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Problem with casting in resin


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G'day.   I bought the Micromark resin casting kit to cast some seats.  I was able to make a master using the rubber latex material,  however,  after the resin casting "cured,"  it came out a powdery white,  and crumbled.  Needless to say the castings weren't usable.  Any guidance on this?  I'm assuming the Parts A and B  were not poured out in specific equal parts. Thanks!

Joe Vattilana

 

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Don't assume the mixing ratio!  Check the box or instructions to verify it.  If you have the mixing ratio correct, you may need to shake both A&B to thoroughly mix them.  Let A&B sit 30 mins, then cast again.  Make certain to mix by WEIGHT and allow for the weight of the mixing cup!  When all else fails, read the directions for use. 😁

 

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IIRC, Micromark's RTV and casting resin are made by/relabeled Smooth-On products.   Most Smooth-On's product mixing ratios are 1:1 (+/- 10%) by volume.    The casting seminars I've attended at the local Smooth-On distributor never have used scales to measure their products.

The most common problem I've seen is where the surface never completely hardens.    This was with older bottles.  The admixtures were not clear.  If one of the two-part mixes is grossly contaminated,  is very cloudy, it may have absorbed atmospheric water/humidity.   The exothermic reaction when the 2 parts are mixed will create water vapor bubbles causing the part to be very porous, not what I'd call crumbly.   I may have seen Alumilite go crumby -- long ago - with a contaminated mix (and IIRC a 10:1 by weight mix).  

Confirm your mix ratios.  Rusty's mixing methods are sound.   If you're making a small batch, go larger.  There will be waste but the mix precision is better

Is your product new?   Is there a shelf date on the label?    If expired, contact Micromark for a replacement.

I deal with the local Smooth-On distributor over-the-counter.     They have experts who can troubleshoot problems and make recommendations.    Check Smooth-On.com for the list of distributors and their locations.    Their website also has good tips and info.

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I got a set from Alumilite that uses about a 10:1 ratio and it kicks (starts to cure) way too fast to mix it properly. Later got the 50/50 ratio stuff under a different brand. It kicks after about 3-4 minutes and has been much easier to use.

 

Edited by VonL
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Thanks for the responses.  I just bought the "package" about a week ago, so it's fresh.  I re-read the directions (surprise there- never did that building models!),  and it's a 1 to 1 mixing ratio by volume.   The only things I can think of is my process- maybe not stirring it completely, etc.   

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When I first started Flagship Models, I made the mistake of not giving both A&B a good shake and allow 30 mins for the air bubbles to settle out.  The castings didn't set properly and had to be discarded.  Since you checked your mix ratio, I would just about guess you didn't give A&B a good shake.

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On 12/21/2019 at 9:29 AM, Rusty White said:

... giving both A&B a good shake and allow 30 mins for the air bubbles to settle out.

QUESTION for Rusty: How do you eliminate/minimize bubbles from the mixing & curing phase? Vibration? Pressure pot? Temperature trick? I've usually taken a fine wire and poked around the mix as it starts to cure and that seems to get most, but not all of them.

???

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With RTV (room temperature vulcanized) rubber for molds, I use a vacuum chamber to degas (remove the excess air) from the rubber before pouring the mold.  Be sure to spray the mold box (or mold for the resin) with a release agent to make removing the mold from the master pattern easier.

Resin is done in a similar manner except I pour the resin into the molds before degassing.  This technique allows the resin to migrate into the smallest details of the mold.

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5 hours ago, Rusty White said:

With RTV (room temperature vulcanized) rubber for molds...

Thank you, sir. I will meditate upon this. Still debating the vac-chamber. I see a couple on Amazon for under 100 bux. Should that do it?

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  • 7 months later...

I did an article for the Journal, volume 18 #2, pages 29-32.

It's how to make a useable and CHEAP vacuum chamber. 

I can send any IPMS member a copy of this article, a part of my job as Historian.

Just email me at IPMS-Q@ipmsusa.org.  Include your IPMS membership number, please, as this service is for members only.

Jim

Edited by jimpearsall
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