Required clamp force for a printed TPU 85A gasket
Reference lid (200 × 150 mm, 700 mm gasket path): 1400–2800 N total. Per latch at the high end: 700 N with 4 latches, 467 N with 6, 350 N with 8 (half that at the low end).
Basis: Reference scenario: 200 x 150 mm lid, continuous gasket on a 700 mm path (own geometry, from the gasket researcher). The numbers are the closing force needed to hold the DESIGN gasket at end of life, not a solid wide bead. Low = 2 N/mm x 700 mm = 1.4 kN; high = 4 N/mm x 700 mm = 2.8 kN. The 2-4 N/mm window is a 1.2-1.6 mm wide, >=2 mm tall TPU 85A rib: at the Garlock seal stress of 1.4 MPa a 1.2 mm rib needs 1.7 N/mm, and at 20 % squeeze a 1.2 x 2.4 mm rib gives ~3.9 N/mm after my shape-factor correction (5.2 N/mm if you scale the Texin curve without it). per_latch values use the HIGH total (2.8 kN / 4, 6, 8); the low-end per-latch figure is exactly half (350 / 233 / 175 N). Rule: per-latch force = line load x latch pitch, so it scales with lid size. Sanity check: I extracted the Texin 985 ASTM D575 table from https://sushengpolymer.com/media/pdf/phFiH7_Texin-985.pdf (2/5/10/15/20/25/50 % = 0.345/1.03/2.07/3.10/4.31/5.52/16.5 MPa); the gasket researcher's arithmetic is correct, and an independent neo-Hookean estimate (E ~12 MPa, shape factor 0.5) lands within ~15-25 % of the curve. ALTERNATIVE (solid 3 x 3 mm bead, own calc): 2.9 kN at 6.7 % squeeze, 6.5 kN at 15 %, 11.6 kN at 25 % total; at 6 latches 483 / 1085 / 1932 N each, which exceeds any printed latch's sustained capacity (my estimates top out at ~300-800 N). Cross-sections: 3 mm round cord 2.2-5.0 kN; 1 mm rib into flat gasket 2.8-6.3 kN (rough). Corner and end latches may carry +20-30 % (assumption). A rigid fixed-travel latch must be preloaded 1.4-2.4x higher at assembly to still hold this load after 30-58 % force loss from 85A compression set at 70 C (Ultrafuse 52 % set), which is why adjustable or spring-loaded latches are favoured. Uncertainty +/-2x on printed-bead stiffness; verify with a force-gauge test on the actual printed bead.
Key numbers
85A TPU compressive load (Texin 985, molded, ASTM D575): 0.345 / 1.03 / 2.07 / 3.10 / 4.31 / 5.52 / 16.5 MPa at 2 / 5 / 10 / 15 / 20 / 25 / 50 % deflection. I extracted these from the datasheet text myself and they match the gasket researcher: https://sushengpolymer.com/media/pdf/phFiH7_Texin-985.pdf . Same sheet: compression set (ASTM D395B / ISO 815) 16-19 % at 23 C/22 h and 40-80 % at 70 C/22 h; flexural modulus 26.9 MPa at 23 C vs 59.0 MPa at -30 C (2.2x). Printed-filament Ultrafuse TPU 85A: set 26 % (23 C/72 h) and 52 % (70 C/24 h), tear 80 kN/m flat vs 18-30 kN/m on edge/upright, break stress 34 vs 10 MPa (researcher-sourced, not re-checked by me): https://www.dream3d.co.uk/wp-content/uploads/2022/12/Ultrafuse_TPU_85A_TDS_EN_v2.5.pdf .
