To avoid the bottom penetration of glossy surface coating silicone, it is necessary to start from four aspects: material selection, process optimization, equipment adjustment, and environmental control, and make systematic improvements in combination with specific operation details. The following are specific solutions:
1. Material selection and pretreatment
Select high hiding power silicone
Color selection: Preferentially use white or dark silicone (such as black, gray), and avoid transparent or light-colored silicone directly for dark substrates.
Opaquer addition: If a light color effect is required, 5%-10% titanium dioxide (strong hiding power) or iron oxide pigment (color adjustment) can be added to the silicone. It needs to be fully dispersed through a three-roll grinder to avoid particle aggregation and light transmission.
Viscosity adjustment: Select the viscosity of the silicone according to the mesh count of the screen (high viscosity silicone for low mesh screens and low viscosity silicone for high mesh screens) to avoid bottom penetration due to too low viscosity.
Substrate pretreatment
Surface cleaning: Wipe the surface of the substrate with alcohol or a special cleaner to remove impurities such as oil, dust, fingerprints, etc. If necessary, use plasma treatment or flame treatment to increase the surface energy.
Primer coating: For dark or rough substrates (such as black fabrics, unpolished metals), first apply a layer of white or light-colored primer (such as water-based polyurethane primer), and then print silicone after drying to enhance hiding power.
Surface flattening: Grind or sandblast rough surfaces (such as coarse-grained fabrics, cast metals) to reduce the difficulty of silicone filling gaps.
2. Process parameter optimization
Screen production and selection
Mesh matching: Select the mesh number of the screen according to the viscosity of the silicone:
Low viscosity silicone (such as 1000-3000mPa·s): Use 80-120 mesh screen to increase the amount of glue.
High viscosity silicone (such as 5000-10000mPa·s): Use 150-200 mesh screen to control printing accuracy.
Screen thickness: Use thick screen (such as 50-75μm) or apply thickening agent to ensure that the thickness of the silicone layer is ≥0.1mm.
Photosensitive adhesive coating: The thickness of the photosensitive adhesive layer on the screen should be uniform to avoid local thinness causing adhesive penetration.
Printing parameter control
Scraper pressure and angle:
Pressure: The silicone can completely fill the mesh without overflowing (usually 0.2-0.3MPa).
Angle: 75°-85°, avoid excessive scraping of the silicone due to a small angle.
Printing speed: Adjust the speed according to the viscosity of the silicone:
Low viscosity silicone: 10-15cm/s, to ensure that the silicone fully fills the mesh.
High viscosity silicone: 15-20cm/s, to prevent the silicone from drying and clogging on the screen.
Back-ink knife pressure: The back-ink knife pressure should be less than the scraper pressure (recommended 10%-20% lower) to avoid silicone backflow causing insufficient thickness in the printing area.
Overprinting and thickness control
Multiple overprinting: For dark substrates or scenes with high covering requirements, 2-3 overprintings are used, and drying is performed after each printing (temperature 80-100℃, time 2-3 minutes), and the total thickness reaches 0.2-0.3mm.
Thickness detection: Use a thickness gauge (such as a micrometer) to randomly check the thickness of the silicone layer to ensure uniformity.
3. Equipment and tool adjustment
Screen spacing setting
The screen spacing is recommended to be 2-3 times the screen thickness (such as a 50μm screen, a screen spacing of 100-150μm), to avoid insufficient silicone transfer due to too large a screen spacing, or poor contact between the screen and the substrate due to too small a screen spacing.
Scraper and ink return knife selection
Scraper material: Use a hard polyurethane scraper (hardness 70-80 Shore A) to reduce silicone deformation.
Ink return knife type: Use an adjustable pressure ink return knife to dynamically adjust the pressure according to the viscosity of the silicone.
Optimization of curing equipment
Curing temperature and time:
Infrared oven: 120-150℃, time 3-5 minutes, to ensure that the silicone is completely cross-linked.
UV curing: If UV curing silicone is used, the light intensity (such as 80-120mW/cm²) and irradiation time (such as 5-10 seconds) need to be controlled.
Hot air circulation: Add hot air circulation function in the oven to avoid abnormal curing caused by uneven local temperature.
4. Environment and operating specifications
Environmental control
Temperature: 20-25℃, avoid low temperature causing increased viscosity and poor fluidity of silicone.
Humidity: 40%-60%, use dehumidifier or air conditioner to adjust to prevent abnormal curing after silicone absorbs moisture.
Cleanliness: Print in a dust-free workshop of Class 1000 or Class 10,000 to reduce dust pollution.
Operation specifications
First piece confirmation: Make the first piece before each batch of printing, test the thickness, hiding power and adhesion of silicone, and then mass produce after passing the test.
Silicone storage: Store in a sealed container after opening to avoid long-term exposure to air and degradation of performance.
Screen cleaning: Immediately clean the screen with a special cleaning agent after printing to prevent the silicone from drying and clogging the mesh.
5. Case verification and effect evaluation
Case 1: Printing white silicone label on black T-shirt
Problem: The bottom is obvious after a single printing.
Solution:
Pretreatment: Apply white water-based primer on the black T-shirt and print after drying.
Process adjustment: Use a 120-mesh screen, overprint twice, and dry at 130℃ for 3 minutes after each printing.
Effect: The thickness of the silicone layer reaches 0.25mm, the bottom problem is solved, and the hiding power meets the standard.
Case 2: Printing gray silicone on transparent PC sheet
Problem: The bottom pattern is visible.
Solution:
Material adjustment: Select high-hiding gray silicone and add 8% titanium dioxide.
Process optimization: Use a 150-mesh screen, print speed 15cm/s, curing temperature 150℃, time 5 minutes.
Effect: The thickness of the silicone layer is 0.18mm, the transmittance is ≤3%, and the bottom seepage phenomenon disappears.
6. Summary and preventive measures
Preventive measures
Standardized process: formulate silicone printing SOP, clarify materials, equipment, process parameters and operation steps.
Incoming material inspection: check the hiding power of silicone, surface flatness and cleanliness of substrates.
Process monitoring: use tools such as thickness gauge and transmittance meter to detect printing quality in real time.
Key control points
Silicone layer thickness ≥ 0.1mm (dark substrate ≥ 0.2mm).
Silicone hardness after curing ≥ 40 Shore A (to ensure wear resistance).
Transmittance ≤ 5% (detected by spectrophotometer).
Substrate surface roughness Ra ≤ 1.6μm (for high-precision printing).