TPU Processing Guide: Injection Molding, Extrusion, Blow Molding and Overmolding

Thermoplastic polyurethane is often selected because it can deliver rubber-like flexibility while still being processed with thermoplastic methods. That sounds simple, but anyone who has worked with TPU in production knows the details matter.

A grade that works well in extrusion may not be right for injection molding. A soft overmolding grade may behave differently from a harder technical molding grade. A part that looks fine during sampling may fail after repeated flexing, abrasion, moisture exposure, or poor drying control.

This guide is written for engineers, molders, extruders, sourcing teams, and product developers who need a practical overview of how TPU is commonly processed and what should be checked before production.

Editorial note: Alliance TPU is an independent educational resource. It does not sell resin, provide official processing conditions, or replace current supplier data sheets. Always verify processing windows with the material supplier and the processor running the job.

TPU processing starts before the machine

Many TPU problems are blamed on the molding machine, extruder, tool, or operator. Sometimes that is correct. But a surprising number of issues begin earlier: the wrong grade, incomplete drying, vague part requirements, or no clear understanding of the operating environment.

Before processing TPU, define:

• the manufacturing method;
• the target hardness;
• whether the grade is polyester-based or polyether-based;
• expected part geometry;
• wall thickness and flow length;
• drying requirements;
• cosmetic expectations;
• end-use environment;
• expected failure mode;
• whether the part must bond to another material.

A TPU processing plan should connect the material, the tool, the machine, and the real use case.

Main TPU processing methods

TPU can be used in several thermoplastic processes, depending on the grade.

Not all TPU grades are suitable for all methods. The process should be known before a grade is selected.

Drying is not optional thinking

Moisture control is one of the first processing questions for TPU. Many TPU grades require drying before processing. Poor drying can lead to defects that are expensive to diagnose later.

Possible moisture-related problems include:

• bubbles or splay;
• surface streaks;
• reduced mechanical properties;
• unstable melt behavior;
• higher scrap rate;
• poor appearance;
• inconsistent part performance.

The exact drying time and temperature depend on the grade and supplier recommendations. Do not guess. Use the current data sheet and confirm that the material handling system can maintain the required conditions.

Injection molding TPU

Injection molding is one of the most common ways to process TPU into finished parts. It is used for flexible covers, industrial components, grips, wheels, seals, footwear pieces, strain reliefs, protective housings, and many other molded products.

What makes TPU injection molding different?

TPU can be more elastic, softer, and more moisture-sensitive than many rigid thermoplastics. This can affect filling, packing, demolding, and part handling.

Key considerations include:

• grade hardness;
• melt flow;
• wall thickness;
• gate size and location;
• venting;
• cooling time;
• surface texture;
• shrinkage;
• demolding forces;
• post-mold part distortion;
• drying discipline.

A soft TPU part may need more careful handling than a rigid plastic part. A thick TPU part may require attention to cooling and shrinkage. A thin or long-flow part may need a grade with suitable flow behavior.

Injection molding checklist

The best time to solve these questions is before the tool is finalized.

Common injection molding issues

Splay, bubbles, or surface streaks

Often related to moisture, contamination, temperature, or shear. Start by checking drying conditions and material handling.

Short shots

May be caused by insufficient flow, poor venting, inadequate temperature settings, restrictive gates, or difficult part geometry.

Flash

Can result from excessive pressure, tool wear, poor clamp control, low viscosity, or parting-line issues.

Sticking in the mold

Soft TPU can be difficult to eject if draft, texture, or ejection design is inadequate.

Warpage or distortion

May come from uneven wall thickness, cooling differences, stress, or part handling before the part stabilizes.

Poor surface feel

Can be linked to material grade, mold texture, processing window, contamination, or moisture.

Extruding TPU

TPU extrusion is common in tubing, hose, cable jackets, film, sheet, profiles, belting, and protective layers. Compared with injection molding, extrusion puts more emphasis on continuous melt stability and dimensional control.

