Alternative to RTV Rubber for molds ?

I've been experimenting with Alumilite to mold some grippers for my bots. Build the prototypes in plastic - cut it down, drill & file
and smooth, etc. Then pour a mold around it using the RTV silicon rubber. My question: Is there a cheaper alternative to RTV Silicon Rubber for making molds ? As I go through several iterations of improvements and learning curve I have already used up the RTV Rubber from the kit but have plenty of the plastic left.
I tried Plaster of Paris (captured detail great, but too dang hard to get the part I created out of the mold). I also tried Silicon caulk from Home Depot (the unpaintable kind that doesn't stick to plastic very well ...) but it didn't cure even in the summer heat for over 24 hours because it was just too thick. Perhaps I could paint layers to build it up but that would takes a loooong time, I would guess.
I know some of the people on here have spent time in the special effects/movie industry so I figured this would be a good starting group for this question. I suspect the answer will be to simply get more skilled at doing this and become more economical with the RTV Rubber!
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Silicone rubber isn't all that expensive compared to other options, and it's what the pros use. If you're using a lot of it the trick is to buy it in enough quantity to save in bulk. Hobbycast.net, for example, sells an eight pound kit for about $100. They throw in a bunch of gloves and mixing sticks as you shouldn't reuse them. Outfits like Smooth-on sell about two dozen types of silicone rubber in gallon sizes (parts A and B) for commercial uses. I guess you could arrange for 55 gallon drums if you need it...
Do check out Smooth-on, as there are silicone rubbers that take a day or two to demold (in average temperature and humidity), and others that take five minutes. You match de mold to de job.
-- Gordon
pogo wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

options, and

to buy

example, sells

gloves and

sell
and B)

drums if

day or

that
I went to Smooth-On and can probably spend a couple of hours just browsing on their site! Hobbycast.net looks interesting too.
Can you explain what is meant by the term "draw" in the world of molding & casting. Such as in this excerpt: "... style concentrates on more dramatic shapes with sweeping curves, deeper draws, larger undercuts, and ..." ( I figured out what "undercut" meant - which was going to be my other question )
Thanks again! JCDeen
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Draw can be one of two things, usually both have interjoined meanings. The depth of a particular mold is refered to as the draw, and also it may pertain to stiffer or more rigid molds. In this case draw refers to the taper of the original part top to bottom. It can be ever so slight but must be present to pull a part from a mold. In deep molds the draw is , and must be greater even though the mold may be flexible. It is especially important if the mold is made of rigid materials, and the part to be created in the mold will ultimately become rigid. This allows for the part to be pulled from the mold because the part will carry a small taper(smaller at the bottom).Undercuts in rigid molding systems require multiple part molds. Especially true in the foundry business-Sand casting patterns are slightly tapered so they may be easily pulled from the surrounding packed sand(so the metal can be poured in the created cavity. As for molding materials-If you buy smooth on or any other two part mediums you MUST mix the VERY accurately. If not the part will be sticky or just stay gummy and never cure. Use a mold release on the original(dont be stingy). Use the right release for the product you buy. I would suggest that you buy a product that mixes by volume not weight also-Unless you have some very accurate scales. If you want to e-mail me I can find you a product that is 1st time user friendly, and will give you the best chance for success. Plan everything carefully-Plan each step. Those materials are expensive and you will waste time and money with sloppy preperation.
Mark
pogo wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
pogo wrote:

Mark has provided some excellent coverage of the terms. As mold making and casting is a world onto itself, I suggest a good book to round out your experimentation. None are "perfect" but they're a start. I found "The Prop Builder's Molding & Casting Handbook" to be among the better books on the subject, though the two-column layout is screwy and the author sometimes jumps around in his explanations. The old TAB book "How to Cast Small Metal and Rubber Parts" isn't bad, either.
There are also some books on casting and injection molding techniques from Lindsay Publishing. You can get these directly from Lindsay or from Amazon. Not sure if you want to build your own vaccum former, but the concepts are interesting.
-- Gordon
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Re-meltable, vinyl. Available in many melting temperatures... ------------------------------------------------------- Ashley Clarke -------------------------------------------------------
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Mon, 17 Jul 2006 23:18:04 -0400 in comp.robotics.misc, "pogo"

Spread one layer of silicone caulk and then after it sets pour a plaster mother mold around it to support it. If needed, embed cheesecloth in the silicone for strength.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote,

too
would
Since I already have the stuff I might as well try that! Good idea! Thanks!
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.