Mostly newbie needs help casting crystal clear 202 plz!

[pbell0000]

Hi,

I got myself deep into a project, deeep! I have an engineering background, done some molding. Decided last minute to make a mold of a complicated glass top used on old vintage gas pumps. We happened to have an original for just a few days and went for it, so we could make replicas in urethane. Made two molds from Mold Star 30 silicone, and then after much playing made some good parts using Crystal Clear 202 urethane doped with their white colorant to make it look milk glass (smooth-on products). So now I'm happily taking orders for product, everybody is excited I can make these, but have had some problems pop up trying to make these in volume.

I shipped out one part to see how it would handle shipping, FAIL! It got their all distorted. Apparently it got hot in shipping or wasnt cured all the way or both. It seemed rock hard when I sent it! I was post curing using light bulbs, not accurately really. So now my questions!:

1. I keep getting eyelashes, small hairs in the parts, bits of dirt. I started wearing gloves, being careful not to hover over the mold post cleaning, but still happens. Ruins a good part, any ideas on preventing contamination post cleaning the mold. Would using mold release help? I've been avoiding using it. Im doing this in a paint both, trying to keep it clean, but it keeps happening.

2. Post curing, ok, so the top I sent probably needed more post curing. I built a hot box to bring it to temp, 160 degrees. The spec sheet for material says, cure time is 90 minutes, but let sit for 8 hours for some reason before post curing, then I'm supposed to post cure for 6 hours in the mold, and then let it sit few hours to cool. I was wondering why wait so long to post cure, can't I just wait until the part is cold (about two hours) and then jump into post curing? Can I shorten the post cure a little too? Having the molds tied up is really killing my ability to get some volume out here. I don't find out if there is a problem until the end of this whole cycle and that really sucks!

3. The spec sheet mentions post curing improves the heat durability of the part. Will it? how much? I am worried about heat since someone may light this with a normal hot bulb and also hot shipping temps.

Anybody know some tricks with crystal clear 202. It molds great just contamination and this curing problems.

Thanks!

[mangozac]

Welcome to RA, Paul!

It sounds like you're already a pretty advanced caster, so not having had much experience with clear resins and post curing I can't give a lot of advice. Here are my comments:

1. This is the first time I've heard of this problem! I can't see why using a mould release would help with this kind of issue, however it should increase the lifespan of the moulds. Perhaps try blasting the mould with some compressed air just prior to pouring the resin?

2. I've never heard of leaving for such a long time before post curing. Sorry but I have no idea why they recommend that! You'd be best to talk to the supplier.

3. Post curing will make the part harder, so one would assume it improves the heat stability. I'd be surprised if it warps that much during shipping (parcels shouldn't get that hot), but putting an incandescent bulb in it might be pushing things...

[pbell0000]

Thanks Magnovac!

I am doing the blowing out the molds with compressed air already, seems like the molds are getting cleaner but still an issue. Keep trying....

As far as the long wait until post-curing, I called the Smooth-On technical rep a few days ago, asking them why? Why are you making me wait so long according to your spec sheets before post curing? All she could do was recite what was in the spec sheet, "you need to wait 8 hours before post curing as stated in the technical bulletin", and then she said "your application may vary". She repeated this multiple times as I probed it deeper. I think I was talking to a human robot! There must be someone at Smooth-on that understands this, Ill try again! I am happy to report post curing seems to really help make these parts rock hard, even when there hot! Thank god, I was getting worried I invested all this time and $$$ and was going to loose it all.

[pbell0000]

So I've been working on these parts last few weeks and ill post here what I've learned, might be helpful for someone else:

I'm post curing crystal clear 202 for 4 hours or more at 140 - 160 F. I'm waiting for the material to cool down from the initial curing but not necessarily waiting 8 hours, seems like its working fine. Seems to be harder and more resilient to heat. This isn't strictly following the specs but like the tech rep said repeatedly "my application may vary". When the material gets too hot it'll make huge blisters like dime size.

Now the eyelash problem, hairs, was driving me crazy. Ended up having a "no place you hands in the mold without gloves" policy. I went as far as wearing long sleeves and taping my sleeves to my gloves. That in conjunction with more thorough cleaning helped a lot. We were seeing hair in like every other shot but not any more.

One thing that's really helping too is that ive learned I can sand and polish these parts, also glass bead blasting is fine too. I can sand out dirt or hair without too much hassle, using 220 / 600 /1500 grits and either touch with a small buffer or hit lightly with glass bead and you can't tell I worked on it.

[mangozac]

Thanks for the update! Glad to hear you've having success with this one!

I'm still surprised that you've having issues with contaminants! Maybe you moult eyelashes at a higher rate than usual

[Fleafa]

Great to hear back about this!
Contaminants aren't something many of us need worry about as we rarely use clear resin. Smaller parts also mean we're less likely to introduce hairs into the mould/mix.
I remember my casts with Smooth-On's clear PU being very flexible for long after casting. This wasn't a big problem for my application, luckily.
I think you've taken the best approach, which is to start with manufacturer's guidelines and then adjust them based on your experiences. Like all tolerance-based guidelines, the manufacturer will err on the safe side as they need their product to do what it says on the tin when those guidelines are adhered to. Ambient temperature and humidity, along with final purpose, will always make the product behave slightly differently for you than others.
Glad you seem to have found the sweet spot for your project.

[pbell0000]

Thanks mangovac and fleafa, maybe I do shed eyelashes at a higher rate! I had no idea how much contaminate is just floating around until started this project! Seems being vigilant about cleaning, blowing off parts and wearing gloves helps a lot. Since my part lights up, all contaminates show unfortunately.

I continue to keep learning. Occasionally we would get trapped bubbles that were big enough to ruin the part. Learned to tilt the mold about 15 degrees while pouring while knocking on the mold occasionally, then when it's near full put it back to flat, seems to avoid that trapped air. Also using small disposable syringes to measure the white colorant we add to be more precise and repeatable, that way each top has the same milk glass look.

Also dawned on me, that since I can sand and buff these cast parts now, I could make a perfect top and make a mold from that. Better than replicating the somewhat worn original glass top. So I sanded and buffed a cast urethane part to near perfection, took a long time, and made new molds from that. If I had to do things over, I would have made a cheap mold off the original glass, then sanded, buffed the first parts, then made new durable molds from those first cast and buffed parts. This must be commonly done, I just didnt really think it through since I was rushed. Well anyway, I have really nice molds and a pretty good procedure now, hope more orders keep coming in... I have to sell off my current inventory before I announce these new molds.

Hopefully, posting this stuff will help someone else not go through the trial and error process I've done. Thanks for the help.

[pbell0000]

Fleafa,

Almost forgot, you are definitely correct about the urethane staying flexible for a long time, weeks it seems. Until we started post curing it was really a problem for our application!

[Fleafa]

Yeah, a long time after casting! Glad you're having more luck with post-curing.