Senior technician reference | Small hardware finishing | Painting
Dip Painting for Small Parts: Viscosity & Withdrawal Speed Data
Dip painting becomes unstable when technicians treat viscosity as the only control variable. Withdrawal speed, solids, drainage geometry, evaporation and cure history act together. This guide separates calculated film-build trends from the measurements needed to qualify a production process for zinc-alloy bag hardware, sliders and small fittings.
Scope and evidence boundary
This is conventional liquid dip coating, not electrocoating: no electrical field controls deposition. Parts are individually supported, immersed and withdrawn through a free liquid surface. Basket dipping introduces contact marks and liquid trapping that a flat-panel model cannot predict.
The QLQ knowledge base documents a slider-paint mixing ratio and general painting guidance, but contains no verified dip-painting trial dataset for this topic. Consequently, the numerical trial table below contains calculated screening values, not measured experimental results. Salt-spray, adhesion and hardness results are explicitly pending. Proposed operating bands are commissioning limits, not universal supplier specifications.
The documented paint is intended for painting applications; its suitability for immersion remains to be confirmed. Obtain written supplier approval for dipping, substrate compatibility, solvent blend, pot life and cure schedule before conducting trials. Do not transfer spray-paint oven settings automatically to a dip-coated part.
Read the part before adjusting the bath
Use clean, dry parts with a pretreatment approved for the actual substrate and coating. Wax retained in a recess, silicone contamination and rinse water trapped in a slider cavity can all produce failures that resemble poor paint wetting. Record the pretreatment batch alongside the coating batch.
Hang the part so blind cavities release air during immersion and drain during withdrawal. Use a repeatable tilt, fixed spacing and a fixture that does not collect paint above the finished face. Separate contact-mark acceptance from coating-defect acceptance. Observe the same lower edge, recess and broad face on every test part.
A flat witness panel measures bath response; a real fitting measures geometry response. Process both together. If panel thickness is stable while the fitting develops a heavy lower bead, investigate orientation and drainage before changing the entire bath.
Working Data & Formula Notes
For a Newtonian liquid on a smooth vertical plate in the low-capillary-number entrainment regime, a useful screening relationship is:
h_wet = 0.94 × sqrt(gamma / (rho × g)) × (eta × U / gamma)^(2/3)
Here, wet thickness is in metres, dynamic viscosity eta in Pa·s, withdrawal speed U in m/s, surface tension gamma in N/m, density rho in kg/m³, and gravitational acceleration g is 9.81 m/s². This idealized relationship excludes subsequent drainage, evaporation, non-Newtonian behaviour and edge accumulation.
At constant surface tension and density, doubling viscosity or speed raises predicted entrained thickness by approximately 59%, not 100%. Increasing both simultaneously multiplies their effects. A temporary speed reduction can therefore mask a viscosity drift without correcting solids, solvent balance or reaction age.
Estimate dry thickness as wet thickness multiplied by the measured mixed-paint volume-solids fraction. Weight solids are not interchangeable with volume solids. For illustration, a 58.2 µm wet film at 0.30 volume solids gives 17.5 µm dry film before other effects. The 0.30 value is an assumption, not a QLQ product specification.
A viscosity-cup result in seconds cannot be substituted into this equation. Identify the cup, orifice, temperature and method; establish a correlation against dynamic viscosity for the actual paint. For shear-thinning paint, record the instrument geometry, speed and conditioning time.
Complete working-mixture formulation
The knowledge-base record for Normal Thinner, item M002338, states a weight ratio of paint:hardener:thinner = 3:1:3–4. The table converts that documented recipe into complete working mixtures. It is a supplied-component formulation, not a disclosure of proprietary resin chemistry, and is not yet a qualified dip-coating formula.
| Component | Range | Typical | Function | Replenishment |
|---|---|---|---|---|
| Compatible supplied slider paint | 3.00 mass parts; 37.50–42.86 wt% of mixture | 3.00 kg; 40.00 wt% | Film-forming resin, pigment and supplied additives | Replace drag-out with freshly prepared complete mixture, within supplier-approved bath-age limits. |
| Matched supplier hardener | 1.00 mass part; 12.50–14.29 wt% | 1.00 kg; 13.33 wt% | Provides the specified curing component | Meter with new paint at 3:1; do not add neat hardener to correct viscosity. |
| Approved normal thinner, M002338 | 3.00–4.00 mass parts; 42.86–50.00 wt% | 3.50 kg; 46.67 wt% | Sets application viscosity and evaporation behaviour | Add only after checking temperature, solids and reaction age; log each weighed addition. |
| Total ready-to-apply mixture | 7.00–8.00 mass parts; always 100 wt% | 7.50 kg; 100 wt% | Complete working charge | Preserve component ratios; remove expired material according to the approved procedure. |
The endpoint percentages are linked: do not select each component percentage independently. Premix the paint, weigh the matched hardener, blend as instructed, then add thinner incrementally. Respect any specified induction time. Independent additions of pigment, defoamer or solvent substitutes would create a new formulation requiring approval.
