In this Instructable I investigated the feasibility of creating my own cast lenses. While not exceptionally difficult, because I was making copies of an existing lens, this process is not trivial and deals with some chemicals that can be kinda nasty. So please make sure to wear safety glasses and gloves at all times!
Step 1: Designing Mold Box & Positive
Designing the Positive
The first step in any casting project is designing the positive. For this project the positive is a glass lens I got on ebay. Having seen the amount of sanding and polishing required for other lens projects, such as Robb's 3D Printed Weird Lenses, I decided to increase the diameter of the lens and create a circular 1/4" hole pattern to give myself flexibility when designing a drill polishing jig (more on this later). All of these pieces were molded using Fusion 360. If you haven't tried this software yet I highly recommend giving it a shot, there is nothing like not having to run parallels on your Mac to get to your CAD program.
Prepare Mold Box
I designed a quick mold box which was one large square for the bottom sized 1in larger in width than diameter of my lens polishing mount and 4 equal sized rectangles the same length as the width of the rectangle but 2 in tall. Once my box sides and lens jig mounts were laser cut on the Epilog Lasers at Autodesk's Pier 9 I hot glued everything together, see photos above for final mold box.
Add Clay for Split Mold
To design the split mold I used some plasticine clay I found lying around our mold making room. You want a clay that wont try or stick to the silicone we about to pour over our mold. once we cover the bottom of our mold box, press the lens and polishing jig into the clay about half of their height, then go around the edges of your mold with a popsicle stick to try and make sharp corners between the clay and mold. This will help give you cleaner edges when pouring resin to cast the mold. This video from Smooth-On explains this process really well.
Step 2: Mold Part 1
Before going any further go get safety glasses and gloves...I'll wait here
Weights and Measurements
Please follow the manufactures instructions for weight or volume ratios as close as physically possible. I calculated the approximate volume of the mold box not including the volume taken up by the clay and mixed about 20% more silicon than I thought I would need.
Pour into Mold
There are tons of recommended mold pouring techniques, personally I create a thin ribbon in a corner of the mold box and let the silicone fill holes and crevices. Take your time, the more effort you put into making proper molds the better your final product will be.
Pull mold under vacuum
The key to getting a good high quality mold is getting all of the air bubbles out of the silicone. Using a vacuum chamber and pulling vacuum until all air bubbles are removed may seem time consuming, this process took me about 10 minutes, but I promise it will pay dividends when it comes to mold quality.
Step 3: Mold Part 2
Demold & Repeat
Once your first mold half dries, use an X-acto knife to cut the hot glue fillets keeping your mold box together, and gently pry open your mold box. The next step is mission critical not to mess up, so go slow! Remove the clay from the mold, but DONT REMOVE THE LENS! If your pieces move during demolding you will get a slightly skewed final piece, which might render your lens useless. Now repeat the steps on mixing silicone, pouring and vacuuming for the other half of your mold.
Step 4: Casting the Lens
Cut Gates and Vents
Now that the mold is complete lets make some lenses! First you need a gate that will get your resin into the mold cavity. I went with a pencil sized gate for the resin and smaller for the vent hold, in retrospect and as you can see from my picture this was not the optimal size. You may have to experiment a little to find whats right here.
I rubber banded the two mold halves back together and began mixing up my clear resin as per the manufacturers specifications. Clear resin is nasty stuff! Some are kinda nasty and some are hazmat suit nasty, so please take the proper safety precautions. Slowly pour your resin into the mold through the resin gate we made earlier and once you cannot fill the mold anymore you are done. Grab a beverage and give yourself a high five for getting this far!
Step 5: Polishing Lens
Removal & Polishing Set Up
Take your lens out of the mold, as you can see mine wasn't awesome, but it wasn't totally a lost either. Polishing lenses by hand, seemed like waaay to much work, so I 3D printed a conical jig that fits into a drill and only requires me to hold sand paper in one hand while the lens rotates beneath. This setup mostly worked, however I recommend spending double the amount of time you think you need at lower grits, really laying a solid foundation for the higher grits and polish. Start with 220 and work your way up to 2000, then jump to polishing compound.
Have fun with your new lens :)