Alumilite RTV silicone shrinking in mold?

[Erik Smith]

I am making a two part mold of a wheel. First attempt taught me what I should do right on the second. I had all the parts laid out fine, tight, smooth seal with clay along mold line. Poured Alumilite quick set RTV silicone on. Placed in pressure pot. When I removed he clay, the mold silicone looks like it shrank and pulled away from the part;

That trough in the mold was not there in the clay. Here is another spot:

Under the metal I used for a pour spout, the silicone is a good 1/8 inch pulled down ( or up, since this picture is the bottom).

Any ideas on why/how this happened? Was this from the pressure pot (40psi)? Bad silicone? Bad mix ratio? Silicone was just purchased new.

I have been disappointed in Alumilite's resin (brown colored stuff - way too thick) is their silicone good stuff?

Edited January 30, 2012 by Erik Smith

[Casey]

Did you de-gass the mold rubber (looks like Alumilite's Quick set, tan (I say it's yellow) in color) before pouring it into the mold and placing in the pressure chamber?

I wonder if the metal you used had some type of adverse reaction to the mold rubber, too. What did you use to clean the metal piece with? I recall reading an article about how to intentionally shrink rubber molds using lacquer thinner or MEK mixed into the rubber, so is it possible something was absorbed by the metal and leached out after the rubber was poured around it? It's hard to tell, but was the only shrinkage which occurred around the metal piece?

I have never had any shrinkage issues using Quick Set, but I usually use it with one-part molds and without pressure or vacuum. If the mix ratio is off, the rubber will harden very quickly or very slowly, but I've never had that happen with QS, as it normally sets in 12 hours or so.

[RyanSilva]

You don't need to degas silicone if you are putting it in a pressure pot. Did you pull the mold out of the pot before the cure time? Most times when you pull a mold out of a pressure pot early you can get some weird pulling off the master/part, mostly due to the pressure changes. Never used alumilite silicones though, Used almost all of smooth-on's line however.

[Erik Smith]

Casey - no, did not degas as I put in a pressure pot. Yes, quick set. I used the metal on the first run and the mold turned out fine - it was my error in placement of vents and sloppy work on the small spokes that caused the first failure - the silicone worked fine BUT was from an older bottle. Shrinkage occurred around the metal on one side and to the right (in first photo) of the piece - on the opposite side of the metal.

Ryan - I left the mold in the pot for 12 hours.

Thanks for the replies guys.

I have "reclayed" the part and poured another batch of silicone last night at 10:00, so I'll give a full cure time in the pot and check the results later today. I didn't really do much different - I did make sure mix was correct and pressure pot was working properly.

"Insanity: trying the same thing and expecting different results" - we will see!

[shucky]

Erik I have no experience with Alumilite quick-set, but I have not heard too many positive comments about it. I've used their HS2 for about 2 years now w/out any issues. I think Casey and Ryan might both be on to something though. If it pulls away again I'd try using a thick styrene rod i/o of the metal your using in the mold. Depending on how many pulls you'd like to get, the quick-set is not very durable in the long run. You may want to think about switching to something else like HS2 or similar from smooth-on. I do have smooth-on's mold star 15 but have not tried that yet. Hope you post your results. Good luck.

[Erik Smith]

Second try turned out fine. I have the other mold half curing now so I'll post the results in a few hours.

I will try some different silicone after this. If I can't figure out why it is doing something weird, I can't do anything to avoid it.

[Erik Smith]

This one turned out fine.

Other side of mold

And the part, a 1/16 Toyota 2000GT wheel

Still baffled on the result from the one mold.

[shucky]

Erik, looks like a real nice result. Thats a solid and clean cast. Nice going.

[Eshaver]

I have attempted using Aluminites products with several problems all over the rainbow . I certainly could / would not recomend anything they make as the resin itself had inconsistent dry times , their rubber is too soft and the overall time used in working with their products has produced inconsistent parts ............... Ed Shaver

[Casey]

Alumilite, like Smooth-On and others, makes many different types of resin and mold rubber, so you can look through their selection and choose the product which best suits your needs.

Quick Set is very firm, so I only use it on parts which have no undercuts and require little flexing of the mold to release the cast parts after they have cured...which means I realy use it. High Strength 3 is my mold rubbber of choice for most parts.