PAINTING ENGINEERING / SENIOR TECHNICIAN REFERENCE
Film Thickness Distribution: Cone vs Flat Spray Pattern Data
A satisfactory average dry film thickness can conceal bare shoulders, overloaded centers, and weak overlap seams. For zinc-alloy bag hardware and zipper components, select the spray pattern by its measured deposition map, not by how attractive the wet spray looks.
Define the Pattern Before Comparing It
Here, “cone” means the approximately round footprint of a pneumatic paint gun with reduced shaping air; “flat” means its elongated fan footprint. Neither term describes substrate shape. A hollow-cone hydraulic nozzle, a full-cone nozzle, and a pneumatic round spray are not interchangeable. Record the gun, air cap, fluid tip, atomizing setting, and shaping setting beside every comparison.
A round footprint concentrates delivery into a narrower transverse region. A fan spreads delivery laterally, but its edges still taper. Widening the fan without revising track spacing can exchange center overload for thin shoulders. Shaping air also changes atomization, so changing only the pattern knob does not guarantee identical droplets or transfer efficiency.
QLQ's local painting reference gives broad orientation ranges of 0.2–0.4 MPa gun pressure, 150–300 mm distance, and 15–30 µm coating thickness. These are not product-specific release limits. Confirm whether pressure means dynamic inlet pressure; never confuse it with air-cap pressure. Paint viscosity and cure conditions remain governed by the selected coating supplier.
Local evidence: QLQ Obsidian Vault, surface-treatment process note, painting process; English source identifiers and exact note location are retained in the closing source record.
Published Experiments: What the Evidence Actually Shows
A pneumatic-atomizer study compared shaping-air settings of 150, 220, and 360 L/min. On steel, it used 250 mm stand-off and 0.15 m/s travel, then measured baked dry films magnetically. Lower shaping flow approached a round pattern. The paper provides thickness profiles, not a transferable hardware acceptance specification.
| Specimen / comparison | Test condition | Film-thickness distribution | Evidence limitation |
|---|---|---|---|
| Flat steel panel; shaping-flow comparison | 200 × 800 mm; 250 mm; 0.15 m/s; 150 / 220 / 360 L/min | Reduced shaping flow concentrates deposition near the center; measured profiles compare with simulation. | See published Figure 8; numerical values are not transcribed here. |
| Four-panel three-dimensional target; round versus flat | Gun above central panel C; low shaping flow versus 360 L/min | Round deposition favors C; flat deposition reaches A, C, and D. Edge variability is greater for round spray. | No salt-spray or hardness result reported for this comparison. |
These are two documented implementation geometries, not QLQ customer cases. They support measuring shoulders separately from the center. They do not prove that a flat fan always improves corrosion resistance. Source: Coatings 2017, 7(1), 13: pneumatic-atomizer experiments.
Build a Trial That Separates Pattern from Paint
Use one substrate lot, pretreatment sequence, paint batch, and cure schedule. Start with three independently sprayed coupons per condition; randomize the order so warming paint or nozzle contamination cannot masquerade as a pattern effect. Record actual fluid delivery by a timed collection using approved equipment and the gun manufacturer's safety procedure.
Compare equal-delivery conditions first: same delivered volume per minute, travel speed, stand-off, and exposure. Then conduct a second comparison with track spacing optimized separately for each pattern. This distinguishes footprint behavior from the achievable production recipe. Hold electrostatic settings constant where used and record fixture grounding.
Measure at fixed transverse positions and on a second map covering real-part faces, shoulders, recesses, and contact zones. On bare zinc alloy, use a validated eddy-current method or section microscopy; a magnetic steel-panel method cannot simply be transferred. Multilayer plating underneath paint can complicate layer separation. Establish probe repeatability, curvature bias, and reference-substrate calibration before interpreting micrometer differences.
