I am very fortunate to have a TAP Plastics store in my area. http://www.tapplastics.com/product/mold_making_materials/casting_products Their website contains good instruction videos for the various casting materials they sell. And, of course, Google offers many other instruction videos with a simple search.
To cast an epoxy part requires first making a mold, a reverse 3D "image" of the part to be cast. A good mold will be easy to make, it will be capable of being used a hundred or so times before "wearing out", and will make cast parts that are almost exactly the same as the model the mold is based on. You can make a mold from urethane or silicone rubber. And, if you use silicone rubber, you can use either tin cured silicone or platinum cured silicone. Platinum cured silicone is by far the superior mold material, because the uncured solution has a very low viscosity, so it easily flows into every detail of the model, and doesn't hold air bubbles in solution, but lets the easily float to the surface. In addition, the finished mold is very strong and isn't affected by the resin you cast the part from. But, it is very expensive for big molds - fortunately this PAR sensor housing is very small.
The details for making a mold are well shown in the TAP Plastics videos, and in other videos on YouTube.
I had a sample housing made by 3D printing, and used that as my model for a mold. After experimenting with one cast part I modified that part slightly, and used that as the model for a final mold.
Since this part is the cosine diffuser, and the epoxy resin is clear, I had to add a white pigment to the epoxy to make white epoxy. TAP Plastics sells that pigment in a tiny little jar. The pigment is similar to toothpaste in consistency. I am using a "smidgeon" spoon to measure the pigment, adding one smidgeon of pigment to 25 ml of resin, to get the proper transmissivity. I determined this by trying both a half smidgeon and one smidgeon. By simple good luck the one smidgeon gives very close to what I need.
This cast housing costs me about $2 apiece, once I have the mold made.
To cast an epoxy part requires first making a mold, a reverse 3D "image" of the part to be cast. A good mold will be easy to make, it will be capable of being used a hundred or so times before "wearing out", and will make cast parts that are almost exactly the same as the model the mold is based on. You can make a mold from urethane or silicone rubber. And, if you use silicone rubber, you can use either tin cured silicone or platinum cured silicone. Platinum cured silicone is by far the superior mold material, because the uncured solution has a very low viscosity, so it easily flows into every detail of the model, and doesn't hold air bubbles in solution, but lets the easily float to the surface. In addition, the finished mold is very strong and isn't affected by the resin you cast the part from. But, it is very expensive for big molds - fortunately this PAR sensor housing is very small.
The details for making a mold are well shown in the TAP Plastics videos, and in other videos on YouTube.
I had a sample housing made by 3D printing, and used that as my model for a mold. After experimenting with one cast part I modified that part slightly, and used that as the model for a final mold.

Since this part is the cosine diffuser, and the epoxy resin is clear, I had to add a white pigment to the epoxy to make white epoxy. TAP Plastics sells that pigment in a tiny little jar. The pigment is similar to toothpaste in consistency. I am using a "smidgeon" spoon to measure the pigment, adding one smidgeon of pigment to 25 ml of resin, to get the proper transmissivity. I determined this by trying both a half smidgeon and one smidgeon. By simple good luck the one smidgeon gives very close to what I need.
This cast housing costs me about $2 apiece, once I have the mold made.