To avoid the problem of uneven mixing of silk-screen silicone, it is necessary to comprehensively control multiple links such as raw material control, mixing process, equipment selection, operating specifications and quality inspection. The following are specific measures and analysis:
1. Raw material control: ensure the stability of basic quality
Strictly weigh the proportion
Use a high-precision electronic scale: the ratio of base glue to curing agent must be accurate to ±0.1% to avoid human estimation errors.
Clear packaging label: separate packaging of silicone raw materials of different viscosities or colors to prevent cross contamination.
Premixed additives: If pigments, flame retardants, etc. need to be added, first pre-mix with a small amount of base glue to form a masterbatch, and then add it to the main material in proportion.
Raw material pretreatment
Reheating treatment: Silicone stored at low temperature needs to be placed in a 25℃ environment in advance to warm up to room temperature to avoid viscosity differences causing mixing difficulties.
Filter impurities: Use a 100-200 mesh filter to filter the raw materials to remove lumps or foreign matter and prevent particles from being generated during stirring.
2. Optimization of mixing process: key operation steps
Choose a suitable mixing method
Mechanical mixing: Planetary mixer or vacuum mixer is preferred to achieve uniform mixing through the composite motion of revolution + rotation.
Manual mixing assistance: In small batch production, first use a scraper to scrape the bottom raw materials along the container wall, and then cross-mix until there is no color difference.
Avoid high-speed shear: The stirring speed is controlled at 500-1000rpm to prevent excessive shear force from causing the silicone molecular chain to break.
Stage mixing
Low-speed premixing: First stir at 300-500rpm for 1-2 minutes to allow the curing agent and base glue to initially infiltrate.
High-speed dispersion: Increase to 800-1000rpm and stir for 3-5 minutes to ensure that the additives are fully dispersed.
Vacuum degassing: After stirring, evacuate to below -0.08MPa for 3-5 minutes to eliminate bubbles.
Control mixing time and environment
Time standardization: Set a fixed stirring time according to the viscosity of the silicone (such as 3 minutes for low viscosity and 5 minutes for high viscosity).
Temperature monitoring: Keep the mixing environment temperature at 20-30℃ to avoid high temperature accelerating curing or low temperature causing viscosity increase.
3. Equipment selection and maintenance: Ensure mixing efficiency
Mixing equipment requirements
Blade design: Use butterfly or frame blades to increase the shear area and avoid mixing dead corners.
Container matching: The diameter of the mixing container should be 1.5-2 times the diameter of the blade to ensure that the raw materials are fully turned.
Vacuum system: Equipped with a vacuum pump and a pressure gauge to monitor the degassing effect in real time.
Regular maintenance
Clean the blades: Clean the blades and containers with solvent after each use to prevent residual silicone from curing and affecting the next mixing.
Check the sealing: Replace the mixer sealing ring regularly to avoid air leakage and insufficient vacuum.
Calibrate the speed: Use a tachometer to calibrate the mixer speed every quarter to ensure that the actual speed is consistent with the set value.
4. Operation specifications: Standardized processes reduce human errors
Develop SOP (standard operating procedures)
Clear the mixing sequence, time, speed and degassing parameters and post them next to the operating table.
Example process:
1. Weigh the base glue → 2. Add curing agent → 3. Premix at low speed for 2 minutes → 4. Disperse at high speed for 4 minutes → 5. Vacuum degassing for 3 minutes → 6. Stand and degas for 1 minute
Personnel training and assessment
New employees need to pass the practical mixing assessment to ensure that they can operate the mixer independently and judge the mixing state.
Periodically retrain and update equipment operation skills (such as the use of new vacuum mixers).
Record and traceability
Record the mixing parameters of each batch (time, temperature, vacuum degree, etc.) to facilitate problem tracing.
Retain a small sample of the mixed silicone, marked with the production date and batch number, for subsequent performance testing.
V. Quality inspection: Discover mixing problems in advance
Appearance inspection
Visual inspection: The mixed silicone should be in a uniform paste without color difference, particles or stratification.
Leveling test: Drop the silicone on a horizontal glass plate and observe whether it automatically leveled into a smooth dot within 30 seconds.
Physical performance test
Hardness test: Use a Shore A hardness tester to measure the hardness of the cured sample, and the deviation should be ≤±5 degrees.
Tensile test: Cut dumbbell-shaped specimens, test tensile strength and elongation at break to ensure compliance with product standards.
Process monitoring
Online viscosity detection: Use a viscometer to monitor the change in silicone viscosity during mixing in real time to determine the uniformity of mixing.
Infrared spectroscopy analysis: Perform infrared scanning on the mixed silicone to confirm whether the chemical bonding between the curing agent and the base glue is complete.
VI. Emergency treatment of common problems
There are still particles after mixing
Cause: The raw materials are not filtered or the stirring time is insufficient.
Treatment: Stop using immediately, re-filter and extend the stirring time to 1.2 times the standard value.
There are still bubbles after vacuum degassing
Cause: Insufficient vacuum or excessive stirring speed to involve air.
Treatment: Check the sealing of the vacuum pump, reduce the stirring speed to below 800rpm, and degas again.
Uneven hardness after curing
Cause: Uneven distribution of curing agent or residual unmixed raw materials in the dead corner of stirring.
Treatment: Discard the batch, check the design of the mixer blades, and increase the low-speed premixing time.
Summary
To avoid uneven mixing of silk-screen silicone, a five-in-one prevention and control system of "raw materials-process-equipment-operation-testing" needs to be established. Through standardized processes, precise equipment control and strict quality inspection, the mixing defect rate can be significantly reduced (usually controlled below 1%), ensuring product performance stability and production efficiency. For high-precision applications (such as medical and electronic packaging), it is recommended to introduce an automated mixing system to further reduce the risk of human intervention.