very thin resin casting

[JohnP]

I am very close to completing a project of creating detailed model bridges via resin castings. I am 95% there except for one final hurdle. So I have turned to this community of modeling masters to pick your brains a bit.

To believably reproduce the typical riveted bridge the parts need to be thin. RTV and urethane resins have worked very well in implementing my designs. The one item left is this- the RTV expands a bit in the mold cavity after multiple contacts with resin, and the RTV itself expands a bit overall. This will ruin the thin parts. I understand the RTV absorbs chemicals out of the resin, but I don't understand the overall expansion.

Question: Does anyone have casting experience and can offer suggestions on what makes RTV swell and how to prevent it? Or has anyone know a source of casting wisdom?

Disclosure: I plan on selling the kits on a limited basis. So this is a semi-commercial question. I hope that doesn't upset the powers that be. But they will be really cool and like nothing available now.

Final question: Three years of developing this bridge modeling technique has left me nearly insane. My spare bedroom has a compressed air line, pressure pot, vacuum equipment, scales, mixing containers, resin jugs, etc. Is it time to turn it into a meth lab or something else profitable?

Thanks, John

[SandiaPaul]

I have done A LOT of molding and casting, not so much for hobby but professionally. The one thing that makes the most difference is the use of high quality molding  casting materials. There are several sources but the one I found that I stuck with was:http://www.bjbenterprises.com/ The super fast cure resins will degrade the rubber much faster. The BJB folk were most helpfull give them a call and ask for advice on the best products for what you are doing.

You are correct that the RTV degrades over time, I would not expect more than maybe 40 casting at the most from a mold. If you are going to do any great quantity design your mold to use as little RTV as possible and just make more molds from your master. Back up your mold with a good sturdy mold box that keeps the mold from expanding. I typically used something like 3/8" or 1/2" acrylic for smallish parts and MDF for bigger ones. Are you pressure casting? I never found pressure casting to be vital unless I was doing something like a clear part where a bubble would ruin it. You will get better parts with pressure though for sure. Keep us updated on your progress!

Paul

[JohnP]

Hey thanks for the answers Paul. I figured there would be somebody reading this forum full of insight by accomplished modelers who knew what I am involved in. You sound like you have certainly been involved in casting.

I am using aluminum mold boxes I make with a 1/2" contact surface. Dowel pins make the alignment. This arrangement is more effort but gives me greater control over wood or styrene. The mold area is already pretty shallow at 3/8" and 1" to 1 1/2" wide. I use that design to minimize rubber costs and increase dimensional control. But your suggestion of using as little as possible may move me to shrink the mold cavity to a minimal size, just enough to contain the part. That may assist with the overall rubber dimensional changes.

Pressure casting is critical to remove air from my enclosed molds. The thin part traces can collect air easily as the resin is injected. I always pressure cast these small parts in a Sears paint pot at about 50psi.

I switched from another big name in RTV/resin to Polytek. There is a gentleman there who is very interested in my project and has helped other model railroad product designers. They have high quality material. I am sure bjb can help me also, I'll have to try them too.

There is very little on the details of resin casting in RTV molds available. Certainly many beginner articles are available. But understanding the detailed interaction of the particular material attributes seems to be a challenge to learn. I recall a consultant article on line where the authors stated a company interested in molded rapid prototyping either needs to spend lots of time in learning the process details or spend lots of money paying others to do it.

With that in mind if any modeler out there wants to try resin casting for a repeating model write a post here or send an email. It is not that hard for small quantities of parts. You can see what the heck I am trying to accomplish in a quantity others can enjoy here: http://www.jpstructures.com/. It's one of those life passions, not for money really but for the accomplishment. I am trying to answer this dilemma- a modeler can by a particular locomotive or car even by the number and date of service but why can't we get any more than one decent plastic bridge and a half dozen wooden bridges?

