Casting problems

I have been having some problems casting some resin parts.
Been casting tires (with part of wheel rim) for a 1/8 scale old time
race car. What a learning experience but also some problems. Casting
the tires in halves, which will clamp around wheel. Tires are over
four inches in diameter, and each half width is about a quarter inch
thick. The RTV mold is 6 x 6 x 3/4 inches. This is the largest
casting mold I have ever worked with.
The first two pours, a day apart, yielded good tires. The third pour,
the resin never hardened, and I ruined the part taking the mold
apart. The fourth pour is still setting up VERY slowly- it has been a
day now, and the resin IS setting up, but it sure is slow. What is
going on here? The first two pours hardened in about 15-20 minutes,
like they say.
The resin was only opened five days ago. I know it has a shelf life,
and absorbs moisture, but this is rediculous. It has been dry here
the whole time, and high pressure. This resin is the regular formula
MicroMark sells, part A & B, 50:50 mix.
Don
Reply to
Don Stauffer in Minneapolis
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"Don Stauffer in Minneapolis" wrote
- Make sure you are mixing properly. Some are by weight, some by volume. I've also experimented by mixing 10-20cc samples of proportions varying slightly to either side of what's recommended to calibrate a good mix with the measuring devices at hand.
- When you get a good mix pour talcum powder into the molds first, slosh it around and shake out the loose stuff, then blow out the molds. This aids filling small details incredibly.
KL
Reply to
Kurt Laughlin
Both temperature and humidity affect setting time. However, it sounds like your problem is in the ratio of catalyst to resin. You might try varying the ratio by adding a little more catalyst to the resin at about 60:40 and see what happens. If you're still having problems, up the ratio a little more. It might not hurt to mix just a small amout and pour it into a small disposable plastic cup (like the ones that McDonalds catsup and hot mustard come in) to make sure the ratio is correct. Also, make sure to mix the two parts completely as, incomplete mixing will also affect the way it sets up.
Hope this helps and good luck with the project.
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Reply to
Bill Woodier
I've never tried the talcum powder trick before, Kurt. I'll have th give that one a try; thanks for the tip.
-- -- -- -- -- "We sleep safe in our beds because rough men stand ready in the night to visit violence on those who would do us harm." George Orwell
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Reply to
Bill Woodier
"Bill Woodier" wrote
I was amazed at the results myself. And, just blow (with your mouth) lightly to remove lumps - you want to leave a dusting over everything.
KL
Reply to
Kurt Laughlin
Sometimes you have to mix what's in bottle A with itself then what's in bottle B with itself before attempting to mix the two parts together.
Reply to
Ron
The trick for me is finding real talcum powder, unscented as that "baby fresh" stench gags me.
Reply to
Ron
"Ron" wrote
Corn starch works too. Buy the white J&J bottle, not the pink or blue.
KL
Reply to
Kurt Laughlin
Thanx for the tip !
I dont or ever have made a mould that big tho...
Reply to
AM
"AM" wrote
The dusting idea is mold-size independent. The smaller the detail being replicated, the better it is.
KL
Reply to
Kurt Laughlin
That indeed seems to have been the problem. After several of you mentioned this, I tried a test mix last night. I had only been rocking the bottles once. The instructions said rock bottle several times, so I really rocked them a number of times. I didn't mix enough to cast the tires, but the mix seems to have set up in a few minutes like it is supposed to, in the cup, and looks fine. I still need to clean the old resin out of mold, and will do a new pour today.
> Bill Woodier wrote: > > > > Both temperature and humidity affect setting time. However, it sounds like > > your problem is in the ratio of catalyst to resin. You might try varying the > > ratio by adding a little more catalyst to the resin at about 60:40 and see what > > happens. If you're still having problems, up the ratio a little more. It > > might not hurt to mix just a small amout and pour it into a small disposable > > plastic cup (like the ones that McDonalds catsup and hot mustard come in) to > > make sure the ratio is correct. Also, make sure to mix the two parts > > completely as, incomplete mixing will also affect the way it sets up. > > Sometimes you have to mix what's in bottle A with itself then what's in > bottle B with itself before attempting to mix the two parts together.
Reply to
Don Stauffer in Minneapolis
Kurt, ever try using micro balloons for your micro-matrix instead of talcum (or cornstarch, which is organic and suspect...)? Regards, Robert Beach
Reply to
Robert Beach
It's not a matrix, the trick to talcum or cornstarch is that they will suck the resin into the tighter areas of the mold. Has something to do with their hygroscopic qualities.
Robert Beach wrote:
Reply to
Ron

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