Parametric Sensory Pebbles

I've included a few sample pebbles, as well as a parametric OpenSCAD file to generate your own.

Design Parameters

OpenSCAD includes a parameter customizer for variables designed for user editing. Most of these are relatively self-explanatory, but there are a few things to keep in mind:

The "Nugget" is the main cylindrical puck, and in most cases will take up the bulk of the silicone. With shore 05 silicone I recommend that the nugget height be at least 5 mm for strength.

The "Nodules" are small cylindrical protrusions from the top of the nugget. They can be disabled completely by setting nodule count to 0. They are placed in a Fermat spiral which curves out from the centre of the puck. The spiral pattern can be adjusted by changing "nodule theta", which is quite sensitive to changes. An angle of 137.5 (the golden angle) gives a pattern similar to a sunflower's seeds, while 137 gives quite a pretty spiral in my opinion.

By default, the height and width of the nodules randomly varies by 0.2, or 20% of their default controlled by "nodule random". Setting this to 0 eliminates this random variation.

For small radius nodules < 1.5 mm, I recommend keeping the nodule height at a maximum of 2/3 of the nodule radius. In my experience, any longer and the friction from the walls is enough that it's difficult to remove the nodules from the mould without tearing.

For ease of use, I've also included some design presets under "sensory_pebble.json.txt". 

Print Parameters

I recommend FFF printing for the moulds, since UV resins can inhibit silicone curing. 

• Layer height: generally as low as possible. Silicone will pick up every layer line, so a large layer height can be used for textured sides if you like.

• Walls: 2-3 minimum

• Top surface - 5+ layers

• Infill: 10% - silicone doesn't expand so the only pressure the mould needs to withstand is the weight of silicone

• Seams: Aligned gives better results if sanding, otherwise random

All other parameters are flexible. I've found that these moulds can print at pretty high speeds since they naturally have little in the way of features requiring slow movement.

Making the pebbles

If you haven't used silicone before, I highly recommend buying a silicone squishy toy making kit. These will include everything you need to get started and the materials are forgiving enough that it should be suitable for kids age ~8+ with adult supervision.

The sample pebbles use about 10ml of silicone each at 100% scale including a small allowance for spills and cup loss, but remember when scaling up the volume increases by the cube i.e. scaling to 200% size in all dimensions would require closer to 80ml. For more accurate estimation, the OpenSCAD file includes a silicone use estimator which will output a baseline for silicone use into the console log. It also includes the option to export a "negative" of the pebble design, which can be imported into your slicer at the same scale as the mould and set to 100% infill - the calculated volume of filament use will then be equal to the actual volume of silicone required for an equivalent mould.

Materials

• Shore 00 Scale Platinum-cure Silicone - I used a store-bought 1:1 "taba squishy" silicone kit, which I think is about shore 05 hardness. The options here are quite flexible, and I suspect you'd get good results even with a softer Shore A silicone.

• Pigment - single colours work great, but you can also swirl in some pigment to get a bit of visual texture

• Cornstarch/Baby powder - At least for me, the silicone mix ends up quite tacky so a thin dusting of fine powder helps the sensory experience. Flocking powder also works but gives a furry texture.

• Soapy water (about 1:5 ratio of dish soap and water) - could also use silicone mould release

• (Optional) Additives - Things like craft pom-poms can add a nice colour and texture variation within a clear silicone, or mica can be used to add textured particles

Moulding

1. (Optional) Lightly sand the inner surface of the mould to reduce layer lines

2. (Optional but helpful) Fill the mould with soapy water, tapping the sides gently to release trapped air from the nodules. Let it sit for 2-3 minutes then tip out just before prepping the silicone

3. Prepare your silicone according to manufacturer instructions.

4. Pour the silicone into the mould. If using multiple colours, I recommend letting the moulded silicone cure for about 1/4 of the recommended curing time before adding the next layer.

5. Allow the silicone to cure according to manufacturer directions. Mine recommended 1-2 hours, but I've found it works best if left in the mould overnight.

6. Carefully remove the silicone from the mould. I find that you get best results by gently pushing down on one side of the pebbles near the mould wall, but you can also gently peel it away from the sides if it gets stuck. 

7. If tacky (and you don't want it to be), apply a layer of powder coating - rub tiny amounts (about 1/8 tsp at a time) of cornstarch or baby powder into the silicone surface, it should be absorbed without a noticeable impact on colour. If you're using flocking powder, just toss the pebble into a small sealing plastic bag with a pinch of powder and shake it for a bit.

The moulded silicone pebbles can be cleaned by gently rubbing with soapy water, drying and then applying a little more of the powder coating if it becomes tacky again.