Food safety is a hot topic in 3D printing, and few options exist that make printed parts truly safe for interaction with food. Check out this post on the 10 Things You Need to Know About 3D Printing and Food for an overview of many of the main considerations.
In this post, you’ll be learning one of the most robust methods for creating parts that are truly food safe. We’ll be making a custom tray that forms ice in the shape of Mount Fuji. This project starts with a Formlabs Form 2 printed master mold that is used for casting a food safe silicone.
For this project, we recommend the following materials:
- Smooth-On SORTA Clear 37
- Vacuum degassing chamber (optional but recommended)
- Disposable volumetric measuring cups (Solo cups work if you don’t have anything more precise)
- Disposable stirring sticks
- 3D printer for making the master mold
Specific design tools and techniques are outside of the scope of this tutorial, but there are a few things you should keep in mind when working with silicone casting.
An undercut is any region that will prevent silicone from being removed from your mold. A useful heuristic to identify undercuts is to visualize the process of removing the casted silicone from your mold.
Any geometry that prevents the silicone from being pulled up and away from the mold is considered an undercut. Mild undercuts can sometimes be overcome when using flexible materials like Smooth-On’s SORTA Clear 37, but more substantial undercuts will prevent your silicone from being removed.
Use Draft Angles
Draft angles are the opposite of undercuts, and these are angled regions that make it easier to remove your casted silicone. Angling flat surfaces by as little as 2 – 3 degrees can make it considerably easier to remove casted silicone from the mold.
On our mold, the walls of the cylinder are angled 2 degrees outwards to ease release. This ensures that when we pull up on the casted silicone, the walls release instead of remaining bonded to the sides of the mold.
Use Vent Holes
Air vents can be useful for allowing silicone to flow into hard to reach areas. Vent holes should be used in all mold regions where air is unable to escape. Trapped air pockets that cannot be displaced will prevent silicone from fully filling your mold.
For this tutorial, we’re using a one piece open mold, so adding air vents isn’t necessary. When working with two parts molds, it’s often critical to design air vents into at least one half of the mold so that air can escape when the parts are combined together.
Opposites *don’t* Attract
The best practice when making molds tends to be using a flexible mold for a stiff casting medium and a stiff mold for a flexible casting medium. In this tutorial, we’re using a flexible casting medium and Formlabs Standard Resin as a hard mold material.
Preparing the Mold
If you’re interested in making your own Mount Fuji ice cubes, grab the design files here! We printed this in Clear Resin on the Formlabs Form 2 to ensure that all of the detail in Mount Fuji resolves successfully.
When using resin printing, it’s important to make sure that the part has been fully washed and cured. Most photopolymer resins inhibit curing of silicone, so sticky prints will prevent your cast from fully curing. With your mold prepared, it’s time to mix and degas the silicone.
Preparing the Silicone
Smooth-On’s SORTA Clear silicone is a two part material that’s mixed in a ratio of one part A to one part B. You could use graduated cylinders for measuring out each part, but a marked Solo cup performed equally well and can be disposed of after use.
Estimate the total amount of silicone your mold will require, and then mix equal amounts of part A and B. It’s best to use a bit more than you think you’ll need, as this material is highly viscous and it’s likely that you’ll lose a bit to the walls of your container.
Thoroughly mix your silicone together for at least 2 minutes. After mixing, you’re likely to notice bubbles throughout the silicone. Vacuum degassing is the best process for removing bubbles from casting materials.
Vacuum degassing setups vary significantly, but the general process involves using a vacuum pump to draw air up and away from the silicone. Degassing your material isn’t critical, and your mold will cure without it, but it’s likely to have a number of bubbles that will affect the surface finish in some regions.
With your mold and silicone prepared, it’s time to pour! A slow pour onto the edge of your mold can help to reduce the likelihood of introducing new bubbles to the silicone. The technique isn’t much different from that of pouring onto the side of a cup to calm a foamy beer.
Fill your mold to the top and allow the Smooth-on SORTA Clear 37 to cure for at least 4 hours. Once the silicone has fully cured, use a thin knife to cut around the perimeter of the mold. This will help to release the silicone from the mold, and you should be able to remove it from there. We were a bit impatient and opted to break a wall of the mold as to remove the silicone more quickly.
After the silicone has been removed, give it an additional 24 hours to fully dry. Then, thoroughly wash the silicone with soap and water to ensure that any residue from the molding process has been removed.
Your mold is ready to go! Ice admittedly doesn’t resolve the resolution of Mount Fuji especially well, and our ice represents traffic cones more closely than the iconic mountain. Nonetheless, we’re excited to see how your molds turn out!
Silicone casting is a great process for medium volume production or one-off food safe projects like this. For high volume production applications, check out this tutorial on 3D printing your own injection molding cores.