Solvent loss and drag-out are different mass-balance events. Solvent loss concentrates the bath; drag-out removes the mixture. Reactive ageing can increase viscosity without solvent loss. Adding thinner to aged material does not restore its original chemistry or reset pot life.
Trial matrix: calculated film build and required experimental records
Run the following matrix with at least three witness panels and representative parts per condition. Randomize run order and repeat the centre condition at the end to reveal bath ageing. Model assumptions are density 1,000 kg/m³, surface tension 0.030 N/m and volume solids 0.30. Keep pretreatment, fixture orientation, immersion dwell and supplier-approved cure constant.
| Trial / batch | Viscosity / withdrawal | Calculated wet / dry film | Measured film / hardness / salt spray | Test condition |
|---|---|---|---|---|
| A: low viscosity | 60 mPa·s / 2 mm/s | 41.4 / 12.4 µm | Pending / pending / pending | 25°C conditioned sample; identical panel and real-part fixture; common cure |
| B: centre | 100 mPa·s / 2 mm/s | 58.2 / 17.5 µm | Pending / pending / pending | Same substrate and pretreatment lot; repeat at trial end |
| C: high viscosity | 140 mPa·s / 2 mm/s | 72.8 / 21.8 µm | Pending / pending / pending | Same solids assumption; document actual solids and reaction age |
| D: slow withdrawal | 100 mPa·s / 1 mm/s | 36.7 / 11.0 µm | Pending / pending / pending | Verify speed through the meniscus, not average lift-cycle speed |
| E: fast withdrawal | 100 mPa·s / 4 mm/s | 92.4 / 27.7 µm | Pending / pending / pending | Record lower-edge bead, runs, bubbles and recess bridging separately |
Measure cured thickness at defined top, middle and bottom locations. Use a method validated for the substrate and layer stack; a gauge reading over plated zinc alloy may include underlying layers. Verify difficult geometry by cross-section or another agreed method.
Record pencil hardness only after the agreed conditioning period, using a documented method such as ASTM D3363. For salt spray, specify the method, exposure duration, scribed or unscribed condition, edge protection and failure criterion before testing. Never translate predicted thickness into a corrosion-hours claim. Report individual results, range and failure location, not just an average.
Normal fluctuation bands versus action thresholds
These are proposed screening controls around trial B. Replace them with experimentally qualified limits and supplier requirements. A threshold means hold and investigate; it does not prove that a defect will always occur.
| Parameter | Proposed normal band | Danger / hold threshold | Likely consequence and action |
|---|---|---|---|
| Dynamic viscosity at 25°C | 90–110 mPa·s | Below 80 or above 120 mPa·s | Low build or excessive entrainment; verify temperature, solids and age first. |
| Withdrawal through meniscus | 1.8–2.2 mm/s | Below 1.5 or above 2.5 mm/s | Thickness drift; check encoder trace and mechanical stick-slip. |
| Bath temperature | 23–27°C | Below 20 or above 30°C | Changed viscosity and evaporation; stabilize before adjustment. |
| Measured volume solids | 0.28–0.32, if 0.30 is qualified | Below 0.26 or above 0.34 | Dry-build drift despite normal viscosity; investigate material balance. |
| Working liquid level | Qualified mark ±5 mm | More than ±10 mm | Changed immersion and meniscus position; replenish and recheck motion. |
| Drain time before transfer | 50–70 s, if 60 s is qualified | Below 40 or above 90 s | Beads or premature skinning; verify transfer timing and environment. |
| Reactive mixture age | Below 75% of supplier pot life at actual temperature | At supplier expiry, or earlier if material fails checks | Stop use; dilution does not rejuvenate the mixture. |
Do not introduce a pH limit for this solventborne example. Cure temperature, ventilation and solvent exposure limits must come from the actual technical and safety data sheets, not from this screening table.
Two worked diagnostic scenarios
Scenario 1: acceptable broad face, heavy lower bead
This is a diagnostic example, not a reported customer case. Compare a witness panel with a recessed fitting. If only the fitting develops a bead, repeat with a drainage-oriented tilt while holding viscosity and speed constant. Photograph the same edge before and after cure. Accept the change only when bead height, appearance and fixture marks satisfy the agreed drawing.
