Warning about Envirotex and RTV molds

macfrank · Post by **macfrank** » Mon Jun 06, 2005 5:05 pm

Envirotex Lite makes a fine two part, 1:1 mix clear resin. I've had no problems using it in Vinyl Polysiloxane (a dental mold rubber material), but I tried casting some parts in an RTV mold (made with the mold material that comes with the Alumilite intro kit), and they just won't fully harden - they have a sticky finish.

Not only that, the parts have a light yellow color, in stead of water (colorless) clear. It's not a mix problem, because I poured the same resin into Polysiloxane molds, and they came out fine.

Just to be sure, I cleaned out the RTV mold, made sure it was clean and dry, and repoured. Same results. The resin poured in the Polysiloxane molds hardened properly, and were water clear. the parts in the RTV mold came out soft, sticky and yellow.

I don't care about the yellow tint, but I don't want them to be sticky. I wonder if painting some hardener on the parts will help?

Frank

Post by **TER-OR** » Mon Jun 06, 2005 9:55 pm

You might try coating it in Future. I've had Envirotex light stay sticky if I try to pour too much. I've heard others say the same thing, build it up in layers if you need a thick part or thick bit of water.

I haven't had troubles with Envirotex light in Oomoo silicone rubber. The parts I cast were pretty thin, though.

Kekker · Post by **Kekker** » Mon Jun 06, 2005 10:13 pm

Could this be a surface inhibition of cure problem? I seem to recall that the catalysts can cause this between some resins/mold materials. Isn't one type of catalyst tin, and the other platinum? If I could say which one the Alumilite one was (I'd bet tin), try the other. Just what you wanted to hear, I'm sure.

I'd go with Ter's suggestion of a coat of Future on the inside of the mold. Of course, you may have some problems getting the Future to properly coat the inside of the mold. Hmmm....

And where do you get the dental mold material? Is that the "sets in a minute" gel stuff used to make molds for crowns, etc? Had a crown put on last year and was thinking about how useful that stuff could be. That and the UV-cure filling material.

Wish I could be more help!

Kev

Shinnentai · Post by **Shinnentai** » Mon Jun 06, 2005 11:38 pm

Oooh! Oooh! Pick me! Pick me!

I've used off the shelf Alumilite RTV for casting W/ EnviroTex epoxy. I experienced the same difficulties at first. The problem is the rubber mold is too "fresh", and is still leaching/ougassing alcohols. For some reason epoxies in general seem to be REALLY, REALLY sensative to this.

You gotta post-cure the mold if you wanna use epoxy resins in it. Put it in the oven at 300.F for a few hours to draw out the evil spirits. After that, your epoxy resin castings will come out clean and clear.

Coating with Future is a good idea in theory, but in practice the Future will just bead up and refuse to stay on the rubber (it was the first thing I tried too). The only way to make the Future stick is to dust the mold first with talc or similar, which kills the clarity of the finished casting. If you do a good post cure on the molds though, you won't need to use a barrier or release of any kind, just pour 'n play.

macfrank · Post by **macfrank** » Tue Jun 07, 2005 1:24 am

Thanks guys. I tried Alumilite in the molds, and they worked fine. I checked the parts tonight and they're slowly getting less sticky, but at this rate it may take a year to fully harden.

I'll try baking the mold, then re-pouring.

Kekker wrote:And where do you get the dental mold material? Is that the "sets in a minute" gel stuff used to make molds for crowns, etc? Had a crown put on last year and was thinking about how useful that stuff could be. That and the UV-cure filling material.

The UCLA Medical Center store is a couple of floors up from my office. They have the Reprosil mold material at a discount. It's still expensive, but great stuff if you want to make a mold in 5 minutes or less, and the Environtex works great with it.

Unfortunately, I'm trying to cast the clear engine parts from the Leif Ericson, and the shape makes it very difficult to make a mold with the relatively thick Reprosil (even in the thinnest version).
The mold I made with the RTV works great with Alumilite. I guess I could pour acrylic into the mold (I know that would work) but the cost in acrylic would pretty much nix that.

Frank

Post by **TER-OR** » Tue Jun 07, 2005 7:41 am

Actually, I was suggesting he coat his tacky casts in Future.

I don't have any idea what would happen with the mold and resin if the mold were coated.

I've got some old molds, I may give it an experiment. I wonder whether I have any old alumilite molds laying around to check that, too.

Most of the clear casts I've done with Envirotex were older molds which had already churned out polyurethane parts. THey weren't fresh.

chiangkaishecky · Post by **chiangkaishecky** » Tue Jun 07, 2005 4:42 pm

Fascinating.
From what I've read on rec.models.scale and the yahoo casting group, envirotex won't fully cure in an RTV mold ... still hafta try plain ol' vanilla RTV molding and resin casting before trying the enviro tho'.

macfrank · Post by **macfrank** » Tue Jun 07, 2005 9:55 pm

My first pour is finally curing - after 72+ hours, it's now only slightly tacky. The parts are still a light yellow in color, though (not a problem for my application). The second batch is still tacky, but I followed Ter's suggestion and bathed them in Future. That'll probably work because Future rules.
I also fired off an email to the Envirotex people. I didn't read every word of the instructions (beyond mix for 2 minutes in equal amounts and don't drink the stuff) so maybe there's a "do not use in RTV molds" somewhere, in 6pt font. It should be in 36 pt font and red, on the front page of the instructions.

