Introduction: For an autoclave-cured sandwich part, density grade and the WH or WH-HT variant should follow part loads, peak cure temperature, pressure, and core thickness, in that order.
Autoclave co-curing turns density selection into a process decision, because the core must carry structural load and survive the cure cycle in the same build. Start with the load path; the technical data supports the choice, and the cure window can override a grade that looked correct on paper. Selection errors often surface late in the build: a core whose compression behavior at 0.7 MPa has not been checked, a panel that finishes heavier than the laminate model predicted, or a 100 mm core that is outside the selected grade’s thickness range. These process-window and availability issues appear after the grade has been fixed. The right selection order reduces them to a short set of questions for a supplier before a trial.
How load paths point to a starting PMI foam density
In a co-cured sandwich, the core transfers shear between skins, resists skin wrinkling under compression, carries local compression under inserts, fittings, and fastener rows, and withstands the compaction pressure of the cure. The starting grade should follow whichever job dominates the part, not the highest number in the range. The WH ladder has four steps. Rifeng 32WH sits at 32 kg/m³ with 0.40 MPa compression and 0.40 MPa shear; 52WH at 52 kg/m³ with 0.85 MPa and 0.80 MPa; 75WH at 75 kg/m³ with 1.75 MPa and 1.35 MPa; and 110WH at 110 kg/m³ with 3.60 MPa and 2.45 MPa. These are typical reference values. Compression, shear, and tensile strength rise together as density climbs. Choosing a grade is therefore choosing a stiffness level plus a mass penalty. The penalty is real because core volume is fixed by the part drawing. A thin fuselage panel with light out-of-plane loading is usually a good starting point for 32WH or 52WH, and staying low keeps the finished assembly light. A stringer run in a pressure bulkhead, a region crowded with inserts, or any zone with concentrated bearing load is where 75WH and 110WH earn their weight. Start at the lowest grade whose typical compression and shear values clear your load case with margin. Then confirm that choice against the cure cycle, because temperature can override a density choice that looked correct on paper.
What 130 °C and 180 °C curing windows mean for WH and WH-HT
The cure schedule determines the variant before density becomes the main question. Standard WH is rated to 130 °C and 0.7 MPa. Heat-treated WH-HT is rated to 180 °C and 0.7 MPa. Both share the same 0.7 MPa pressure ceiling, so temperature is the discriminator. Settle it early; it narrows the choice to one material family and prevents a density grade from being approved for the wrong process window.
1. Standard WH covers 130 °C cure peaks without an added heat-treatment step
A 120 °C or 130 °C peak cure temperature fits standard WH directly. If your recipe stays at or below that peak, standard WH is the straightforward choice: no additional treatment step, no extra handling, and a process route your router already describes. This is common for many industrial and aerospace sandwich parts. The risk comes when a later cycle change raises the peak and the standard grade stays in the build plan. Freeze the peak cure temperature at the start of the material review, then check every autoclave recipe that will run the part. A 130 °C limit is a process control point, not a line to test with a one-off trial.
2. WH-HT reaches the 180 °C cure window after a defined heat treatment
For a 180 °C co-cure at 0.7 MPa, specify WH-HT. Heat treatment is a separate process step that must be planned before layup. Decide whether Rifeng supplies the foam already treated or whether your plant runs the treatment before layup, then write that step into the process router so untreated foam never reaches a 180 °C autoclave. Heat distortion temperature is reported as a typical value of 200 °C or above across the range, but the rated cure window controls the cycle: 130 °C for standard WH and 180 °C for WH-HT. The distinction belongs on the purchase drawing, the material certificate request, and the work instruction so engineering, procurement, and the autoclave team work from the same variant.
How autoclave pressure and thickness tolerances affect density choice
During a 0.7 MPa dwell, the selected grade’s compression behavior matters. If your cycle runs close to the 0.7 MPa ceiling, include that in the data request and ask for the compression behavior of the specific grade at that pressure rather than assuming the whole range behaves alike. Pressure can move the starting grade upward when the lighter option’s typical compression data does not leave enough margin for the process, even if in-service loads alone would allow it. Thickness availability narrows the ladder further. Standard boards are 2500 × 1250 mm for 32WH, 52WH, and 75WH, and 2160 × 1100 mm for 110WH. Available thickness ranges are 3–110 mm for 32WH and 52WH, 3–100 mm for 75WH, and 3–90 mm for 110WH, with a thickness tolerance of ±0.2 mm. A 105 mm core therefore rules out 110WH immediately. A thick part with tight skin fit may need a machined or thermoformed pre-shaped core rather than a plain board. Thickness tolerance matters because core height sets the sandwich depth and the bondline your skins see. If you need to hold ±0.2 mm across a curved part, raise that requirement early. Boards, CNC-machined blanks, and ready-to-use pre-shaped cores are different supply options to confirm with the supplier.
Conclusion
Density selection for autoclave co-curing follows a fixed order: read the dominant load path, set the cure temperature to 130 °C standard WH or 180 °C WH-HT, then check pressure and thickness availability against the chosen grade. That order avoids two costly mistakes: a grade that does not match the autoclave pressure and a grade that cannot be supplied at the required thickness. Before a first trial, bring the load case, peak temperature, dwell pressure, thickness range, and target density to Rifeng. As a PMI foam core manufacturer, Rifeng can review those inputs against the WH ladder and typical value data. As an aerospace PMI foam supplier, Rifeng can also confirm the WH-HT treatment route and thickness availability before the run.
FAQ
Q: How do I choose between 75WH and 110WH for an autoclave-cured sandwich part?
A: Base the choice on the concentrated loads first. 75WH offers typical values of 1.75 MPa compression and 1.35 MPa shear; 110WH raises those to 3.60 MPa and 2.45 MPa. Move to 110WH when insert pull-out, bearing at fittings, or high-pressure compaction at 0.7 MPa calls for the extra stiffness, and confirm that the required thickness falls within its 3–90 mm range.
Q: Does standard Rifeng WH support 180 °C co-curing?
A: Standard WH is rated to 130 °C and 0.7 MPa, which covers 120–130 °C cycles. A 180 °C co-cure requires the heat-treated WH-HT variant, rated to 180 °C and 0.7 MPa. Specify the variant explicitly on the purchase drawing so the treatment step is not lost between engineering and procurement.
Q: What pressure and temperature limits should I discuss for WH-HT?
A: Start with peak cure temperature, dwell time, ramp rate, and autoclave pressure. WH-HT is rated to 180 °C and 0.7 MPa, so any cycle approaching either figure deserves a direct discussion of compression behavior at that pressure. Also settle how and where the heat treatment is performed, and confirm the needed thickness within the 3–110 mm, 3–100 mm, or 3–90 mm range for your chosen grade.
Sources / References
AC 20-107B - Composite Aircraft Structure | FAA
No comments:
Post a Comment