Key Considerations for Thermal Paper Coating Process

 

 

I. Thermal Paper Coating Slurry Control (Core Source)

 

1. Dispersion and Stability

 

- The particle size D50 of the thermal liquid material should be 0.5～0.8μm to prevent pigment/developer agglomeration, which will result in spots and white spots.

 

- Continuous low-speed circulation stirring is required; prolonged standing is prohibited. If the machine is shut down for more than 2 hours, the pipes and troughs must be emptied and cleaned to prevent the slurry from drying and clogging the nozzles/dies.

 

- The slurry must be continuously filtered through a 10～20μm bag filter; damaged filter bags should be replaced immediately.

 

2. Stable Physicochemical Parameters

 

- Temperature: 23～28℃, fluctuation ≤±2℃;

 

- Viscosity and solid content should be tested at least twice per shift; pH of water-based thermal paper coating slurry should be controlled at 6.0～8.0.

 

- Appropriate addition of defoamer is crucial; air bubbles inside the slurry can cause pinholes and missed areas in the coating, resulting in blank lines in the print.

 

3. Slurry Transportation

Avoid right-angle bends and dead angles in the conveying pipeline to reduce material retention; clean the pipeline regularly to prevent dried particles from mixing into the coating.

 

II. Substrate (Base Paper) Pretreatment & Tension Control

 

1. Strictly control the moisture content of the base paper to 5%～7%. Excessive moisture content can cause bubbling during coating and yellowing of the paper later; insufficient moisture content makes the paper brittle and the coating prone to cracking.

 

2. Stable Tension Throughout the Line

 

- Set segmented tensions for unwinding, coating, and rewinding; rewinding tension should be 20%～30% lower than unwinding tension to prevent back adhesion (coating transfer to the back of the paper). - Severe tension fluctuations directly cause uneven coating thickness, transverse streaks, and paper web wrinkles.

 

3. Dust Removal from the Base Paper Surface: Use a brush + electrostatic eliminator. Paper dust and dirt falling on the wet coating layer will form permanent defects.

 

III. Core Operating Points of the Coating Head (Spraying Unit)

 

1. Precise Control of Coating Amount (Most Important Indicator)

 

- Base Coating: 3～6 g/m²; Thermal sensitive(color development) Layer: 3.5～6 g/m²; Top Coating (Protective Layer): 1.5～3 g/m².

 

- Transverse and longitudinal coating amount deviation control ≤±0.2 g/m²; Too thin coating → faint printing; Too thick → decreased sensitivity, easy adhesion.

 

2. Equipment Maintenance

 

- Slit head/spraying chamber cleaned every shift; dried slurry accumulation will cause periodic vertical streaks and missed coating.

 

- Back roller and pressure roller kept smooth and free of dried coating; roller surface damage directly replicates defects onto the paper surface.

 

3. Speed ​​Matching: Production line speed, paste supply flow rate, and coating gap must be synchronized; sudden increases/decreases in speed are strictly prohibited to prevent abrupt changes in coating amount.

 

IV. Drying System Control (Determines the shelf life and color development performance of thermal paper)

 

⚠️ Direct high-temperature baking is prohibited; gradual temperature increase drying is mandatory!

 

Recommended hot air segmented temperature zone settings: Inlet 75～85℃ → Middle section 90～105℃ → Outlet ≤115℃

 

1. First, slowly evaporate surface moisture at a low temperature to prevent surface skinning and internal bubbling;

 

2. The maximum temperature must not exceed 120℃ for extended periods! High temperatures will cause premature reaction of the thermal color developer, resulting in yellowing and aging of the finished product before printing.

 

3. Regularly clean clogged air nozzles in the drying oven; insufficient local airflow will cause dry spots and color differences.

 

4. After drying, the paper must be cooled by cooling rollers before rewinding; high-temperature rewinding easily leads to back adhesion and coating damage.

 

V. Workshop Environmental Requirements

 

1. Temperature: 23±2℃; Relative Humidity: 45%～60%RH

 

2. High humidity environment: The slurry easily absorbs water, the coating turns white, and the weather resistance of white paper deteriorates; excessively low humidity causes severe static electricity, resulting in spots and dust adsorption.

 

3. Keep the coating area clean and as dust-free as possible; dust is the main source of coating spots and pinholes.

 

VI. Common Defect Prevention (Key Points in Production)

 

**Defect Phenomenon | Key Control Points**

 

**Uneven Color Development, Dark and Light Stripes:** Tension fluctuations, insufficient slurry mixing, partial die blockage, uneven drying airflow

 

**White Spots, Pinholes:** Slurry air bubbles, filter failure, substrate dust, unstable spraying pressure

 

**Yellowing of Finished Paper:** Drying temperature too high, abnormal slurry pH, excessive moisture in base paper

 

**Coating Peeling, Poor Scratch Resistance:** Insufficient drying (residual moisture), binder ratio, compatibility between primer and topcoat

 

**Back Adhesion During Rewinding:** Incomplete drying, excessive rewinding tension, insufficient cooling

 

 

VII. Safety and Changeover Procedures

 

 

1. When changing product type or coating (primer → heat-sensitive layer → topcoat), the entire slurry tank, pump, pipes, and die must be thoroughly cleaned. Cross-contamination between different slurries can severely damage the heat-sensitive color development system.

 

 

2. Before starting the machine, run a test run to check the coating uniformity. Use the first 30 meters as waste material to inspect the coating condition. Only proceed with formal production after confirming there are no defects.

 

3. Online Timed Sampling: Take samples from the beginning, middle, and end of each roll of paper to test coating amount, printing color density, and aging resistance.

 

VIII. Mandatory Requirements for Multi-Layer Coating Sequence

 

The following sequence must be followed: Primer → Sufficient Drying and Cooling → Thermal Layer → Sufficient Drying and Cooling → Top Coating Protective Layer. Applying the next coating layer directly without sufficient drying will cause interlayer solubility, significantly reducing thermal sensitivity and shortening shelf life.

 

For more questions about thermal paper production technology, please feel free to contact us for consultation and exchange.

 

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Jack.liu@jiaqimat.com

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