Custom TPU Injection Molding Factory

What TPU actually is

TPU is a flexible engineering elastomer used for gaskets, soft-touch overmoulds and wear-resistant components. Can be either ester-based (better oil resistance) or ether-based (better hydrolysis resistance). The tool runs cooler than hard engineering plastics.

Chemically, TPU belongs to the Thermoplastic polyurethane - engineering family. The polymer chain structure — linear and semi-crystalline — is what sets its shrinkage behaviour, temperature ceiling and melt flow. Like every thermoplastic, its properties are the result of processing history as much as of the resin grade itself: slow cooling increases crystallinity and brittleness, fast cooling locks in an amorphous structure and a different shrink. Resin suppliers offer TPU in commodity, reinforced and specialty grades, and the first selection step is to pick the right family before settling on a specific TDS.

Processing window

The typical process window runs with a melt temperature around 180-10 °C and a mould temperature around 30-0 °C. Inside that band the resin fills, packs and ejects predictably. Outside it, the failure modes are specific rather than vague.

Above the recommended melt temperature, TPU can degrade thermally — the symptom is silver streaking, a mottled surface and a measurable loss of impact strength that the part carries into service. Long residence time in a large barrel running small shots is as damaging as a direct temperature overshoot, because the resin sits at temperature longer than intended. Below the lower bound, the melt freezes before it has fully packed the cavity: the part shows visible weld lines, flow marks around the gate and a surface that reads differently across the flow path.

Drying is 3- h at 80 °c. TPU absorbs moisture readily from ambient air, and moulding it wet produces silver streaks that look like overheat and cannot be fixed by lowering the barrel temperature. 3- h at 80 °C is the normal starting window. The resin supplier TDS always takes precedence over a generic figure on drying and moisture content.

Shrinkage and dimensional tolerance

Design the tool to 0.8-1.2 % shrinkage and expect the actual figure to sit at a specific point inside that band rather than the middle. The value is set by wall thickness, flow length, gate position, packing pressure, hold time and mould temperature. Thick sections shrink more because there is more melt to contract; long flow paths lose pressure before the cavity is packed, so the far end shrinks more than the gate end.

TPU is semi-crystalline, which means shrinkage is directional — flow-direction shrinkage differs from cross-flow shrinkage. That is why a single global shrink factor is a starting point, not an answer. On a long, thin part, the difference between the gate end and the far end can exceed the tolerance you are trying to hold. The fix is either to gate where the pressure can reach the whole part, to add a packing profile, or to accept that the tight tolerance belongs on the features near the gate and not on the overall length.

We cut the first sample with a nominal allowance and measure the first shots on a CMM before touching the cavity. Correcting a tool from measured parts is cheaper and more reliable than correcting it from a data-sheet range, and the measurement record is what the next tool for the same grade starts from.

What TPU asks of the mould steel

The default steel family for TPU is P20 / 718H. That choice is driven by three properties in combination: abrasiveness, mould temperature and the risk of chemical attack.

Abrasiveness is Low. The resin carries no abrasive filler, so the gate, runner and vent lands are not eroded by the melt, and a pre-hardened block is normally sufficient. A replaceable gate insert is still good practice for any tool that may later switch to a filled grade.

Mould temperature matters. TPU wants a mould at 30-0 °C, and a tool designed for a different temperature will run unevenly when set to it. Cooling channels need to be placed where the heat actually concentrates — at thick bosses, at the gate, and where the part joins the runner system — not spread evenly around the cavity. A tool that cools the thin walls and the thick boss at the same rate will hold a flat part and still show a sink mark on the boss.

Applications where TPU wins

TPU wins on strength-to-weight ratio and heat resistance. It is chosen where the part needs stiffness and impact resistance above what commodity grades deliver. Common application families include automotive interior and exterior components, appliance and consumer housings, electrical connectors, industrial structural parts and medical device components.

Where TPU gets replaced is usually clear. When the part has to survive aggressive solvents or continuous UV exposure, a different family takes over. When the temperature ceiling of TPU is exceeded, a high-temperature polymer steps in. When the resin itself is corrosive — PVC or a flame-retardant grade — the tool steel changes before the resin does.

Design guidelines

• Wall thickness. Keep nominal walls in the 1.5–3.0 mm range for most grades; thicker walls invite sink and shrink, thinner walls risk incomplete fill. Avoid abrupt thick-to-thin steps.
• Ribs. Rib thickness around 0.5–0.6 of the adjoining wall keeps the rib from sinking the face. Space ribs so the gaps between them can be cooled and vented.
• Draft. A minimum of 1° on smooth walls and 1.5–2° on textured walls. Add more where the draw is deep; the part drags most where the texture is deepest.
• Fillets. A fillet at least 0.5 × wall thickness at internal corners avoids a stress concentration that turns an impact into a crack.
• Bosses. Use a wall around 0.6 of the nominal wall, add gussets rather than thickening the boss, and keep the boss height under about 2.5 × diameter.

Cost and cycle-time effect

TPU sits in the middle of engineering thermoplastics on price; the premium over commodity grades buys temperature and stiffness, not a long cycle. The cost triangle is material price, cycle time and yield. A cycle that is a few seconds shorter on a high-volume part is worth more than the material saving, because the press hour is the dominant cost.

Yield is where the cheap resin can become expensive. Wet resin produces silver streaks and scrap; a cold mould produces weak weld lines that fail in the field; a badly vented tool produces burn marks that need rework. None of these are material costs, and all of them are more expensive than the few percent saved by buying a lower grade.

Common selection mistakes

• Using a global shrink number without a measurement loop. The nominal allowance is a starting point, not an answer. Correct from the first measured shots.
• Specifying TPU outside its temperature ceiling. Service temperature above the rated HDT changes the part behaviour over its lifetime, even if it passes first article.
• Ignoring abrasion when the resin is filled. A pre-hardened block in a glass-filled job wears at the gate long before it shows bulk wear.
• Not reading the resin TDS for drying and moisture. Drying is not optional for a hygroscopic grade, and skipping it shows up as a surface defect that looks like a moulding problem.