media into the details, too much blocks.

Joe, warm (80 - 90 degrees) your mold and cure in a warm place, that can accelerate the cure. Yours may be old on shelf when bought.

John

One trick that used when casting was to vibrate the mold to help get the air bubble out of the mold. I used one of those vibrating massagers, pressed against the board the mold was laying on. The vibrations helped bring the bubbles to the surface. But then I was also casting larger parts, in a open topped mold, so the finished part would have a flat back for mounting.

I think what makes this so tuff, is the size of the part. Steps on an O gauge engine are fairly thin and small and have places where the air gets trapped. I will drill some more holes. I like the idea of a plug to add. I did not know how to order thinner material. I saw them cast a thin part with this stuff and thought it would be best. The original regular stuff in the kit came out slightly better.
I also swapped mold material for higher strength silicone that would not tear when removing these pilots. It has drawbacks of it’s own. It does not hold the original shape as well. It’s tuff to drill as it just returns to it’s shape after the bit is removed. I probably need to recast this mold with something to create these holes better. There’s probably a median where you find the best strength vs. flex.
If I order a third time, I will try their thinner casting resin and maybe, a mold material in the middle of both properties.
I’m casting tongues for my roadrailers as suggested by Marty.

If you are using a two piece mold, you do need a vent channel to let the forced air get out. Also a shape on top to pout into. Like the sprues on a model kit. I find the talc to work fine. just bang the mold some to get all the excess out.

What exact product would you guys use to make a mold of a whole G scale diesel shell? Would you cast it in pieces? open cast the sides and top?
Just wondering if I can make copies of my SD70ACe I’m building.

I would make casts in sections. The larger the cast, the more that can go wrong. Making a one piece shell mold, and trying to make it (the shell) thin has a lot of potential problems, just waiting to happen. Even as a 4-5 part mold keeping the spaces in the right area, sagging of the mold under it’s own weight and trapped air and etc. besides using a very large amount of expensive RTV.

If I had to do it for hire…… I would build the master with casting in mind, and make each section to fit together. And then make a bunch of smaller molds containing separate sections. Some molds would be made from softer RTV to allow removal of complex parts, and others would be from rather hard compound for durability and mold stability.

Just My $.02 worth.

Thank you for the advice. I’m trying to dive in before I’ve figured out all the issues. I may have access to a vacuum chamber to try in my next session.
I wish I hadn’t assembled that engine before making casts of it. It would have been much easier.
Just a thought, would it work if the silicone mold had two casts. Inner shell for capturing the details and then pour the more rigid over that? Would they bond? I’ve got a lot to learn before the cost forces me out. It’s tough to dabble at these prices. If the engine was perfect, I’d probably seek out the local plastics company for production. Maybe down the road. They do place ads for smaller projects.
WD40 worked on a small test as a release. I’m not sure of the drawbacks yet. I should have made sure the baby powder was fully removed and taken more care. The more I try, the sloppier I get. I like to walk away and regain some patience.

I built a few 1/29th gondolas in the past using the silicone rubber molds . I built the cars using 3 silicone molds . The side , the end and the bottom were each cast seperatly . My molds are one piece (open face) molds so after I pour the resin I had to put a piece of glass that was sprayed with mold release carefully over the mold. I would tilt the glass and lay it slowly on top of the mold all the time making sure no major air bubbles were trapped under the glass. These are fairly large parts so I usually had to go back and fill in any voids with more resin after the part was cured. Then I just had to grind the part smooth. You also want a resin that is tolerant to hot sunlite . I used two different types of resin for my parts. Alumalite regular withstands the heat fine but its viscosity is somewhat thick and you only have 90 seconds to pour it . That includes mixing time . I used that mixed with their aluminum power 50%/50% . The metal added more heat diflection as well as giving the part a little extra weight. For the larger bottom piece I used Alumilite White resin mixed 50%/50% aluminum powder . The white is water thin and you have 3 minuites to mix and pour. The powder didn’t seem to change the viscosity much to me. Use a resperator if you decide to add metal powder , bad stuff. I’ll post some pic’s of the molds if you want.

Great info, thank you! I would like to see the mold pics, sure!
I like the idea of the glass. I was going to air pour the sides and then I would have that lip around the edges that would need surfacing.
I don’t understand fully, why you chose the two different resins? I thought I’d use just the Alumilite White resin for the whole thing. I didn’t know what the metal powders were for.

I wont be using the regular anymore . It’s okay for small molds cuz it has a very short pot life but it was a real pain to mix it and then try to carefully pour it into the mold before it suddenly set up . The regular will tolerate the heat of the sun better than the white if you don’t add the metal powder . I add the metal powder to the white cuz it brings up the heat deflection charateristics of the cast parts meaning it wont get soft and droop in the hot sun . I would recomend the white as it has a 3 minuite pot life and its much thinner so it will get into all the little cracks . I built one gondola out of the Alumilite white with no aluminum powder added as a experament. I painted it black and left it outside for a few hours in the hot sun to see what would happen. The car did get soft and plyable. Got to admit that I liked the way the car had a noticable swayback . Thats okay for a gondola as many of them get that way over the years but for other cars i’m not so sure the swayback effect would be desireable. I could eliminate the swayback if I wanted by putting the car on its back on a piece of glass and heating it with a hairdrier. My opinion is to use the white and add the metal powder if you plan on using the cars outside . I didn’t need a vacuum chamber to mix it eather . just do you best not to introduce any airbubbles to the resin when you mix it. Ill post some pics later , gotts go to work.