FDM 3D Printed Molding and Surface Finishing

From our group assignment, I learned that both 3D-printed molding and machining molding have their own advantages and disadvantages. Personally, I found the 3D-printed molding process to be more enjoyable. While the machining process tends to be more high-precision and potentially faster (assuming all the digital and physical settings are correct), numerous technical issues can arise, making it more stressful and intimidating for me.

On the other hand, 3D-printed molding, despite taking longer, allows more room for customization and adjustments according to our needs. This makes the process much more enjoyable and provides room for experimentation. Therefore, I decided to go with this experimental 3D-printed molding process for this week’s assignment.

Designing for 3D Printed Mold

For this assignment, I plan to design a multi-purpose trinket dish that doubles as both a jewelry tray and an incense holder. I aim to design an organic shape that allows for various interpretations.

Inspiration

Designing Incense Holder/Trinket Dish Mold on Onshape

Considering the 3D printing process tends to smooth out small details, I’ll simplify the design to minimize these intricacies. Instead, I’ll emphasize curves for a visually appealing result.

• Start by making sketch

• Extrude

• Fillet Edges

Fillet the inner circle (radius value = 10 mm)

Fillet the outer circle (radius value = 5 mm)

Here, I experimented with the design by setting the fillet value quite high to make the shape more playful. Since I positioned the inner circle off-center, closer to the edge, applying the high fillet value resulted in uneven edge heights. This was unexpected but turned out beautiful!

• Final result

3D Printed Molding Process (Positive)

Printing the Master Pattern (Positive Part)

Slicing with Ultra-Detail Layer Height

In order to ease the surface finishing process later, we have to aim for the layer height to be seen as minimally as possible. Here, I used the ultra-detail setting, with a layer height of 0.07 mm.

The downside is that it would take quite a long printing time. For this piece, it took 2 hour 12 minutes to print.

Printing post-processing: remove any stringings

For the 3D printing mold, we’re printing the positive part as the master pattern as well as the container. The idea is to make the mold container reusable for other designs.

Remove stringings or any other build up post-printing with cutting plier and exacto knife

Surface Finishing

1. Filling with UV Resin + Baby Powder

First, mix UV resin with baby powder and stir it until achieving thick-cream consistency.

With a sponge brush, coat the surface of the mold in a thin layer and even it.

2. Curing with UV Lamp

Next, cure the resin-powder layer coating with a UV curing lamp. The process is really fast, it only takes seconds for it to cured.

3. Sanding

After the resin has cured, the next step is to sand the mold using sandpaper. Start with 240-grit sandpaper and gradually work up to 1000-grit. The sequence of sandpaper grits I used is as follows: 240, 320, 400, 600, 800, and finally 1000.

Everytime you snd, feel the texture of the surface with your fingers. And if you can still fill some bumps/rough texture, sand it further.

Do the 3 steps above repetitively until we feel there’s no bump/ rough texture left, in order to achieve the smooth surface finish.

This surface finishing process, from coating with UV resin to curing under a UV lamp to sanding, will take the most time. In total, I spent around an hour on this process. However, I believe I was quite excessive with the sanding. It might not be necessary to use all the grit sizes; you could potentially jump from 240 to 400 to 600, which would make the process much faster. Even though it takes longer, I found the process to be quite meditative, allowing for more control over the smoothness. However, if you’re a beginner, it can be challenging to know when the surface is smooth enough.

Anyway, once the surface feels smooth, you can proceed to the next step: making the silicone mold!

Silicone Molding Process

Because in the end I would like to cast hard materials, like eggshells composite. Therefore, the mold has to be soft (Silicone).

Tools and materials needed:

• Master Model
• A mold container to hold the model and silicone
• Silicone rubber RTV-48 (two-part silicone mold-making kit)
• Mixing containers and stir sticks
• Measuring cups
• Scale (for accurate measurement)

Here’s the step-by-step process:

• Prepare the Master Model and the Mold Container

  

  The model is the 3D-printed master model (positive part) that I already made and finished. For the mold box, I 3D printed a separate container. It is designed this way (separated into two components) so that the container can always be reused for different master models as needed.

  

  To assemble them, stick masking tape on the back of the master mold, as well as the inner surface area of the mold container. Then, join them together with super glue.

  This way, when we’re done using the mold, we can simply peel off the masking tape, and both the mold and the container will be ready for reuse with other master models.

  If all is well with the mold container assembly, clean the model and the mold box and make sure it is dry. Any dust or debris can affect the mold.

• Apply Mold Release Agent: Baby Powder

  It is highly recommended to apply a mold release agent to the model and the interior of the mold container. Here, I use baby powder. This will help in easy removal of the mold from the mold container.

• Measure the Silicone Rubber

  The Silicone Rubber that we use here is the RTV-48. The typical mixing ratio for RTV-48 is 5% (100:5), means 100 parts silicone base (part A) to 5 parts catalyst (part B) by weight. But after experimenting, the sweet spot we found is 4% ratio (100 silicone base : 4 catalyst).

  

  To determine the weight of the silicone base (part A) that we need, we first had measure the volume of the mold container’s negative part by filling it with water. Then, we measured the part A weight by matching the volume of the water. Based on that, we just had to calculate the 4% of that part A weight for the catalyst (part B).

• Mix the Silicone Rubber

  Once all the parts are ready, next step is mixing! Stir slowly and thoroughly to ensure the catalyst is evenly distributed and minimize air bubbles. But also don’t take too long because the silicone might starting to set.

  

• Pour the Silicone

  Slowly pour the mixed RTV-48 silicone into the mold. Try to avoid trapping air bubbles. After pouring, vibrate or gently tap the sides of the mold container to help release any trapped air bubbles.

• Wait for the Silicone to Cure and Demold

  Let the silicone to cure. Typically it will take 24 hours to fully cure at room temperature. But with this mixing ratio, it took around 3-4 hours to fully cure mine.

  

  Ensure the silicone has fully cured before demolding.

Silicone Mold Result

Looking good!

🖍️ Minor Error: When pouring, I did not account for the small hole, resulting in the silicone mixture not reaching the bottom. I should have poured the silicone mixture into the hole first before pouring the rest of the silicone.