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Compression Set vs Pressure Redistribution: Why an Orthotic Pad Keeps Its Cushion
An orthotic or prosthetic pad can feel perfect on the bench and still fail in service, because the bench tests softness while service tests recovery. Two material properties decide whether a pad keeps its cushion: compression set, the permanent thickness a foam loses after sustained loading, and force-deflection, the curve that decides whether the foam spreads a load or concentrates it.
A foam can be excellent at one and poor at the other. This guide explains both, shows why a soft-feeling foam can still bottom out, and reads them off the data sheet, with everything framed at the material level only.
Two properties that decide whether a pad lasts
An orthotic or prosthetic pad can feel perfect on the bench and still fail in service, because the bench tests softness while service tests recovery. Two material properties decide whether a pad keeps its cushion: compression set, which is how much permanent thickness the foam loses after sustained loading, and pressure redistribution, which is how well the foam spreads a concentrated load across a larger area.
A pad can be excellent at one and poor at the other, and reading both off the data sheet is what separates a pad that lasts from one that flattens.
- Compression set (ASTM D395): the permanent deformation remaining after a foam is held compressed and released. A low set means the foam recovers its thickness after loading; a high set means it stays thinner, so the cushion is gone.
- Force-deflection (ASTM D3574): the curve of how the foam resists compression. A flatter curve at low stress spreads load and redistributes pressure; a steep curve concentrates it.
- Density and recovery: the grade properties that, together, set how the foam carries repeated loading and returns to shape between cycles.
Why does a pad that feels soft still bottom out?
Initial softness and durable cushioning are different things. A foam can have a low initial firmness, so it feels soft when you press it, and yet have a high compression set, so after weeks of sustained loading it loses thickness and the cushion disappears. Once the foam has taken a set it sits thinner, its remaining travel is short, and it bottoms out under the same load it used to absorb.
Pressure redistribution depends on the foam having travel to spread the load; a flattened foam cannot redistribute anything, so the load concentrates exactly where the part was meant to relieve it. The design answer is to read compression set and force-deflection together: pick a resilient grade with a low set for any layer that must keep its cushion, and reserve the very soft, higher-set grades for short-life or non-load-bearing uses.
The numbers are grade-specific and live on the maker's data sheet.
Reading the data sheet: what each number is and is not
| Property | What it tells you | What it does not tell you |
|---|---|---|
| Compression set (ASTM D395) | How much permanent thickness the foam loses after sustained loading | The service life on a person, which depends on the loading profile and duty |
| Force-deflection (ASTM D3574) | How the foam resists compression and whether it spreads or concentrates load | The comfort or fit, which are finished-product matters |
| Initial firmness / durometer | How soft the foam feels unloaded | Whether it keeps that feel after weeks of loading, which is compression set |
| Density | A proxy for load capacity and durability | The recovery by itself; density and set are read together |
| Recovery time | How quickly the foam returns to shape between cycles | Anything about the contact pressure distribution, which is force-deflection |
A worked reading: matching the grade to the duty
Consider a load-bearing comfort layer under daily use. Here a low compression set is the governing property, so the reading favors a resilient microcellular polyurethane that recovers its thickness cycle after cycle, even if a softer foam feels nicer on the bench. Now consider a light, intermittently loaded interface pad. Here a softer, higher-set grade may be acceptable because the loading is brief and the cushion is not asked to survive sustained compression.
In both cases force-deflection is read to confirm the load spreads rather than concentrates, and density supports the load capacity. The grade is chosen from the duty and the data sheet, not from how it feels in the hand, and the finished-product fit is confirmed by the device or orthosis maker.
Duty to Cushion-Durability Reading Direction
For a sustained load-bearing layer, weight compression set so the foam keeps its thickness; a resilient microcellular polyurethane with a low set fits. Confirm compression set (ASTM D395) and force-deflection (ASTM D3574) on the data sheet.
Illustrative only; confirm against current material data and a drawing review. This selector returns which property to read and a material family direction, not a service life, a durometer or any orthopedic or clinical claim. Compression set (ASTM D395), force-deflection (ASTM D3574) and density are read on the maker's current data sheet, and the finished product and its fit belong to the device or orthosis maker.
Design and converting considerations
- Read set and force-deflection together. Low set keeps the cushion; the right force-deflection spreads the load. A foam needs both for a load-bearing comfort layer.
- Do not specify on initial firmness alone. A soft feel with a high set flattens; confirm compression set per ASTM D395 for any layer that must last.
- Reserve soft, higher-set grades for light duty. They are fine where loading is brief and the cushion is not asked to survive sustained compression.
- Layer to keep travel. A firm base preserves the comfort layer's travel so it can keep redistributing load; bottoming out is a stack problem as much as a grade problem.
- Process fit. Grades are die-cut, skived and laminated; send the load case and the contour so the stack can be matched.
