Here is a trap worth naming early: TPU and TPE run on standard injection molding machines, look like ordinary pellets, and mold into ordinary-looking parts. So a team that knows ABS or polycarbonate assumes an elastomer is just "soft plastic" and reuses the same instincts. The instincts are wrong in a few specific, predictable ways — and those ways are exactly where the defects show up.
This is a materials-side contrast, not a molding tutorial. The point is to show how the material behaves differently, so the right TPU-specific parameters — which come from the elastomer's own datasheet, never a generic rigid-resin table — get used. For the deeper single-topic pieces, see the TPU processing guide and processing window & common defects.
Editorial note: This is an independent educational guide. The rigid-resin figures here are used only as a contrast backdrop; TPU values are illustrative of published grades. Always use the actual grade's datasheet for processing parameters.
Why an elastomer is not just a soft plastic
A rigid resin and a thermoplastic elastomer are built for opposite jobs. The rigid resin is meant to hold its shape and resist bending; the elastomer is meant to flex and recover. That difference in what the material is shows up in how it behaves in the barrel and the mold. Four areas drift far enough from rigid-resin habits to cause real trouble: drying, flow, mold temperature and cooling, and ejection. Take them one at a time.
Drying: TPU plays by stricter rules
Rigid resins span the whole range of moisture sensitivity. Commodity resins like polyethylene and polypropylene barely need drying. Hygroscopic engineering resins — nylon (PA), polycarbonate, ABS — do need drying, commonly to a target on the order of 0.1 to 0.3 percent moisture, often with a dehumidifying dryer.
TPU sits at the strict end and then some. It is strongly hygroscopic and is typically dried to below 0.02 percent — roughly an order of magnitude tighter than a typical engineering-resin target. The mechanism is the same family of problems a molder already knows from nylon (don't load undried resin, use a dehumidifying dryer), but the tolerance is smaller and the consequences — splay, bubbles, and quiet strength loss from in-barrel hydrolysis — arrive faster. The habit to drop: "I dried it like I dry my engineering resins." TPU needs more. The full treatment is in drying TPU before molding.
Flow: shear thinning and the TPU exception
Many materials shear-thin: push them harder (higher shear rate) and their viscosity drops, so they rush into thin walls and long flow paths. A lot of TPEs do this strongly, which is part of why soft styrenic compounds fill delicate sections so well.
TPU is the notable exception. Urethane-based materials do not shear-thin the same way, so the "just push it harder and it'll fill" reflex does not pay off as expected. A thin-wall design that a rigid resin or a styrenic TPE fills easily can short-shot in TPU at comparable settings. Practically, that means more attention to wall thickness, gate size and location, and fill speed when moving a design into TPU — it will not bail you out with easy shear thinning.
Mold temperature and cooling
Rigid resins run a wide spread of mold temperatures, and hot molds (often well above 80 °C for engineering and high-temperature resins) are common to get surface finish and crystallinity right. TPU generally runs relatively cool molds — frequently in the 20–60 °C range depending on grade and part. The cooling behavior of a soft, flexible part also differs from a rigid one, and a soft part pulled too hot will distort more readily because it has little stiffness to hold its own shape on the way out.
Ejection: soft parts do not pop out
This is the difference that surprises rigid-resin molders most. A rigid part is stiff enough that ejector pins push it cleanly off the core. A soft elastomer part is not — it can deform under the pins instead of releasing, and it tends to cling to mold surfaces. The same softness that makes the part useful makes it awkward to demold.
The implication is that ejection has to be designed for a soft material: larger ejector contact areas to spread the load, gentler ejection, attention to draft, and awareness that surface finish and texture affect release. A soft grade that feels great in the hand can be the one that fights you on the floor, and that is a material property, not a machine fault.
