Flash over?

[td]

In several posts I've read, people have reffered to when the PR flashes or when it starts to flash over. I'm wondering what this is reffering to..... Is it when the PR starts to solidify or at the height of the exothermic reaction when the mold is almost to hot to pick up?

I am using silicone molds and a chat with the experts at ETI suggested upping the catalyst by 25 to 50% to get rid of the "extreme tackiness". Although it helped considerably with the tackiness it does set up much quicker and then later comes the heat.

Any thoughts?

Thanks,

Terry Hefner
Dallas, TX

td is me!

 

[jwalt191]

I think the flash over is referring to when the resin starts going from a liquid to a gel state. Upping the MEKP would help the tackiness, but
i have found that it makes the cured resin awflly brittle. I have even added more than the recomendations, ad wound up with a block of cured resin with cracks running through it. Just my $.02 worth.

 

[its_virgil]

The cracks and fissures are from the heat buildup. More MEKP means more heat and the only place for the heat to eacape is through the exposed surface of the casting. For thick pours you can pour in layers and not be able to tell were the layers are. Also, the thicker the cast the less MEKP that is needed because of the heat buildup.

For my snake skin blanks I use 3 drops per ounce and have excellent results using the molds made by gadget. When I was casting in blocks I had the cracks and fissures show up from time to time. And I do think that more heat caused the finished casting to be brittle. Some disagree.

The surface exposed to air will be tacky or even have a very thin layer of unhardened resin but it will dry in a short time. I set the casting out in the sun and it dries and hardens very quickly.

Do a good turn daily!
Don

 

[cozee]

I agree with Don about heat causing brittleness. I have gone through a more PR than I cared to waste simply testing resin to catalyst ratios and resulting temperatures for different molds I use. Large ratios of catalyst can produce extreme heat and every pour that experienced this excessive heat was very brittle when turned. Some had even gotten so hot they effected the colors of the pigments. Decreasingly smaller mix ratios do produce less brittle castings. Also, the colorants used will also effect these ratios. Some colorants we've used in the dungeon while "experimenting", even with high mix ratios of catalyst, still yielded rubbery blanks!

Another way to speed up set up time with out adding extra catalyst is to pre-warm the resin and catalyst before mixing, especially if the ambient air temps are below 70 degrees. I do this in my curing/drying box also.

Also, another factor in curing of your pours is the age of the resins. The older they get (after they've been opened) they can cause problems with curing.



(curing/drying box = old refrigerator with controlled venting and heat.)