Cast Resin Parts

[Bill Stone]

I've not seen much discussion here about casting parts with resin in RTV molds. I'd really like to encourage you scratchers and bashers to try it if you haven't.

The photo shows my first stab at it. I was amazed at how easy it was, how my first attempt came off without a hitch, and how perfect this is for creating multiple copies of those parts you can't find or can't afford. I used the RTV and resin sold by Micro-Mark. I simply followed the brief and clear instructions.

This is a box car door. I made 2 RTV molds so I could pour 2 parts at a time, cast 10 doors, and all but 1 came out perfectly. The molds show no deterioration. I imagine I could cast 50 or 100 parts in each --- if I wanted to. I reiterate: This was my first attempt at resin casting. If I did it so easily, anyone can. I'm planning now to cast an entire box car!

The photo is self explanatory: At the left is the original pattern (with a low "box" added around it to retain the RTV.) The pattern was made of wood, a little bit of paper, a little cardboard, and some paper clip wire. I sealed it with spray shellac.

On the upper right is the RTV mold that I had poured onto the pattern. The lower right is one of the cast doors, painted.

 

[Steam Donkey]

Hi Bill,

I'm glad you posted this thread, I've been thinking about casting with RTV molds. Is it possible to cast an entire skeleton log car (minus the fiddly undercarriage parts and trucks of coarse ) ?

If the resin is not heavy enough to meet the NMRA minimum car weight requirements, I wonder if one could use a different casting material, perhaps some type of soft white metal?

Has anybody out there tried this kind of thing before? I've never done anything like this before, but for a bit of extra effort, it sounds like a great way to duplicate an item in a real hurry!

Stan

 

[Bill Stone]

Hi Stan,

Micro Mark also sells low-melting-point metals that they say can be cast in RTV molds. I haven't tried it.

Another approach might be to cast the end bunks (or whatever they're called) alone (1 smaller and simpler mold - cast 2 pieces for each car) and then use a square brass (or other metal) rod for the center beam.

Don't know if that would give you enuf weight, but I suspect it would come close.

Another advantage, you could build cars of varying length by using longer/shorter beams.

Bill S

 

[jon-monon]

Pretty cool, and I looked up the cost like you said is very reasonable. I have several things I would like to cast. Missing box car doors and some box car ends (long story). I wonder what's special about their RTV and resin, compared to say marine or 2 ton epoxy and standard RTV gasketmaker...

 

[Steam Donkey]

BINGO!!!! Bill, that's perfect! I'm gonna give it a try, looks like Mico Mark will have something that will work. Thanks for the tip!

Oh, this is a tale about cast-a-ways.....dum de dum.....

 

[jon-monon]

Oh, ya, do you need a release agent on the mold (when you make the mold off the original or when you do the copy)?

 

[Bill Stone]

Stan..... Let us know how it works out.

Jon..... I wondered about release agents. I was told that you only need them if you're making a two-part mold, so the RTV doesn't stick to the other RTV half. My bro-in-law is a sculptor who does a lot of casting. He said to spray the pattern with a tiny bit of WD-40, and then wipe it off. I did that, whether it needed it or not. Worked perfectly. I didn't use any release between the RTV and the resin.

Another guy told me that if I ever do a two-parter, Vasoline (petroleum jelly) is all you need for release agent between the two RTV halves.

The greatest sounding idea, that I learned at a clinic at a recent NMRA Regional, is to paint the inside of the RTV before pouring the resin. The idea is the paint adhears to the resin, and you get a finished part out of it, and there is no covering up of detail by painting. Sounds perfect. I'm going to try that the next time.

Bill S

 

[Blake]

Actually, I have been using silicone rubber and poly urathane resin for making parts for military models. I will be using it for Model rring too. I highly reccomend going to a company called Smooth On (link below) They have a starter kit for under $30 that gives you 2 quarts of rubber and 2 quarts of resin plus an in depth how to book. I suggest trying the Oomoo rubber (no joke, it's really called Oomoo!) and the Smooth Cast 305. Neither have any odor, they are low viscosity and de-gas themselves.

http://www.smooth-on.com/

 

[t. alexander]

Bill, thanks for the post. The door came out great. Painting the RTV before casting sounds like a neat trick.

t.

 

[Vic]

Hi Bill, I've been using a RTV mold for years to cast "blasted stone" tunnel portals. I made six for myself and probably 50 or so for some friends over the last 10 years or so and the mold is still going strong

 

[Lighthorseman]

Blake...

I looked at the Smooth-On site, and am wondering which starter kit is the one to look at? There seem to be two; urethane and silicone.

Please advise!

Also, are there any forums like "The Gauge" but are geared toward military modelling?

Thanks!!

