Rubber setting too fast - humidity?

OdysseySlipways · Post by **OdysseySlipways** » Tue Jul 22, 2008 10:19 am

I thought maybe it was the older batch of rubber being the cause of the rubber setting up too fast, but i started using a new tub today and i only did 5 simple, small molds, so pouring time was about 10 minutes if that.

but the rubber was turning into bubblegum before i was done and now it looks like th emolds i started may not completely level off and may be a loss.

I was getting a bit warm in the basement as my damp shirt was one of two clues. so i checked the temp down there. it's only 75, but it feels way warmer than that, so i am thinking humidity.

i have my one large dehumidifier runing 24-7 and am getting about 5 gallons a day from it (used to water the garden).

any other ideas? it's not the mixure as i know it's not (i always skimp just a hair on the catalyst).

I'll have one of the two new doors closing off the area from under the pourch up tomorrow sometime, so that will cut down on a lot of any heat produced from the concrete porch and coragated metal (concrete was poured over it - used as part of the form).

for now i am bringing the tub of rubber and catalyst upstairs to chill off a bit more, i hope that helps for right now.

and to think, in the winter i had to bring the tub up to keep it warm so it would cure propperly........

Joseph Osborn · Post by **Joseph Osborn** » Tue Jul 22, 2008 2:45 pm

Tin-cure? Humidity really affects it. Platinum, not so much. I have to do all my mixing and pouring in the house this time of year. Also, your "new" tub could've been sitting on somebody's shelf for a while before it got to you.

OdysseySlipways · Post by **OdysseySlipways** » Tue Jul 22, 2008 3:19 pm

not sure what mine is, it's Smooth-cast 321 rubber.

as to it sitting, i can't say i have had a problem from SC yet. i tey have a habit of being out of stock once in a while, so i don't think it's sitting onthe shelves that long.

USS Atlantis · Post by **USS Atlantis** » Tue Jul 22, 2008 4:32 pm

OdysseySlipways wrote:not sure what mine is, it's Smooth-cast 321 rubber.

as to it sitting, i can't say i have had a problem from SC yet. i tey have a habit of being out of stock once in a while, so i don't think it's sitting onthe shelves that long.

Uh Chris - SC-321 is a Resin, not a Rubber - want to check on that number and get back to us?

Smooth-On's normal rubbers are Oomoo and Mold Max for Tin-cure and Smooth-Sil for Platinum-cure - there are others, but those three are the main pouring-mold rubbers

BTW: I use Oomoo-30 for my molds, and haven't had an issue with the humidity aspect yet - but my basement usually stays cool and dry no matter what the weather is so that might be it

OdysseySlipways · Post by **OdysseySlipways** » Tue Jul 22, 2008 4:53 pm

Doh!

your right, I use Mold max 10

USS Atlantis · Post by **USS Atlantis** » Tue Jul 22, 2008 5:21 pm

Ok, Mold-Max is a 10:1 mix ratio

If you get too much of Part-B, it can have the tendency to set up faster - but knowing you, you're probably measuring to within an inch of it's life

Also, the 10:1 Tin-cures are more affected by humidity than the 1:1 Oomoo

Have you considered Oomoo, or have you already tried it and moved to Mold-Max?

Only other thing I can suggest is going to the Smooth-Sil Platinum-cures

910 and 920 are 1:1 mix, 930, 940 and 950 are 10:1

The Smooth-Sil line has the same hardness as the Mold-Max, but a higher tear strength (102 vs 90 for the SS-910 vs MM-10)

Price is comparable - 1qt of SS-910 (1pt each of A and B) is $28 vs $24 for 1 qt of MM-10

2 Gallons of SS-910 (1 gallon each of A and B) is $174 vs $182 for 2 gallons of MM-10

Something to think about - FWIW: When I run out of my Oomoo-30, I'll be getting SS-920 as a replacement

Joseph Osborn · Post by **Joseph Osborn** » Tue Jul 22, 2008 5:28 pm

It's been my experience that Mold Max 10/20/30 ages very rapidly after it's exposed to humid air. If your bucket is contaminated by humidity, I don't know of any way to rescue it. I wish I did, because right now I have about a half a quart that's only good for make very simple & quick molds.

Even though they cost more, I usually buy the "trial" sizes so that I know it'll get used before the rubber gets too old or too contaminated to mix. I'm lucky in that I have a supplier who's close enough to ship stuff at ground rates but it gets to me overnight.