Elastomer vs rigid resin at a glance
| Behavior | Typical rigid resin (e.g., ABS, PC, PA) | TPU (elastomer) |
|---|---|---|
| Drying need | None to moderate; ~0.1–0.3% target for hygroscopic grades | Strong; typically < 0.02% target |
| Shear thinning | Often present | Weak — does not shear-thin like many TPEs |
| Mold temperature | Wide; often hot for engineering grades | Often relatively cool (~20–60 °C) |
| Ejection | Stiff part pushes off cleanly | Soft part can deform / cling; needs gentle, spread-out ejection |
| Part stiffness leaving the tool | Holds shape | Flexible; distorts more easily if too hot |
| Heat / residence sensitivity | Varies | Sensitive; degrades with excess heat or residence time |
| Source of parameters | Rigid-resin datasheet | Elastomer datasheet (never a rigid-resin table) |
Shrinkage and dimensional behavior
Shrinkage is grade-specific for both material classes, so the headline is "use the real number," not a generic rule. The pattern worth knowing is that reinforcement changes the picture sharply: a glass-reinforced TPU shrinks much less and more directionally (lower along the flow, higher across it) than a soft unreinforced grade. Carrying a shrinkage value over from a familiar rigid resin is a recipe for a tolerance miss — the elastomer grade's own data is the only safe input. The reinforced end is covered in glass-fiber reinforced TPU.
What it means for part design
These material differences feed straight back into how a part should be designed. Wall thickness interacts with TPU's weaker shear thinning; ejection interacts with softness; shrinkage interacts with grade and reinforcement. In other words, choosing an elastomer is not only a material decision — it changes the geometry that will mold well. For the buyer-and-design view of how those geometry choices affect what can actually be made, PlasticsTechnologyAlliance.com has a useful primer on how part design affects moldability.
Bottom line
TPU and TPE run on the same presses as rigid resins, but they are different materials and they behave differently in four ways that matter: they need stricter drying (TPU often below 0.02% versus 0.1–0.3% for hygroscopic rigid resins), they do not shear-thin the way many materials do, they usually run cooler molds and leave the tool soft, and they resist clean ejection. The single most important habit to drop is reusing rigid-resin assumptions — including generic processing tables. Pull TPU's parameters from the elastomer's own datasheet, design the part for soft-material behavior, and the material runs well.
For grade-level processing data on TPU, BASF's Elastollan documentation is a solid reference.
FAQ
Is processing TPU the same as processing a rigid plastic?
No. While TPU runs on standard injection molding equipment, its behavior differs from rigid resins in several ways: it is strongly hygroscopic and needs stricter drying, it does not shear-thin like many materials, it releases from the mold differently because it is soft, and it is sensitive to heat and residence time. Treating it like a rigid resin is a common source of defects.
Does TPU need more drying than rigid resins?
Usually stricter. Many engineering rigid resins dry to a target around 0.1 to 0.3 percent moisture, while TPU is typically dried to below 0.02 percent. Some rigid commodity resins barely need drying at all, whereas TPU almost always does and to a tighter limit.
Why does soft TPU fill thin walls less easily than expected?
Because urethane-based materials do not shear-thin the way many TPEs and some rigid resins do. Many materials drop sharply in viscosity at high shear and rush into thin sections; TPU does not show that effect as strongly, so thin-wall filling needs more attention to gate, speed, and wall thickness.
Why are soft molded parts harder to eject?
A soft, flexible part can deform under ejector pins instead of pushing off cleanly, and it can cling to mold surfaces. Rigid parts are stiff enough to be pushed off, so ejection design for soft elastomers needs larger contact areas, gentler pressure, and attention to surface and draft.
Do elastomers shrink differently than rigid resins?
Shrinkage varies by grade for both, but elastomers and rigid resins behave differently, and reinforced grades shrink less and more directionally than unreinforced ones. Always use the specific grade's data rather than assuming a generic value.
Can I use rigid-resin processing tables for TPU?
No. Generic processing-condition tables for rigid resins do not apply to TPU. TPU's drying, melt temperature, flow, and ejection behavior are specific to elastomers and to the individual grade, so its parameters must come from the elastomer's own datasheet.
Related: TPU Processing Guide → · Processing Window & Common Defects →