Flow marks are wavy surface defects that appear near the gate of plastic molding parts. Characterized by concentric circular resin flow traces centered on the gate, these defects compromise the surface quality of plastic molding parts and may affect their functional performance. Flow marks primarily result from uneven temperature distribution, rapid plastic solidification, turbulent flow or cold slug accumulation at the gate, and insufficient material replenishment during the holding stage. Their formation is closely related to injection molding machines, molds, materials, and operators’ parameter settings—all critical factors in producing high-quality plastic molding parts.

Temperature inconsistencies in the plastic or equipment directly impact melt fluidity and solidification, leading to flow marks on plastic molding parts.

Inadequate injection or holding pressure fails to press the solidified plastic skin tightly against the mold surface, leaving flow traces along the melt flow direction. Increasing pressure can effectively resolve this issue.

Short residence time of plastic in the barrel results in low melt temperature, preventing full compaction. Even if the mold cavity is barely filled, the plastic cannot be sufficiently pressed during holding, leading to flow marks.

Excessively short cycle time limits sufficient heating of the plastic in the barrel, causing low melt temperature. Extending the cycle time ensures thorough heating, reducing the risk of flow marks on plastic molding parts.

Low barrel temperature leads to insufficient melt temperature. This prevents the injection and holding pressure from pressing the solidified skin tightly against the mold surface, resulting in flow marks. Gradually increasing the barrel temperature improves melt fluidity and reduces defects.

The melt should reach the ideal temperature when exiting the nozzle at the barrel end. Low nozzle temperature causes rapid melt cooling, forming flow marks. Appropriately increasing the nozzle temperature addresses this problem.

Injection mold design and performance directly influence melt flow and filling, making them key factors in flow mark formation for plastic molding parts.

Low mold temperature causes the melt to cool rapidly, preventing the solidified skin from being pressed tightly against the mold surface. Raising the mold temperature ensures uniform cooling and tight adhesion of the plastic skin.

Small gate, runner, or sprue sizes increase flow resistance, disrupting smooth melt flow. Use CAE simulation software to optimize the dimensions of these components for reduced flow resistance.

Poor mold ventilation hinders melt filling, as trapped air blocks the melt front from pressing the solidified skin against the mold surface. Add vent grooves at appropriate positions in the flow path to ensure smooth air escape.

Material properties and operator practices also play significant roles in the formation of flow marks on plastic molding parts.

For mold cavities with a large flow length-to-wall thickness ratio, plastic with good fluidity is essential. Choose materials with optimal fluidity (without causing flash) to ensure sufficient filling force.

Lubricant content should generally be controlled below 1%. For parts with a large flow length-to-wall thickness ratio, precise lubricant dosage is critical to ensure the solidified plastic adheres tightly to the mold surface and avoids flow marks.

Operators opening or closing the molding machine door too early or too late during the cycle causes process instability. Strengthen operator training to emphasize the importance of consistent cycles and foster good operating habits, ensuring stable quality of plastic molding parts.

Resolving flow marks on plastic molding parts requires a comprehensive approach, addressing temperature control, mold design, material selection, and operator practices. By optimizing injection/holding pressure, adjusting barrel/nozzle/mold temperatures, improving mold ventilation and component dimensions, selecting high-fluidity materials, and standardizing operator actions, manufacturers can effectively eliminate flow marks. For plastic molding parts with high surface quality requirements, continuous process monitoring and refinement are essential to maintain consistency and meet customer expectations.