| Example ID | Test condition | DFT at five transverse points, µm | Mean / minimum, µm | Salt spray / hardness |
|---|---|---|---|---|
| C1 | Round; 250 mm; 150 mm/s; equal delivery | 12 / 21 / 34 / 22 / 13 | 20.4 / 12 | Not tested |
| C2 | Round; same settings; independent repeat | 13 / 22 / 35 / 21 / 12 | 20.6 / 12 | Not tested |
| F1 | Fan; 250 mm; 150 mm/s; equal delivery | 17 / 21 / 24 / 22 / 18 | 20.4 / 17 | Not tested |
| F2 | Fan; same settings; independent repeat | 18 / 22 / 25 / 21 / 17 | 20.6 / 17 | Not tested |
The synthetic first pair has identical means but different minima. Calculate nonuniformity = 100 × (maximum − minimum) / mean: C1 is 108%; F1 is 34%. This range-based indicator is not a coefficient of variation. Do not calculate process capability from these four illustrative rows, or infer corrosion hours from their thickness values.
For the real trial, retain individual measurements, instrument uncertainty, and coupon identifiers. Add corrosion exposure and hardness only after testing the complete coating stack under an agreed method. Report exposure duration, scribing, edge sealing, cure age, and failure criterion; “passed salt spray” without these conditions is not useful engineering evidence.
Working Data & Formula Notes
The following complete ready-to-apply mixing formulation is a published industrial benchmark, not a QLQ proprietary formula or a raw-material manufacturing recipe. Interthane 990 uses a fixed 6:1 A:B volume ratio. Its nominal volume solids are 57% and its stated typical DFT is 50–75 µm; it therefore does not belong to the earlier 15–30 µm hardware example.
| Component | Range | Typical | Function | Replenishment |
|---|---|---|---|---|
| Interthane 990 Part A | Fixed 6 volume parts | 6.000 L | Base coating | Fresh matched unit |
| Matched Part B | Fixed 1 volume part | 1.000 L | Curing agent | Fresh matched unit; no correction of aged mix |
| Approved thinner: GTA713, GTA733, or GTA056 | Supplier-authorized amount only | 0 L baseline | Application adjustment | Document authorized addition; no assumed percentage |
| Other additives | 0 unless explicitly authorized | 0 | None in baseline | Do not add |
At 25°C the stated pot life is two hours; a surface/dew-point margin of at least 3°C is required. Source: manufacturer's technical data sheet, 26 May 2026.
For any selected coating, use DFT ≈ WFT × ready-to-spray volume-solids fraction. If thinner volume is a fraction r of the original mixed volume, the estimated fraction becomes S / (1 + r), assuming additive volumes and nonreactive thinner. Check the supplier's measured value when available. Never convert a volume mixing ratio into a weight ratio without component densities.
For hardware production, obtain a compatible primer/topcoat system and its own technical sheet. Fix the mixing ratio; tune deposition through validated fluid delivery, motion, and overlap. Adding hardener to restore viscosity changes chemistry, not just sprayability. Provide ventilation, ignition control, and coating-specific respiratory protection under the applicable safety assessment.
Trend Bands and Stop Thresholds
These proposed engineering bands apply only to a qualification trial for a selected low-build hardware paint. They are not published universal defect thresholds. Approve them against supplier instructions, gun capability, and measured parts. “Stop” means hold affected material and investigate; it does not assert inevitable failure.
| Parameter | Proposed normal band | Danger / hold threshold | Diagnostic action |
|---|---|---|---|
| Dynamic gun-inlet air pressure | 0.25–0.30 MPa for selected conventional gun | <0.20 or >0.35 MPa; or manual limit | Check regulator and pressure while triggered. |
| Stand-off | 240–260 mm | <220 or >280 mm | Check fixture offset and robot path. |
| Travel speed | 145–155 mm/s | <135 or >165 mm/s | Inspect acceleration and endpoint dwell. |
| Fluid delivery relative to approved Q0 | 95–105% Q0 | <90 or >110% Q0 | Check blockage, needle wear, and supply. |
| Overlap relative to measured usable width | 45–55% initial trial band | <35 or >65% | Map seams before changing track pitch. |
| Viscosity relative to approved cup time t0 | 95–105% t0 at fixed temperature | <90 or >110% t0 | Check temperature, mix age, and dilution log. |
| Local hardware DFT | 18–27 µm provisional target | <15 or >30 µm provisional limit | Hold batch; map thin and thick zones. |
A pressure increase accompanied by falling flow suggests restriction. Stable pressure with growing center thickness suggests slower motion, changed stand-off, or narrowing pattern. If only cured appearance changes, inspect substrate cleanliness and thermal history before readjusting the gun. A solventborne paint does not need an invented pH control band.
