Injection Molding Weld Marks and Solutions
Injection molding weld lines are linear marks formed on or within a plastic part due to the meeting of two or more melt stream fronts. They are common cosmetic and performance defects in melt molding, particularly prominent in complex parts or molds with multiple gates. They are caused by poor fusion of the melt stream fronts after cooling. As the melt flows through the mold cavity, it can split when encountering obstacles such as cores and holes. The split melt then rejoins after bypassing them. By this time, the stream fronts have partially cooled, reducing the diffusion capacity of the molecular chains and preventing complete fusion, resulting in a weld line with weak mechanical properties. For example, when producing a plastic housing with multiple reinforcing ribs, the melt streams split at the ribs and then merge, forming a noticeable weld line at the base of the ribs. This not only affects the appearance but can also reduce the impact strength of that area by over 30%.
The severity of weld marks is influenced by a combination of material properties, mold structure, and process parameters. Regarding materials, plastics with poor flowability (such as PC and POM) are more prone to visible weld marks than plastics with good flowability (such as PE and PP) because the flow front cools faster in the former, hindering molecular diffusion. Furthermore, glass fiber reinforcement can disrupt fiber orientation at the weld line, further reducing bond strength. In mold design, improper gate placement (resulting in long melt diversion distances), poor venting (resulting in air inclusion at the confluence), and uneven cooling systems ( resulting in large temperature differences at the flow front) can all exacerbate weld marks. Regarding process parameters, slow injection speeds (overcooling the flow front), low melt temperatures (resulting in insufficient molecular activity), and insufficient holding pressure (resulting in inadequate compaction of the weld area) can all make weld marks more noticeable. For example, an ABS part with an injection speed of only 30 mm/s had a tensile strength at the weld line that was 25% lower than the original part, far exceeding the normal 5-10% difference.
The key solution to material issues lies in improving the melt’s fusion properties. Selecting a material grade with better fluidity, such as increasing the melt flow rate (MFR) of PC from 10g/10min to 15g/10min, can prolong the time the flow front remains in the molten state, promoting molecular fusion. Adding compatibilizers or plasticizers is also effective. For example, adding 5% styrene-butadiene copolymer to a PC/ABS alloy improves interfacial compatibility between the two phases and increases weld line strength by 15%. For glass fiber-reinforced materials, reducing the glass fiber content (from 30% to 20%) or using short glass fibers (length ≤ 0.2mm) can reduce fiber resistance to molecular diffusion. Adding 0.5% silane coupling agent can also enhance the interfacial bonding between the glass fiber and the resin, indirectly improving weld line performance. Furthermore, thoroughly drying the material (for example, keeping the moisture content of PA66 below 0.05%) to prevent moisture-induced melt degradation can also reduce weld line defects.
Optimizing mold design is key to eliminating weld marks, focusing on reducing melt diversion and promoting fusion. Properly positioning gates ensures symmetrical melt flow paths and shortens the distance between flows after diversion. For example, switching from a single gate on a rectangular part to a diagonal double gate ensures the weld mark is located along the central symmetry line and maintains a melt temperature difference of less than 5°C at the point of convergence. Adding a weld mark eliminator (such as a localized heater) at the weld mark location raises the cavity temperature in the convergence area by 10-20°C, slowing down the cooling of the flow front. For example, installing a heater at the weld mark on a car dashboard and controlling the temperature at 200°C (50°C above the mold’s normal temperature) makes the weld mark invisible to the naked eye. Enhanced venting capacity can be achieved by creating a 0.03-0.05mm deep vent groove at the melt convergence point to remove trapped air and volatiles, preventing bubbles from hindering fusion. For large plastic parts, sequential valve gates (SVG) are used to control the opening and closing order of each gate so that the melt is filled in sequence, avoiding the simultaneous confluence of multiple streams and fundamentally eliminating weld marks.
Weld line quality can be significantly improved by adjusting process parameters, requiring precise control based on the weld line type. For cold welds (where the flow fronts converge after complete cooling), the melt temperature should be increased (e.g., from 200°C to 220°C for PP) and the injection speed should be increased (from 50 mm/s to 80 mm/s) to maintain the flow front in a molten state. Simultaneously, the mold temperature should be increased (e.g., from 60°C to 80°C for ABS molds) to reduce the cooling rate. For weak welds caused by insufficient pressure, the holding pressure should be increased (from 60% to 80% of the injection pressure) and the holding time should be extended (by 1-2 seconds) to compact the weld area and promote molecular chain interlacing. Pulsed injection technology, which periodically varies the injection speed (e.g., alternating between 80 mm/s and 50 mm/s) during injection, enhances mixing at the flow front through melt disturbance, resulting in a 20% improvement in weld line strength. For example, when producing polyoxymethylene gears, the injection speed is changed from a constant 60mm/s to a pulse mode of 80-50-80mm/s, and the holding pressure is increased to 100MPa. The impact strength at the weld line is increased from 5kJ/m² to 7kJ/m², approaching the performance of the original material.
