Resin Casting - RTV Molds

I don't always need to get hit in the head with a rock, but sometimes it helps!

I am building a 1/50 Gokstad Longship, and trying to make the rope thingie which acts like a deadeye on the front stay.  Every silicon mold I made (3) would leave small air pockets in the tight areas behind the cleat.  After our discussion here, I poured just enough material to cover the detail.  I then worked out the air bubbles with an Xacto #11 blade.  There were 2 bubbles which were literally jammed because of the small size.  After removing all of the bubbles from the detail itself, I then finished pouring the rest of the silicon letting material flow around and over the initial material.  The finished mold is perfect.  Once again, "Real old dog, new trick!"

Thank you all for helping me through this and I hope that this discussion will help some other resin casting 'newbie' like me.

Big Wick wrote:

Ed Have you used modeling clay for molds?  I have both the non-drying and 'sculpty', any thoughts?

You need to be careful with clays.   Some modeling clays contain sulfur which will cause cure inhibition.    I don't think the plasticine never-hardening clays are too good.  Test & see what happens.   I made a master of some Sculpey, baked it, the used that as a part master with no problems.   I also have some Clean Clay product which is recommended as a molding material

Ed

Have you used modeling clay for molds?  I have both the non-drying and 'sculpty', any thoughts?

Big Wick wrote:

Ed The lower the number (cps) the thinner? ...or is it the other way round?

Yes, the lower the number the less viscous (thinner). 

Hey, a little science never hurt anybody .... http://www.vp-scientific.com/Viscosity_Tables.htm

Ed

The lower the number (cps) the thinner? ...or is it the other way round?

Ed

I am new to this resin casting business.  I'll give it a try, nothing ventured, nothing gained!  Thanks for your help.

What is the basic viscosity of the MicroMark rubber without the thinner?  What is the viscosity when thinned?  

You will find that most Tin Cure and Platinum Cure silicone RTVs are 20K CPS and up.   For reference, water is 1 CPS.   Smooth-On's OOMOO products are ~4200 CPS and are excellent rubbers for the model maker.  They also look to be less expensive than the stuff at MicroMark.

Ed

Micro-Mark shows a RTV thinner in their catalog.  It states that you can use up to 15% for making molds for thin castings.

Big Wick wrote:

Has anyone used a RTV thinner to eliminate air bubbles in very small molds?

RTV Thinner?  Not aware of any such animal.   There isn't a product identified as such on the Smooth-On website  http://www.smooth-on.com/default.htm.  Smooth-On is one of the leading manufacturers of RTV & resin. 

One of the recommended process is to vacuum the RTV after mixing/before pouring.  Then pour the rubber from a little extra height so that it forms a long string & entrains less air.   Pour into the lowest part of the mold and allow it to flow up & displace air as it rises.  

I don't have a vacuum pump but I do have a compressor for the nail gun.   I have also had some success in putting the RTV under pressure (30 to 50 pounds) to squeeze down the bubbles to minimum size.