Common failure modes
| Failure mode | Root cause | Reading correction |
|---|---|---|
| Cushion flattens after a few weeks | A high-compression-set foam used in a load-bearing layer | Reselect a resilient low-set grade; confirm compression set per ASTM D395 |
| Pressure concentrates instead of spreading | A steep force-deflection grade where redistribution governs | Choose a flatter low-stress curve; read force-deflection per ASTM D3574 |
| Soft feel, short life | Specified on initial firmness alone | Read compression set, not just firmness, for any lasting layer |
| Comfort layer bottoms out | No firm base to preserve its travel | Add a structural base so the comfort layer keeps travel and keeps redistributing |
| Over-built light-duty pad | A premium low-set grade where loading is brief | A softer higher-set grade is acceptable for intermittent, non-load-bearing use |
How H-O Products helps
H-O Products is a precision die-cut converter serving medical and clean-component programs. For an orthotic, prosthetic or rehab pad we convert resilient microcellular polyurethane, polyurethane foam, crosslinked polyethylene and EVA into layered, skived, contoured parts, and help you read the maker's compression-set and force-deflection data so each layer matches its duty.
As an ISO 9001:2015 certified organization we name a foam family only where it is catalog-verified, and we defer every grade-specific number to the manufacturer's current technical data sheet.
We do not produce the raw foam and we do not make the finished orthosis; the finished product and its fit belong to you.
Choosing foams that keep their cushion?
Tell us how each layer is loaded and what it must do, and the material-level documentation you need. We will help you read the maker's compression-set and force-deflection data, recommend a grade direction, and die-cut the stack on your geometry.
Samples typically ship in 3 to 5 business days for common die-cut configurations; standard production runs about two weeks; special orders are quoted to the job.
Family-owned in Winsted, Connecticut since 1971 · ISO 9001:2015 certified · OEMs worldwide send us a drawing — and get back their part.
Related reading
ApplicationOrthotics, Prosthetics & Rehab (application page)Read →
GuideConform, Cushion or Structure? Choosing Materials for Orthotic and Prosthetic PaddingRead →
GuideHow to Spec an Orthotic or Prosthetic Pad: What to Send H-O for a QuoteRead →
IndustryMedical & clean components overview (industry hub)Read →
Frequently asked questions
What is compression set and why does it matter for an orthotic pad?
Compression set, characterized per ASTM D395, is the permanent thickness a foam loses after it is held compressed and released. For an orthotic pad it decides whether the cushion lasts: a low set means the foam recovers its thickness after loading, while a high set means it stays thinner and the cushion is gone. It is a material property on the maker's data sheet. It describes the foam, not a service life on a person, which depends on the loading and is a finished-product matter.
Why does a foam that feels soft still bottom out?
Initial softness and durable cushioning are different. A foam can feel soft because its initial firmness is low and still have a high compression set, so after sustained loading it loses thickness, its remaining travel is short, and it bottoms out. A flattened foam also cannot redistribute pressure, because redistribution needs travel to spread the load. Reading compression set and force-deflection together, not just initial firmness, is what avoids this. The numbers are grade-specific and on the data sheet.
Is pressure redistribution a clinical claim?
No. On this page, pressure redistribution refers to a material's force-deflection behavior, characterized per ASTM D3574, which describes whether a foam spreads or concentrates a load. That is a material property, not a clinical, orthopedic or therapeutic outcome. H-O Products describes padding materials only and makes no efficacy or fit claim; the finished product and its assessment belong to the device or orthosis maker.
Request a quote for your orthotic or prosthetic pad
Send H-O your load case and contour—engineering helps you read the maker's data sheet, recommends a grade direction per layer, and die-cuts the stack on your geometry.
Last updated · Reviewed by H-O Products engineering (ISO 9001:2015 certified converter, Winsted, CT).
References
- ASTM D395, Standard Test Methods for Rubber Property: Compression Set. ASTM International.
- ASTM D3574, Standard Test Methods for Flexible Cellular Materials: Slab, Bonded, and Molded Urethane Foams. ASTM International.
- ASTM D2240, Standard Test Method for Rubber Property: Durometer Hardness. ASTM International.
- ISO 10993-1, Biological Evaluation of Medical Devices, Part 1: Evaluation and Testing Within a Risk Management Process. International Organization for Standardization (material-level designation).
- Manufacturer technical data sheets for the named foam families; compression set, force-deflection and density values are confirmed on the maker's current TDS.
This article describes material families H-O Products converts and cites published standards by designation (ASTM D3574 and ASTM D395 for urethane force-deflection and compression set, ASTM D1056 for cellular rubber, ASTM D2240 for durometer, ISO 10993 and USP Class VI as material-level designations). It makes no patient, clinical, therapeutic, orthopedic, healing or efficacy claim of any kind, and describes padding and cushioning materials only.
The material maker characterizes and documents a grade on its current technical data sheet; the device or orthosis maker owns the finished product, its fit and its regulatory file. H-O Products is a precision die-cut converter, not a raw-material producer and not a medical-device or orthotic manufacturer; this is not, and does not imply, an ISO 13485 certification, an FDA device registration, a cleanroom certification or a biocompatibility certification.
The only certification claimed is ISO 9001:2015, which governs the converting process. Grade-specific values are confirmed against the manufacturer's current technical data sheet and an engineering drawing review. All parts are custom manufactured to order and minimum production run quantities apply.