Seeking advanced help on resin casting

Still got that long sig, eh?

OK, choosing the right material means digging through the tech specifications on the companies websites. Two of my favorites are Smooth On

and Freeman Supply The have tons of info on the various strength and wear capabilities on their websites, usually in an .pdf doc. If you're already casting and are familiar with what you're using (and what's going wrong with it!) then compare it to other materials and see what might work better. Bubbles- hoo boy! They could be from a dozen different reasons- bad mold design, the material you're casting with, etc.. some needs to be pressure cast or vacuumed to come out OK. You might need to use a thinner (less viscous) material to get rid of the bubbles, but it might not have the strength when cured. You might get away with something having a long pot life to give the bubbles more time to escape? But you do need to make sure the mold has enough vents in the right place to get them out period. Check out Freeman's website, they have a very good DVD that covers a lot of the basics plus some more advanced stuff, as well as some minor troubleshooting. And consider investing in a cheap Harbor Freight pressure paint pot to use as a pressure or vaccum container. Rig it up with a scavanged compressor from an old refrigerator and it will help a lot! You can save yourself a lot of headaches with a little mechanical assistance. Good Luck!

BTW, may I ask what you are casting? Smooth on is very helpful if you call them directly with problems, they have a good tech archive oonline and great phone help!

snipped-for-privacy@netscape.net spake thus:

Thanks; bookmarked.

One of the most important choices I need to make is the casting material. At this point, I really don't know what's best.

Yes, I've heard about casting under vacuum to eliminate bubbles. Worth considering.

The other trick I've used successfully (at least with molding materials, like RTV silicone) is to use a cup with a small hole in the bottom and let the material drip out this way. Eliminates most bubbles, but I tried it with casting material and it didn't do very well, as the material had bubbles galore throughout.

In answer to the question in your other reply, I'm trying to cast windows--correctly-scaled ones in HO, which means extremely thin parts. Of course, the other part of the puzzle is creating a mold in the first place.

From what I know now, the materials available range from various kinds of plastic resins (polyester, etc.) to epoxies. What do folks here know about these materials? I need something pretty strong which will allow a very small cross-section. I tried the polyester I have (from Tap Plastics), but it seems it doesn't set hard enough--kind of rubbery. Could be that I mixed the catalyst in the wrong proportion, though.

If I may make a recommendation- Try Smooth On's Smooth Cast 300 or 305- same product, just different pot life. The finished plastic is pretty durable, not quite styrene but not bad. I had a lot of problems when I started (with Aluminite) and was about to chuck the works out the window. The 300 really made a difference. And the trial size is only $20 for roughly two pints. And avoid the drip method with plastic- you will trap more air in the mold. Are you using a one part or two part mold? For a one part mold I use a paper cup to pour directly into the mold so the pour is touching the liquid- maybe even sloshing it slowly around a bit to cover. A two part mold I do the same on both sides, then do a quick flip to bring the halves together. You don't want a bunch of excess, you do want a good channel for the excess to flow out of and some sort of index in the mold (so the halves align correctly) is very important!

A couple of tips:

Make your mold 45 degrees from vertical so there are no horizontal muntins.

Run a fill sprue into the bottom of the mold cavity (A "J" shaped sprue from the top) and vent from the top.

Dust the mold with talc before use.

While I'm not an expert, I've done some casting.

And I think that it will be very difficult (if not impossible) to case true scale H0 scale windows. The parts will be too thin in cross section for the resin to be able to flow through. Even "water thin" resin would be too viscous.

Also, resin at that thickness (or should I say "thinness") is extremely brittle. And even the smallest air bubble would cause break in the casting.

Pressure casting might hold some promise. Also, mybe make a simple one piece flat open mold. Fill it with resin, then cover it wits a flat non-stick cover to squish most of the extra resin out. When set, sand it to get rid of the flash.

What you are trying to do would be best done by high pressure injection molding.

That is my 5 cents... Peteski

Peter W. spake thus:

That's just what I was planning (hoping) to do: cast just the window parts (frame, mullions and muntins) in a 1-piece mold with a flat cover.