Important extrusion factors include:

• grade designed for extrusion;
• melt strength;
• temperature profile;
• screw design;
• line speed;
• die design;
• cooling method;
• puller control;
• wall thickness;
• surface finish;
• long-run consistency.

Tubing and cable applications may require different TPU grades even when the hardness looks similar.

TPU extrusion checklist

A tube that looks good on the line can still fail if the chemistry is wrong for the fluid or environment.

Blow molding TPU

Some TPU grades can be used for blow molded flexible parts, bellows, boots, and hollow shapes. Blow molding places special demands on melt strength, parison behavior, wall distribution, and trimming.

Questions to ask early:

• Is the grade suitable for blow molding?
• Can the parison hold shape?
• Will the wall distribution be acceptable?
• Does the part need repeated flexing?
• Is low-temperature flexibility required?
• Will the part contact oil, water, or chemicals?
• How will the part be trimmed and inspected?

Blow molding is not simply injection molding with a different tool. It requires grade and process review from the start.

TPU overmolding

TPU overmolding is used when a product needs a soft-touch surface, grip, protective layer, seal, cushioning, vibration damping, or hard-soft assembly.

Typical examples include:

• tool handles;
• protective device covers;
• strain reliefs;
• grips;
• automotive interior or protective parts;
• sporting goods;
• electronics housings;
• knobs and control surfaces.

The main risk is assuming adhesion will happen automatically. It will not.

What determines TPU overmolding success?

TPU overmolding depends on the whole system.

For critical parts, design the overmolded assembly so that mechanical interlock supports adhesion whenever possible.

Processing TPU by application type

The same word "TPU" can mean very different processing choices depending on the part.

Design choices that help TPU processing

A good design can make TPU easier to process. A poor design can create problems no material grade will fully solve.

Helpful design practices include:

• avoid unnecessary thick sections;
• maintain reasonable wall thickness transitions;
• use adequate draft for molded parts;
• design gates for flow and appearance;
• consider texture carefully for soft parts;
• avoid sharp internal corners where flexing occurs;
• include mechanical interlock for overmolding where possible;
• define acceptable cosmetic standards early;
• test real parts, not only plaques.

TPU often performs well when the design respects its flexibility. Problems happen when a part is designed like a rigid plastic part but expected to behave like an elastomer.

Production questions to ask before approval

Before approving a TPU process, ask:

1. Has the material been dried according to the supplier's recommendation?
2. Is the selected grade intended for the chosen process?
3. Has the processor run similar hardness and chemistry before?
4. Are cosmetic requirements defined clearly?
5. Is the tool designed for flexible part demolding?
6. Are key dimensions stable after cooling and conditioning?
7. Has the part been tested in its real environment?
8. Is there a plan for moisture control during production?
9. Are scrap causes being tracked?
10. Is there a backup grade if supply or performance changes?

If these questions are not answered, the project is still in development, not production readiness.

Common TPU processing mistakes

Mistake 1: Treating the data sheet as a process plan

A data sheet gives guidance, not a complete production recipe. Machine, tool, part geometry, and operator practice still matter.

Mistake 2: Underestimating drying

TPU moisture problems can look like processing problems. Drying should be verified before changing too many machine settings.

Mistake 3: Choosing the grade too late

If the tool is already built around the wrong material assumption, the grade selection becomes damage control.

Mistake 4: Ignoring the processor's experience

A resin supplier may know the material, but the processor knows the machine and tool behavior. Both matter.

Mistake 5: Assuming one TPU grade can cover every part

Different hardness, chemistry, flow, and additive packages exist for a reason.

Bottom line

TPU processing works best when material selection, part design, tooling, drying, and manufacturing method are treated as one system. The most reliable projects do not begin with "Can you run TPU?" They begin with a clear definition of the part, process, environment, and proof required for approval.

For early projects, the practical path is:

1. define the application;
2. choose the likely process;
3. narrow the chemistry and hardness;
4. confirm drying and processing requirements;
5. involve the processor early;
6. test real parts under real conditions.

That approach will prevent more problems than any single machine adjustment.

Related: TPU Applications in Automotive, Footwear, Cable, Tubing and Industrial Products →