Infill density and firmness: how to choose the hardness of a 3D-printed insole
With traditional materials, you choose an insole's hardness mainly through the material: EVA of one density or another, polypropylene of a given thickness. With a 3D-printed insole the logic changes: with a single TPU filament you can make soft insoles, firm ones, or insoles with several hardness levels at once. The key is the infill.
This article goes deeper into choosing that firmness. The basic print settings are covered in how to 3D print orthotic insoles in TPU.
Why infill sets the hardness
An FDM-printed part isn't solid. It has outer walls, solid top and bottom layers and, inside, an infill: a repeating structure with a pattern and a density (the percentage of the internal volume filled with material).
When the patient steps down, it's mostly that internal structure that deforms. Lower density means more empty space, so the insole gives more. Higher density means more material to spread the load, so it gives less.
The filament's hardness (typically TPU Shore 95A) sets the range; density picks the point within that range.
Four firmness levels
In Podia3D, firmness is prescribed in clinical terms and translated into gyroid infill density:
| Firmness | Density | Starting guidance |
|---|---|---|
| Soft | 10% | Lighter patients (< 60 kg) or those who prioritise cushioning |
| Medium | 16% | Everyday use, 60–85 kg (default) |
| Firm | 24% | 85–105 kg, or when control matters more than cushioning |
| Extra firm | 33% | > 105 kg, impact sports or maximum rearfoot control |
These are starting points. The final feel also depends on the specific filament (two 95A TPUs from different brands don't behave the same), the insole thickness in each zone and the number of walls and solid layers. That's why it's worth printing a sample of each level with your profile and keeping it in the clinic as a reference.
Pattern matters too
At the same density, the pattern changes the behaviour:
| Pattern | Behaviour |
|---|---|
| Gyroid | Similar in every direction; good balance of cushioning and support. The usual choice for TPU |
| Honeycomb | Stiffer and stronger, less cushioning. Useful for firm bases |
| Cubic | Strong in all three axes, good all-round behaviour |
| Grid | Quick to print, stiff in two axes |
| Concentric | Follows the outline; very flexible, interesting for soft zones |
| Lines | Lightest and fastest, with minimal resistance |
Without a specific reason, start with gyroid and only change density: it's easier to reason about and to reproduce.
Per-zone firmness
This is the big advantage of 3D printing. The slicer can apply a different density (and pattern) to one part of the piece using modifiers. In practice, zone prescriptions tend to follow the anatomical bands of the foot:
- Rearfoot (roughly the first 32% of the length): soft to cushion the heel (heel spur, plantar fasciitis); firm for more control.
- Midfoot / arch (32–58%): firm to support the arch, which is the usual case; soft if the arch is sensitive.
- Forefoot (58–100%): soft to offload the metatarsal heads (metatarsalgia).
Working with predefined zones, rather than "painting" hardness point by point, has advantages: it's the language of podiatric prescription, it mirrors identically between left and right feet, and it's easy to document.
Base layer and corrections with their own hardness
Two finer tools:
- Base layer. A bottom layer a few millimetres thick with a different firmness, for example a firm base under a softer upper part. It adds stability without giving up cushioning where the foot rests.
- Corrections with their own firmness. Each added element can carry its own hardness: soft metatarsal pads or heel cushions, extra-firm wedges, firm reliefs around the area being offloaded.
How to validate your firmness table
- Print a test insole per level with your final profile and filament.
- Label it and compare it by hand with your usual materials: it's the quickest way to calibrate your judgement.
- If you change filament brand, redo the samples.
- Document which density matches each level in your process: it's part of your manufacturing documentation.
How Podia3D does it
In Podia3D you choose overall firmness, per-zone firmness (rearfoot, midfoot and forefoot), infill pattern and, if needed, a base layer with a different hardness. Each correction has its own default firmness (soft for pads, extra firm for wedges…), which you can change.
All of that travels in the 3MF project as infill modifiers, for Bambu Studio, OrcaSlicer, PrusaSlicer or Cura. No need to rebuild the zones in the slicer.
Podia3D is a prototype in its pilot phase and is not certified as medical device software. You can try it now in your browser, no sign-up needed: you start with a sample case loaded, and when you want to export the 3MF or STL, we just ask for your email.