was wondering something; author uses special scales and vacuum method for casting from Smooth On. I use the Smooth On simple 1:1 no special scales or vacuum. Seems to work OK. Just wondering what the value is in the more elaborate stuff (I don't think I saw an answer in the article).
nice advice on buying from Smooth On direct. One of my batches from a Smooth On outlet was old and lumpy (RTV) so I used it up fast. Also, RTV is pricey; wonder if there's a cheaper substitute.
The RTV I'm using doesn't seem prone to bubbling; and, it withstands heat as I've been metal casting all day with it
thanks Ray, I came to the same conclusion. I believe the trick is to use as much filler as possible to avoid excessive RTV usage. For example, a concrete outer support shell, a sponge filled with RTV for inside filling and so on. I also have used the reverse side of RTV molds to make a second mold. Here's a photo of a tipper and the tipper mold. Note the gears I put into the top to get double usage. Also, old RTV can be used as filler as RTV clings to Rtv fairly well, esp if it was poured recent. I also cast molten metal in my molds and no signs of wear yet.
If mold wears out (e.g. crack), you actually can repair it sometimes with new RTV rubbed in
squish mold of tipper:
resulting tipper body (other parts incl wheels cast separately)
I read the article after reading your post. Depending on the RTV you are using you can end up with some pretty wacky by "volume" mixtures. I use Dow Corning's High Strength 3 for it's ability to cast deep undercuts without ruining the mold. The "volume" measure for it ends up being 2 ounces RTV to every 1 (small provided) scoop of catalyst. Alumilite has done all the figuring to make sure this is a proper 10:1 by weight ratio between the catalyst and the RTV. Depending on the type of RTV used will dictate wether or not it needs to be degassed before pouring into the mold. HS3 doesn't need to be degassed, but, I do notice tiny air bubbles at the very top surface of the mold. These are not a problem for what I do since they are nowhere near the actual part. If you used say Dow Corning's Silastic series RTV's you would need to degass them before pouring into the mold because they trap air bubbles easily while being mixed. Another thing to think of is if the RTV is a platinum or tin cure. The platinum cures tend to have a problem with inhibition sesitivity, like certain modeling clays, and most adhesives will cause the inhibition and the RTV won't cure right next to the items it's reacting with. If you want a real high strength mold that will last a long time once it's made that does well with multiple types of casting materials you best bets are going to be looking into some of these platinum cure RTV's and they really need to be degassed to get rid of the air trapped in them.
All said it seems like a very good article so far, I can't say I've seen any misinformation given in it yet.
Jack
Jack, thanks for input. I do noticee among those who cast is that people pretty much stick to a single method that satisfies them.
I may try some other types of methods and materials, such as what you mention.
I joined the casting Yahoo group the author mentions but after 4 days they have not run my post or allowed me to post photos or files. Guess it takes approval from a moderator who might be vacationing.
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