Silicone mold designA "positive" is a copy of the thing you'd like to make. This can be made in clay, 3D printed, or just be an object you already have.
A "negative" is anything that has the outside surface of what you're making: it is a mold, and you can cast something by filling it with silicone.
The surface finish of the negative mold will dictate the surface finish of the cured silicone. I am currently working on a comparison of 3D printed mold finishing techniques.
3D-printed molds do not have a fantastic surface finish without processing. Sanding and expoxy coatings like XTC-3D are used to improve the surface finish of the mold in order to get a more desirable final silicone surface finish.
Remember that your mold should be at least 2 parts- 3 or more if it is large. Consider features on the mold to help clamp it together, and to help take it apart after casting. Small cut-outs on the side, at least 5mm deep, will help as spots to use a flathead screwdriver to pry it open.
Filling the mold can be done in two ways. Most commonly a mold will be made with a single hole in the top, for silicone to be poured into. This is easist, and is possible with parts with one flat surface. Injection molding is more complex, and requires the use of a syringe.
In addition to casting in a mold, it is also possible to mandrel dip silicone, as shown in this video of silicone being poured on mandrels to shape it for breast implants. Another example of this process is the manufature of latex gloves.
A vacuum chamber is useful to "de-gas" the silicone prior to filling the mold, as silicone naturally holds many air bubbles. There are a variety of DIY vacuum chamber plans that work great.
- ALL3DP on 3D Printed Molds
- Blog entry on printing a positive, making a negative out of silicone, and using that negative to finally make a cast resin part
- Tabooless article on making dildos
- Venus O'Hara article on making dildos
- Brandie Harris video on dual density toys
- SexToyCollective video on dual density toys
- XTC-3D, a commonly used epoxy coating for 3D prints
- Smooth-on video taking a 3D-printed positive, smoothing it with XTC-3D, then making a silicone mold and producing a final resin positive
- Makerbot article on sanding 3D prints
- ALL3DP article on smoothing 3D printed PLA
- Fictiv article on finishing 3D printed parts
- Blender tutorial on creating a mold from an STL
- Blender forums on subtracting a part from a larger volume to produce a negative model
- Makerbot on silicone molding with 3D-printed positives
- YouTube video on a DIY vacuum chamber
- Hackaday on a DIY vacuum chamber
- Make Magazine an a DIY vacuum chamber
- Rapid and Low-cost Prototyping of Medical Devices Using 3D Printed Molds for Liquid Injection Molding, Journal of Visualized Experiments
- Proto3000 article on 3D printed molds for injection molding