Flexible parts that work: TPU and silicone for gaskets, grips and dampers

Ask most engineers what a 3D printing service makes and they'll picture rigid parts — brackets, housings, jigs. The flexible stuff — gaskets, grips, seals, vibration dampers — still gets sourced the old way: catalogue extrusions cut to length, or a moulding quote that only makes sense at ten thousand units. That's a gap worth closing, because printed elastomers have got quietly good. We print flexible parts most weeks, and the difference between a part that works and a squashy paperweight is mostly knowing which of two very different materials to reach for.

Two routes to a flexible part

The first route is TPU — thermoplastic polyurethane — run on our FDM printers. It's a firm, tough rubber at around Shore 95A: think roller-skate wheel or boot sole rather than rubber band. It resists abrasion, is genuinely hard to tear, and shrugs off oils and greases, which is why it turns up in grips, feet, bumpers and wear parts.

The second is Silicone 40A, a true silicone we run through our SLA printing service on the Formlabs Form 4. At Shore 40A it's properly soft — closer to an oven glove than a shoe sole — and it behaves like the moulded silicone you already trust: stable from roughly −25°C to 125°C, excellent recovery after being squashed, and chemically docile. It's real silicone, not a rubbery plastic imitating one.

Where TPU earns its keep

TPU is the default when a part needs to be flexible and tough at the same time. Typical jobs: machine feet and anti-vibration pads, protective bumpers and edge guards, cable and hose strain reliefs, rollers, tool grips, blade covers, and straps or clips that flex thousands of times without whitening or cracking. Printed TPU parts are near-impossible to snap, and layer bonding is excellent — so, unusually for FDM, you rarely need to think about load direction.

The trick most people miss is that stiffness is a property of geometry as much as material. A solid block of 95A TPU is surprisingly firm; the same part printed with a tuned internal lattice compresses like a much softer rubber. We use this constantly for dampers and pads — telling us the feel you want ("squashes a few millimetres under thumb pressure") is more useful than naming a Shore number.

When only real silicone will do

TPU has two blind spots: it's never truly soft, and it doesn't love heat. On FDM the practical hardness floor is around Shore 90–95A, and above roughly 60–80°C TPU starts to relax and creep. Long-term sealing is the other weakness — clamp a TPU gasket for months and it takes a set, and the seal slowly gives up.

That's where Silicone 40A takes over: gaskets and soft seals that stay springy for years, drip-proof covers and gaiters, dust boots, soft-touch pads, and anything that lives near heat. Compression set — the tendency of an elastomer to stay squashed — is dramatically better in silicone, and resisting it is precisely a gasket's job.

Designing a flexible part that actually works

A few rules carry most of the weight. Wall thickness sets feel: below about 1 mm, TPU walls turn floppy and hard to print cleanly; 2–3 mm flexes; 5 mm and up reads as solid. For seals, design in a compression feature — a rib, bead or O-ring-style bulge 10–25% taller than the gap it fills — rather than relying on a flat face. Give squashed material somewhere to go: rubber is incompressible, so a gasket groove needs around a quarter of its volume spare. And for dampers, remember the lattice trick above — tell us the load and we'll suggest one.

If the part you need already exists — a perished seal, a crumbled grommet, a grip long out of production — this becomes a reverse engineering job: we measure what's left (or the groove it sat in), rebuild the geometry in CAD, and print it in the right elastomer. The same thinking extends to soft tooling: where printing the final part directly isn't the answer, we print mould tooling and cast instead, as in our eight-part mould for a silicone bust.

A representative example

To make the numbers concrete: an outdoor sensor enclosure with a perished rectangular seal, the original supplier gone and no drawings. We measure the groove, rebuild the seal in CAD with a proper compression bead, and print twelve in Silicone 40A. A job of that shape typically lands around £100–150 all in, most of it the one-off CAD work — repeats reorder at £4–8 a part. These are illustrative figures for a small seal, not a quote; your geometry sets the price.

What we can and can't do

The honesty section, as usual. We don't supply certified parts: nothing certificated for food contact, medical use, potable water or gas sealing — our elastomers are engineering-grade, and we won't paper over that. We can't print truly soft TPU: below roughly Shore 90A the material buckles rather than prints on FDM, so "softer than a skate wheel" means silicone. High-pressure hydraulic and pneumatic seals are out — printed elastomers suit gaskets and low-pressure sealing, not 100-bar glands. Silicone parts are capped at roughly 200 mm on the Form 4. And while silicone's chemical tolerance is broad, continuous immersion in hot or chemically treated water is something we'd rather test and confirm than promise off a datasheet. If a standard catalogue O-ring fits your groove, buy the O-ring — printing earns its keep on the shapes catalogues don't cover.

How to start

For TPU, upload your model for an instant online quote and pick TPU as the material. For Silicone 40A — or anything where you're not sure which side of the line your part falls — send a short enquiry with a photo or model and we'll come back with the material call, any geometry tweaks that make it work better, and a price, usually the same day. Most flexible parts are small, and small parts move through our queue fast.