Casting Silicone Using 3D-Printed TPU Molds

My grandma really likes silicone cookware, but she often complains about how expensive it is. I initially thought about buying her a silicone spatula and a couple of silicone cake molds. Then I remembered that I own a 3D printer and could use it to create molds for casting liquid silicone. This could be cheaper than buying several ready-made items, and any leftover silicone could also be used in future projects.

For example, if I needed a custom seal between two surfaces, I could cast it instead of ordering sealing rings and putting the project on hold while waiting for them to arrive. A homemade seal may not be as precise or reliable as a professionally manufactured one, but it could still be useful for low-risk applications if it performs its intended function.

Casting silicone usually comes in two components: a base and a curing agent. These must be measured and mixed according to the ratio specified by the manufacturer, as not all products use a 1:1 ratio. The mixture is then poured into the mold, left to cure, and removed once it has fully solidified.

Before beginning the casting process, make sure to wear suitable gloves and protect the working area with a disposable covering. The uncured components feel oily and are difficult to remove using ordinary soap. Dish soap helped slightly in my case, while acetone was ineffective. Isopropyl alcohol worked best for cleaning my tools, although you should always follow the cleaning and safety instructions provided by the manufacturer. Before washing the mixing container, wipe away as much uncured silicone as possible using a paper towel.

It is also essential to select the silicone according to its intended application. Not every silicone is certified as food-safe or suitable for skin contact. Carefully read the product description before purchasing or using it. For food-contact applications, the silicone must be explicitly certified for this purpose and must cure completely before use.

As a first test project, I made a small heart with my name written on it. The name is intentionally misspelled as “Nergas.” It was extremely hot outside while I was working on the project, so the joke is that “Nergas” is the evaporated form of “Nergis.” As stated on the bottle, the silicone cured in approximately 24 hours. Its viscosity was low enough to flow into the small gaps between the letters, and the overall shape came out as expected. The mold and final result are shown below.

Although it is not immediately noticeable on the picture, the surfaces that were in contact with the TPU mold remained wet, while the surface exposed to air cured properly. This most likely indicates cure inhibition caused by an incompatibility between the silicone and the TPU mold or one of its additives. 

An incompletely cured part should not be used for food-contact applications, and simply cleaning the wet surface is not sufficient to make it safe. However, to test the mold for potential future use, I cast a spatula. I will not use it for cooking or give it to my grandma. Instead, I will repurpose it as a brush. The STEP files for the spoon and the handle are available for download. The part named spoon should be printed using TPU, and the part named handle should be printed with a more rigid material, like PLA. You can see the casting process and the final part below.