This is a reusable mold designed for two-part RTV silicone casting compounds and putties. The mold creates a silicone sock which covers the heat block to insulate it. There are two separate files for both the left and right extruder hotends. I have not actually explored the thermal performance of this insulating method compared to other solutions, though, so I can't guarantee that it is better, but it sure looks cooler!
This is version 1 of this, so please feel free to submit makes and feedback. I made the model in OnShape, a nice online CAD package. You can view it there and even export it if you have an OnShape account (it's free). I also uploaded Solidworks part files for it, if you want another format, let me know. These files also include parts and an assembly for the hotend itself.
Edit version 1.1:
Created an air gap between the silicone sock and the heat block. Previously, the silicone was 2mm thick, now it is 1mm thick with standoffs which create a 1mm air gap. The sock is slightly more difficult to release from the mold, but it should insulate well. Filleted the bottom of the sock.
OnShape link V1.0: https://cad.onshape.com/documents/755775ac5ac5e8ceb9784b7d/w/7a709f9d2c30f75e0e5a75b8/e/5cf270a76a18d9ef2997f353
OnShape link V1.1:
V1.0: Initial release
--> V1.1: Create an air gap between the silicone and heat block for better insulation.
V1.2: Improve the mold design to make extraction easier.
- Print the mold. Any rigid material is fine, but print it fairly sturdy. 3 shells and 25-50% infill recommended. If you're using pourable silicone, maybe a flexible filament would work, I haven't tried it.
- Get some silicone. Look for high temperature RTV silicone casting compound or putty. I have tried two types: TAP Plastics platinum cure (translucent white, 260°C), and Castaldo Quick-Sil putty (green, 315°C). The TAP Plastics stuff works well, but it's a bit expensive. The Quick-Sil putty is available through McMaster and Amazon, can be gotten in small (and cheap) quantities, cures super fast, and is rated for higher temperatures. However, it is infused with mineral oil, which will smoke a bit when heated until it burns out. I didn't use any release agent on either of these, but it probably would have made extraction easier, so feel free to try it.
- Make sure the two mold parts fit together easily. If the little pillar on the top part melted a bit during printing or something, just file/sand it until it fits loosely, it's only there to index the pieces when mating. The top piece should slide down far enough that the pillar is flush with the small hole on the bottom of the bottom piece. The rounds on the ends of the top piece should be touching the walls of the bottom piece.
- Prepare your silicone. You'll need about 15 grams of it. Follow the instructions. If using the Castaldo Quick-Sil putty, you must move quickly once you start mixing!
- Pour the silicone. It is meant to fill the mold until the silicone is flush with the top of the mold, or slightly lower. For low viscosity stuff like the TAP Platinum, you can just put both mold pieces together and slowly pour the silicone in. For high viscosity or putty, you'll have to pour/pack some in the bottom (and walls for putty), then squish the second part in, then fill it until it's flush with the top of the mold. For the putty, I used a small arbor press to push the mold together, but probably any hard surface will do if you put your weight into it.
- Wait. Let the silicone cure for the full curing time. I heated my printer's build plate to 50C and set the mold on top of it to help it cure.
- Extract the silicone. I used the back side of a hobby knife to run along the edges of the mold to pull the silicone off the walls. Then, I used some vise-grips and pliers to pry the two sections apart (see photo). If you used release agent, you'll probably have an easier time.
- Clean up the silicone. Some very minor cleanup might be needed. Most importantly would be the hole for the nozzle, you'll probably have a small ring of flashing that needs to be removed. Tweezers work well. Also, if your silicone was taller than the top of the mold, you'll have to shave the top down, as it won't fit under the heatsink. If you used the Castaldo putty, you might want to wait a day or two for it to "sweat" out its mineral oil, otherwise it will smoke when heated until it's gone. Not harmful, but a bit annoying.
- Put it on your hotend! The silicone sock splits in the back and top, so you will be able to slide it over the heater block without removing anything, although removing the side cooling fan will make it easier. You're done.