Actually, a variation that could work would be to cast the window with its glass in a 1-piece mold, then paint the frame parts afterwards. But this would require a really flat backing, which would be hard for me to do. (Not difficult for a professional moldmaker, I'm sure.)

Yeah. I'm trying to do them one better on a shoestring.

Jason Davies spake thus:

Hmm; I've tried to picture this a dozen ways and failed. Must be suffering from density. How would I do this? And how could I use this to cast a window with both mullions and muntins?

D "perplexed" N

For the bubble problem, are the bubbles in random places or are they always in one place? If the former, get a bell jar and vacuum all of your resin after mixing to get the bubbles out. You'll be supprised at how much the resin will bubble when you put it under vacuum so make sure that you put the resin in a large enough container. For the bell jar, you don't need the high vacuum diffusion pump, just a decent rotary pump so your costs won't be all that much. Pressure casting will also help to get the resin into all of the places if you are having problems with bubbles in consisten spots and you may have to modify the molds so that the air can get out of those places. I found that in plastic injection molding that sometimes you need to run a slight depression to certain areas of the mold to get a full flow on long thin sections to get the air out. You don't need much, just a wipe with 600 grit paper to get the necessary space with aluminum.

-- Why do penguins walk so far to get to their nesting grounds?

I was assuming a two-part mold, with the window cast vertically and corner-up.

Basically oriented as a diamond rather than a square.

Jason Davies spake thus:

Nope; I was planning to cast it flat in a 1-piece mold with a flat cover. Not even sure how you'd cast it "vertically": can you explain how that would work? I'm having trouble visualizing this. And why would you want to do it that way?

On Tue, 06 Dec 2005 00:50:48 GMT, David Nebenzahl purred

If you would like assistance and have a legitimate email address, perhaps I can help. I don't like flooding a public forum with tech info few want which uses their bandwidth.

cat

On 8 Dec 2005 00:04:59 -0800, "Peter W." purred

If you get a chance drop by Bandai and catch their trick of molding parts in several colours. Now that is some tricky molding.

cat

Yes, I've seen those sci-fi robot part trees and yes, I'm very impressed!

I've seen similar techniques used on come common household items. I don't recall which at this point, but I remember thinking that it was made just like those Bandai models.

Peteski

Gee, i thought that's what this forum was here for. I'd love to read what you know...

Exactly, its not tech info that wastes our bandwidth but rather the floods of inconsequential chit chat in between the valuable bits. Taking such discussion off line frustrates those who would like to hear it and also cuts out other possible contributors who may have useful ideas to add. It also defeats the object of the group, please make your contributions on here. Keith

On Sun, 11 Dec 2005 04:45:27 -0500, Big Rich Soprano purred

Not knowing exactly what he needs help with, do you really want me to post what may be several dozen pages on various techniques we use and have developed over time? It would bore almost all except those directly involved silly and just waste bandwidth, not to mention my time (without not knowing the specific problems, which involves asking questions, etc I would have to write a lot which would be of no use to the poster since he has some knowledge already but how much i do not know). In some cases it would be best to send some .PDFs we have but these do not post in the group so would be useless here. Besides if someone wants help but won't extend the effort to get what they need, then why should I waste everyone's time with a big public reply.

cat

On Sun, 11 Dec 2005 10:22:56 +0000, Keith purred

Okay then how do I post all the images and .PDFS in a message? no way? Well then there is why I have not posted here. I have a lot but it is formatted to be downloaded from a Yahoo (yecch) group.

cat

I wouldn't consider it a waste of bandwidth nor a waste of your time to share your discussion on this group. Even the conversation to pin down the details of what the asker is in need of. You might be surprised at what information would be considered useful by various people on this group. Considering the volume of posts dedicated to the frivilous slapping matches, I doubt you could generate enough overhead bandwidth to compare to what's been posted here before. However if you have links to those PDFs you might want to share the urls. It's your call.

Jb