Application Note | Polymer Foam

Polymer Foam Surface Characterization

Roughness, Cell Topography and Defect Volume of PE Packaging Foam and Window Insulating Foam with 3D Non-Contact Profilometry

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Row of blue polyethylene packaging foam profiles, their cut ends showing the foam's cell structure

Research & Experimental Testing

Davide Morrone & Walter Alabiso

Visual Design & Editorial

Andrew Shore

Introduction

Polymer foams are cellular materials, a polymer matrix filled with gas cells. Polyethylene (PE) foam is widely used to protect goods in packaging, and foam strips and profiles seal and insulate windows. Their surfaces are formed by cell walls and open cells, so their topography is set by the cell structure: cell size, cell-wall folds, cracks and the depth of the open cells.

NANOVEA characterized two such foams, a blue PE packaging foam and a black window insulating foam cut in cross-section after failure, with a portable 3D non-contact profilometer, from topography and functional parameters to roughness and defect volume.

Why Measure Foam Surfaces Without Contact

Foams are soft and compressible. A contact stylus loads the cell walls it is meant to measure and can deform them, while an optical pen measures height without applying any force. The cell structure also makes foam surfaces deep and steep: the scanned areas in this study span more than 850 µm in height over a few millimeters, which calls for a sensor with a large Z range and fine lateral steps.

The same measurement serves quality control and failure analysis: photo-simulations and 3D renderings show the cells, the Abbott-Firestone curve separates peaks, core and valleys, profile and areal parameters quantify the roughness, and volume analysis sizes defects such as cracks.

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Measurement Objectives

Two polymer foam samples were measured with the NANOVEA JR25 3D non-contact profilometer and a PS3 optical pen. Each surface was reconstructed topographically (photo-simulation, 3D rendering, Abbott curve), and its roughness parameters were determined: profile parameters in accordance with ISO 21920 on the PE foam, areal parameters in accordance with ISO 25178-2 on the window insulating foam. The PE foam was also scanned bent, to inspect its internal cracks.

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PE Foam: Sample and Parameters

Blue polyethylene packaging foam strip on a wooden table — Blue PE foam for packaging (one sample).

Parameter	High-accuracy area scan	Series of profiles
Optical pen	PS3	PS3
Z-scan range	1400 µm	1400 µm
X-distance	4.5 mm	16.5 mm
X-step	1.0 µm	1.0 µm
Y-distance	4.5 mm	1.0 mm
Y-step	1.0 µm	100.0 µm
Averaging	1	1
Measurement type	Direct	Direct
Acquisition mode	Single frequency	Single frequency
Acquisition rate	400 Hz	400 Hz
Light intensity	100 %	100 %

Table 1: Test parameters for the PE foam.

PE Foam: Topographical Reconstruction

The following picture shows the 2D photo-simulation of the scanned area with artificial light settings.

Grayscale photo-simulation of the PE foam surface showing folded, irregular cells, with a 1 mm scale bar — Fig. 1: 2D photo-simulation of the PE foam with artificial light settings.

A false-color 3D rendering (normalized in XYZ) of the scan is presented below.

False-color 3D rendering of a 4.5 x 4.5 mm area of PE foam with deep dark-blue valleys between cells, height range 866.71 micrometers — Fig. 2: False-color 3D rendering of the PE foam (normalized in XYZ): X = 4.512 mm, Y = 4.501 mm, Z = 866.71 µm.

Functional Parameters

The Abbott-Firestone curve reveals the functional parameters Spk and Svk (ISO 25178-2). Svk (reduced valley depth) represents the portion of the material that functionally acts as tiny pools to retain oils and liquids. Spk, the reduced peak height, is a measure of the peak height above the core roughness, representing the portion of the material that functionally acts as the initial contact area (peaks of high stress concentration that can easily be abraded).

Abbott-Firestone curve of the PE foam with its Sk, Spk, Svk, Smrk1 and Smrk2 values — Fig. 3: Abbott-Firestone curve and functional parameters of the PE foam (ISO 25178-2).

The following chart shows graphically the peak-valley distribution relative to the mean plane based on the functional parameters derived from the Abbott curve (valleys in purple, mean plane in green, peaks in orange). The height thresholds c1 and c2 were calculated based on the definitions of Spk and Svk.

