Casting AC&Y 3900 Series 50' SS Box Car

[wazzou]

Following along closely due to a couple of similar projects.  I did have very good success at flat casting sides only but the resin and/or mold rubber shrank some.  (Alumilite)

@DKS, if you haven't already, can you post some information about your pressure pot?
Also, when you are casting your crawlers for instance, I would assume that you have a mold with several crawler bodies and tracks to mass produce, not just one mold...correct?
That would certainly be the benefit of the way these are mastered, by laser, in that they would be all first generation and could be gang cast.

[DKS]

@DKS, if you haven't already, can you post some information about your pressure pot?
Also, when you are casting your crawlers for instance, I would assume that you have a mold with several crawler bodies and tracks to mass produce, not just one mold...correct?
That would certainly be the benefit of the way these are mastered, by laser, in that they would be all first generation and could be gang cast.

My pressure pots come from a professional painting equipment supplier; they are painting pots modified (sort of) for casting. I highly recommend them because they are quality, durable pots, as opposed to the ones a lot of people get from Harbor Freight. While the HF pots themselves are OK, the lid clamping fittings are awful. I further modified my pots with thrust bearings to make the clamping process easier, as well as better valves.

My crawler molds are made from an assembly of three first-gen masters. Thus, each mold yields three complete crawler kits. That said, I cast so many of the suckers that I'm almost always making new molds, and since I have multiple pots, I can make molds and make castings simultaneously. Right now I have about a dozen good production molds, so I can cast 36 crawlers in one cycle.

Regarding shrinkage, Mold Max 20 shrinkage is .001 in./in., and Smooth Cast 310 shrinkage is .0065 in./in. In other words, very low.

[wazzou]

My pressure pots come from a professional painting equipment supplier; they are painting pots modified (sort of) for casting.

Yikes $$$.  That's an investment.

[DKS]

Yikes $$$.  That's an investment.

Yes, and it's paying off nicely.

[haasmarc]

Thanks for the info guys!

[JoeD]

You are also going to need a Vacuume pump to draw the air out of the RTV, and a chamber.  Otherwise, when you put pressure on the molds with the resin inside, you get all sorts of fun distortions.  You'll need a pump that can pull at least 28 inches of mercury to get all the air out...those food vacuume pumps will not do it, nor will the hand pumps you get at the auto parts store.    David is right about the Harbor Freight pressure pots...they don't work well.   I have a couple of old Autoclaves that the steam jacket wore out on.  Ask  your doctor or dentist, they generally toss these out because it's cheaper to get a new one than fix an old one...or at least that what I was told.  I spend an afternoon one day in Orlando and drove around the back of medical buildings and gathered up 8 Autoclaves...they were glad to be rid of them.  Strip off all the steam stuff and put a pressure release valve and a gauge and you have a really nice unit to do production in. 

Joe

[peteski]

Regarding shrinkage, Mold Max 20 shrinkage is .001 in./in., and Smooth Cast 310 shrinkage is .0065 in./in. In other words, very low.

We have already been through this with Bryan, no in another thread. 

EDIT: Fixed typo.

[DKS]

You are also going to need a Vacuume pump to draw the air out of the RTV, and a chamber.  Otherwise, when you put pressure on the molds with the resin inside, you get all sorts of fun distortions.  You'll need a pump that can pull at least 28 inches of mercury to get all the air out...those food vacuume pumps will not do it, nor will the hand pumps you get at the auto parts store.

Actually, you can do without a vacuum pump quite easily. Instead of applying a vacuum to molds, I apply pressure. I have pressure-cured all of my molds at the same pressure as I do my castings (~75 psi). The molds come out perfectly bubble-free, and they have never distorted while casting under pressure. This is a good thing, too, because creating a 28-inch vacuum is much costlier than applying 75 pounds of pressure. Plus, it means I can use any of my five pots for either task.

[wazzou]

Yes, and it's paying off nicely.

Of course it is and you know what I meant.  I'm just saying that it isn't necessarily for the casual hobbyist, that's all.

[JoeD]

Cool, will try it again that way.  We tried in the past and still had problems with surface distortion.  If there's an easier way to do this I'm all for it..

[JoeD]

All this stuff we are talking about is geared for production or complicated stuff.  I have a friend who produced his own line of 1/700 ship models called Coursair Armada and he did work just as complicated as ours but never used a pressure pot...tiny stuff, flawless.  He used slower resins, 15-30 minute sets and picked out the bubbles with a toothpick.  First Century stuff, but his quality was second to none.  So for the casual guy just making a few parts here and there, get a sample kit from Smooth-On and have fun!

Joe

Of course it is and you know what I meant.  I'm just saying that it isn't necessarily for the casual hobbyist, that's all.

[wazzou]

We have already been through this with Bryan, no another thread. 

  

This is what you should focus on trying to do more often.   

[wazzou]

So for the casual guy just making a few parts here and there, get a sample kit from Smooth-On and have fun!

Joe

Been there and done all of that.  I don't have any issues in that respect.  I am simply trying to learn of a better way to cast more difficult scratchbuilt masters of things like a freightcar, for instance. Often, I'd like more than one but don't want to go through all the tediousness to construct more than the one build for the master.  Isn't that generally why all casting is done.

[DKS]

Of course it is and you know what I meant.  I'm just saying that it isn't necessarily for the casual hobbyist, that's all.

Yes, naturally. But there are alternatives...

Been there and done all of that.  I don't have any issues in that respect.  I am simply trying to learn of a better way to cast more difficult scratchbuilt masters of things like a freightcar, for instance. Often, I'd like more than one but don't want to go through all the tediousness to construct more than the one build for the master.  Isn't that generally why all casting is done.

If you're already using higher-quality materials, then the next logical step is to improve the tools at your disposal. Pressure casting is certainly one of the easiest ways to improve quality--significantly--but of course that comes at a price. You might be able to find a more economical pressure pot than the pro paint jobs I use. Maybe for low volume the Harbor Freight model might do; just be prepared to replace the clamping hardware at some point. Pumps are pretty cheap, and you can find those at big box stores.

[peteski]

  

This is what you should focus on trying to do more often.   

I will try my hardest Bryan - thanks for your input.

You know, if you you have such a problem with shrinkage  , just make your masters oversize.  Since the shrinkage rates are clearly indicated, you should be able to use some basic math to figure out how much large to make the masters. Um, IIRC, I think that you are using commercially produced items as masters so what I mentioned might not be feasible.

I hope that I was focused enough for you.