Introduction: Density and compression are the two numbers that most often decide which low-density PMI foam grade ends up on a sandwich core drawing.
Open a PMI foam datasheet for the first time and the density column looks like the simplest thing on the listing, while the compressive strength column looks like the most important. In practice, the two are read together. 32WH and 52WH are the two lowest-density grades in the Rifeng WH coarse-cell family, and they sit close enough on the ladder that choosing between them is mostly a question of how much compression a core has to carry for its weight. What follows explains what the two labels describe, how density shapes the typical compressive response, and how to use published typical values without turning a small coupon result into a promise about a finished part.
What the 32WH and 52WH labels actually describe
The number in front of the WH suffix is a nominal density in kilograms per cubic metre: 32WH is listed at 32±7 kg/m³ and 52WH at 52±12 kg/m³. That is simply how much polymer mass sits inside a given volume of foam. The WH part names the family — coarse-cell, closed-cell rigid PMI structural foam, the grade group built for high-adhesion sandwich cores where resin anchoring at the skin interface matters. The two density bands sit next to each other in the low-density segment without overlapping, so a density check alone is usually enough to tell which grade is in hand. On any single board, expect the real value to fall somewhere inside the band rather than exactly on the nominal number. The remaining columns tell a more useful story. Typical compressive strength is 0.40 MPa for 32WH and 0.85 MPa for 52WH; typical tensile strength runs 1.00 MPa against 1.65 MPa; typical modulus 35 MPa against 80 MPa; typical shear strength 0.40 MPa against 0.80 MPa; and shear modulus 12 MPa against 30 MPa. Heat distortion temperature is listed as ≥200 °C typical for both, so the choice between them is a stiffness and strength question rather than a thermal one. The two grades also share a board format of 2500 × 1250 mm, thickness from 3 to 110 mm and ±0.2 mm thickness tolerance, which keeps cutting and machining plans identical. Grade names here are specification labels, not quality tiers.
How density changes the typical compressive response in the low-density range
Density is the variable that ties the low-density grades together, because it sets how much polymer is packed into each cubic metre and, with it, how the cell walls carry load. Moving from 32WH to 52WH adds roughly 60 percent more mass per unit volume, and the typical compressive strength more than doubles along the way. That is not a straight-line relationship, and it is why the gradient matters more than any single figure: same family, same cell type, different point on the stiffness curve.
- Mass per volume. A 52 kg/m³ grade carries about 20 kg more polymer in every cubic metre than a 32 kg/m³ grade. Across a full panel, and especially through a thick core, that difference multiplies quickly, so it shows up in finished part weight rather than staying a datasheet detail.
- Cell-wall thickness. The added polymer mostly lands in the cell walls, which is why the 52WH walls resist local buckling better and why typical modulus climbs from 35 MPa to 80 MPa. Denser cells plus thicker walls give a stiffer core at the same nominal cell size.
- Compressive deformation. Under the same face pressure, a 32WH core reaches its typical 0.40 MPa level earlier and flattens further before the load path settles; a 52WH core holds its shape to roughly twice that level. In practice that sets how much a core can be squeezed during cure or in service before the sandwich loses thickness control.
- Test-method caution. A compression figure comes from a defined specimen, platen and loading rate, and a small coupon samples a small patch of foam. A coupon result describes how that sample behaved; the full panel, with skins, edges, inserts and a real load path, behaves as its own structure.
How engineers should read typical values without overfitting a coupon result
A familiar working pattern looks like this: an engineer pulls 32WH and 52WH up side by side, sees 0.40 MPa against 0.85 MPa, picks the lighter grade to hit a weight target, then orders a coupon to confirm the call. Ordering the coupon is the right instinct; reading it needs a little care. Typical values are central values from a test population, and real foam sits inside a density band, so one specimen can land near the low end of 32±7 kg/m³ while another lands near the high end of 52±12 kg/m³. Comparing a single 32WH coupon with a single 52WH coupon can mix a grade effect with a density spread effect. Run several specimens per grade and compare medians before concluding which grade is stiffer. It also helps to keep the test method attached to the number. Compressive strength measured on a thin slice at a fixed loading rate is a different quantity from the load a cored panel carries through skins, radii and potted inserts, and the same holds for shear values taken from a core coupon. The sound way to use 0.40 MPa and 0.85 MPa is as a ranking and a starting point for laminate sizing, then to treat the actual sandwich — skin layup, core thickness, edge close-out, process pressure — as its own structure to be checked. Where a purchase needs a floor value rather than a typical one, that belongs in the specification agreed with the pmi foam core manufacturer, because a datasheet typical value and a contractual minimum are two different documents. Aerospace programmes rarely stop at a datasheet either: ISO 9001 supplies the process-control and traceability baseline most suppliers work inside, and FAA advisory material shows how formally aviation writes those manufacturing quality expectations down.
Conclusion
Comparing 32WH and 52WH is a density exercise first and a strength exercise second, and that order holds whether the material comes from one aerospace pmi foam supplier or another. The 32 kg/m³ grade is the lighter option with a typical 0.40 MPa compressive strength; the 52 kg/m³ grade roughly doubles that strength and stiffness while adding meaningful mass per cubic metre. Both share the coarse-cell, closed-cell build, the ≥200 °C typical heat distortion temperature, and the 2500 × 1250 mm board with ±0.2 mm thickness tolerance. Use the values to rank and to size, test a few coupons per grade, and let the finished sandwich design confirm the choice.
FAQ
Q: What is the typical compressive strength difference between 32WH and 52WH PMI foam?
A: The published typical values are 0.40 MPa for 32WH and 0.85 MPa for 52WH, so the denser grade carries roughly twice the compressive strength of the lighter one. The gap of about 0.45 MPa is large in relative terms even though both grades sit in the low-density segment. Read it as a ranking that helps you size a core, and confirm the number you actually need through coupon testing on the batch you plan to use.
Q: Are PMI foam density and compressive strength values guaranteed minimums?
A: They are typical values, which means they describe the centre of a tested population rather than a contractual floor. The density ranges of 32±7 kg/m³ and 52±12 kg/m³ already tell you that individual boards vary within a band, and the strength figures follow that spread. If a design needs a guaranteed minimum, state it explicitly in the purchase specification so it is agreed and verified separately from the datasheet.
Q: How should engineers read 32WH and 52WH data without overfitting a small coupon test?
A: Test several specimens per grade and compare medians instead of single results, because one coupon can sit anywhere inside the density band. Keep the test method visible next to the number, since platen, specimen thickness and loading rate all shape a compression result. Use the typical values to rank the two grades and to start laminate sizing, then check the real sandwich — skins, core thickness, edges and process pressure — as its own structure.
Sources / References
ISO 9001 - Quality Management Systems | BSI UK
AC 150/5345-10F - Specification for Constant Current Regulators Regulator Monitors (Cancelled)