Map of the PE foam surface split into valleys in purple, mean plane in green and peaks in orange, with projected areas and volumes — Fig. 4: Peak-valley distribution of the PE foam relative to the mean plane (valleys in purple, mean plane in green, peaks in orange).

Texture Isotropy

The following chart shows the texture isotropy of the surface.

PE Foam: Roughness Evaluation

A series of 11 parallel profiles was acquired along the X direction, and the roughness parameters were calculated in accordance with ISO 21920. A Gaussian L-filter with a cut-off of 8.0 mm was used (edge effects compensated), as specified for surfaces with 10 µm ≤ Ra ≤ 80 µm.

Fig. 6: Profile series and ISO 21920 roughness parameters of the PE foam (S-filter 2.5 µm, F-operation leveled, L-filter 8 mm, evaluation length 2 λc).

Parameter	Description	Mean (µm)	Std dev	Min	Max
Ra	Arithmetic mean absolute height	53.698	5.917	44.287	63.876
Rq	Root mean square height	85.250	11.516	70.323	105.554
Rp	Mean hill height	113.713	15.674	90.769	138.489
Rv	Mean dale depth	510.385	72.784	417.603	683.403
Rz	Mean height	624.098	78.423	509.640	785.184
Rsk	Skewness	−2.681	0.326	−3.287	−2.201
Rku	Kurtosis	12.091	2.346	9.415	17.273

Table 2: ISO 21920 roughness of the PE foam over 11 profiles (Rsk and Rku have no unit).

Internal Cracks in the Bent Foam

The blue foam piece was also scanned in a bent position. The high-accuracy scan revealed some internal cracks. It is possible to quantify the volume of the defects, as shown below.

JR25 optical pen above the blue PE foam held bent in a vise

3D rendering of the bent PE foam surface with dark streaks of internal cracks

Figs. 7–8: The PE foam scanned in a bent position (left) and its 3D rendering, X = 3 mm, Y = 3 mm, Z = 682.463 µm (right).

Height map of the bent PE foam with one crack region highlighted and its volume and maximum depth — Fig. 9: Volume of a crack defect: 9721920.070 µm³, maximum depth 557.118 µm.

Window Insulating Foam: Sample and Parameters

Small black window insulating foam piece, a cut cross-section, on a wooden table — Black foam for window insulation: cut cross-section after failure (one sample).

Parameter	Value
Optical pen	PS3
Z-scan range	1400 µm
X-distance	3.0 mm
X-step	3.0 µm
Y-distance	3.0 mm
Y-step	3.0 µm
Averaging	1
Measurement type	Direct
Acquisition mode	Single frequency
Acquisition rate	400 Hz
Light intensity	100 %

Table 3: Test parameters for the window insulating foam.

Window Insulating Foam: Topographical Reconstruction

The following picture shows the 2D photo-simulation of the scanned area with artificial light settings.

Photo-simulation of the window foam cross-section showing a dense field of closed polygonal cells, with a 750 micrometer scale bar — Fig. 10: 2D photo-simulation of the window insulating foam with artificial light settings.

A false-color 3D rendering (normalized in XYZ) of the scan is presented below.

Grayscale 3D rendering of the window foam cross-section over 3 x 3 mm, height range 850.225 micrometers — Fig. 11: False-color 3D rendering of the window insulating foam (normalized in XYZ): X = 3.044 mm, Y = 3.019 mm, Z = 850.225 µm.

Functional Parameters

Abbott-Firestone curve of the window foam with its Sk, Spk, Svk, Smrk1 and Smrk2 values — Fig. 12: Abbott-Firestone curve and functional parameters of the window insulating foam (ISO 25178-2).

Map of the window foam split into valleys in purple, mean plane in green and peaks in orange along the cell walls, with projected areas and volumes — Fig. 13: Peak-valley distribution of the window insulating foam (valleys in purple, mean plane in green, peaks in orange).

Texture Isotropy

The following chart shows the texture isotropy of the surface.

Window Insulating Foam: Roughness Evaluation

The examined foam shows an Sa value greater than 10 µm. From a metrological standpoint, according to ISO 25178-2, the proper cut-off for the Gaussian L-filter to allow separating the contribution of waviness to roughness should be 8.0 mm. This, however, exceeds the lateral dimensions of the scanned areas. Therefore, the values of the roughness parameters presented here were evaluated on the primary surface, where only the S-filter for microroughness (λs = 2.5 µm, ½ cut-off) was applied.

