Resin Casting

[johnh35]

Silwhite

And why do you prefer it over the others? From the quality of your castings I would presume you have had a lot of experience so it would be interesting to know

[wazzou]

Wazoo:   Wazzou (fixed it for you)

 Alumilite Regular shrinkage is 0.005, SmoothOn Smooth-Cast 300 is 0.01, Smooth-Cast 310 is 0.0065, Smooth-Cast 380 is 0.0025 (all in in./in.) Alumilite doesn't seem all the terrible compared to the popular SmoothOn resins.  What is the acceptable shrinkage value for you?

It depends on what you're casting.  For freight car parts, when trying to mate to others, yes that is too much.
For example, if you're making a 50' car and the roof casting turns out to be 48'9", that is unacceptable.
For DPM walls or gas pumps or other details, not a deal breaker but still more than I'd like.
As I get some time later this week, I'll shoot some photos of masters compared to castings with measurements and we'll see what you think.

[peteski]

It depends on what you're casting.  For freight car parts, when trying to mate to others, yes that is too much.
For example, if you're making a 50' car and the roof casting turns out to be 48'9", that is unacceptable.
For DPM walls or gas pumps or other details, not a deal breaker but still more than I'd like.
As I get some time later this week, I'll shoot some photos of masters compared to castings with measurements and we'll see what you think.

Sorry for misspelling your user name. I meant no disrespect --  just rushing again. 

If we take Alumilite .005 in. / in. shrinkage, 50' car is 3.75" in N scale.  A roof casting (assuming no RTV shrinkage) would result in 0.01875" shrinkage (which is 3 scale inches).  That would be 49'9".
If the rest of the car was also cast out of the same resin then the shrinkage woudl be the same on all the parts.

In any case, which resin do you use which has no appreciable (in my words) shrinkage?

[wazzou]

Sorry for misspelling your user name. I meant no disrespect --  just rushing again. 

If we take Alumilite .005 in. / in. shrinkage, 50' car is 3.75" in N scale.  A roof casting (assuming no RTV shrinkage) would result in 0.01875" shrinkage (which is 3 scale inches).  That would be 49'9".
If the rest of the car was also cast out of the same resin then the shrinkage woudl be the same on all the parts.

In any case, which resin do you use which has no appreciable (in my words) shrinkage?

It's worse than that...much worse.   In most of the cases, I am utilizing a cast roof, adapted to scratch built sides and/or underframe, ends or some combination of that.  On some cars, I have mastered sides to fit to OEM roofs and ends.  I engineered the sides to shrink what I thought they should, however it was much more than anticipated.
When Athearn released the NACC, boxcars, it was the first 50' Stanray diagonal panel roof available, so I wanted to cast it and the PS roof, because they would separate from the body.  When my castings were all shorter and narrower than I required them to be, I purchased 10 undecorated NACC kits.
Each kit came with both roof examples so it was a 2 for 1. 

As far as which resin I now use, that is the reason I'm so interested in this thread.  I'd like to learn of some new tools and tricks.

[Spades]

The Trainboard had a great thread on resin casting http://www.trainboard.com/grapevine/showthread.php?127607-Resin-casting   I used the Robert Ray method,  From Tap Plastic Platinum Silicone, a 50/50 mix ultra low viscosity silicon RTV for the molds, and TAP Quick Cast.  From Randgust I use the fiberglass sryinges from West Marine to fill the molds.

Greg

[peteski]

It's worse than that...much worse.   In most of the cases, I am utilizing a cast roof, adapted to scratch built sides and/or underframe, ends or some combination of that.  On some cars, I have mastered sides to fit to OEM roofs and ends.  I engineered the sides to shrink what I thought they should, however it was much more than anticipated.
When Athearn released the NACC, boxcars, it was the first 50' Stanray diagonal panel roof available, so I wanted to cast it and the PS roof, because they would separate from the body.  When my castings were all shorter and narrower than I required them to be, I purchased 10 undecorated NACC kits.
Each kit came with both roof examples so it was a 2 for 1. 

As far as which resin I now use, that is the reason I'm so interested in this thread.  I'd like to learn of some new tools and tricks.

So what you're saying is that the manufacturer's specs for shrinkage are way understated?  I find that difficult to believe.  Maybe something else in your casting process is causing the shrinkage?  The RTV rubber also shrinks...

Since you still haven't mentioned what you use for non-shrinking resin, should I assume that you haven't found it yet?

[wazzou]

Since you still haven't mentioned what you use for non-shrinking resin, should I assume that you haven't found it yet?

You strike me as one of those guys that while having a conversation is just waiting for an opportunity to talk rather than one who listens.

Read the last paragraph of my previous post.

[peteski]

You strike me as one of those guys that while having a conversation is just waiting for an opportunity to talk rather than one who listens.

Read the last paragraph of my previous post.

Um, thanks for your clear and direct answer Bryan. What is clear and direct to you is not to me. Sorry!
I listen when what I hear seems to make sense or when I think I can learn something.  Good luck with your search for a very low-shrink resin.

[kornellred]

Just thought I would mention that RTV rubbers also have a shrinkage factor as well as the various resins.  This needs to be taken into consideration, since your products will be twice removed in size from the master.  This obvious reality might have been implied in the various responses, but vitriol tends to obscure facts.

