Every product with a seal, a grip, a bumper, or a flexible boot forces the same choice: make the soft part from a true thermoset rubber cured in a compression press, or from a thermoplastic elastomer injected on a standard molding machine. Both squeeze between your fingers. They behave completely differently after a year in service.
The short version: thermoset rubber wins on performance, thermoplastic elastomer wins on manufacturing. Start by asking whether the application genuinely needs the performance, because if it does not, TPE is cheaper, faster, and far easier to combine with rigid parts.
Two Different Materials Pretending to Be One
Thermoset rubber — natural rubber, EPDM, nitrile, neoprene, fluoroelastomer, solid silicone — is an uncured compound of polymer, filler, and curatives. Heat and pressure in the mold trigger vulcanization, forming permanent crosslinks. The reaction is irreversible: the part cannot be remelted and scrap cannot be reground.
Thermoplastic elastomer — styrenic block copolymers, TPU, TPV, and copolyester grades — is a blended or block-structured plastic that softens with heat and resolidifies on cooling, over and over. No chemical cure, ordinary injection equipment, ordinary cycle times, regrind allowed.
Both are specified on the same Shore A scale, which is where people assume equivalence. A 60 Shore A EPDM and a 60 Shore A TPE feel alike in the hand and behave nothing alike under sustained load, as Shore hardness explained makes clear.
Compression Set Is the Decisive Property
If a soft part is squeezed and expected to push back a year later, this is the number that matters. Compression set measures how much of an applied deformation is permanent after a defined time at temperature.
A good nitrile or EPDM compound shows roughly 15 to 25 percent set after 22 hours at 212°F. A typical styrenic TPE runs 40 to 70 percent, with TPU in between. Practically: install a TPE gasket under bolt load and seal force decays as the part takes a permanent impression of the flange. This is why static seals, lip seals, and anything under long-duration clamp load stays in thermoset rubber, as covered in O-ring selection and gland design.
Where the part is squeezed briefly and released — a push button, a grip, a bumper, a cable boot — compression set barely matters and TPE is entirely appropriate.
Temperature and Chemical Limits
- Silicone rubber: −80°F to 400°F, outstanding UV and ozone resistance, poor with fuels and oils, biocompatible grades available.
- EPDM: −50°F to 300°F, excellent weather and water resistance, poor with petroleum oils. The default outdoor seal.
- Nitrile (NBR): −30°F to 250°F, excellent oil and fuel resistance, poor ozone resistance.
- Fluoroelastomer (FKM): to 400°F with broad chemical resistance, at several times the cost of nitrile.
- Styrenic TPE: realistically −40°F to 160°F, weak against oils, softens noticeably in a hot car.
- TPU: to 175°F, exceptional abrasion and tear resistance, hydrolyzes in hot humid air unless a polyether grade is specified.
- TPV: to 275°F with compression set close to EPDM — the bridge grade for TPE processing with rubber-like recovery.
Liquid silicone rubber sits apart: a thermoset that injects on dedicated equipment with fast cycles and no flash, giving rubber performance with plastic-like economics. When it wins is set out in liquid silicone rubber molding.
Tooling and Cycle Time: The Economics Flip
Compression molding uses a simple two-plate tool. Uncured preforms are placed in the cavities by hand and cure under heat and pressure. Tooling is cheap — typically $2,500 to $12,000 for a multi-cavity tool — but the cycle is two to eight minutes, and every part comes out with flash to be trimmed or cryogenically tumbled. Labor content is high and per-part cost stays high forever.
Injected TPE uses a conventional injection mold, costing more up front — $9,000 to $45,000 for a production steel tool — but cycling in 20 to 50 seconds with no flash and no operator. The tooling logic is the same as any thermoplastic, including the choice in aluminum versus steel injection molds and the cost structure in injection molding costs.
The crossover generally arrives between five thousand and thirty thousand parts a year. Transfer molding sits between the two: a rubber process with a runner system and better dimensional control, tooled at about twice compression cost, useful for parts with inserts.
Overmolding Is Where TPE Pulls Away
If the soft material must bond to a rigid part — a grip on a tool handle, a seal molded onto a housing rim, a strain relief on a connector — TPE is dramatically easier. Compatible grades bond chemically to polypropylene, ABS, polycarbonate, or nylon during a second shot, giving one component with no adhesive, no assembly labor, and no gap for water. Two-shot molding on a rotary machine makes it a single automated operation; insert molding does it with two presses. Both are explained in overmolding and two-shot injection molding.
Bonding thermoset rubber to a rigid substrate needs a chemical bonding agent on a cleaned, primed surface, and the process window is narrow. Either way, design mechanical interlocks and undercuts into the interface so the joint survives even if the chemical bond degrades.
Recyclability, Regrind, and Sustainability Claims
TPE runners and rejects regrind back into the process, typically at 15 to 30 percent without property loss. Thermoset scrap cannot; it is ground into crumb or landfilled, and sprue and flash losses on a compression job can reach 20 percent of shot weight. An all-thermoplastic product recycles as a single stream far more easily than one with a bonded rubber component, which matters if you carry environmental claims or ship into take-back markets.
A Practical Decision Rule
- Sustained compression, high heat, or exposure to oils and aggressive chemicals? Thermoset rubber, with the polymer chosen by the specific exposure.
- A grip, bumper, cushion, boot, or soft-touch element bonded to a rigid part? TPE, overmolded.
- Rubber-grade recovery with plastic-grade throughput? TPV, or liquid silicone if temperature or biocompatibility demands it.
- Annual volumes under a few thousand? Compression molding's cheap tool probably wins regardless.
Specify the Soft Part Before the Hard One Is Frozen
Wall sections, draft, bond surfaces, and gate positions on the rigid part all depend on which route you take, and reversing the decision after tooling is expensive. Projects House selects elastomers and molding routes for US clients and manages qualification through a global manufacturing network. Send your part geometry, service temperature, and expected volume through our contact form.