Post-Molding TPU: Annealing, Welding, Bonding & Machining

Most of the TPU literature — including most of this site — stops at the moment the part leaves the mold. In practice, a lot of parts are only half done at that point. Seals get annealed to improve recovery. Films and tubes get welded. Soft pads get glued to metal. Logos get printed. Prototype stock gets drilled and milled. Every one of those operations works on TPU, and every one of them has a rule or two that differs from rigid plastics — usually because of the same toughness and elasticity you chose the material for.

This guide collects the downstream operations in one place. It assumes the molding side is already handled — for that, see the TPU processing guide.

Editorial note: This is an independent educational guide. Annealing conditions, welding behavior, and machining parameters are grade-dependent; the values here are illustrative of published TPU guidance. Confirm against the current supplier documentation for your grade.

The part is molded — you are not done

Secondary operations fall into four buckets, and a given part may need none or several:

• Heat treatment — annealing to finish developing the material's properties.
• Joining — welding TPU to itself, or bonding it to other materials.
• Decoration — printing and painting.
• Machining — drilling, turning, milling, grinding, and cutting.

The common thread: decisions made back at grade selection (hardness, lubricant package) and at molding quietly decide whether these steps go smoothly. The most expensive way to learn that is to discover, after tooling, that your chosen grade cannot be painted.

Annealing: the cheapest property upgrade

Freshly molded TPU has not finished becoming itself. The phase structure that gives the material its properties keeps developing after molding, and a heat treatment accelerates and completes that process. The published recommendation is straightforward: about 20 hours at 100 °C in a circulating-air oven.

Published comparisons of annealed parts against parts conditioned at room temperature for up to 35 days show the payoff, most visibly in compression set — the recovery property that matters for seals and load-bearing parts. An annealed part can reach better, more stable properties than weeks of room-temperature aging deliver. Two practical cautions come with it:

• Soft or thin parts with low dimensional stability should be supported during annealing, or 20 hours of oven heat will leave them deformed.
• Extruded products are annealed only in specific cases — the treatment is mainly an injection-molding finishing step.

If a TPU seal or cushion is failing a compression-set requirement by a small margin, annealing is the first thing to try before changing the grade — it is an oven and a day, not a requalification. The property itself is covered in TPU compression set & resilience.

Welding TPU: matching method to part

TPU is a thermoplastic, so it welds — the trick is matching the method to the part form and hardness. Published guidance sorts it cleanly:

• Injection-molded parts — hotplate, ultrasonic, high-frequency, or friction welding. Ultrasonic works best on the harder grades; soft grades damp the vibration that ultrasonic welding depends on.
• Semifinished parts and profiles — hotplate and friction welding, plus hot-gas welding.
• Films — thermal sealing, heat-impulse welding, and high-frequency welding give the best results.

Whatever the method, weld strength comes down to two variables: temperature high enough for genuine plastic flow but below the decomposition temperature, and pressure that drives the molten boundary layers to flow and diffuse into one another — and then holds the joint while it solidifies. And a safety note that is easy to skip: welding TPU generates vapors, so adequate extraction is required at every welding station.

Welding methods by part form

Adhesive bonding: TPU to TPU, TPU to metal

When welding is not an option — dissimilar materials, no access for a horn or hotplate — adhesives take over (distinct from molding a soft layer directly onto a rigid part, which is covered in overmolding with TPU/TPE), and the chemistry pairing is well established:

• TPU to TPU: elastic polyurethane-based adhesives have proven successful. Like bonds like, and the elastic adhesive flexes with the joint.
• TPU to metal and other hard materials: epoxy resin adhesives are the standard choice, with systems available specifically for the purpose.

Two requirements decide success before any adhesive is opened. The surfaces need the usual pretreatment, and — the one that surprises people — good adhesive bonding is only achievable with lubricant-free TPU grades. Internal lubricants that make molding and demolding easier migrate to the surface and quietly defeat the adhesive. If the part will be glued, that constraint belongs in the grade selection, not in a troubleshooting meeting after the bond fails.

Printing and painting

The same lubricant rule governs decoration: lubricant-free grades can be printed or painted, using printing and painting systems offered for polyurethane. A grade with an internal lubricant package will shed ink and paint for the same reason it sheds adhesive.

So the practical sequence is: decide at the start whether the part will ever need printing, painting, or gluing — and if yes, specify a lubricant-free grade even though it may demold a little less easily. Trading a slightly harder ejection for a surface that accepts decoration is almost always the right trade, because the alternative is a grade change after tooling.

Machining: tough material, hot edges

TPU can absolutely be machined — drilled, turned, milled, ground, punched, cut — but its exceptional toughness and tear strength mean it does not machine like a rigid plastic. Soft, elastic material deflects away from a dull edge instead of shearing cleanly, and it converts cutting friction into heat that the polymer cannot tolerate. Hence the two iron rules:

• Keep cutting edges correctly sharpened. A dull tool tears and smears instead of cutting.
• Cool aggressively. Excessive heat generation must be avoided — compressed air or emulsion cooling should always be provided.

The elasticity also shows up in the dimensions. Drilled holes come out smaller than the drill's nominal diameter, because the material relaxes back after the cut — for grades up to about 80 Shore A the reduction is roughly 4–5%. Hollow drills give better accuracy, and the drill should be raised frequently with efficient cooling. In turning, tools with smaller-diameter cutters than metalworking practice reduce cutting force and heat; in milling, fewer blades on the cutter head help chip formation; grinding wants a high-porosity, open-structure wheel with low hardness (grain size 60–80) to prevent heating.

Machining parameter reference

Published guideline values for machining TPU:

Treat these as starting points and expect the soft end of the hardness range to be the fussiest — the harder the grade, the more conventionally it machines.

Planning post-processing into the part

The pattern across all four operations is that they are won or lost upstream:

• Need a better compression set? Plan the annealing oven into the process, with support fixtures for floppy parts.
• Joining TPU to TPU at volume? Pick a welding method that matches the part form and a hardness that suits it (harder for ultrasonic).
• Gluing, printing, or painting? Specify a lubricant-free grade on day one.
• Machining to final dimensions? Budget the elastic spring-back (under-size holes) and the cooling setup.

None of these steps is exotic. They just need to appear in the specification at grade-selection time instead of being discovered at the end of the line.

Bottom line

TPU's life does not end at the mold. Annealing — about 20 hours at 100 °C — finishes the material's property development and is the cheapest fix for marginal compression set. Welding works across hotplate, ultrasonic (harder grades), high-frequency, friction, and film-sealing methods, governed by temperature, pressure, and vapor extraction. Bonding pairs polyurethane adhesives with TPU-to-TPU joints and epoxies with metal, and both bonding and decoration demand lubricant-free grades. Machining rewards sharp tools, aggressive cooling, and an allowance for elastic spring-back. Decide which of these the part needs before the grade is locked, and the downstream steps become routine instead of rework.

For grade-specific guidance on annealing, welding, bonding, and machining TPU, BASF's Elastollan processing documentation is the reference this guide draws on.

Related: TPU Processing Guide → · TPU Compression Set & Resilience →