[peteski]

Just thought I would mention that RTV rubbers also have a shrinkage factor as well as the various resins.  This needs to be taken into consideration, since your products will be twice removed in size from the master.  This obvious reality might have been implied in the various responses, but vitriol tends to obscure facts.

I clearly mentioned (not just implied) that possibility couple of posts above yours - it was not acknowledged. 

[johnh35]

This thread was useful and informative and it would be nice to keep it that way.

I would like to hear about experiences with different resins as related to demold time, especially as related to shells. Do you prefer the faster resins, or something slower? I find that the faster resins tend to shrink more and I seem to have problems with bubbles. CR600 takes about 30 minutes and seems to work good. I have tried longer curing resins (Smooth On 322 IIRC) and didn't really see any advantage, What say ye?

[LV LOU]

John,I usually use 322.I like the ability to take my time,use a toothpick to work resin into some areas,and I also am usually casting way more than one or two things at a time.It's just a less rushed process..I mean,really,what does it matter if demold time is one,or two hours? Most resins still take 24 hours before they're completely hardened..

[randgust]

There's a very real strategy in lengthening mold life by being very attentive to demold time, at least with CR600

If you use a predominance of multipart, closed molds, with fill gates like I do, and hold them together with rubber bands.  The rubber bands can still easily pull out of the CR600 at 30 minutes.  Later than that and they're cast in for good, cut them apart.

At about an hour, I pull molds apart, except for the final interior shell insert.  This lets me get a good look at it, but more importantly the flash is still flexible and soft; even it if does get into hairline cracks on the outside visible view, it doesn't damage the mold like it would if you let it set for several hours.

After several hours, any interior filler molds that have to be popped out are safe to pull without distorting or damaging the shell.

On the shrinkage factor.... there must be some for CR600, but I sure can't prove it.  I can lay my parts beside my scratchbuit masters and the dimensions are still right-on.  Most of my parts have to fit other commercial parts (conversions) so there's relatively little room for error.  My Climax parts have to fit right onto the Kato frames, same with the CF7 parts for the Atlas frame.  Works fine.   I'm not saying it doesn't exist because I've met Wazzou and we've compared notes, I've seen his process and parts, and wow, yeah, there's shrinkage there he's had to plan for and design around.   That's one of the reasons I'm so unwilling to try other stuff, all I know is that what I'm doing works.  I went into this completely ignorant about designing for shrinkage and it worked anyway!

Another thing I've evolved into is a lot of 'sacrifical space' at the 'bottom' of a shell or part - which is typically cast upside down.  The tendency for any bubbles to end up at the 'top' of a mold is a lot easier to deal with if you put about a 5' band of .020 around the top edge, and put the air bleed sprues off of that.  It may come out looking like swiss cheese, but that's fine, it all gets cut off anyway.  You need to design, from the start, the sprues, etc., that are going to get cut off and evaluate how that mold is going to fill and displace the air out of it.  That's sometimes harder than scratchbuilding the part to begin with.  I've had to go back and redesign molds and masters repeatedly as I've learned more how this actually behaves in use.

That's also why anybody that asks somebody to 'borrow' a scratchbuilt model to use as a master doesn't have a clue what that means.  Most people don't even recognize my masters when they see them because of the interlocks of sacrificial panels, sprues, and gates that get built onto them.   That scratchbuilt master is never usable as a model again.

[johnh35]

Another thing I've evolved into is a lot of 'sacrifical space' at the 'bottom' of a shell or part - which is typically cast upside down.  The tendency for any bubbles to end up at the 'top' of a mold is a lot easier to deal with if you put about a 5' band of .020 around the top edge, and put the air bleed sprues off of that.  It may come out looking like swiss cheese, but that's fine, it all gets cut off anyway.  You need to design, from the start, the sprues, etc., that are going to get cut off and evaluate how that mold is going to fill and displace the air out of it.  That's sometimes harder than scratchbuilding the part to begin with.  I've had to go back and redesign molds and masters repeatedly as I've learned more how this actually behaves in use.

I utilize the same concept. I typically take a shell and attach a "base of 0.060" X 0.125" styrene around the bottom edge to allow a place for the air to escape to. TypicallyI use a fill hole at one end, and vents both sides mid-way and a vent at the other end. I typically put the mold on a sheet of glass and tilt it to where the fill side is lower. When building the mold, I use lego blocks to form the box around the master, with 4-6 pegs to ensure the inner and outer mold halves are correctly positioned/oriented. A well designed mold is the most important aspect of casting. I have also noticed that if I heat the mold under a lamp prior to pouring the resin, the resin tends to set up better, especially when there is more humidity.

[wazzou]

Just thought I would mention that RTV rubbers also have a shrinkage factor as well as the various resins.  This needs to be taken into consideration, since your products will be twice removed in size from the master. 

No doubt about it.  That's harder to quantify through measurement than the resulting castings that can be compared directly to the master. 
Therefore it's difficult to tell if the mold is more responsible or the casting resin. 
All I know for sure is, I wasn't happy using the Alumilite products for the projects I had intended.

YMMV