Anyone else have trouble with Smooth-Cast Onyx curing too fast?

red4

Sr Member
I've done 3 tests using 30 milliliters each time (15ml of Part A, and 15ml of Part B). The first mix started to cure right in the cup after about 10 seconds of stirring. The second test was a lot more cooperative, but it didn't give me enough time to burp the mold. I usually shake and squeeze the mold with the resin inside in order to make bubbles escape through the pour spout and ventilation ducts, but the Onyx cured too fast for this. The third test was only slightly less terrible than the 1st; I filled the entire mold in time, but it gelled while I still had the cup tilted. When I demolded that test, the exterior of the resin was solid; but when I snapped off the sprues after a couple hours of curing, the newly exposed areas were soft like gum. They did eventually fully harden after a day.
 
I've never used the Onyx resin. I've always wanted to give it a try. Its hard to believe how fast it is setting. How long are you mixing it for?

To prolong the pot life, try chilling the two parts in the fridge before mixing it. This should help delay the reaction. But this may cause condensation which is bad for the resin.
I suggest trying a small batch first (don't refrigerate the whole bottles of resin. You don't want to ruin it all).
Let us know if it works.
 
I've never used the Onyx resin. I've always wanted to give it a try. Its hard to believe how fast it is setting. How long are you mixing it for?

To prolong the pot life, try chilling the two parts in the fridge before mixing it. This should help delay the reaction. But this may cause condensation which is bad for the resin.
I suggest trying a small batch first (don't refrigerate the whole bottles of resin. You don't want to ruin it all).
Let us know if it works.

I was mixing for about 20 seconds. I've now gotten in the habit of mixing it for less time. This results in marbling, which is mostly noticeable on smooth, flat areas.
 
You have to mix the containers a lot before you cast. Then pour out your portions, let them settle a little, and mix. I never cast over 70 degrees F and never more than 8oz total at a time unlese its a little on the chilly side. Also of you get the ratio wrong it can cure much faster or slower depending on the error. I also never mix less than a 2oz batch at a time.
 
I didn't pay attention when I bought it, but I've now learned that there is a Fast version and a Slow version, and I unknowingly bought the Fast one. It says so right on the package, which I didn't read carefully at first. I also tried chilling the separate parts in the refrigerator for 10 minutes. I'm not sure that helped anything. I couldn't tell any difference in the curing speed, or the resulting castings.
 
I have used the Smooth-on Onyx fast that you speak of, simply because it has a deflection temp of about 250 degrees. basically the temp it starts to get soft at after it has been cured. important for a model boat sitting in the back of a pickup truck on a hot summer day here in colorado. dashboard temps in the truck have reached 140 degrees easily. the slow onyx only has a 212 degree deflection temp. in comparison, the classic smooth-cast 305 has a deflection temp of only 120 degrees.

two things that i have noticed:

1) mix/stir the two separate parts thoroughly before pouring them into their individual measuring cups. the first batch of onyx fast i ever purchased needed to be mixed with a paint stick for about 20 minutes, there was a giant clump of solid material at the bottom of the container.

2) the combination of your texas heatwave (109!!) plus the humidity of 30 plus percent is what is killing you. the humidity more than the temp. remember that all this stuff is basically expecting temps of 68 degrees and humidity of about 10 percent. the closer you can get to that, the better off you will be. i squeek by because the humidity where i live almost never hits double digits, and my air-conditioned basement stays around 65 in the summer, and drops to about 60 in the winter.
 
Oh boy, I don't know what to do with this stuff anymore. I cast something, and it seemed to be fine. Then a few days later it expanded and cracked open like a corpse. The exterior was a solid, brittle shell, while the interior was hollow and gooey - but it was only hollow because it expanded. I could still press it back into its original shape, but doing that just made it crack more. I can't control or anticipate anything about this Onyx stuff anymore. This stuff is way more hassle than it's worth.
 
I had a pretty poor experience with it overall -brittle, prone to weeping, and even some foaming. Not that I didn't have some successes, but I wouldn't use it again.
 
I did fine with it as long as it was pressure cast otherwise it would blow up like a black sponge. But only with new batches, after aging a bit it would become very brittle.

I had the Specialty Resin on hand for 2.5 years before I ever opened it, and it's been working flawlessly.
 
Oh boy, I don't know what to do with this stuff anymore. I cast something, and it seemed to be fine. Then a few days later it expanded and cracked open like a corpse. The exterior was a solid, brittle shell, while the interior was hollow and gooey - but it was only hollow because it expanded. I could still press it back into its original shape, but doing that just made it crack more. I can't control or anticipate anything about this Onyx stuff anymore. This stuff is way more hassle than it's worth.

i have had that happen... poor mixing. this stuff is extremely finicky about proper ratio and good mixing. like i said earlier, the individual parts need to be mixed very well with a paint stick, there was tar like goo at the bottom of the bottle that needed to be mixed in the first time i opened the bottle.
 
i have had that happen... poor mixing. this stuff is extremely finicky about proper ratio and good mixing. like i said earlier, the individual parts need to be mixed very well with a paint stick, there was tar like goo at the bottom of the bottle that needed to be mixed in the first time i opened the bottle.

I believe you, and I've tried. But the non-pigmented part has such a horribly pungent smell that I can't stir it in a room with closed windows, and it is so hot in Texas that if I open a window, it will heat up the whole room. I have to put on a breathing mask, and mix this stuff in a walk-in freezer.
 
I believe you, and I've tried. But the non-pigmented part has such a horribly pungent smell that I can't stir it in a room with closed windows, and it is so hot in Texas that if I open a window, it will heat up the whole room. I have to put on a breathing mask, and mix this stuff in a walk-in freezer.

i believe you... i have had my wife yell at me when i sand or cut it as well... the odor that comes from cured material when i have to shape it drastically is horrible. the struggles we go through for our hobbies :)
 
This thread is more than 5 years old.

Your message may be considered spam for the following reasons:

  1. This thread hasn't been active in some time. A new post in this thread might not contribute constructively to this discussion after so long.
If you wish to reply despite these issues, check the box below before replying.
Be aware that malicious compliance may result in more severe penalties.
Back
Top