ISO 25178-2 primary surface height parameters of the window foam — Fig. 15: ISO 25178-2 height parameters of the window insulating foam on the primary surface (S-filter 2.5 µm, F-operation leveled).

ISO 25178-2 parameter	Value
Sa, arithmetic mean height	72.173 µm
Sq, root-mean-square height	96.526 µm
Sp, maximum peak height	270.274 µm
Sv, maximum pit depth	579.589 µm
Sz, maximum height	849.863 µm
Ssk, skewness	−1.347
Sku, kurtosis	7.539

Table 4: Primary-surface height parameters of the window insulating foam (from Fig. 15).

Comparing the Two Foams

Parameter	PE packaging foam	Window insulating foam
Sk (core roughness depth)	106.793 µm	204.642 µm
Spk (reduced peak height)	23.637 µm	66.732 µm
Svk (reduced valley depth)	237.529 µm	156.849 µm
Smrk1	7.869 %	6.607 %
Smrk2	87.282 %	85.528 %
Threshold c1 / c2	68.204 / −38.600 µm	116.357 / −88.236 µm
Valleys / core / peaks (projected area)	12.718 / 79.413 / 7.869 %	14.473 / 78.921 / 6.606 %
Texture isotropy	58.983 %	81.886 %

Table 5: Functional parameters and texture isotropy of the two foams (ISO 25178-2, from Figs. 3 to 5 and 12 to 14).

Conclusion

The NANOVEA JR25 with a PS3 optical pen characterized two polymer foams without touching them. On the blue PE packaging foam, the high-accuracy area scan resolved the folded cell structure, the Abbott-Firestone curve gave Sk 106.793 µm, Spk 23.637 µm and Svk 237.529 µm, and the 11-profile series gave Ra 53.698 ± 5.917 µm in accordance with ISO 21920 with an 8.0 mm cut-off. Scanned bent, the same foam revealed internal cracks whose volume could be quantified.

On the cut cross-section of the failed window insulating foam, the scan resolved its closed cells, with Sa 72.173 µm on the primary surface and a texture isotropy of 81.886 %, against 58.983 % for the PE foam.

Fast 2D and 3D scans allow measurement of any region of interest, and because the optical pen identifies the sample’s features without damaging or interfering with it, the method suits both quality control and R&D on soft, cellular materials.

Frequently Asked Questions About Foam Surface Measurement

How do you measure the surface roughness of a foam?

With a 3D non-contact profilometer: an areal scan reconstructs the topography for areal (ISO 25178-2) parameters, and a series of parallel profiles gives profile roughness per ISO 21920. Here a PS3 optical pen with a 1400 µm Z range measured both foams.

Why measure foam without contact?

A foam is soft and its cell walls deform under load, so a contact stylus can change the surface it is measuring. An optical pen applies no force, and its Z range follows the deep valleys of open cells; the PE scan spanned 866.71 µm in height over 4.5 mm.

Which cut-off should be used for foam roughness?

For profiles with 10 µm ≤ Ra ≤ 80 µm, ISO 21920 specifies a Gaussian L-filter with an 8.0 mm cut-off, used here on 16.5 mm profiles (Ra 53.698 µm). When the scanned area is smaller than 8 mm, as on the 3 mm window-foam scan, parameters are reported on the primary surface with only the 2.5 µm S-filter.

What do Spk and Svk say about a foam surface?

From the Abbott-Firestone curve, Spk (reduced peak height) is the peak material above the core that acts as the initial contact area, and Svk (reduced valley depth) the valley material that can retain liquids. The PE foam gave Spk 23.637 µm and Svk 237.529 µm, a surface dominated by deep valleys.

Can a profilometer find cracks and defects in foam?

Yes. Scanning the PE foam bent opened up internal cracks in the high-accuracy scan, and a volume analysis quantified one defect at 9721920.070 µm³ with a maximum depth of 557.118 µm.

Which NANOVEA instrument was used?

The NANOVEA JR25 portable 3D non-contact profilometer with a PS3 optical pen (1400 µm Z range), scanning at 400 Hz with steps down to 1 µm.

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