What Is TPE (Thermoplastic Elastomer)?

I got my first TPE job wrong in 2009. Customer wanted a soft grip on a cordless drill housing. We ran Santoprene 101-64 over glass-filled nylon substrate. The parts looked fine coming off the press. Three weeks later the customer called. The TPE was peeling off in sheets.

Turned out we had the tool temperature too cold. The nylon skin froze before the TPE hit it. No bond formation. Scrapped 8,000 parts.

TPE is not rubber. This distinction matters. Rubber vulcanizes. Once you cure EPDM or silicone, the molecular chains crosslink permanently. You cannot remelt it. TPE has physical crosslinks instead of chemical ones. The hard blocks in the polymer chain act like temporary anchor points. Heat breaks those anchors. Cool it down, they reform. You can regrind TPE runners and put them back in the hopper. Try that with LSR.

Two-shot molding with TPE means you are injecting two different materials in sequence. First shot is usually a rigid substrate. PC, ABS, PP, nylon. Second shot is the TPE overmold. The trick is getting them to stick.

Teknor Apex publishes bonding charts for their Monprene grades. So does Kraiburg with their Thermolast K line. I keep printed copies in my office because the online versions change and sometimes the old data was more useful. The charts show which TPE grades bond to which substrates without primer or adhesive. PP to TPE-S works. ABS to TPE-S does not work. You need TPE-U for ABS.

Melt temp on Santoprene runs 380 to 420 depending on the grade. I have seen guys crank it to 450 trying to get better flow on thin walls. Bad idea. The material degrades. You get black specks and the Shore hardness drifts.

Wall thickness is where new engineers mess up. They design 0.020 walls because that is what they do on PC parts. TPE does not flow like PC. You need 0.050 minimum for anything structural. I have gotten away with 0.035 on short flow lengths but it is risky.

The two-shot process requires the substrate to still be warm when the TPE hits it. Rotary platen presses handle this well. Substrate shoots at station one, platen rotates 180 degrees, TPE shoots at station two. Total cycle maybe 35 seconds for a medium-sized grip. Some guys use transfer robots between two single-shot presses. Works fine but the substrate cools during transfer. You compensate by running the TPE hotter or heating the substrate cavity.

Shore hardness selection depends on what the part does. 40A feels like a pencil eraser. 70A feels like a shoe sole. 90A feels almost rigid. Most handheld grips land between 50A and 70A. Automotive armrest skins run softer, maybe 45A.

Chemical resistance is the other consideration. Styrenic TPE grades like Kraiburg Thermolast K swell in gasoline and brake fluid. For under-hood automotive you need TPE-V like Santoprene or TPE-U like Desmopan. Cost difference is real. Styrenic runs maybe $2.50 per pound. TPE-V is $4 to $6. TPE-U can hit $8 for specialty grades.

I stopped quoting jobs under 25,000 annual volume for two-shot TPE. The tooling cost does not amortize. A two-shot mold runs $80,000 to $150,000 depending on cavitation and complexity. Below 25k pieces you are better off with insert molding or post-applied grip tape.

Regrind percentage matters. Virgin TPE bonds better than regrind. Most material specs allow 15 to 20 percent regrind in the blend. Push it to 30 percent and bond strength drops. I tested this on Dynaflex G2703 a few years back. At 25 percent regrind the peel strength fell from 12 pli to 8 pli. Parts still passed spec but barely.

The medical device guys have their own concerns. USP Class VI is table stakes now. Cytotoxicity testing, extractables studies, lot traceability. GLS Thermoplastic Elastomers has a whole medical portfolio with the documentation package. Teknor Apex Medalist line is the other option. Lead times on medical grades run longer. Plan for 6 to 8 weeks.

Gate vestige on TPE is ugly. The material strings when the mold opens. You get a little nub that has to be trimmed. Put your gate where nobody sees it or plan for secondary deflashing. Tab gates work better than pin gates for controlling vestige height.

Two-shot TPE is mature technology at this point. The material suppliers have figured out the formulations. The machine builders have the process dialed. Most failures I see come from design engineers who spec the wrong material pairing or tooling engineers who get the gate location wrong. Read the bonding charts. Run the DoE on tool temp. The rest takes care of itself.