Views: 0 Author: XINYITE PLASTIC Publish Time: 2025-10-15 Origin: Site
The causes of fiber floating and flow marks in PP GF30 (30% glass fiber-reinforced polypropylene) products are complex, involving multiple dimensions such as materials, processes, and molds. The following is a detailed analysis based on industry experience and technical data:
As an inorganic material, glass fiber has weak interfacial adhesion with PP. If the glass fiber surface is not treated with a coupling agent (e.g., silane-based) or no compatibilizer (e.g., maleic anhydride-grafted PP, PP-g-MAH) is added, the glass fiber tends to separate from the matrix and expose on the surface. For example, when the addition amount of PP-g-MAH is less than 3-5wt%, the interfacial bonding force is insufficient, significantly increasing the risk of fiber floating.
Excessively long glass fiber (e.g., long fiber >1mm) is prone to breakage under shear force, forming short fiber agglomeration; excessively short glass fiber (<0.5mm) is difficult to be effectively coated by resin. Both situations lead to fiber floating.
Uneven mixing or unreasonable screw configuration design (e.g., insufficient shear section) results in poor glass fiber dispersion. Local areas with excessively high glass fiber concentration will break through the surface and cause fiber floating.
Although PP has low hygroscopicity, the glass fiber surface may absorb moisture. If drying is not performed at 70-90℃ for 2-4 hours, moisture will volatilize at high temperatures, causing flow marks and weakening the bonding between glass fiber and resin.
Low melt temperature (below 180℃) increases PP viscosity, making it difficult for glass fiber to be fully coated and raising the risk of fiber floating; excessively high temperature (above 275℃) may cause glass fiber degradation and PP oxidation, resulting in flow marks and reduced mechanical properties.
Low mold temperature (below 40℃) causes the surface resin to solidify rapidly, and internal glass fiber is squeezed to the surface due to shrinkage stress; excessively high mold temperature (above 80℃) may lead to excessive crystallization and increased surface roughness.
Low injection pressure or slow injection speed results in insufficient melt filling, and glass fiber accumulates on the surface due to flow resistance; excessively fast injection speed (e.g., over 250cm/s) triggers the "fountain effect," causing glass fiber to remain with the surface melt.
Insufficient holding pressure (below 50MPa) or short holding time (below 10s) leads to insufficient replenishment, and glass fiber is exposed due to shrinkage rebound; excessively high holding pressure (above 120MPa) may intensify glass fiber orientation and form local fiber floating.
Excessively high screw speed (above 150rpm) causes excessive shearing of glass fiber, breaking it into short fibers; low back pressure (below 10MPa) results in uneven glass fiber dispersion and excessively long residual length.
Improper screw configuration design (e.g., lack of a mixing section) leads to uneven plasticization and aggravated glass fiber agglomeration.
Too small gate size (e.g., diameter <1.2mm) causes the shear rate to exceed 8000s⁻¹, forcing glass fiber to the surface; unreasonable gate position (e.g., far from thin-walled areas) leads to long flow paths and concentrated glass fiber orientation.
Too thin runner diameter (below 4mm) or high surface roughness (Ra >0.8μm) increases flow resistance and causes glass fiber retention.
Insufficient depth of mold venting grooves (below 0.02mm) or blockage causes gas to entrain glass fiber and float upward, forming silver streaks or fiber floating on the product surface. Especially for complex structural parts, additional venting grooves should be added at weld line positions.
Mirror polishing (Ra <0.1μm) makes glass fiber more likely to adhere and expose; insufficient surface hardness (e.g., no nitrided steel or PVD coating) accelerates mold wear and causes glass fiber to scratch the surface.
Excessive use of mold release agent or mismatched types (e.g., silicone-based) weakens the bonding between resin and mold, promoting glass fiber exposure.
Severe wear of the screw or barrel leads to uneven plasticization and reduced glass fiber dispersion; an excessively large nozzle aperture (above 4mm) causes melt pressure loss and affects glass fiber coating.
When environmental humidity exceeds 60%, the glass fiber surface easily absorbs moisture; excessively low workshop temperature (below 15℃) causes rapid mold cooling. Both situations increase the risk of fiber floating.
Add certain percentage of anti-floating agent to improve the compatibility and dispersion of glass fiber and PP.
Temperature: Control the melt temperature at 220-250℃ and increase the mold temperature to 60-80℃ to delay surface solidification.
Pressure and Speed: Adopt segmented holding pressure (e.g., 90MPa/2s → 80MPa/3s → 70MPa/5s) and control the injection speed at 100-150cm/s to balance filling and orientation.
Screw Parameters: Reduce the rotation speed to below 100rpm and set the back pressure to 15-20MPa to enhance plasticization.
Gate and Runner: Increase the gate diameter to 1.5-2mm and use fan-shaped or submarine gates; expand the runner diameter to 6-8mm and polish it to Ra <0.4μm.
Venting: Open venting grooves with a depth of 0.03-0.05mm at the end of the cavity, or use a split mold to enhance venting.
Surface Treatment: Adopt matte polishing (Ra 0.8-1.2μm) for the mold surface and perform hard chrome plating or nitriding to improve wear resistance.
Regularly inspect the wear of the screw and barrel and replace them in a timely manner; adjust the nozzle aperture to 3-4mm to match material fluidity.
Control the proportion of recycled material to ≤20% and fully mix it with new material; maintain environmental humidity at 40-50% and workshop temperature at 20-25℃.