LINE LOAD (own calculation = D575 stress x bead width; 3 x 3 mm bead has shape factor ~0.5, close to the D575 button's 0.57; treat as +/-2x for printed beads because fill pattern alone changed 100%-fill 85A peak force 1315 vs 2587 N, https://reference-global.com/2/v2/download/article/10.2478/aucts-2024-0004.pdf). Solid 3x3 mm bead: 4.1 N/mm at 6.7 % squeeze, 6.2 at 10 %, 9.3 at 15 %, 12.9 at 20 %, 16.6 at 25 %. On a 700 mm path (200x150 mm lid) that is 2.9 / 4.3 / 6.5 / 9.0 / 11.6 kN. Rule of thumb about 3.1-5.5 N/mm per mm of bead width at 15-25 %; an independent neo-Hookean check with E ~12 MPa (https://plahunter.pages.dev/data-sheets/ninjatek/tpu-ninjaflex) and shape factor gives 2-6 N/mm per mm width, i.e. within ~15-25 % of the datasheet curve. Narrow 1.2 x 2.4 mm rib: 1.7 N/mm at 7 % up to 5.2 N/mm at 20 % (Texin-scaled, conservative-high; my shape-factor correction gives ~3.9 N/mm at 20 %). 1 mm rigid rib pressed 1 mm into a flat 3 mm gasket: ~4-9 N/mm (rough, +/-2x). 3 mm round cord (Lindley, E ~14 MPa): 3.1 N/mm at 15 %, 4.8 at 20 %. Hollow or low-infill printed bead: assumed 0.3-0.6x solid, unmeasured.
Seal thresholds
Elastomers seal at about 1.4 MPa (200 psi) assembly stress, recommended minimum 4 MPa for 70A (Garlock, https://legacy.garlock.com/sites/default/files/documents/en/Gasket Assembly Stress Non-metallic - Aug 2022.pdf ); 1.4 MPa is only ~6.7 % squeeze on the Texin curve. IP67 water head is ~9.8 kPa (own calc), negligible, so leaks come from FDM surface roughness and layer-line paths, not pressure. Squeeze guidance: 20-30 % for solid rubber (Elasto Proxy https://www.elastoproxy.com/how-much-compression-do-rubber-gaskets-require/ , KRAIBURG https://www.kraiburg-tpe.com/pl/node/1281 ); Parker static max ~30 %, minimum 0.2 mm absolute squeeze, gland fill 60-85 % (https://nfc.arizona.edu/sites/default/files/2022-02/parker_o-ring_handbook.pdf ); about 10-12 % residual squeeze needed (https://www.nrc.gov/docs/ML1419/ML14195A258.pdf ).
What this means for latches
(1) Stroke is tiny: 0.20-0.75 mm for a 3 mm bead, the same order as FDM warp and hinge play, so every design needs a printed hard stop beside each latch (height ~0.75-0.8 x free bead) plus adjustability or a series spring. (2) Compression set: 52 % set at 70 C means a rigid fixed-travel latch loses ~30-58 % of its force after hot sun; a steel spring about 4x softer than the gasket cuts that to ~7-10 % (own calc: loss = set x ks/(kg+ks)). (3) Cold: modulus is 2.2x at -30 C, so closing force at fixed stroke may be 1.5-2x higher at -20 C. (4) Latch count is limited by rim bending, not latch strength: pitch L = (384 EI d / w)^0.25 with E = 1.8 GPa (assumed), bow d = 0.15 mm, w = 3.7 N/mm gives ~28 mm for a flat 3 mm lid edge, ~110 mm with a 20 mm skirt, ~154 mm with a 30 mm box rim (own calc); stiffen the rim before buying a stronger latch. (5) Recommended gasket: 2-4 N/mm line load (1.2-1.6 mm wide, >=2 mm tall rib, 20-25 % squeeze, printed flat at 100 % infill, ironed land, hard stops). Per-latch force then equals line load x latch pitch (e.g. 100 mm pitch = 200-400 N). Solid 3 mm beads at 15 %+ need >1 kN per latch on 6 latches and are beyond what any printed latch holds for months. Only printed-latch-plus-85A-TPU design found: MakerWorld Tough Case 2385298 (PETG snap latches, no force or IP data): https://www.makerworld.com/en/models/2385298 . No source certifies any printed TPU gasket to IP67.