Step 61 : Casting Parts
Time to complete : 30 hours.
Total time to date : 408 hours.
I decided to spare no expense and purchase a very high end injection system in order to cast my parts with. This "system", otherwise known as a turkey baster, cost me JUST ABOUT $2.00 !! I modified the baster with a brass end piece that would fit the holes in my rubber molds nicer.
The product that I used to cast my parts is called Smooth Cast 305, a Smooth-On product from the same Sculpture Supply Store that I got my RTV material from. It is a resin that mixes 1:1 and is very easy to work with. It has a "working time" of 7 minutes according to the label, but it's closer to 5. After that, it starts to gell up pretty quick. I found that mixing a cup of each, for a total of 2 cups was the most I would mix up at one time. Any more than that and I found it was starting to gell before I could inject it into my molds. ....depends how quick you are I guess. A 2 gal kit of Smooth Cast 305 cost me $120. Castings could be demolded in about 1.5 hours, but they are still a bit soft. Total cure of the castings took about 3 hours. The cured resin is white....very white, easily visible through the semi transparent rubber molds.
Casting of the parts is where you find out how good your molds are. I found I had to modify my molds after a few trial attempts to cast the parts in order for the parts to be produced bubble free. It was just a matter of adding some venting channels where needed. Knowing what I know now about making molds and casting parts, my next go at making molds will be a little different and produce a better mold. The last mold I made, which was for my fore dive plane armatures, was the best mold and produced the best parts. I figure that's no coincidence.
The actual casting of the parts is simple. Put a hard back on your rubber mold, clamp secure, and then get the liquid resin in the mold. In my case, it was a matter of using my high end injector to force the resin into the molds through the small channels. My injector technique worked very well. You want to make sure that you don't clamp the hard back to hard as to distort the mold. You just want to clamp to hold the two mold halves securely against each other. Once the mold was full, I would keep forcing the resin in, allowing some resin to seep out the vents to ensure I was getting a full casting and eliminating any trapped bubbles. Once the curing time has elapsed, the two mold halves are separated (no release agent required), and the casting removed. After removal, the excess resin sprue was trimmed off. Without question, this is a very effective way of reproducing parts.
I attempted an experiment which I did not have a lot of confidence that it would work. I tried to cast my deck railings using the same exact rubber mold techniques as the other parts. I felt that the small diameter brass rod that I used to make the master railings would be too small to produce an effective mold. I was very pleased to discover that in fact I was able to cast these railing parts. The savings in time is huge. It took me about 2 hours to solder the railing out of brass rod, ....and about 2 min to cast the railing out of resin. The math is simple. The final casted railing will require some fine tuning on behalf of the modeller, but in all, I was pleased with the results.
Once the parts were casted, I cleaned off the flash, trimmed, filled any minor flaws with plastic putty, sanded them, and prepped them with a first coat of primer. This process was time consuming but produced a more refined part.
A modeller assembling the kit should have very little work to do on the parts to bring them up to a finished standard. This kit was intended to be a toy to play with in the lake/pool, and not a museum static display model. In total, I casted 142 parts, which is enough for 6 kits, plus 6 extra pieces that I will be using as templates.
The "Time to Complete" includes all 142 parts for 6 kits with all the work I did to the parts after casting. Take this into account when looking at the 30 hours.
I am now going to be making molds and castings of the tower, putting my knowledge to good use.
More to come.
"I don't have anything else planned for this afternoon." - Lt. Col J.O.E. Vandeleur
A Bridge Too Far (1977)