It has been a couple of months since I've created any molds, and this morning, I mixed up a batch of HS3 to pour over my masters. It the past, it has always poured very thinly and leveled very nicely. However, today it started to cure extremely fast. It started getting sticky and clumpy even while I was mixing it in the mixing cup - nothing like when I last used it. I mixed it in my basement, with temps in the 70's and normal feeling humidity. It's been about two hours since I poured it into the molds and looks to be completely cured. I haven't pulled out the masters, but I have low hopes for a successful mold.
Any ideas what might have caused this and what I can do to avoid this in the future? Could tiny amounts of water in the mixing cup cause it cure so quickly? I opened the container about 9 months ago and has been stored in dry basement. Is there anything else that might have caused it the problem?
Questions.......hopefully get this issue resolved...
Just confirming that you have had successful mold pours from this batch aprox. 2 months ago and that the same container was first opened 9 months ago...correct? How long did you mix it for before you noticed the sudden change in consistency Did you mix by volume or by weight What was the temperature of the masters...room temp. in the 70's, Hot, Cold etc. etc. What was the condition of the mixing cups....wet,dry ....if wet was it water or some other liquid.
I've messed around with adding a couple grams extra catalyst to speed the curing process up, but nothing to this extent as far as cure time goes. As far as water goes, it would slow down the overall curing process if anything, but not the mix time as you would still have that short working time to deal with. I'm curious to hear how the mold did turn out....if you can ,post some pictures, they may offer up some more clues as to what happened.
Brian, Thanks for the questions. I opened the package in November. I made a few small molds at that time. The last time mold I made was in January.
Everything was done in my basement. Air temp was mid-low 70's. Don't know about humidity, but we run a dehumidifier and it "felt" normal. Masters were ambient temp.
For this one, I estimated that I needed approx 200ml so I measured 200ml of tap water, transferred it to my mixing cup, made a mark at the height so I would know how much rubber to pour into the cup. Then I emptied the mixing cup and used a paper towel to dry the mixing cup. It is possible there were a couple drops still in the cup. I poured the rubber into the cup to my mark, on my digital scale. At this time, I got interupted for about 10 minutes and let it sit on the workbench (I had not added the catalyst yet). I then added the catalyst mixing 10:1 by weight and stirred.
I had done this a number of times already and felt pretty compfortible with the process and assumed I had a bunch of time. I stirred slowly using a wooden stick and at approx the two minute mark it started to feel very "thick". I began pouring into my mold and it started somewhat thick, and did not flow as was the case previously. By the time I was finished, I had to scoop it out of my mixing cup and put in the mold. It was the consistancy of sillyputty. I thought maybe it would self level over time. As evident in the first photo, it did not level. I put everything on a flat surface and placed them in the garage (as there is a discernable smell). This was probably 10-15 mins after initial mixing and the residual rubber in the mixing cup had already set. Outside temp was in the upper 80's today and quite humid.
I checked everything approx an hour after and the rubber seemed pretty solid. I waited until tonight (10 hours after pouring) and demolded. It seems to have set okay. However the rubber did not flow around all of the details and, there are large voids around and between the masters. But, frankly, it is much better than I expected. I'll have to create some resin castings to see how good it captured the detail of each individual piece. See photo 2.
I still have a small amount remaining in the container, and plan to put forms around the molds and pour more rubber to try to fill the voids. I figure I have nothing to loose at this point. I'll provide an update on what happens.
....This one has me perplexed..........let us know if you come up with anything, particularly if you have the same issue with mixing a smaller batch for in filling the voids. I'll have to do some research on this end and if nothing else, recreate what you did to find out the cause since we know what the effect is.
I mixed up another small batch and tried to pour the silicon to fill the small voids in the fist mold. This time, I ensured to use an absolutely dry mixing cup. I mixed the silicon for about 2-3 minutes until it was fully mixed and then began to pour into the small voids. All seemed to be okay until about the 5 minute mark where the silicon mixture began to become thick and stringy consistancy. I had very mixed results filling the voids.
Since then I have poured some resin to test the molds, and there are a number of places where the silicon did no pick up the details of the originals or there are large voids around the masters. All-in-all, I think this was a failure. I will have the purchase some new silicone and create new molds.