Add tiered PVC lab checks: per-lot benchtop, in-house workhorses, external qualification panel

docs/quality-automation.md

@@ -138,6 +138,57 @@ the lot ID follows every record pressed from it:
   and verification of the virgin/regrind ratio at the extruder; regrind
   batch IDs are logged like any other lot
 
+### The PVC lab, in tiers
+
+**Per lot, in-house from day one** (benchtop instruments, all with data
+export for the traceability gate):
+
+- Moisture content: halogen moisture analyzer or Karl Fischer titration;
+  minutes per sample, and the most common cause of noisy surfaces
+- Heat stability: oven strips at pressing-relevant temperatures watching
+  discoloration over time; the formal methods are ISO 182 (Congo red /
+  conductivity), tracking HCl evolution
+- Density: checked against the compound spec; feeds record weight
+  consistency
+- Color: spectrophotometer L\*a\*b\* against the reference, plus gloss for
+  black compound
+- Granulate physicals: sieve analysis for particle size distribution, bulk
+  density, dust/fines content
+- Ash content: muffle furnace burn-off as a filler-loading drift check
+- Pressed-plaque inspection for specks, gels, and contamination (shareable
+  with the optical inspection investment)
+
+**In-house as the lab matures** (the PVC-specific workhorses):
+
+- Torque rheometry (Brabender class): fusion/gelation time and torque,
+  the standard predictor of how a PVC compound behaves in the extruder;
+  plain melt-flow index is less informative for PVC than for other
+  thermoplastics because the material is shear- and heat-sensitive
+- DSC: glass transition, additive signatures, and the record-relevant
+  application of measuring the degree of gelation of a pressed record,
+  which correlates with surface noise and durability, turning DSC into a
+  process-tuning tool as well as an incoming check
+- FTIR: identity fingerprint; record compound is classically a vinyl
+  chloride/vinyl acetate copolymer rather than pure PVC, and FTIR verifies
+  the copolymer and catches unannounced formulation changes
+
+**External lab, per supplier qualification** (not per lot):
+
+- K-value / dilute solution viscosity (ISO 1628-2): molecular weight, the
+  headline spec for record-grade resin since it governs flow into the
+  groove
+- XRF elemental analysis: stabilizer system identification (Ca-Zn or tin
+  acceptable, legacy lead rejected) and heavy-metals compliance for export
+- TGA: full compositional breakdown (plasticizer, filler, carbon black)
+- Volume/surface resistivity: static behavior, which becomes dust
+  attraction on finished records
+
+The tiers define the protocol: the per-lot gate runs moisture, heat
+stability, color, density, and granulate physicals plus the release test
+pressing; a new supplier additionally gets the full external panel and
+pressing trials. This is the working method for qualifying Thai and
+regional compounds against the European references.
+
 ## Packaging QC
 
 Packaging comes from local carton partners, so QC splits into an incoming