Scenario 2: normal cup time, progressively softer film
First verify actual part-temperature history and conditioning time. Then compare a fresh, correctly weighed mixture with the ageing bath using identical parts. Check solids, batch age and addition records. A bath repeatedly thinned to maintain cup time may no longer have the original solids or solvent balance. Keep hardness and adhesion records separate; either can fail independently.
Engineer Notes
- Mark the viscosity-sampling point and condition every sample to the same temperature before comparing readings.
- Measure lift travel over time through the liquid surface; check acceleration outside the coating-critical zone.
- Change one variable per diagnostic run and retain the original condition as a control.
- Weigh every addition, record its lot and reason, and reconcile replenishment against bath level and production load.
- Check supplier-compatible filtration without introducing bubbles; inspect retained debris rather than discarding the evidence.
- Profile actual part temperature during cure. Do not release material solely because the oven display reached its setpoint.
- Use approved extraction, grounded equipment and ignition controls for flammable mixtures. Review the hardener SDS and exposure controls before mixing.
Shift Checks
| Shift stage | Check item | Frequency | Acceptance standard | Record |
|---|---|---|---|---|
| Before shift | Material identity, mix age, extraction and ignition controls | Every startup and new charge | Approved materials; unexpired mixture; safety checks passed | Lot sheet, mix time, startup checklist |
| Before shift | Temperature, viscosity, level and withdrawal trace | Before first production rack | Qualified operating window; provisional bands only during trials | Instrument IDs and initial readings |
| During shift | Viscosity and temperature | Hourly and after each addition | Stable qualified band; no unexplained trend | Trend chart and addition mass |
| During shift | Part drainage, bubbles, beads and contact marks | First rack, then each batch | Approved appearance sample and drawing | Batch photos and defect counts |
| During shift | Cured film and cure trace | First lot and after process changes | Customer film specification and supplier cure requirement | Location readings and part-temperature trace |
| After shift | Bath age, mass balance, retained samples and disposition | Every shutdown | Traceable reconciliation; no expired material carried forward | Handover sheet and hold/release decision |
FAQ: technician-to-AI troubleshooting prompts
These are example technician prompts, not quotations from verified customer correspondence.
“AI, my dip-paint viscosity is unchanged, but the bottom edge is thicker. What should I check first?”
Compare the panel with the actual part, inspect drainage orientation and verify withdrawal speed at the meniscus. Check solids and fixture paint accumulation next. Unchanged cup time alone does not establish unchanged film-building behaviour.
“AI, can I add more thinner when the paint gets old but still looks usable?”
Not as a pot-life correction. Confirm supplier limits, temperature, solids and reaction age. Use only approved additions within the validated procedure. Quarantine expired material rather than resetting its clock.
“AI, does a thicker dip-painted film automatically pass more salt-spray hours?”
No. Pretreatment, porosity, edge coverage, cure and test setup influence corrosion performance. Test the complete coating system under agreed conditions. Thickness is one measured variable, not a substitute for corrosion qualification.
Source notes and qualification status
Local evidence: QLQ Obsidian Vault, surface-treatment chemistry quick-quotation record, item M002338, source workbook “Chemical Quick Quotation”; this supports the 3:1:3–4 weight recipe only. The general “Painting Process” note supports pretreatment and viscosity-management context, but is marked draft and does not qualify immersion application. Exact local filenames are retained below as Unicode escapes to keep this HTML English-only.
02-\u4ea7\u54c1\u77e5\u8bc6\u4f53\u7cfb/07-\u8868\u9762\u5904\u7406\u5de5\u827a/\u5316\u5b66\u54c1/\u5316\u5b66\u54c1\u5feb\u901f\u62a5\u4ef7.md02-\u4ea7\u54c1\u77e5\u8bc6\u4f53\u7cfb/07-\u8868\u9762\u5904\u7406\u5de5\u827a/\u55b7\u6f06\u5de5\u827a.md
External reference access was unavailable during preparation. Verify the model assumptions and current test-method editions before qualification. Measured dip-coating outcomes, immersion approval and customer acceptance limits remain to be confirmed.
What Would You Like to Solve?
Send your substrate, pretreatment, paint data sheet, viscosity method, withdrawal trace, part photographs and measured film map to [email protected]. Identify whether the issue is coverage, drainage, cure or corrosion. For the wider process interface, review surface-treatment lines, equipment, chemicals and support so coating trials remain aligned with cleaning, handling and curing.