casting cracked why?

[tommyd]

I'm new to casting. just poured a casting and it cracked. this is what I did poured three layers about 1/4" thick let each layer jell be for pouring another. mixed 5drops of hardener to 1oz of resin. set out on table temp was about 85deg at the time. after third layer was jelled it cracked. any itdes?

 

[Glass Scratcher]

Too much internal heat.

use less MEKp with each additional layer.

 

[stevers]

I agree, too much heat.
Don't understand why you were pouring in layers. Silmar 41 can be poured as thick as desired.

 

[Blind_Squirrel]

I agree, too much heat.
Don't understand why you were pouring in layers. Silmar 41 can be poured as thick as desired.

Ditto.

 

[jason_r]

I generally agree heat can be a problem, but wonder if it would be a problem
at 3/4" total. But cutting the MEKP is certainly a good start.
I just cast a 4" diameter x 3.5" cast in 2 swirl pours with no problems. (in AZ, so it
was about 95F when I poured). But then I cut the MEKP to 2 drops/oz and
it still gelled in 20 min.

What I suspect happened/contributed here is that casting in layers was the problem.
Your lower layer may have setup and started shrinking, then you poured the upper layer on it.
The upper layer chemically bonded to the already-shrunken lower layer, but then as it cured the
top layer shrank- until the whole thing cracked to relieve the stress.

So I'd:
1) Not cast in layers unless needed. All the same color- do 1 pour.
2) Use less MEKP
3) If you cast in layers, pour the 2nd layer as soon as possible so the layers
are as close as possible in shrinkage.

 

[Skye]

So I'd:
1) Not cast in layers unless needed. All the same color- do 1 pour.
2) Use less MEKP
3) If you cast in layers, pour the 2nd layer as soon as possible so the layers
are as close as possible in shrinkage.

All good advice, but really there's no need to cast in layers at all, so most of that good advice is probably moot.

Just pour one layer, ignore the directions about thicknesses on the can. I go for anywhere from 4 to 8 drops per ounce and have never had cracks like that.

 

[Russianwolf]

I do 3/4"+ x 8 x 8 castings with 7+ drops per ounce and have not had any problems at all.

I do them in a single pour. In fact I'll be doing a solid white cast tonight when I get home so that I can get a fountain pen out the door this weekend.

The only reason for doing layered castings is if you are embedding something in the resin.

 

[tommyd]

thanks for all the info. I will try pouring all at once next time. I thought putting blanks in the resin was like embedding things.

 

[Darley]

IMHO to much MEKp 3 drop per 1oz you generated too much heat and like other say pour in 1 lot

 

[Russianwolf]

thanks for all the info. I will try pouring all at once next time. I thought putting blanks in the resin was like embedding things.

Not really, you have the little blocks to support the tube, so they can't sink to the bottom.

The layer is if you want something to appear unsupported and completely surounded by resin. Yoyo Spin does that type quite a bit for his "do-hickies" I think he calls them.