Many hobby robot builders cast their own tires to get the perfect shape and grip. Silicone rubber (often RTV caulk) is a popular choice because it’s cheap, elastic, and easy to mold. Hardware-store silicone sealant can be cast over 3D-printed wheel hubs to make custom tiresinstructables.com

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. In fact, hobbyists report that silicone tires give much better traction than improvised solutions like inner-tube strips, making robots easier to steerinstructables.com

. One reason is that silicone stays flexible (even at small sizes) while still being durable. Using RTV (room-temperature-vulcanizing) silicone means you don’t need special curing ovens – it cures at room temperature by absorbing moisture. This lets makers work entirely at their workbench with readily available materials.

Why RTV Silicone? Flexible, Grippy, and Hobby-Friendly

RTV silicone caulk is widely used because it hits a sweet spot for hobbyists. It’s a one-part (single-component) material that cures in air, so you just squeeze and shape ithackaday.com

. High-quality silicone wheels (often 30–60A Shore hardness) are used in robotics for their combination of grip and durability. For reference, Smooth-On notes that Shore 70A is as hard as a car tire, whereas ~30A is very soft and flexiblesmooth-on.com

. Robot tires typically sit in the middle: soft enough to conform to the floor for traction, yet hard enough to carry weight without squashing. (Many combat/ sumo robot wheels use mid-range hardness around 40–60A.) Too soft (e.g. 10–20A) can actually reduce stability at speed, while too hard (>70A) gives poor gripsmooth-on.com

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Compared to other rubbers, silicone is chemically inert (won’t melt or emit oily plastics), and can tolerate temperature extremes. For simple molds, the flexibility of silicone makes it easy to peel off the cured tire. (By contrast, harder materials like epoxy or urethane can be brittle or hard to demold.) One caution: cheap household silicone can be very thick (making detail pickup tough) and smells of acetic acid (vinegar) as it curescommunity.ceramicartsdaily.org

. But for many small wheels, it’s an affordable solution.

Shore Hardness: Balancing Grip and Wear

Silicone’s Shore A hardness describes how soft or stiff it is. Lower numbers (e.g. 20A–40A) are gel-like and tacky, while higher (60A–80A) feel like hard rubber. Smooth-On’s hardness chart illustrates this: a Shore 70A rubber is “as hard as a car tire” and very stiff, whereas Shore 30A is quite flexiblesmooth-on.com

. For robot wheels, you generally want something in between: soft enough to bite into surfaces, but not so soft that the tire deforms excessively under load or wears out instantly. Many hobbyists find the sweet spot around 50–60A (similar to skateboard wheels or office chair wheels) for good traction and longevity. This matches common practice: for example, small robotic wheels often come in that range for a mix of grip and durability. If you’re unsure, err on the softer side and prototype – softer tires give better traction (on smooth floors or carpet) but may reduce top speed and durabilitysmooth-on.com

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Coloring the Silicone: Paint vs. Pigment Powder

It’s tempting to mix in acrylic paint to color silicone (and many DIYers do it). A small drop of inexpensive acrylic paint per batch can tint clear silicone without obvious curing problemshackaday.com

. The paint even acts as a visual mix indicator. However, acrylics are not designed for silicone; large amounts can interfere with cure. In contrast, silicone pigments or mica powders are formulated to blend with silicone. They are essentially inert mineral colorants that won’t upset the cure chemistrynaomidlynch.com

. Silicone pigments are sold by mold-making suppliers and give richer, more consistent colors with minimal effect on properties.

In practice, using a little acrylic paint (as shown in some guideshackaday.com

) works for a quick pastel tint, but if you want strong or opaque colors, specialized silicone pigment powders (sometimes called “silc pigment” or “fuse fx colors”) are better. These pigments are designed to stay in suspension and won’t “bleed” or settle during cure. (One effects artist notes that silicone pigments “must be used in a base” and generally won’t inhibit cure unless grossly overdosednaomidlynch.com

.) In short: a few drops of acrylic can color a batch for testing, but for high-quality tires use dedicated pigment powders or silicone-compatible dyes.

Additives: Cornstarch and Glycerin

Two common household additives can help when mixing RTV silicone:

• Cornstarch: Stirring fine cornstarch powder into silicone caulk (often in a ~5:1 silicone:starch volume ratio) is a well-known “Oogoo” hack. Cornstarch absorbs moisture and spreads it through the silicone. This dramatically speeds up cure, even deep in the moldinstructables.com
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• . It also turns the mixture into a thick putty, which can be molded by hand. For example, adding ~50% cornstarch by volume made a batch cure in under an hourhackaday.com
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• . However, corn starch also alters the rubber properties: it makes the cured material slightly harder and less tacky. As one builder notes, “the more cornstarch, the less grippy the tires will be”instructables.com
• . So there’s a trade-off: you get fast, strong curing (useful for thick or large tires), but the result can be less soft. For small robot tires, a light starch load (just enough to ensure cure) is recommended, then test grip.

