This Instructable shows how to create a 6-button silicone rubber keypad. If you are not knowledgeable about silicone rubber keypads, the Wikipedia article provides a pretty good description. I recommend you read it before proceeding.
This Instructable contains STL files you can print on a 3D printer, both for the mold to create the silicone rubber keypad and also for the top and bottom plates that hold the keypad in place. It is intended to be incorporated into designs that contain a processor such as an Arduino or a Raspberry Pi, but is not a standalone device.
Step 1: Prepare the Mold for the Keypad
Print the Mold
Use a 3D printer to print the two attached STL files. The resulting objects are the two halves of the keypad mold. I printed them in ABS plastic, but I see no reason that PLA or other common 3D printed materials will not work just as well.
Apply Mold Release
Once the two parts of the mold are printed, spray them with mold release. Mold release does for the molded object what greasing a cookie sheet does for cookies; it makes it easier to remove the object. A light coating is sufficient. I recommend you spray in a well-ventilated area.
Step 2: Silicone Rubber
For my keypad I used the silicone rubber mold-making compound Oomoo 30. We are not using it to make a mold; we are using it for the keypad itself. It comes in two separate parts that are normally a liquid. When you mix them together, the resulting substance will harden into silicone rubber. It is fully cured after 6 hours.
You will not need very much of it for this project. I used two large craft sticks to spoon some from container A and some from container B into a paper cup. I tried to get as close as I could to an equal volume of liquid from each, but did not measure.
Step 3: Mix the Silicone Rubber
Keep stirring until it is all the same color, and then stir some more. It does not harden quickly, so take a few seconds to make sure the compound is completely mixed before proceeding.
Step 4: Pour the Silicone Rubber
Before You Pour!
The mold top will only fit onto the mold bottom one way. Before you pour, figure out which way it goes together.
Pour the silicone rubber into the bottom mold and add the top mold. The silicone rubber compound is similar to honey in consistency. When you put the mold top on, it will push some of the rubber out of the wells in the bottom mold, and will likely flow out and get a little messy.
I apologize for the picture being out of focus.
Step 5: Prepare for Hardening
One of the long edges of the assembled mold has an open slot between the top and bottom of the mold. Turn the mold so this is facing up. Use a clamp or vice to hold the mold together, but not a lot of pressure is required. Spoon some more of the silicone rubber compound into the slot at the top, so the mold is entirely full,
Leave the mold to cure. It will be pretty hard after a couple of hours, but I let mine cure for the full 6 hours the manufacturer recommends.
Step 6: Remove the Keypad From the Mold
After the keypad is completely cured, release the mold from the clamp, pry off the mold top, and carefully pull the keypad out of the mold bottom.
Step 7: Print the Keypad Frame and Mount the Keypad
Print the two attached STL files on a 3D printer to create the top and bottom plates. I recommend you print the top plate with no raft, which provides a better finish.
Place the keypad onto the bottom plate with the posts going into the holes in the keypad. It only goes on one way, so figure out which way that is before you put it on. Then add the top plate. I had to enlarge the holes in the top plate slightly for the buttons to fit through comfortably.
Why the Frame?
In order for the keypad to work correctly, only the part supporting the button should bend when you push the button. The frame prevents parts of the keypad that are not supposed to distort from bending when you push a button; it holds them securely in place.
Step 8: Hooking It Up
Each of the buttons on its bottom has a rectangular platform, circled in red in the picture. To make the keypad functional, add a layer of conductive rubber 1-2 mm thick to these platforms. This Instructable shows how to make conductive rubber. The conductive rubber should be flat so it can make a good connection to a circuit board.
Underneath the keypad bottom plate you will need a circuit board with contacts that the conductive rubber can short together. Normally people use a layout similar to what is shown in the circuit board picture, where the contacts look like interlaced fingers.
When the conductive rubber is pressed down across the contacts, it causes the two contacts to be connected together so a processor (an Arduino or Raspberry Pi, for instance) can detect that the button has been pressed. This Instructable describes how to connect a button to an Arduino.
Step 9: Further Exploration
My first experiment with doing a silicone rubber keypad was to create one large button. The attached STL files are for the mold to create the button. There are a variety of things I did better on the 6-button keypad, but the big button is easy to work with physically. A quarter is included in the picture for scale.