Do you know why some surfaces of silicone strips are not smooth?
Formation mechanism of surface defects of silicone strips
1.Poor quality of raw materials
(1) Substandard base rubber quality: If too much cheap and inferior low molecular weight ring bodies (D3-D6) are used in industrial-grade silicone raw materials or the process is not refined, resulting in uneven dispersion of fillers, it will lead to an increase in surface precipitates after vulcanization, and the surface of the silicone weather strips will be uneven or the color will be uneven. Experimental data show that when the ring body content exceeds 0.8%, the surface stickiness of the product increases by 47%.
(2) Unbalanced compatibility of additives: In order to reduce costs, some factories will use cheap anti-yellowing agents, flame retardants and other modifiers to be compatible with the base rubber. It is unreasonable to control and use curing agents during the raw material mixing process. The amount of curing agent added is not completely cured, resulting in the surface of the product becoming sticky. Therefore, if a larger proportion of modifiers is mixed in the raw materials, it is easy to cause the silicone weather strips to be not durable and have poor elasticity.
2.The factory’s machines are not advanced and the vulcanization process is rough
(1) This depends on the machines purchased by the factory. Some factories use relatively backward production lines, which will lead to large proportion errors and the surface of the weather strips is not fine enough.
(2) Due to machine problems, the mixing uniformity is not enough: when mixing on a double-roll mill, if the mixing time is not ≥15 minutes, the Fourier infrared spectrum shows that 2-3% of undispersed curing agent agglomerates will remain. This will cause the surface of the weather stripping window to be rough. During the kneading period, the curing agent and silicone are not evenly stirred, and the mold is cured while curing, and after the product is vulcanized, the hardness and hardness difference will easily cause the product to deform.
3.There are problems with the quality process of the sealing strip mold
(1) When the machine is cleaned, the mold is not cleaned and the mold is not smooth enough. When the mold is not cleaned, there will be some impurities on the surface, which makes the surface of the sealing strip rough. Principle: Due to the excessive roughness of the mold surface: the Ra value of the mold cavity should be controlled at 0.2-0.4μm. EDX spectral analysis shows that when Ra>0.8μm, the micro-tears generated at the moment of demolding increase the surface friction coefficient by 0.3-0.5.
(2) Residues in the mold will cause uneven products and make the surface of the product rough. Principle: Release agent residue: When the amount of silicone oil release agent exceeds 2mg/dm², a nano-scale oil film (10-50nm) will be formed, which will be oxidized to form viscous siloxane oligomers after thermal curing.
4.Disadvantages of subsequent treatment
(1) Insufficient secondary vulcanization: When the post-curing treatment at 200℃×4h is not performed, the residual volatile content reaches 1.2-1.8%, which is 3-4 times higher than that of qualified products. The main components of GC-MS detection are methyl triketone (MTO) and cyclotetrasiloxane (D4).
(2) Surface coating defects, caused by failure to spray hand oil in time: When the plasma treatment does not reach the critical surface tension of 46-52mN/m, the adhesion of the UV coating decreases by 60%, and the standard deviation of the coating uniformity observed by AFM reaches ±15nm.
The silicone strips produced have the problem of rough surface. It is necessary to find out the cause and correct it in time to avoid the output of a large number of defective products, resulting in material waste and affecting production efficiency.
Industrialization Solution
Raw Material Preprocessor Technology
Use twin-screw extruder (L/D=40:1) for premixing, set five-zone temperature control (80-120℃ gradient), and cooperate with online near-infrared (NIR) monitoring system to ensure filler dispersion ≥98%.
Intelligent Vulcanization System
Integrated PLC-controlled injection molding vulcanization equipment, mold temperature controller accuracy ±0.5℃, pressure sensor (0-25MPa) and infrared temperature measurement (±1℃), to achieve precise vulcanization of A/B rubber at 170-180℃ (t90 time control ±3s).
Mold maintenance plan
(1) Perform EDM spark trimming every 500 molds to maintain the cavity surface roughness Ra≤0.4μm
(2) Use PVD coating technology (CrN/TiAlN) to increase the mold hardness to HV2800 and extend the service life by 3-5 times
(3) Establish a mold cleaning log, use a special cleaning agent with pH7.5±0.2, and an ultrasonic cleaning frequency of ≥2 times/shift
Post-processing process package
(1) Design a stepped secondary vulcanization furnace: 80℃×1h→120℃×1h→200℃×2h, gradient heating rate 3℃/min
(2) Introduce atmospheric plasma treatment (APPJ), working parameters: argon flow rate 15L/min, power 800W, The processing speed is 0.5m/min, which increases the surface energy to 72mN/m
(3) Nano-spraying system: using a 0.15mm atomizing nozzle, the solid content of the silicone-fluorine modified polyurethane coating is 65±2%, and the film thickness is controlled at 8-12μm
Quality control system
Online detection: installing a machine vision system (5 million pixel CCD+LED coaxial light), which can detect surface defects ≥50μm
Laboratory analysis: weekly sampling for DMA testing (strain 0.1%-10%, frequency 1Hz), ensuring that the tanδ value is in the range of 0.02-0.05
Durability test: 1000 hours of aging in a simulated kitchen environment (85℃/85%RH), requiring hardness change ≤±5 Shore A, mass loss rate ≤0.8%