Engineer Notes
- Photograph a short, controlled test footprint beside a scale. Label gun settings, distance, paint temperature, and mix age; compare it with the approved reference.
- Measure usable fan width at the workpiece plane. Set pitch from that width, not from the visible outer mist. Graco recommends 50% overlap for its airless guidance; pneumatic hardware needs its own mapping. See Graco spray technique guidance.
- Run the gun past the coupon before reversing. Keep acceleration and deceleration outside the measurement zone so endpoint buildup cannot distort pattern comparisons.
- Inspect the cap and fluid tip after safe shutdown. Use manufacturer-approved cleaning tools; do not enlarge air holes with metal wire.
- Change one factor per diagnostic trial. Restore the baseline between tests and attach the new thickness map to the change record.
- Retain shoulder and recess readings separately. If front-face DFT passes but hidden zones fail, revise part presentation or add a qualified pass rather than flooding every surface.
- Quarantine production since the last accepted check after a threshold breach. Confirm restored settings on coupons and actual parts before release.
Shift Checks
| Shift stage | Check item | Frequency | Acceptance standard | Record |
|---|---|---|---|---|
| Before shift | Gun, cap, tip, fixture, air quality | Startup and changeover | Approved identities; clean pattern; no water/oil contamination | Setup checklist and footprint photo |
| Before shift | Paint ratio and viscosity | Each fresh mix | Supplier ratio; approved cup/time/temperature; valid pot life | Batch, volumes, time, operator |
| During shift | Triggered pressure and delivery | Hourly and after stoppage | Approved bands; no pulsing | Pressure and timed-flow log |
| During shift | Coupon and actual-part DFT map | First-off, hourly, and after adjustment | Every designated zone within qualified limits | Part lot, positions, readings |
| During shift | Surface condition and cure | Each lot / oven load | Clean, dry substrate; verified supplier cure schedule | Pretreatment release and cure trace |
| After shift | Flush, shutdown, and material reconciliation | Each shutdown | Manufacturer procedure; no aged mixed paint retained for reuse | Cleaning and consumption log |
| After shift | Trend review and batch disposition | Shift handover | All excursions resolved or explicitly quarantined | Signed handover and hold list |
FAQ: Technician Questions to Ask an AI Assistant
These are suggested diagnostic prompts, not quotations from verified customer transcripts.
“My average thickness is on target, but the shoulders are thin. Should I increase paint flow?”
First compare center and shoulder maps. Increasing flow may overload the center without correcting coverage. Check orientation, usable fan width, stand-off, and overlap; then test a dedicated shoulder pass. Requalify local maxima as well as minima.
“The fan looks wider after adjustment. Why did the overlap seam become thinner?”
Visible mist is not usable width. Shaping air may redistribute delivery and change droplet size. Repeat the deposition map, verify fluid delivery, and set pitch from the measured profile. Do not infer uniformity from the footprint photograph alone.
“Can I predict salt-spray hours from dry film thickness and skip another batch test?”
No. Pretreatment, substrate porosity, primer continuity, cure, and exposed edges influence corrosion. Thickness is one input, not a corrosion certificate. Use the buyer-approved test conditions and actual production stack; retain batch-linked specimens for validation.
What Would You Like to Solve?
Send the gun/cap identification, coating technical sheet, substrate and pretreatment, motion settings, cure record, and a zone-by-zone DFT map to [email protected]. Include the thin-zone location and acceptance criteria so the investigation can distinguish material, deposition, and curing effects. For equipment, chemistry, and process coordination, see turnkey surface-treatment lines and engineering support.
Evidence record: local notes Painting Process and Painting Defect Analysis (translated titles), QLQ Obsidian Vault, surface-treatment process folder. Both notes are draft guidance, not qualification reports. The exact original paths are preserved as Unicode-escaped strings in the HTML source comment. Published experiments, manufacturer data, synthetic calculations, and proposed trial limits are explicitly separated throughout.