Figure: Cornstarch powder (left) is a common kitchen additive. Mixing it into silicone (after dispensing caulk) speeds cure from the insideinstructables.com

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• Glycerin: A few drops of glycerin (available as dish soap or cosmetic glycerol) act as a flow improver. Hobbyists report that adding a teaspoon of glycerin to the mix thins the silicone slightly, making it pour or press into molds more smoothlyhackaday.com
• . The effect on curing is minor – glycerin mainly helps the silicone flow into fine mold details. It can also plasticize the cure slightly, but in very small doses (a few drops per batch) this is negligible. We found only one clear source for glycerin usehackaday.com
• , but it confirms this practical tip.

In short, cornstarch = catalyst-thickener (faster cure, less flexible), glycerin = thinner (better flow, minimal cure change). Both are optional: you can make tires without them, but they help in tricky cases.

Avoiding Bubbles: Mixing and Degassing

Trapped air bubbles in your silicone will ruin the tire surface and weaken the rubber. Degassing (pulling a vacuum on the mixed silicone) is the ideal solution: under vacuum, bubbles expand and pop out, leaving a clear rubber with higher tear strengtheasycomposites.co.uk

. However, a vacuum chamber isn’t common in hobby shops. If you lack a vacuum pump, here are workarounds:

• Mix gently and thoroughly: Stir the base silicone and catalyst (for 2-part) or the caulk with additives slowly to minimize whipping air. Pour or knead slowly. As Hackaday advises, scoop the mix into the mold “while trying to avoid creating air pockets”hackaday.com
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• Let it rest: After mixing, let the silicone sit in its cup for a few minutes. Tiny bubbles will rise to the surface and pop. Skim off any froth before pouring.
• Layered pouring: Pour a little silicone into the mold first, allow large bubbles to escape, then add more.
• Use a pressure pot (if available): Unlike a vacuum, a pressure pot squeezes air bubbles to invisibility. If you have a small air compressor and tank rated for a few tens of PSI, curing your filled mold under pressure will compress remaining bubbles. (This is especially common in resin casting.) Under pressure, any trapped air stays as microscopic voids that don’t weaken the part.
• Final tapping or vibration: After pouring, gently tap or vibrate the mold to encourage trapped air to float out.

The reason this matters is documented by moldmaking experts: a degassed silicone has no air pockets and shows a “completely clear appearance,” with “improved tear strength”easycomposites.co.uk

. In practice, even without a vacuum, taking time to minimize bubbles (stirring slowly, tapping, etc.) greatly improves tire quality.

RTV vs. Two-Part Silicone Rubber

There are two broad classes of silicone for molds: one-part RTV caulk (the stuff we’ve been describing) and two-part mold silicone (often labeled “platinum-cure” or “tin-cure” silicone). For DIY tires, both work, but each has pros/cons:

• One-part RTV (caulk): This cures by exposure to moisture in the air. It’s cheap and easy (just squeeze out and mix if using additives). But its cure starts at the surface and moves inward. In thick sections or in a closed mold, the inner silicone may cure very slowly or not at all, because moisture can’t penetrate. Hackaday warns that thick applications of silicone caulk “have difficulty fully curing when applied thickly, especially when sealed into a mold”hackaday.com
• . That’s why cornstarch is often added. Also, RTV silicones can be sticky until fully cured and may not pick up very fine detail as sharply as specialized rubbercommunity.ceramicartsdaily.org
• . In summary, RTV caulk is easy and accessible for quick castings, but it requires care with thickness and cure time.
• Two-part silicone rubbers: These come as a base and a catalyst that you mix (usually 1:1 by weight or volume). Brands like Smooth-On (Mold Star, Dragon Skin) or other mold silicones are in this category. They cure by chemical crosslinking, not humidity. The benefit is uniform cure throughout even in thick parts, and generally better mechanical strength/elongation (if you choose the right formula). A mixed two-part silicone typically sets in minutes or an hour, giving more consistent hardness and detail fidelity. Because of that, two-part silicones are preferred for professional molds and sealed or deep cavities. For example, one user notes that mold-making silicone “tends to be more rigid [or controllable]” and doesn’t rely on air to curecommunity.ceramicartsdaily.org
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For hobby tires, many makers simply start with RTV because it’s cheap ($3–5 per tube) and easy to handle. If you need very durable tires or extremely large/hollow molds, upgrading to a 2-part silicone kit is wise. The two-part silicones allow you to design complex closed molds (like hollow tyres or very thick treads) without worrying about incomplete cure. But they are more expensive and require precise mixing.