Envirotex is otherwise great stuff - easy to mix and use. The other clear resin that I've used was incredibly sensitive to the mix ratio (1:10 exactly) and stank something awful. My fallback is acrylic powder (water clear, easy to use, insensitive to mix ratios, cures fapidly. It does stink, though), but unless/until I find a cheaper source for the stuff, it's not an option.

Owen E Oulton · Post by **Owen E Oulton** » Wed Jun 08, 2005 7:45 pm

As I understand it, the problem is an incompatibility between epoxy casting resin and RTV rubbers. It's just chemistry. Not only does epoxy have the curing problems cited above, but epoxy will very quickly cause the RTV moulds to break down. I don't think the difference between tin- and platinum-catalysed RTVs has any bearing on this issue.

One thing you can try with your existing castings to get them to cure is called "post-curing" which simply amounts to heating them after they come out to the moulds. The amount of heat required varies with the type of resin, but generally speaking 120° Fahrenheit for 10-16 hours should help.

macfrank · Post by **macfrank** » Wed Jun 08, 2005 8:22 pm

I'll try heating, but I'm done with this experiment. The Future didn't help; those parts are still tacky. The ones that did eventually harden enough so that the surface was dry enough to handle turned out to be extremely brittle - more so than regular Envirotex; cutting off some very thin slash caused some damage to one of the parts. cutting off similar flash from parts cast in the Reprosil molds was not an issue.

The Envirotex people were not particularly helpful - "don't use that mold" was their reply.

I noticed that Alumilite sells a clear casting resin; has anyone used that? I know that the Alumilite works just fine in these molds... because the RTV came from them!

Frank

Shinnentai · Post by **Shinnentai** » Wed Jun 08, 2005 10:07 pm

Owen E Oulton wrote:As I understand it, the problem is an incompatibility between epoxy casting resin and RTV rubbers. It's just chemistry. Not only does epoxy have the curing problems cited above, but epoxy will very quickly cause the RTV moulds to break down.

Not to be to objectionable, but I don't believe that is at all correct.

I'm not that up on the chemistry mind you, but I've certainly not had these troubles when working with the materials myself. I like the durability of epoxy resins, and I use them almost exclusively. I've had RTV discolour from use with epoxy, but not noticably degrade. The cure inhibition issue only occurs with fresh molds. After the mold has been post cured, or even if it's just been sitting on the shelf for a few months, this issue goes away. Clean castings; no goopy surface, no yellowing, no brittleness, no unduly ruined molds.

I have cast a reasonable amount of stuff using epoxies and RTV, most of that using the off the shelf Alumilite kits.

I think there's a lot of people out there who just give up on the whole idea after having one type of bad experience, usually resulting from a frankly illogical up front assumtion that it should all be the same as casting urathane

. No R&D, just jumping to assume the idea is worthless after the first few failures, then telling everyone else who asks that it's worthless.

I've done it. It's easy. It works.

macfrank · Post by **macfrank** » Fri Jun 10, 2005 4:33 pm

OK, I tried caoting the second set of Envirotex with Future - that made sense; the Future would harden and act as a barrier over the sticky resin. Well... that set is still tacky, and if anything, the Future made it worse.

I also baked the molds at 350F for 2 hours. After it cooled down, I cleaned it well with some acetone folled by water. I then dried it out. It couldn't get any cleaner. I poured in some Evirotex and got exactly the same result - a yellow, sticky casting.

Now I'm going to try the Alumilite clear resin. I couldn't find it anywhere in L.A. (at least within driving distance) so I ordered it from Hobby Engineering. They look like a cool web store.

Sparky · Post by **Sparky** » Fri Jun 10, 2005 10:51 pm

Sorry you had to find this out the trial method. We found this out when I tired to use the envirotex light to cast some parts (the PL TOS bridges) from Tony's molds. I have used it fine with smooth-on's oomoo to make early castings of the cargo shuttle bays They just take forever to cure.

And I think after 6 months the parts lost their tackyness.

BTW TCS sold me some of smooth-ons trial size crystal clear at wonderfest this year, so they can still get the small packs.

Shinnentai · Post by **Shinnentai** » Sat Jun 11, 2005 12:30 am

Why would I lie?

This was a recent R&D casting. The mold is one I made about a year and a half ago. It was originally a stylized relief lion's head for a Harry Potter themed project I was helping out with. It has since become a tesbed mold for trying out new methods & materials. In this case, I was trying to see what sort of finish would result from dusting the mold with Pearl-ex powder (650 Micropearl). Half the mold was dusted, and half was not, it turned out to be a very subtle effect, and is not readaly discernable in these pics.

This was one of the first molds I tried casting epoxy in, with much the same initial results MacFrank is currently having. Obviously this has been overcome though.

