In the injection blowing process, weld lines on the surface of finished products are a key defect that affects both appearance and mechanical properties. They arise from insufficient interface fusion when multiple melt streams converge within the mold cavity. To effectively control weld line strength, a systematic solution must be developed encompassing material selection, mold design, process parameter optimization, and the integration of supporting technologies.
Material selection is fundamental to controlling weld lines in the injection blowing process. Highly fluid materials reduce resistance to melt flow, promote diffusion and entanglement of molecular chains, and thus enhance weld strength. For example, low-viscosity resins or modified materials can reduce melt viscosity, resulting in tighter molecular bonding at the convergence interface. Furthermore, adding an appropriate amount of lubricant or flow improver to the raw material formulation can further reduce melt surface tension and enhance interfacial fusion. For reinforced materials, fiber dispersion and orientation must be optimized to avoid stress concentration caused by fiber concentration at the weld line.
Mold design directly influences the flow path and convergence of the melt during the injection blowing process. The number and location of gates are critical factors. A single gate design prevents the confluence of multiple material streams and reduces weld marks. If multiple gates are required, the layout should be optimized through CAE simulation to ensure that the melt fills the cavity with a balanced speed and pressure. The runner cross-section should be appropriately designed based on the melt viscosity to avoid excessive flow resistance that causes a rapid drop in melt temperature. A cold well in the weld mark area can guide the melt front to converge there, preventing defects in the main cavity. Furthermore, the surface roughness of the mold cavity must be consistent to avoid uneven melt filling speeds due to differences in frictional resistance.
Precise control of process parameters is key to improving weld mark strength during the injection blowing process. Melt and mold temperatures must be controlled in tandem. A high melt temperature reduces viscosity and prolongs the molecular chain activity time, while a high mold temperature slows the melt cooling rate and promotes interfacial fusion. Injection pressure and speed must be coordinated. High injection pressure enhances melt compaction at the confluence interface, while high injection speed reduces the cooling time of the melt front and reduces heat loss. Holding pressure is as important as holding time. Sufficient holding pressure compensates for melt shrinkage during cooling and increases density in the weld area. For thick-walled parts, pulsed holding can be used to apply additional pressure at critical stages to strengthen interfacial bonding.
Optimizing the venting system can prevent weld marks from worsening due to gas entrapment. If mold venting is poor, gas can be compressed in the weld area, forming bubbles or coking spots, further reducing strength. Therefore, venting slots or venting needles should be strategically placed around the weld to ensure timely gas escape. For parts with complex structures, vacuum entrainment can be used to accelerate gas evacuation through negative pressure, reducing defects in the weld area.
The application of specialized process technologies offers innovative solutions for controlling weld marks in the injection blowing process. Sequential valve gate gating controls the opening and closing of the gate, allowing the melt to fill the cavity sequentially, preventing the confluence of multiple streams and thus eliminating weld marks. The combination of a hot runner system and rapid heating and cooling mold temperature technology can raise the mold temperature above the material's glass transition temperature before injection, maintaining fluidity during melt confluence and allowing for rapid cooling and finalization after injection, ensuring a balanced appearance and production efficiency.
Structural optimization can minimize the impact of weld marks on product performance. Uniform wall thickness design avoids thermal equilibrium differences between thick and thin areas, reducing the occurrence of weld marks in thick areas. Weld marks can be placed in non-critical areas, or reinforcing ribs can be used to redirect flow, shifting defects to low-stress areas. For transparent products, surface treatment can be used to conceal weld marks and improve appearance.
