February 16, 2011 at 9:06 pm #2229Ed KrausharCVRS Member
I have cast some small parts in zinc or polyester resin using sacrificial wood molds, plaster or RTV silicone molds but have never been happy with the mold making. I found a solution to making the molds for plastic parts on a site called the "Instructables".
The silicone molds I made in the past were made using construction type RTV silicone from the hardware store. Yes, I know you can get proper casting silicone but it is expensive and requires ordering and shipping. I am out in the country so I just cannot go out and pick it up and what about shelf life of it with infrequent use.
The solution appeared in a mixture called "OOGOO". No, I did not think up the silly name. Basically it is a mixture of common silicone caulk and corn starch, cheap and easy to get. The clear silicone I used came from a discounter at $1.00 per tube. I won’t go into the mix details, they can be found at–
For the trial I choose a trim part from my Theriodyne TF5 radio. One of the two pieces of trim was broken and missing a piece. The overall part size is 1 3/8" by 5 1/4" and is a thin framework. The good original part–
To make the mold I stuck the part to a 1/4" thick piece of plastic with two sided tape to hold it in place. I used a paintable mold release on the part and the plastic. The Oogoo was mixed up taking care not to incorporate too much air and was spread over the part. A second piece of plastic was placed over the sandwich and pressed down to spread the mixture out.
After curing overnight the sandwich was pulled apart. This shows the back of the pattern part in the mold. The mold when cured is very flexible and easily cut and trimmed.
To cast the part I used ordinary unfilled polyester resin from the automotive store. For a filler I used some melamine micro-beads left over from a boat project. Othe fillers or none could be used, that is just what I had. A little burnt umber pigment was added to colour the mix. The catalyzed resin was poured into the Oogoo mold and carefully worked into the details with a sharp object to assist in getting air bubbles out of the mold. After curing the polyester resin the flexible mold is easily peeled back releasing the molded part.
The cast parts–
The center part is the part fresh from the mold with the flash (excess material) attached. The other parts are trimmed and painted. Gloss paint on the left and flat on the right. The photo does not show the detail well.
I was impressed with the detail that the mold picks up from the pattern part. At first I thought the castings were rough but examining the original part showed that it is rough also but does not show it as well as the high gloss casting. Professional materials may work better but for only $2.00 worth of material I was happy with the mold and the results.
Ed.February 18, 2011 at 4:58 pm #2233
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