July 25, 2011

How2: Smooth-On's Mold Star 16 FAST Rubber

So this next How2 video tutorial is a guide to using Smooth-On's new rubber, Mold Star 16 FAST, a quick-set version of their Mold Star 15 SLOW rubber. With a 6-minute pot life (that is, 6 minutes of ideal working time after mixing) and boasting a 30-minute cure time, I jumped at the chance to try this stuff out. We're finishing up our Dragon*Con projects right now, and there isn't really anything else that needs to be molded, so I bought some really just to play around with. However! When I originally molded up the belt buckle to my soon-to-be-posted Monarch's Henchman project, the Rebound 25 rubber didn't cure right over a majority of the surface area. I could produce usable castings, but I had to work them a little with some glazing putty. Not a big deal, but it's giving me a good opportunity to remold the part as an applicable demonstration piece.

I had a feeling that it was more of the model's fault than the Rebound's when it came to the curing issue, so I scrubbed down the part with some Simple Green (a degreasing cleanser) and did a small material test to see if the part would be usable for the How2 video tutorial I would later make. This served two purposes: 1) obviously it allowed me to see if the part would prevent the new rubber from curing, and 2) it would allow me to get some hands-on experience with the new rubber so I could learn about it prior to filming the video. I'll just go ahead and say that scrubbing the part down did clean it up and the rubber did cure. It's not as exciting as it is enabling. LOL. Anyways, below are my initial thoughts on the rubber, and I'll be directly comparing it to Smooth-On's Rebound 25 silicone rubber.

Both Mold Star 16 and Rebound 25 are platinum cure silicone rubbers. So if you're used to using Rebound 25, you'll find that it is compatible with nearly all the same casting materials. Note: If you want to use Thi-Vex thickener with it, don't bother - it is NOT compatible with Mold Star 16. The term "platinum cure" refers to it being an addition type of curing process as opposed to a "tin cure" process, which is a condensation curing process. Note: If you are familiar with Smooth-On's Oomoo product, that is a tin cure rubber. The first thing I noticed as I opened the bottles is that this rubber is a lot less viscous that Rebound 25. The viscosity is stamped right there on the material data sheet, but having endured the agonizingly slow process of scooping and mixing Rebound 25 for so long, the viscosity of the Mold Star 16 is surprising and relieving. You would be better off pouring each component into your mixing cups with MS16. I would compare it to the consistency of warmed chocolate syrup, where as the consistency of Rebound 25 would best be compared to cold honey. As such, it's also a lot quicker to mix up - it has to be, considering the 6-minute pot life. Another plus to the low viscosity is that any trapped air bubbles are released to the surface of the rubber a lot faster and with a lot less effort.

Regarding the 6-minute pot life, I have to say that, while the 30-minute curing time is very tempting for use on larger models like helmets and armor, such a short working time really does limit it to smaller items like detail pieces, or smaller, smooth items that wouldn't require extensive brushing over. As such, I would really only recommend this rubber for block molds, so don't bother asking about any other applications because we're not going to recommend any (we don't want you wasting time, money, or rubber on our account!). Regarding the 30-minute pot life, in my material test I found that short curing time to be more of an estimate than a calculated time stamp. My test cured in approximately 35-40 minutes, though I admit I lost track of time when I found I needed to make dinner. So don't undercut your time here. Go at least 30 minutes, but get in the habit of shooting for 45 minutes just to be safe. Despite the added 50% curing time, that's still hours shorter than Rebound 25.

Tear resistance-wise, it's comparable to Rebound 25. I'm sure there are minute differences and the numbers are sure to be different on the material data sheet, but it felt like it'd tear at about the same elongation that Rebound would. It's certainly a softer shore hardness than Rebound, though, and you can cut chunks of it out with your fingernails a lot easier than you can with Rebound. So be careful when demolding the stuff if your model is complex. As a whole block of rubber, it's respectfully durable, but if you try tearing off a small chunk on purpose, you'll work at it a lot less than you would on Rebound.

Our How2 video tutorial shows the mixing process and application of Mold Star 16 FAST in a simple block mold. As stated before, the part being molded is a Monarch's Henchman belt buckle (if you're unfamiliar with the Monarch and his Henchman, go watch The Venture Bros.). At the time I've written this, I have yet to film the thing, so fair warning: it may be scripted. EDIT: It was not.

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