I manage purchasing for a 150-person industrial service company—roughly $450,000 per year across 8 vendors. I am not a materials engineer. I'm the person who gets called at 4:45 p.m. because a seal failed and the maintenance team needs a replacement by tomorrow morning. That's why I had to learn the practical difference between thermoset, thermoplastic, and elastomer. Not from a chemistry textbook. From purchase orders.

When I first started in this role, I assumed "thermoset vs thermoplastic vs elastomer" was a three-way choice, like choosing between three vendors. It's not. Two of those words describe how a polymer responds to heat. The other describes how stretchy it is. Mixing them up is expensive.

What This Comparison Is (and Isn't)

If you search "thermoset vs thermoplastic vs elastomer," you'll find a lot of chemical definitions. I'm going to compare them the same way I compare suppliers: heat behavior, mechanical behavior, supply chain, and lifecycle. If you're an engineer, this will feel oversimplified. If you're a buyer, this is the part that saves you from a $2,400 expedite charge.

Dimension 1: What Heat Does to Each Material

Thermoplastics melt when heated. Thermosets cure when heated, and then they don't melt again. Elastomers are defined by their ability to stretch and recover—and they can be either thermoset or thermoplastic. That's the part that surprised me. A cure-cast thermoset rubber and a thermoplastic elastomer (TPE) can both behave like rubber, but their heat limits are completely different.

The fastest way to choose is to ask: does this part need to survive a one-time cure, or does it need to be melted down and re-processed later?

PTFE is a good example. PTFE is a semi-crystalline thermoplastic, but it doesn't melt-flow like a common plastic. That's why we machine PTFE rods instead of injection molding them. If a drawing calls for Rundstab PTFE, that means PTFE round rod—not sheet, not tube. I know that because I once ordered the wrong product form and the shop called me before cutting anything. That phone call saved us a re-order.

Conclusion: If the part must not remelt in service, thermoset is the safer side. If it must be machined or welded, thermoplastic is the practical side. For elastomers, check the continuous service temperature on the datasheet, not just the material family name.

Dimension 2: Elasticity and Mechanical Behavior

This is where the term "elastomer" gets confusing. An elastomer is not a third family; it's a behavior. A silicone foot on an instrument is an elastomer: press it, it deforms, release it, it bounces back. A PTFE washer is a thermoplastic: it will deform under load and stay deformed—that's called creep. A thermoset like phenolic is rigid; it will crack before it stretches.

From the outside, a silicone foot and a PTFE bumper look like they could be swapped. The reality is their failure modes are opposites. The silicone foot is chosen because it returns. The PTFE part is chosen because it doesn't stick and it resists chemicals. Swap them, and both fail.

Another number to ask for is compression set. A gasket with high compression set stops springing back, and then it leaks. That matters more than the durometer reading. I use ASTM D1418 nomenclature mostly to avoid writing "rubber-like material" on a purchase order. The letter code—NBR, EPDM, VMQ, FKM—forces me to name the polymer.

Conclusion: If the part must return to shape, choose an elastomer. If it must hold a rigid shape, choose a thermoset or a rigid thermoplastic. And don't assume that a part with rubbery feel is an elastomer. TPEs are not the same as crosslinked rubber.

Dimension 3: Processing, Machining, and Lead Times

As a buyer, I care about how I'm actually going to get the part. Thermoplastic rod stock is easy for a machine shop to cut, weld, or rout. That's why PTFE rods exist. When I need custom spacers, I order PTFE round rod—often listed on European drawings as Rundstab PTFE—and our shop can machine it the same day. A thermoset part is usually molded or cast. Once it cures, you can't soften it to rework it. An elastomer gasket is usually die-cut from sheet or molded, and it has a shelf life.

I use the DuPont website to confirm whether a material can be machined, welded, or molded before I choose a product form. It saves us from a lot of "I thought that came in a rod" conversations.

Conclusion: If you need speed and local fabrication, thermoplastic is easier. If you need a formed part with stable structure, accept the thermoset mold and cure lead time. For seals, order the elastomer with the right hardness and compression set, and buy a spare.

Dimension 4: Maintenance and Lifecycle Cost

This is where prevention beats cure. Materials don't usually fail at the moment you order them. They fail later because the environment was wrong or maintenance was skipped.

For example, the DuPont Rubber Saver Gasket Conditioner sounds like an afterthought. It's a preventive step that we apply to elastomer gaskets during scheduled maintenance. It keeps the rubber from drying out and taking a permanent set. I used to think it was a cleaner. It's not. If you apply it to a cracked gasket, it won't save you. Apply it before the crack appears. That's the lesson I learned after replacing a $14 gasket with a $900 service call.

A silicone foot on a machine should also be checked for compression set. When it starts leaving a flat spot, it has already gone too far. The same logic applies to PTFE seals: PTFE doesn't dry out like rubber, but it creeps under load. A PTFE seal should be inspected for extrusion, not just cracking.

I don't have hard data on industry-wide material failure rates. But in our 2024 audit, 3 out of 11 late shipments were caused by wrong or vague polymer specifications. That's enough to make me verify.

Conclusion: The cheapest material is the one that keeps running. A maintenance checklist is the cheapest insurance you can buy.

Which One Should You Order?

This advice comes from a service company with predictable orders. If you're designing a high-volume consumer product, the manufacturing economics will change the answer. But for industrial MRO and custom parts, this is how I think about it:

  • Thermosets: Choose these for parts that must not deform or melt under heat and chemicals, and where you don't plan to recycle the material. Think structural housings and high-temperature composite parts.
  • Thermoplastics: Choose these for machined, welded, or injection-molded parts where lead time and reusability matter. PTFE rod—Rundstab PTFE—is my go-to chemical-resistant, low-friction stock for custom spacers and seals.
  • Elastomers: Choose these for seals, gaskets, vibration mounts, and soft-touch parts. A silicone foot is a good example: it's not a structural plastic, it's a rebound part. It needs the right polymer for temperature, chemical, and UV conditions.

If you're not sure, buy a datasheet before you buy the material. As of January 2025, I start on the DuPont website and pull the official data sheet for every polymer I order. Fifteen minutes of verification now beats a five-day correction later. That's the whole comparison, really. Not thermoset versus thermoplastic versus elastomer as winners. Find the right behavior for the job. Document it. Done.