Silicone or Silicone? Choosing the Right Material for Your Application (and What 'Casting Silicone' Actually Means) - Forward polymer systems for demanding manufacturers

Let's clear something up first

When someone says "silicone," they could mean a dozen different things. And when they ask about "casting silicone," I've found they're usually looking for one of two very different answers—depending on whether they're making a plastic duck for a bath toy or a gasket for an industrial machine.

I'm a quality compliance manager in the rubber and plastics industry. I review specifications for roughly 200 unique items a year—everything from o-rings to foam insulation to the sealant holding a skyscraper's windows together. And here's something that surprised me early on: the word "silicone" gets thrown around like it's one material. It's not. Not even close.

What most people don't realize about 'silicone'

Here's something vendors won't tell you: what gets marketed as "silicone" in the hobbyist space—mold-making kits, baking mats, phone cases—is often a different formulation than what we'd call silicone in an industrial spec sheet.

And is TPU silicone? No—it's not. Thermoplastic polyurethane (TPU) and silicone are chemically different animals. But they compete in similar applications, especially for parts that need flexibility and durability. The confusion usually comes because both can feel rubbery, and both get used in casting processes. But they behave very differently under stress, heat, and over time.

I still kick myself for a project in 2022 where I assumed a TPU casting was "close enough" to silicone for a vibration dampener. It wasn't. The part failed at 60°C. If I'd specified platinum-cure silicone from the start, we'd have saved $4,000 in rework.

Scenario A: You're casting a prototype or low-volume part (like a toy or model)

If you're making a plastic duck or a custom mold, you probably want a two-part addition-cure (platinum) silicone. These are the silicones you mix in a cup, pour into a mold, and cure at room temperature. They're what most hobbyists and small-scale manufacturers mean when they search "casting silicone."

Never expected how finicky these formulations can be. Turns out, platinum-cure silicones are sensitive to contamination—sulfur, latex, even certain clays can inhibit curing. I had a batch fail completely because we used a mold release agent that wasn't compatible. The surface stayed tacky for three days.

For this scenario:

Choose a low-viscosity, addition-cure silicone (shore hardness 10-30A for soft parts, 40-60A for more rigid ones)
Verify compatibility with your mold material and release agents
Expect to pay $40-80 per kilogram for quality platinum silicones (pricing as of January 2025)

Scenario B: You need an industrial silicone rubber for a functional part (gaskets, seals, o-rings)

This is where the material choice gets serious. If you're specifying a silicone rubber for an o-ring or a gasket, you're not just buying a material—you're buying a spec.

Industrial silicone rubbers are typically categorized by: — Or rather, they're categorized by cure system first, then hardness, then temperature range.

What I mean is: a high-consistency rubber (HCR) silicone for compression molding is a different product entirely from the liquid silicone rubber (LSR) used in injection molding. And neither is the same as the pourable stuff you use in a hobby mold.

For functional parts, I'd recommend:

LSR (liquid silicone rubber) for high-volume, precision parts like o-rings
HCR/peroxide-cure for larger, simpler parts where cost is a factor
Always specify the durometer (Shore A) and temperature rating—most industrial silicones handle -60°C to 200°C continuously

Per ASTM D2000, silicone rubbers are classified as GE/GS materials. Verifying that your supplier meets the specific line callout is where my team catches about 15% of first-delivery rejections. In Q1 2024, we rejected a batch of seals because the compression set was 42% against our spec of 30%. The vendor claimed "within industry standard." Our customer wasn't interested in industry standard—they wanted the spec.

Scenario C: You're comparing TPU to silicone (and you're not sure which fits)

Here's the surprise: TPU often wins on tear strength and abrasion resistance. Silicone wins on temperature range and chemical resistance. But the decision isn't always obvious.

If you need a part that flexes repeatedly without tearing—like a bellows or a diaphragm—TPU might outlast silicone by a factor of 2-3x in some applications. But if that same part sees oil, solvent, or high heat, silicone is the safer bet.

I ran a blind comparison with our engineering team last year: same part geometry, one in TPU, one in 50A silicone. 80% of the team identified the silicone as "more premium feeling" without knowing the difference. The cost difference was about $0.08 per part on a 20,000-unit run—$1,600 total for measurably better perception in that specific application.

How to figure out which one you need

Here's the framework I use with our procurement team:

What's the operating temperature? Above 100°C consistently? Go silicone. Below -40°C? Also silicone.
What's the part geometry? Complex undercuts? TPU or LSR can handle injection molding better than cast silicone.
What's the production volume? Under 500 units a year? Cast silicone is your cheapest path. Over 10,000? LSR or TPU injection molding will win on unit cost.
Is this a cosmetic part? Silicone typically has better color consistency and a softer touch. TPU can look more "plastic."

The fundamentals haven't changed—silicone is still my go-to for thermal stability and flexibility. But the execution has transformed. New formulations in the last five years have closed the gap between traditional silicone and specialty TPUs in ways that make the old rules of thumb less reliable.

Side note on the Dow question: If you landed here because you searched "how did Tony Dow die" or "Dow Jones Sustainability Index," I can't help you with those—but I can tell you that Dow (the materials company) has been investing heavily in sustainable silicone technologies. If I remember correctly, their circular silicone initiative targets 100% recyclability for certain sealant products by 2025.