MacFrank: Two things- first is that two hours might not've been enough; I usually go for around six, although I admit I don't time it prescisely. Second is DON'T USE ACETONE! Acetone is the stuff used to dissolve uncured and partially cured epoxy for cleaning up drips & overaplications. If you're post cure worked, you effectively undid it by applying acetone to the rubber.

Don't let these jokers get you down. I keep hearing it can't be done, and yet I keep doing it. 8)

Sparky · Post by **Sparky** » Mon Jun 13, 2005 10:57 am

Here's alook at something we tried with envirotex, it's an oomoo mold, but before I had a vacuum pump to degas. So the neatest parts on the master came out with bubbles in the mold. . .

http://www.kc6sye.com/techmages_2_11_04.html

These casts were not <A HREF="http://www.kc6sye.com/techmages_4_24_04.html" target="_blank"> really thin</A> but they remained soft for many months.

macfrank · Post by **macfrank** » Mon Jun 13, 2005 1:16 pm

Sparky wrote:Here's alook at something we tried with envirotex, it's an oomoo mold, but before I had a vacuum pump to degauss..

Ah! so magnetic fields are a problem too?

Shinnentai - I believe you, it's just that I reached my "cost limit" (like the cops in THX1138) and I'm going to wait for the clear Alumilite before I destroy this mold. After that, I'll try the clear oomoo. I'll still use the envirotex, but only in the molds that I know will work without baking.

Sparky · Post by **Sparky** » Mon Jun 13, 2005 2:06 pm

Sure its, all black magic in the final analysis.

The point is that the envirotex lit worked fine in the oomoo molds. The cure time is, after-all, 48 hours for 'thin' pours over a table top like surface.

With smooth-on's crystal clear resin I even have cure issues with thick pieces. We just made a cockpit for a 1/32 scale mech and the main pieces are still tacky, 5 days post pour. While the smaller pieces from the same mold are completely cured.

jack wendt · Post by **jack wendt** » Mon Jun 13, 2005 9:37 pm

there is an easy solution, but i don't think anyone will like the answer.
what about vacuforming the piece in question?
by using plastic cups, you can vacuform clear, green, blue, and red.
for instance, the red buzzred collectors of any enterprise can be vacuformed with the correct color suitable for lighting.

macfrank · Post by **macfrank** » Mon Jun 13, 2005 10:16 pm

jack wendt wrote:there is an easy solution, but i don't think anyone will like the answer.
what about vacuforming the piece in question?
by using plastic cups, you can vacuform clear, green, blue, and red.
for instance, the red buzzred collectors of any enterprise can be vacuformed with the correct color suitable for lighting.

Jack, I thought about vacuforming, but these parts are pretty complex. It's unlikely that the clear plastic would reach into all the corners and if it did (because of the shape) the original part would be trapped!

I'm also not going to risk an original LE engine to the heat from softened plastic.

Sparky · Post by **Sparky** » Tue Jul 11, 2006 12:16 pm

Umi_Ryuzuki · Post by **Umi_Ryuzuki** » Tue Jul 11, 2006 1:23 pm

I have had similar problems with certain polyester resins in RTV.
One of the explanations from the supplier was that the resin was an "air inhibited cure". So from what I can gather from that, was that the RTV was allowing enough air throught the pores to inhibit the resin from curing.

Also, there was enough shrinkage in the polyester resin to create an air gap between the mold and the part. The result was a sticky cure with "strands" of resin landing on the piece as it shrank and pulled away from the mold. I have Never used polyester clear cast again....

macfrank · Post by **macfrank** » Fri Jun 29, 2007 11:56 am

Smooth-on Oomoo rubber won't inhibit Envirotex clear resin from curing!

I have pictures and a description this thread

The only disadvantage is that the Oomoo is stiffer than the Alumilite (Dupont) RTV I'd used before, but I'm not sure that this is a bad thing.

Frank

modelnutz · Post by **modelnutz** » Sat Jun 30, 2007 7:19 pm

Macfrank..FYI...both alcohol ( the stuff leaching out of your tin cure mold) and acetone are effective solvents for epoxy. Either of these materials in your mold will cause the issues you've been dealing with.
Also, epoxy is a heat set resin and silicone acts as a heat sink..robbing heat from the resin thereby causing cure issues.

I have a suggestion for you based on my clear casting experiences...
Pre-heat your molds ( 160 deg. for 1 hour )....mix the resin, pour the resin/epoxy, apply pressure ( if pressure casting { recommended )let cure the recommended time, then post cure at 160 degrees for 2 to 4 hours. I also found the best results were had when I used a platinum cure silicone.

Umi...you might have been getting tacky casts for the same reasons...tin cure just causes cure issues with polyester resins.
I did a zillion embedded items in a clear poly resin and they worked really well once I learned to use platinum cure silicone and post cure at 100 deg. in a light hot box. If your interested I can give you the supplier for the best of the resins I tried.

This process was essential when casting Crystal Clear resin from smooth-on. Unless the people at Alumalite say differently..this should work for you.

Mac..If you find that your still not getting the results...send me a clean set of castings and I'll mold them up in some plat cure and cast a few sets for you. ( I can even tint them red if you'd like....

)

Modelnutz

(Ed Holt)