 

[Blake]

The kit number is 99941 and it is the Silicone rubber. Don't buy it online, call them. The service staff is very informative. I use the Smooth Cast 305 plastic as it has a longer pot life. Once the molds cure you bake them in your oven at 200 degrees F for about 4 hours. Even while bakeing this stuff has a low odor and it won't mess up your oven. You get a how to book that covers way more than you will need to know. The main reason I chose these materials is that they are mixed by volume. Most of the others are mixed by weight and require a triple beam scale. The ones that mix by volume are more forgiving. I use clear plastic disposible bathroom cups for measuring the rubber and the plastic (niether material has any effect on these cups). I mix the rubber in a small food container (rubbermaid, tupperware ect.) as once the rubber cures you can just peel it off. I mix the plastic in a medium clear platic disposible cup as it is harder to get off of the food containers. Lay some newspaper down on your work bench as some drippings may hit the work surface. Trust me on this, just get the stuff and do it and don't be aprehensive about it. I put off doing it for a while because I was a bit unsure of what to expect. When I finally did it, I kicked myself because it was so straight forward and simple. I don't know of any forums about casting, but you will probably find that you won't need one. Good luck and keep me posted on what you are doing. If you have more questions I will be more than happy to help you out. That goes for anyone else who is interested.

 

[Bill Stone]

Another supplier is Bragdon Enterprises at:

www.bragdonent.com

I haven't used their stuff, but he is the guy who gave the clinic at the NMRA regional. Was really knowledgeable, and I suspect would be very helpfull to customers with questions. They also sell stuff for scenery.

I agree on the "mixing by volume" preference. That is one of the reasons I used the Micro-Mark stuff. In fact, unlike some, their stuff mixes 50-50, so it would be pretty tough to get it wrong.

Vic: I would imagine that you're going to wind up with a lot more "friends" wanting tunnel portals!

Bill S

 

[roryglasgow]

Thanks for the great info, guys.

This all dredges up something I've had in the back of my mind for a couple of years now: building my own locomotives. One of the major reasons that I haven't attempted this is because I don't know how to get ahold of some of the parts that I need.

Would resin be suitable for making gears? Or would it be too soft or brittle?

Also, could low-temp alloys be used for making wheels or gears? What about Crown Pewter?

I'm looking at a Micro-Mark catalog, and without thinking this through too much my mind is saying "I can do this." Anyone have any experience with these things?

Thanks!

 

[Bill Stone]

I asked Bragdon about wheels. He said he didn't see any reason why it wouldn't work, but hadn't done it himself.

I don't know about gears. My first thought is that this resin isn't nylon, and perhaps it would not be tough enough/slippery enough to last as a gear. But I could very well be wrong. Actually it would be so simple to cast a gear, and only would cost a few cents worth of RTV and Resin, that it would be a great thing to try. I'd run and do it right now, except I haven't a test track to run the experiment on to see if the gear would last.

I do recall that in one of the model rail magazines (RMC???) a year or so ago, there was an article about scratch building a small rail bus or truck, in which the author resin cast his spoked drive wheels, and told how he did it. I recall that he had metal tires, and cast the wheels right into the tires. But I cannot recall if this was a static model, or something that ran. My one concern on this process is that the resin will shrink (although very slightly) as it sets, and so would the tire wind up a little loose?

But speaking of gears: I remember years ago reading an article in an engineering magazine by this guy who used screw threads to make one-off worm and gear trains for prototype equipment. The worm was no problem at all, but making the gear was a little tricky --- using a tap to cut the threads on the concave edge of the gear blank. (without breaking the tap.....)

Bill S

 

[Bill Stone]

Oh --- about the low temp alloys. My wild guess is that they would not be suitable, but perhaps someone has tried it, and knows better.....
Bill S

 

[roryglasgow]

The shrinkage issue occurred to me, too. I was wondering if it would cause problems with making the gears--if they might come out distorted.

I've seen resin shells made for N-scale locomotives, but I've never seen "homemade" gearboxes. I suppose I could try it sometime, but I think you're right about the durability and slipperyness of the plastic.

The wheels are still turning... Thanks for the info!

I wonder what the custom builders use...

 

[Vic]

Hey Rory, Go back to the Smooth-on site and click on prototyping. Apparently the have resins that are suitable for making gears. I didn't look too far so don't know if they are talking about casting blanks to machine gears from or what?

 

[roryglasgow]

Thanks for the tip, Vic. I looked in that section. It appears that SC-322 might work. One of its suggested applications is "plastic mechanical parts." It has a long curing time, but it has minimal shrinkage (0.0005 in/in).

Eager Plastics (http://www.eagerplastics.com/) has a questionnaire you can fill out. They'll tell you what plastics might fit your application. They sell Smooth-on products, too.

 

[Bill Stone]

Regarding cast gears as opposed to machined gears. Assuming one could get it to cast in precisely the right shape (a pretty good bet) cast gears would theoretically be stronger than machined gears of the same material. It has to do with the molecules, grain, and etc.

I found the the RTV and casting resin I used shrank very little. From the original pattern to the final resin casting --- and that's through two processes, the RTV and then the resin --- only about 0.002 inch in 1.00 inch. Hardly enough to worry about for our purposes, even for gears and worms. And now that I've figured that out, I'm more inclined to believe that metal-tired drivers and other wheels could be done with this resin.

Bill S