Thanks, John

[SandiaPaul]

Wow your stuff looks great!
It sounds like you are doing all the right things, it's funny I did most of my mold boxes with dowel pins but I was reluctant to delve into that in my response as a lot of "model railroaders" don't have access to the tools to do that. Its really the best way to go, as you can set up and break down faster and more accurately. I got around the need for pressure at lot of times by a system where I poured with the mold connected to a long vinyl tube and a funnel located 1' -2' above the mold so I got a pretty decent head and some pressure behind it. It was a waste as far as material goes because you lost all the material in the tube, but it was a good tradeoff as far as material cost vs. good castings went, plus it eliminated the extra step of the pressure pot. For your thin casting though you prob. need the pressure.

As you have found out pressure works great, its just another step in the precess and when you are dong it for a profit its nice to have fewer steps!
There is little info out there on casting, at least for producing quality castings in a decent quantity. I really learned by doing, and doing over till I got it right.

In looking at your project, I think you too probably have reached the the next logical conclusion, injection molding! An aluminum tool will last a long time and not move at all.  Of course its a whole different ballgame, but take a look at the Tichy stuff for example, its all done with aluminum tools(at least it wa a few years ago when I talked with the owner(can't recall his name now) His castings are great and all done with a inexpensive cnc and alum tools.

Have you tried hard plastic tooling, say epoxy or other urethanes?  15 years ago or so I played with some ideas I tried to emulate injection molding with resin. I think this could work but it is opening a pandoras box as far as experimenting with techniques! the right release agent will be critical here. I had very good results with:
http://www.miller-stephenson.com/release_001.htm  I used this release on the rtv too and never had problems releasing anything, even stuff with moderate undercuts.

Good luck with your project it sure looks nice so far!

Paul

[JohnP]

Goodness Paul you are thinking along the lines I am headed I believe. Right now itt is frustrating because the parts you see on the bridge I can reproduce 6-8 times before the dimensions get too wonky. So close and yet so far. I am trying a controlled shimming of the mold boxes to see if there is an upper limit of the RTV expansion. Plus I have A60 rubber to try, should be more stable then the A45.

I have future plans to CNC into aluminum molds the parts with no undercut. Of course I need a milling machine first so it is on the distant horizon. The hard cast molds are an angle I brushed off but they may be a good transition. I don't mind using the release but the removal of it seems difficult, but it needs to be done so paint will stick, especially PollyScale etc.

Your funnel idea is very inventive and useful, other modelers should take note and try it. I set-up for pressure casting a while back and am used to it. I even made a three-tier tray like a desert server to hold the filled molds and drop it in the tank 9-12 molds at a time.

My current job is as a process technician at a place with out-of-control processes. Same as I have done other places- fix other people's processes. What do I do when I get home? Figure out my own processes. I guess I must really love getting things done right!

Thanks for the nice words,
John

[Ray Dunakin]

Wow, your bridge looks great!!

[JohnP]

Thanks Ray. I made a 7th grade science fair project about bridges. I started college in civil engineering but when I found I would not be working on riveted steel (reinforced concrete is so boring and ungraceful) I switched to electronics. So this project represents an expression of my long interest in riveted iron and steel bridges. Of course railroads use the most awesome of steel bridges so it is all a great fit.

The potential for detailed, skilled modeling in the bridge genre is tremendous. For example, brake systems and rigging on freight cars are similar but different over the years and manufacturers. Or the roof may have x number of panels, flat, embossed, whatever. On riveted iron trusses the method of attaching the floor beams to the vertical beams varied considerably even though the general look is there. So the opportunities are enormous for understanding a new subject to model in bridges.

John

[Ray Dunakin]

I also am fascinated by riveted steel bridges (though I'm no expert on them). I just love the way they look -- they're so intricate. And they have a lacy, delicate look despite their strength.

I want to build a through-truss bridge for my 1/24th scale outdoor layout someday. I'm still trying to figure out how to do it. Styrene would be easiest, but probably not durable enough for long-term use on such a fragile structure that will be subject to occasional bumps and knocks. Brass would be better but much more difficult to work with, and also much more expensive.

