View Full Version : Clear Casting Resins and Molds

03-26-2007, 01:54 AM
Obviously I am new here, but I have thoroughly enjoyed reading through many of the threads. I am in the early stages of designing and building a fairly large product (roughly a 12in cylinder ~ 30in tall) made from clear resin. I had planned on building the molds from a combination of metal/wood/fiberglass and RTV silicone for hand pouring a clear finished product. The product will be low volume (<1k units/year) and is aimed at aquarium hobbyist. I have built smaller molds from all of the above materials for various resin castings in the past, but never anything this large; and I have never cast clear resins. Because optical clarity is fairly important, I am certain that I am going to waste a lot of resin in the ‘learning process’

I am familiar with vacuum de-gassing, slow cycle times and post cures. Selecting a resin is an area where I could really use some input. There are polyester resins, epoxy-resins and urethane resins that all purport to be ‘crystal clear’. I have cast non-clear versions of all of the above, and my best results were with epoxies and urethanes. Constant contact with water and potentially ozone will probably preclude polyester based resins, though if this is a bad assumption, please point this out. The wall thickness of the parts will be ~1/4in to 1/2in. Structurally there will be little stress or pressure on the finished product.

Additionally if any here think that building molds suitable for acrylic (eg. Perspex) might be a viable option even in the early stages, then I would be interested in hearing the ‘pros’ and ‘cons’ of that route. My assumption is that the cost of the molds would initially preclude this option for such a low production volume. I can fabricate molds suitable for casting resins, thus keeping the costs reasonable, but the industrial quality molds that Perspex would require are likely beyond my capabilities.

This project will be more on the DIY level than an industrial approach to begin with. I have a mill-drill, a couple small lathes and other basic shop tools, but nothing that would impress any machinist. My goal is to use low-tech, low-cost molds for the first year or two of the product, and then if volume justifies it, moving to a higher production mold/process.

Thanks In Advance for any input,


03-26-2007, 08:25 AM
Here is web site you might want to look at. http://www.smooth-on.com/

Lots of advice on-line and great people on the other end of the phone! I'll bet they will have some answers for you.


03-26-2007, 09:40 AM

Thanks for the link. I have been to their site and reviewed their product lines, but I have not contacted them directly yet; I will do that today.

Thanks for the response,


03-26-2007, 08:07 PM
In many cases the finish of your mold affects the surface of your part. you will have to work with a material which mixes easily and pours well to avoid trapping bubbles. The problem with silicon is it picks up everything and leaves the marks. You cant clean it up.
Many resins will dull or yellow over time. With an aquarium part you really need to be picky about what resin you pick.
Just a thought to keep in mind.


03-26-2007, 11:40 PM

Obviously RTV has its up-sides and its down-sides. I am considering it viable for rapid, cost-effective mold replication; I view the short life of the molds an acceptable cost. The actual parts are rather large, thin-walled parts. My plan is to build the mold infrastructure from rigid materials and then 'pour' an initial part from a machinable material, then finish machine the part and polish it. Once the 'plug' is highly polished I plan on using the RTV to build up the mold. The plug can then be used over and over to create more molds.

Resins. Yes, this is my biggest concern. I realize this forum is primarily the domain of mold making professionals, and as a DIYer I am seriously limitting the help that I can recieve here, not because of an unwillingness on the part of the members, but because I simply will not have the tools, skills or resources to 'do it right'. It is my hope that some here have some experience bridging the gap between 'hobby solutions' and 'industrial problems'. Obviously if there were a greater demand for this product then a larger, better financed company would be sending out bids to members of this forum to have professional quality molds built. But I digress; if any here have used products like Crystal Clear from Smooth-On or other clear casting resins for large, thin-walled parts I sure would appreciate tips and suggestions. Additionally information about both negative and positive results with respect to yellowing, water absorption and chemical clouding from ozone with respect to particular resins or families of resins would be a huge help.



03-28-2007, 05:31 PM
You will have to talk to each manufacturer of the resins to get specifics. I doubt if anyone sould have that specific knowledge about the products. Each manufacturer is going to tell you why theirs is the best blah blah blah....
I would say you should contact the resin makers to find out what resin you need to use first before worrying about the mould material. You may have to use wierd resin. Worry about the mold stuff after you commit to a resin or two. Then play to see results. Ask specifics about thickness versus clarity, coloring and 'wah' effect of thickness variations. You are not going to have a tight contol on a lot of this as you would in a steel mold.


03-29-2007, 03:06 PM
I have used a different method. I use positive pressure to cast my rapid prototype parts. I have been doing it this way since 1990. I can even use the clear resins (urethanes) made by BCC in Franklin, IN. send me an email I will help. gsilberberg@progressive-plastics.net

03-29-2007, 10:26 PM
What is wrong with just buying plexiglass tubes for the applaction?Cheaper in the long run.

03-30-2007, 01:33 PM

The final shape is conical, not cylindrical, and there is not a good way to re-form acrylic or plexiglass from stock materials. Additionally, by molding, numerous machining, fitting and gluing steps can be averted ultimately producing a superior product with less labor, but thanks for the suggestion.


04-07-2007, 04:46 PM
The Best Aliphatic Clear Casting Resin in made by BJB Enterprises in Tustin, CA. It's pricey though (around $7-$8/lb). Hydroseal POlymers in Riverside, CA makes a nice low cost Clear Urethane Casting Resin.

Chuck Reamer
04-20-2007, 09:56 PM

if any here have used products like Crystal Clear from Smooth-On or other clear casting resins for large, thin-walled parts I sure would appreciate tips and suggestions.

There is a plastic supplier next door to me, own by the company I am renting space from. We casted a 4"x12" long cylinder of Crystal Clear, its was casted around a big stapler with a chain cast in it (It was a gag keychain for a going away or retirement). It turned out pretty good, very few bubbles and very smooth. The finished product was incredible clear looking, even with a 4" dia. We cast it inside of a clear PVC tube (I think it was PVC, but not %100), so the surface was really nice.

Was not very hard when cured, had a tiny bit of give to it.

Hope that helps