In the process of silicone screen printing, the selection of suitable silicone and diluent needs to be comprehensively considered from four dimensions: application scenario, physical properties, chemical compatibility, and operation process. The following is a specific plan and analysis:
1. Selection of silicone type: based on application scenario and performance requirements
Selection by temperature resistance
High temperature environment (such as aramid fabric printing): select addition type silicone, the temperature range can reach 200-300℃, and there is no small molecule release after curing, avoiding deformation or volatilization of toxic substances at high temperature.
Normal temperature or low temperature environment: condensation type silicone can be selected, which has low cost, but poor temperature resistance (usually ≤150℃), and needs to be adjusted according to the actual upper limit of use temperature.
Selection by hardness requirements
Soft touch (such as clothing printing): select low hardness silicone (such as Shore A 10-30 degrees), and further reduce the hardness by adding silicone oil dilution (ratio ≤10%), but it should be noted that excessive dilution will reduce wear resistance.
High-strength support (such as industrial seals): choose high-hardness silicone (such as Shore A 60-80 degrees), use it directly or add a small amount of filler (such as fumed silica) to enhance the hardness.
Selection by special performance
Food contact (such as children's tableware): Food-grade silicone must be selected, which complies with FDA or GB 4806.11-2016 standards to ensure non-toxicity, odorlessness, and high temperature resistance.
Flame retardant requirements (such as fire suit logos): Select flame-retardant silicone with an oxygen index of ≥28% and pass UL94 V-0 certification.
Conductive requirements (such as electromagnetic shielding): Select conductive silicone, add silver powder or carbon black filler, and the resistivity can be as low as 10⁻³Ω·cm.
2. Diluent selection: based on silicone type and process requirements
Universal diluent
Silicone oil: Applicable to all types of silicone, it can reduce viscosity and improve fluidity, but it will reduce hardness (for every 10% silicone oil added, the hardness decreases by about 5-10 degrees).
407 printing water/white oil: commonly used for printing silicone dilution, moderate volatility, but attention should be paid to fire prevention and explosion prevention (flash point ≤30℃).
Special diluent
Special diluent for addition type silicone: such as hydrogen silicone oil, can participate in the curing reaction to avoid a sharp drop in hardness after dilution, but the cost is high.
Special diluent for condensation type silicone: such as dimethyl silicone oil, the amount added needs to be controlled (≤5%), otherwise it may affect the curing speed.
Dilution ratio control
Thick lines/thick printing: the ratio of diluent to silicone can be 1:1.3 (such as embossed silicone) to ensure filling.
Thin lines/thin printing: the ratio of diluent to silicone can be 1:0.9 (such as liquid silicone) to avoid flowing.
Covering layer: low viscosity and high gloss are required, and 120% of the weight of diluent can be added (such as the case of Chaopai craft), but the leveling time needs to be extended to prevent bubbles.
3. Key matching principle: compatibility of silicone and diluent
Chemical compatibility
Avoid using condensation silicone diluent for addition silicone, otherwise it will inhibit the curing reaction (resulting in non-curing or incomplete curing).
If the diluent contains elements such as nitrogen, phosphorus, and sulfur (such as certain organic solvents), it will poison the platinum catalyst of addition silicone and must be strictly prohibited.
Process matching
Room temperature vulcanization (RTV): Use volatile diluents (such as white oil) to facilitate rapid drying.
Heat vulcanization (HTV): High boiling point diluents (such as silicone oil) can be used to avoid pinholes caused by rapid volatilization at high temperatures.
Environmental protection and safety
Prioritize the use of odorless, low-VOC diluents (such as food-grade silicone oil) that comply with ROHS and REACH standards.
Avoid using diluents containing carcinogens such as benzene and toluene (such as crude benzene) and use environmentally friendly alternatives (such as isopropyl alcohol).
4. Practical cases and data support
Silicone screen printing on aramid fabric
Silicone selection: addition type high temperature resistant silicone (temperature resistance 250℃), hardness Shore A 40 degrees.
Dilution scheme: Add 5% silicone oil to reduce viscosity to 6000 centipoise, with 1% curing agent, baking temperature 150℃/10 minutes, adhesion reaches 4B grade (Baige method).
Children's silicone tableware
Silicone selection: food grade addition type silicone (FDA certified), hardness Shore A 30 degrees.
Dilution scheme: Add 3% silicone oil to adjust fluidity, vulcanization temperature 120℃/30 minutes, ensure non-toxic, odorless, and high temperature resistance.
Tide brand clothing printing
Silicone selection: high transparent addition type silicone, hardness Shore A 20 degrees.
Dilution scheme: Add 150% weight of 407 printing water when diluting, add 5% color paste when adjusting color, cover the surface twice, vulcanize at room temperature for 24 hours, gloss ≥90%.
5. Common problems and solutions
Silicone does not cure
Reason: The diluent contains impurities or is incompatible with silicone.
Solution: Replace the special diluent and mix it strictly in proportion (such as 10:1 silicone: curing agent).
Foaming after printing
Reason: The diluent evaporates too quickly or bubbles are introduced during stirring.
Solution: Use a vacuum degassing machine to treat the mixed silicone, or extend the leveling time.
Hardness does not meet the standard
Reason: Excessive diluent addition or incomplete curing.
Solution: Control the dilution ratio (≤10%) and extend the vulcanization time (such as extending the addition type silicone to 24 hours).