[JohnP]

Ray, you could use ABS plastic which I believe is more sunlight resistant than styrene. Or Plexiglass too. Properly bonded with a solvent method it will hold up well. Also, I am using continuous thin steel bars on the floor stringers for this On3 truss bridge. That means the track and trains will be supported even if the truss fails due to damage or un-bonding. You could assemble a bridge floor grid of brass or aluminum (easy to work) and allow the truss structure to be a semi-functional model.

When I am independently wealthy I will be cutting the molds for this bridge in 1:48, 1:20.3 and maybe 1:24 on my future CNC machine. It's good to have goals!

John

[marc_reusser]

John,

When you are independ. Wealthy you could do CNC milled slide mold dies for injection molding...

A fellow modeler/mfr I know used to do all his parts/kits in resin, and has now shifted over to injection molding. The detail is really crisp, consist., & beautiful, and the production quantity are much greater/faster......so probably a time vs start-up cost break at some point. Listening to him, it seems like the time to do the CAD & Die Milling is where the biggest expense comes in. (once you get over the hurdle of purchasing the machines)

FWIW.....If interested I can try and find out the name of the auction site, but I have a couple of friends that have begun equiping a machine and cabinet shop with new and nearly new top of the line machines, and they are in many cases paying less than 10-cents on the dollar for them.....most have been from places like Nevada & Arizona, where the economy has tanked, but all the equipment is new from the recent boom, and now they need to liquidate...only drawback is they need to drive a truck out to pick the machines up.  (Before people on the forum get upset, and call this bottom feeding in a bad economy, let it be known that their new shop is creating jobs for skilled tradesmen here.)

MR

[JohnP]

Marc, I have money for rubber, resin and a few bicycle thingies right now. And good beer too, that even comes before resin. And bikes. And I am in VA but the darn weather makes me wish I was in New Mexico.

I think with the time it has taken me to develop masters and apply the huge number of rivets, and now figure my way through this rubber issue, making hard molds for resin with a CNC machine would maybe have taken the same time. I see the hard aluminum mold for liquid resin the next step, Injection mold pressures and temps are so high that is another realm that takes experience. Someday, for sure, I would like to have a nice line of molded bridge kits for everybody.

There are table top CNC mills and enough forums out there to learn quickly but that would be yet another hobby-within-a-hobby. Any time compressors available surplus?

John

[John McGuyer]

When you are independently wealthy, you could make probes on your CNC 5-axis machining center for your sinker EDM to burn your molds.

John

[JohnP]

Oh yes John that day will come; it is in the master plans.

Meanwhile, I was in the basement tonight at an unheated 48 degrees F working on mold boxes using a cheapo Ryobi benchtop drill press and relying on a Craftsman lil' belt sander bench rig to perform my machine work. I guess the tools get the job done for now, the precision work is done in my upstairs room.

What I am conjuring in my head is an X-Y stage that moves a sheet of styrene under a small pneumatic rivet embosser head. I could make any pattern, any spacing on my masters. That may come before the EDM and the 5-axis. Actually, my father was a life long machinist (back when you needed to know how to count dial turns and account for backlash) so an old 'murican Bridgeport would be to my taste for custom mold box creations.

[Hauk]

What I am conjuring in my head is an X-Y stage that moves a sheet of styrene under a small pneumatic rivet embosser head. I could make any pattern, any spacing on my masters. That may come before the EDM and the 5-axis. Actually, my father was a life long machinist (back when you needed to know how to count dial turns and account for backlash) so an old 'murican Bridgeport would be to my taste for custom mold box creations.

This might be old news for you, but is it something like this you are thinking about?
Bill Box´CNC riveter

Regards, Håvard H

[John McGuyer]

It was a very happy day when I got my three axis DRO on my mill and no longer had to count turns or allow for backlash.

John