problematic resin casting

JadedMonk · Post by **JadedMonk** » Sat Jul 25, 2009 8:20 pm

Despite a bit of practice, I am having trouble my resin casts lately. The resin has a 30 min pot life. I'm pulling from a shallow open face mold and get numerous small bubbles forming close to the detail surface. Close to, mind you, but not actually breaking the detail. If I sand, at all, I expose too many pinholes to make the cast worth while. This batch of resin was purchased from MicroMark more than a year ago, but the seal wasn't broken until recently. The humidity has been pretty low, around 20%. Vacuum and pressure casting aren't options right now. I've been pouring from quite high in order to get the bubbles to break. I used a release agent on the first of the five pulls. I am going to try corn starch next. I haven't had any luck finding talc.

So, am I overlooking anything? Has my resin past it's shelf life?

Thanks in advance,

JM

en'til Zog · Post by **en'til Zog** » Sat Jul 25, 2009 8:42 pm

A friend of mine bought several sets of resin material from Micro-Mark, and each had problems. It may be that MM lets it's resin sit on the shelf too long to begin with - it had begun to separate even just after my friend bought it. Add a year on YOUR shelf...

I think MM uses Smooth-On, so I'd try and by directly from them - Smooth-On that is.

Joseph Osborn · Post by **Joseph Osborn** » Sat Jul 25, 2009 8:44 pm

You have plenty of pot life, so try this procedure instead of pouring from up high in a thin stream. Dust your mold with baby powder or cornstarch (I favor baby powder myself) and shake out the excess. Mix the two sides of your resin very slowly so as not to introduce a lot of air into the batch. Use a round stirrer, not something flat like a popsicle stick. Let the mixed resin stand for a few minutes to let any bubbles rise to the surface. Tap the cup a little if you want. From about 1" or less, pour a little bit of resin into the mold to coat the surface and then wait a little bit, then pour a little more and wait a little bit. Tap the side of the mold and pop any bubbles that come to the surface. Continue filling just a little at a time until the mold is full, then leave it to set. Pouring resin from a height is not like pouring rubber-- resin is much thinner and breaks into droplets that tend to create a foam where they smash into the mold. Your resin should still be good if you've just recently opened it and it has not been subjected to humid air for very long. I hope this helps, but you can get far more consistent results by using basic pressure casting.

Stu Pidasso · Post by **Stu Pidasso** » Sat Jul 25, 2009 8:53 pm

Joseph Osborn wrote:Dust your mold with baby powder or cornstarch (I favor baby powder myself) and shake out the excess.

Joe's right, it helps a lot. But make sure your baby powder is TALC, not the other stuff. Talc=good, other stuff=bad. I buy my talc from Wal-mart (under the name Equate baby powder) or Target, in the baby section. I get a big ol' 22oz. bottle for like 2 bucks. Usually the cheap store-brand is talc, the Johnson&Johnson is not. Hope this helps.

Where are you located? If you're in the US, PM me, maybe I can help.

JadedMonk · Post by **JadedMonk** » Sat Jul 25, 2009 9:20 pm

Excellent advice guys, thanks! I'll try Joe's procedure next and check my nearest walgreens (or target) for economy brand baby powder.

Pressure and vacuum casting are on my wish list.

Stu Pidasso · Post by **Stu Pidasso** » Sat Jul 25, 2009 9:51 pm

Good luck! By the way, got pics of the affected molds/parts?

JadedMonk · Post by **JadedMonk** » Sun Jul 26, 2009 4:55 pm

http://www.cognitivedissonant.com/graph ... CN0357.JPG[/img]

I've got to learn to allow for more space around the walls too.

The slow pour worked very well. I tried dusting the mold with cornstarch, but may have removed too much. Sadly, I think my resin may be past it's prime as it doesn't cure to the solid mass I'm used to, but more a semi-dense foam. Time to back to Smooth-On, I guess.

modelnutz · Post by **modelnutz** » Sun Jul 26, 2009 5:57 pm

You can always add more materials to the side walls.
Place your mold on a flat panel ( perhaps with a few spots of super glue to keep it down ) and build new walls around the mold.
( Hot melt and foam board work very well here )
Keep a space ( 1/4 to 1/2 inch ) between the mold walls and the new mold box.
pour more silicone into the space and let cure.

Note...if there is any chance that mold release could be present on the joining surface...wash first with Iso-Alcohol to remove it.

Ta-Da ! Thicker side walls.

Silicones are real good about sticking to themselves.

PS..cornstarch will not do the same trick as Talc....you really need to get some baby powder with talc listed in it's ingrediants.

Also, the foaming may have been caused by traces of moisture in the cornstarch....do a small mix in a dixie cup to test the resin...if it cures OK...then it's the C-Starch

TazMan2000 · Post by **TazMan2000** » Mon Jul 27, 2009 12:50 am

I also have heard if you microwave your mold for a minute or two (depending on the power of your microwave) then dust it with baby powder seems to improve the castings. Remember to blow out excess powder.

If that doesn't work, I would try another resin such as Alumilite.

modelnutz · Post by **modelnutz** » Mon Jul 27, 2009 9:31 am

Yup... heating the mold drives away surface moisture...thereby reducing/eliminating foaming as well as reducing the viscosity ( improving the "flow-ability ) of the resin a bit.

it also aids in the curing of the urethane.

All good things fer sure.

chasd25 · Post by **chasd25** » Fri Aug 14, 2009 6:40 pm

TazMan2000 wrote:I also have heard if you microwave your mold for a minute or two (depending on the power of your microwave) then dust it with baby powder seems to improve the castings. Remember to blow out excess powder.

If that doesn't work, I would try another resin such as Alumilite.

I don't know about the microwave for heating it up, we put ours in a fairly large toaster oven to heat them up.

I try and steer clear of the Micro Mart resins. There are much better, and affordable brands out there.

Smooth on makes some excellant starter packages, and they're resins are top notch. Hobby Silicone house brand is good stuff, BJBm Vagabond, Polytek, Douglas and Strugess, Reynolds, all make excellant urethane resins.

I tend to stick clear of the Alumilite. Never liked it. It stinks, and seems a bit hard on the molds (mold life wise), in my experience, there's better brands out there.