casting resin

[mighalpern]

Hello:  just wondering if anyone had some advise on using the 2 part casting resins.  I am trying to make a few duplicate wall section for my kit bash building and am using a wall as a template.  i made an RTV mold and that came out great, all the detail I wanted, but this thin casting is coming out soft.  I have verified that the mix is 50/50 and on the second one warmed up the mold, but still very soft.  I have placed them in the oven for 10`15 minutes at about 200 degree to kick them to completion ( like instructions recommend) but no use.  any advise
??
thanks
Miguel

[Spades]

https://www.therailwire.net/forum/index.php?topic=37486.0

[peteski]

You forgot to mention which brand and type of resin you are using. 

In my experience (Alumilite and SmoothOn), urethane resins (especially in thin cross-section) will take a long time (like a day or two) to harden. The part might stay flexible for some time (even if heat cured during the initial setting).  The flexibility also depends on the hardness of the resin itself (that is usually mentioned in the technical specs).

How thin is the part? 0.030" or 0.300" for example?

[mighalpern]

yes its alumilite  brand and maybe .060" to .080" thick
I am going to try preheating the oven and warming the mold, pour the casting resin in and then slide it back into the oven to help kick it evenly.  The other two sections are still very pliable and I don't think heating it 1 hour after pour helped. 

[peteski]

yes its alumilite  brand and maybe .060" to .080" thick

The other two sections are still very pliable and I don't think heating it 1 hour after pour helped.

Those are still pliable after few days?  That sounds like you didn't get the ratio of A and B parts correctly.

[wazzou]

Those are still pliable after few days?  That sounds like you didn't get the ratio of A and B parts correctly.

Agreed.  The easiest way to do this is by using a scale that can measure in grams.

[mighalpern]

i used very small measuring cups, 5 mL each  but the part A resin I think might be spotty.  The bottle was crusted over when I opened it.  ( used about 6 months ago )
I have a small scale and will try that next.
thanks

[peteski]

Crusty?  That is not a good sign. It might be too old. The brown liquid also needs to be stirred or shaken right before pouring it out for mixing - it seems to separate.

[mighalpern]

I did shake and when I tried to pour nothing came out
so I squeezed the bottle and found that a complete disk had formed and dried on the surface> I cracked it some more and shook it it up, poured smooth, no chunks, but who knows if it was good.  Will buy a new set of bottles/ product and try again.  Maybe Bragdon casting resin from my LHS.
thanks
Miguel

[peteski]

Unfortunately all 2-part urethane resins do have limited shelf life. Humidity also greatly decreases their useful life.

This kills me - I usually cast very small parts and I end up throwing away about 90% of the resin unused, because it goes bad before I have a chance to use it. And I even use the gas blanket spray which purges the (moist) air out of the resin containers.

[daniel_leavitt2000]

Anything crusty will not work as intended. I have also found that if you have soft parts, changing the +-5% would help. Usually this is a tad more activator to resin, though I have needed to do the opposite.

It would be my guess that if some of the resin is no good (as evidenced by being crusty), then you have a higher activator to resin ratio than a 50/50 mix would indicate. Try remixing with 5% more resin to activator than you did last time and see what happens. Worst case is you use the rest of your bad resin anyway.

Maybe my resin thread should be listed as "Best of"?

[mighalpern]

update:
ok so the old casting resin is from 2014 so definitely past its prime, bought some new stuff at Micheals and brought out my mini scale. I did the measuring by the graduations on the cups and then weigh them.  i have the scale on grams and 2 digit accuracy, and they both weigh the same.,  anyway i preheated the mold with my heat gun and poured the mixture in and then added the heat gun over the mold.  Pretty cool to watch the process go.  I think the heat definitely helped with the thinness and providing the energy to continue the reaction at the far edges.  So far 4 casting all pretty stiff, and good enough to be able to cut up into the stripes I need for my scratch building





many thanks for all the replys

[peteski]

Good to hear Miguel!

[daniel_leavitt2000]

Is that concrete and brick structure scratch built? It looks fantastic.

[peteski]

Is that concrete and brick structure scratch built? It looks fantastic.

Looks Walther-ish to me (with some added cast-resin curtain walls). But if it is scratch-built then I'm even more impressed.