Resin Casting

[Doc Frankenfield]

trust me if you are going to use either heat or pressure when you cast resin, you realy need to degas the RTV. the airbubbles will expand in the rubber and cause all sort of issue, distortion, hollows on te pwice etc.
Tin cure RTV can be affected by moisture but that is simpy curing time as it is partially air cure. Platinum cure RTV will not be affected at all by moisture. RTV is viscuss enough that it seldom absorbs moisture in the air, Polyester resins on the other hand will suck atmospheric moisture up like crazy;  rule of thumb if it is very humid, don't even open the resin let alone try and pour it!
 as far as using a Shop vac, it doesn't really draw enough most commercial RTV requires 19 inches of Mercury vacuum,but it is certainly better than nothing.

not trying to sond like I know it ll but have been working with this stuff since '81.. ( Dow Corning J)

[pnolan48]

Well, we disagree, but I'll yield the floor since I've been working with it since only '91.

All I know is that platinum RTV that has been sitting on the shelf in an opened container, even with inert gas upon closing, can make a lousy mold.

[Doc Frankenfield]

"All I know is that platinum RTV that has been sitting on the shelf in an opened container, even with inert gas upon closing, can make a lousy mold.
[/quote]"
the stuff has a shelf live which is shortened upon exposure, it isn't moisture getting in it is volatiles  getting out.
platinum has the shortest lifespan / shelf life. Tin is far more forgiving and be forced with additional activator or accelerator. with tin heat can also rescue out of date material.  generally speaking RTV is only good for avout six months after manufacture..

BTW if one is only dong a quick one off, dental alginate and Nail acrylic is a quick way to do small parts, even shells.

[peteski]

BTW if one is only dong a quick one off, dental alginate and Nail acrylic is a quick way to do small parts, even shells.

Tried it - nail acrylic powder resin/liquid hardener (also used by dentists for temporary crowns) swells up my silicone RTV molds.

[Doc Frankenfield]

nail acrylic : cast into alginate.. tin cure silicone will swell, platinum Like rtv J will hold up. cheaper silicones like SMooth on will degarde and well after a while with just about any solvent.

[randgust]

I've noticed a HUGE increase in mold life using Micro-Marks' mold-release spray.    It's some kind of silicone.   It does help the part release, but more importantly, seems to protect the mold from whatever chemical reaction embrittles them over time.

I use the spray, then talc lightly, blow clean,  then inject mold.   I'm getting well over 50 parts per mold now.   It takes longer for mold prep, but it takes a lot longer to make a good mold than go through these steps.

[sd75i]

  Has anybody cast truck trailers or containers?  Do you make them solid?  Can you cast them hollow?  Any pics?  Thanks

[randgust]

I'm making 40' waffle-sided 1960's XTRZ trailers - mostly because I really wanted them and nobody else has anything close.

I do the frame/bottom as one piece and the top, ends and sides as another piece; hollow.

See http://www.randgust.com/prod071.htm

You don't do shells as solid pieces; you usually work upside down, fill the sealed body with mold rubber, let it cure, flip it over, and pour up to it in a second pour, then do the outside(s) as additional molds.   The trick, if there is one, is designing multi-part molds in different sections so you aren't stressing out molds pulling parts out of them (leading to premature mold damage) and also on planning, well in advance, how feed gates, sprues, air bleeds, etc. are going to take the air out during casting (either pouring or injection).

Something relatively simple like this is a good starter project.    If you're good on CAD you can do Shapeways; I did this one in resin because the artifacts on a model like this would just go nuts and you'd never be able to sand it with those sides.  Resin still has an advantage because you really can get glass-smooth surfaces to reproduce properly. 

[Angus Shops]

Solid body containers would be fine, but think about the weight. Two solid containers or trailers on an already properly weighted car will be pretty heavy. In addition, solid body containers or trailers on a flat car might result in a loaded car with a high centre of gravity, which might be unstable. On the other hand, my 'Canadian slab side hoppers' have a solid car body with applied sides and roof, and a few other parts added, and the total weight is almost perfect.

I once tried to make a 'lightweight' solid resin casting (coal loads for hoppers) by adding a good proportion of Woodland Scenics ballast into the resin before pouring into the mould, but it seemed to cause the resin to expand as it cured (foamed up). The finished surface of the casting was excellent, but the the excess needed to be sanded off to obtain a flat surface on the one side. That would be ok if that surface was not visible. I didn't pursue this idea any farther , but I might have some merit...

Geoff

[sd75i]

  Looks good.  Thanks for all your info.  I'm gonna try and give it a shot!

[randgust]

I've been experimenting with using Shapeways parts as masters as well.   The material itself - after a Bestine bath to remove the wax - is porous enough to retain air that forms tiny surface bubbles when you use molding rubber.    Masters have to be primed, or sealed, or something - to make it work.    I've had a mixed-bag there as well, as some attempts to prime the Shapeways FUD have resulted in warpage and even some documented shrinkage of the printed master.   I haven't tried the newest extreme FUD yet but looking forward to it.

[jimmo]

I think, from experience, that even filling in the gaps with the same resin formulation will not work well. The new resin may not stick well to the old resin, even if exactly the same formulation.

Pete is correct, once a resin has cured it becomes resistant to it's own kind. The only success I have had with using resin to fill holes in cast resin parts are if the hole has enough internal area to hold the new resin in place. I have however, used CA and kicker to fill holes. The best remedy for holey parts is to cast them without holes in the first place.

[jimmo]

essential for a good quality casting is:  pressure pot for the resin stage, vacuum chamber for the RTV (silicone phase)
 proper prep and cleaning  / polishing of the master
  while smooth on is great for consumer use, there are far better, more durable materials that cost far more.

 drop me a line I can probably help you out with doing the work for you.
 here and also can be reached at raven@ravendesigngroup.com

Seems that everyone has an opinion about the best way to make resin castings. For the first couple of years in the model-making business I struggled with trying to find a suitable casting resin as well as mold rubber. After trying several manufacturer's products I stumbled upon the Smooth-On line of resins and molding materials. Contrary to what some may think, these products are geared to the professional with affordable sample quantities so one can try out several types without going broke.

When I discovered that several of my fellow modelers and manufacturers were also using these products, I felt as though my search was over. One well-respected modeler/manufacturer (not going to mention any names but his initials are DKS) gave me some great advice about what kind of device I could use for my casting challenges. The hotly debated subject of pressure or vacuum has been around for as long as I have been aware of such things. Simply put it was recommended to me to purchase a 2-1/2 gallon pressure pot and use that to make both molds as well as my resin castings. Since that time I have been able to make flawless castings regardless of atmospheric conditions.

Of course there are more expensive resins and mold rubbers out there but who has the time and money to try everything?

[daniel_leavitt2000]

My casting equipment is finally out of storage. If you would like me to cast the parts, PM me.

[jimmo]

NOT trying to hijack this thread
BUT I have a question...
I received two resin cast boiler shells for a steam locomotive
each shell has gaps in it
is there a remedy other than putty?

This is for anyone who has purchased items that were cast in resin that have gaping holes in them. Request your money back. There is no excuse for selling such crap. These people need to either get their act together or get out of the business.