Variable Infill in TPU: Four Hardnesses from One Filament

It depends on which hardness we mean. The hardness of the material never changes: Filaflex 95A is 95A on the spool and in the finished part. It's measured on the Shore A scale, defined by the ISO 868 standard, which runs from 0 to 100: the higher the number, the harder the material. If you want the full picture of how the scale works, read our complete guide to Shore hardness.

What does change is the hardness you feel when you press the part. That depends on its internal structure. A low-infill zone holds more air than material, gives way easily and behaves like a much softer material. A dense zone pushes back and feels firm.

The figure from our demo: one Filaflex 95A part, four infill zones from 7 % to 25 %, and durometer readings from ~25 to ~40 Shore depending on the zone. The material is the same from start to finish. Geometry makes the difference.

This is worth knowing to avoid the wrong conclusions. Durometer readings on a printed structure vary with the part's geometry and wall count. On a low-infill zone, the instrument measures the response of the structure, not the true hardness of the material. That's why we always give an approximate range rather than exact figures: it helps you compare zones with each other, not certify a value.

This is the map we used in the demo with Filaflex 95A. Treat it as a starting point: depending on the shape and size of your part, you may need to adjust a few points up or down.

The rule is simple: less infill, softer zone; more infill, firmer zone. The filament is the same across the whole table.

The key tool is the modifier. A modifier is a volume you place inside the part, with its own settings. Everything inside that volume prints with those settings, and the rest of the part prints with the general ones. That way, a single body can hold four different behaviours.

Bambu Studio and OrcaSlicer

Right-click the part and choose Add modifier. You can use a basic shape, such as a cube or a cylinder, or load your own volume. Place and scale it so it covers the zone you want to change, then change the infill density in the modifier's settings.

PrusaSlicer

The process is equivalent. Right-click the part and choose Add modifier mesh. Then add the infill density setting to that volume and give it the value you need.

Assign a Density to Each Volume

1. Decide which zones of the part should be firm and which should be soft.
2. Create one modifier per zone and fit it to its shape.
3. Give each modifier its infill percentage, using the table above as a reference.
4. Use the same infill pattern in every volume so the transitions stay clean.
5. Slice and check the preview layer by layer to confirm each zone has the density you expect.

For flexible parts we recommend gyroid infill, for two reasons:

• Consistent behaviour: gyroid responds the same way in every direction, so a soft zone deforms evenly wherever you press it. That uniformity is exactly what a cushioning part needs.
• Easy to print: it's a continuous path with no sharp changes of direction, which matters a lot when the filament is soft. Fewer starts and stops mean steadier extrusion and fewer problems.

This is the most common mistake. Perimeters and top and bottom layers stiffen the part considerably. If you use too many, the outer skin masks the infill and every zone ends up feeling the same, even though they're different inside.

To make the difference between zones noticeable, use:

• 1 to 2 walls at most.
• Few top and bottom layers, just enough to close the part.

If your part shows no difference between zones, check this first.

These are the settings we used to print the demo, with a 0.4 mm nozzle. They're a good starting point for any variable infill part.

Don't skip the drying. TPU absorbs moisture, and damp filament gives parts with a poorer finish and less predictable behaviour, which shows especially in low-infill zones.

If your printer is compatible with the Filaflex 2.20 mm system, you can print flexible up to 3 times faster.

The underlying idea changes how you design: hardness stops being something you choose when you buy the filament and becomes something you decide when you design the part. You place soft zones where cushioning is needed and firm zones where support is needed, all in a single print, with no assembly and no bonding of different materials.

This is how 3D printed shoes and insoles are made, and the same principle works for:

• Grips: ergonomic handles and grips.
• Protection: protective parts and padding.
• Saddles: comfort where the body rests.
• Wearables: parts in contact with the body.

Filaflex 95A suits these uses because it's skin-contact safe and offers 500 % elongation.

Variable infill gives you a wide range of feel from one filament, but the starting point is still the hardness of the material. If your application needs even softer zones, you can apply the same technique with a lower-hardness filament. The Filaflex range steps down like this:

A material like Filaflex 70A Ultra-Soft already starts from a much softer base. Softer filaments do need more care when printing, and the settings in this article are tested with 95A.

Does infill change the Shore hardness of TPU?

No. The Shore hardness of the material is fixed: Filaflex 95A is always 95A. What infill changes is the hardness you feel in the part, because that depends on its internal structure. With little infill, the zone behaves like a much softer material.

What infill percentage makes TPU softer?

The lower, the softer. In our Filaflex 95A demo, 7 % gives a very soft zone and 25 % a firm one, with 12 % and 18 % in between. Combine it with few walls so the difference shows.

Does it work on Bowden printers?

Filaflex 95A is compatible with Bowden printers and doesn't need a heated bed, which makes it a good starting point for trying variable infill on almost any printer.

Does it work with any TPU?

The principle is general: in any flexible filament, less infill gives a softer zone. The percentages and settings in this article, however, are tested with Filaflex 95A. With another material, use them as a guide and run your own tests.

The best way to understand it is to print it. A simple plate with four zones at 7 %, 12 %, 18 % and 25 % will show you in an afternoon how the material responds and which percentages fit your project. For that first test, Filaflex 95A is the easiest flexible in the range to print.