When plastic melt fills the mold cavity, if the temperature of the leading edge of two or more melts is not exactly the same when they meet, these melts cannot fully fuse, and a linear groove will be generated at the merging point, thus forming a weld line, as shown in Figure 3-14.
Figure 3-14 Schematic diagram of weld line formation
The causes and solutions for weld lines are as follows：
The melt temperature is too low
Low-temperature melts have poor flow characteristics and are prone to weld lines. If weld lines appear on the inner and outer surfaces of a plastic part in the same location, it is often due to poor welding caused by excessively low melt temperature. To address this, the temperature of the barrel and nozzle can be appropriately increased, or the injection cycle can be extended to raise the melt temperature. Simultaneously, the flow rate of cooling water in the mold should be controlled to appropriately increase the mold temperature. Generally, the strength at weld lines is poor. Local heating of the corresponding area in the mold where weld lines occur can improve the local temperature of the weld area and thus increase its strength. If a low-temperature molding process is necessary due to special requirements, the injection speed and injection pressure can be appropriately increased to improve melt flow characteristics. A small amount of lubricant can also be added to the raw material formulation to improve melt flow properties.
As shown in Figure 3-17, gate designs with fewer branch flows should be used as much as possible, and gate locations should be carefully selected to avoid inconsistent filling speeds and interruptions in the material flow. Single-point feeding should be used whenever possible. To prevent weld lines from forming when low-temperature melt enters the mold cavity, a cold slug well can be provided within the mold while simultaneously increasing the mold temperature.
poor mold venting
First, check if the mold vents are blocked by solidified melt or other objects, and check for foreign objects at the gate. If carbonization points still appear after clearing the blockage, add vents at the mold's material collection point. Alternatively, you can accelerate material flow by repositioning the gate, appropriately reducing the clamping force, or increasing the venting clearance. In terms of the injection molding process, auxiliary measures such as lowering the material and mold temperatures, shortening the high-pressure injection time, and reducing the injection pressure can be taken.
Improper use of release agent
In injection molding, a small amount of mold release agent is generally applied evenly only to areas that are difficult to demold, such as threads. In principle, the amount of mold release agent used should be minimized.
The plastic part has an unreasonable structural design
A plastic part with excessively thin walls, significant variations in wall thickness, or too many inserts can all cause poor melt welds, as shown in Figure 3-18.
When designing the shape and structure of a plastic part, ensure that the thinnest part is greater than the minimum allowable wall thickness during molding. Furthermore, minimize the use of inserts and strive for a more uniform wall thickness.
Figure 3-18 Example of the effect of plastic part wall thickness on weld lines
Other reasons
High moisture or volatile content in the plastic raw material, uncleaned oil stains in the mold, cold material in the mold cavity or uneven distribution of fiber filler in the melt, unreasonable mold cooling system design, excessively rapid melt cooling, excessively low insert temperature, excessively small nozzle orifice, insufficient plasticizing capacity of the injection molding machine, and large pressure loss in the plunger or injection molding machine barrel can all lead to varying degrees of poor melt cohesion and weld lines, as shown in Figure 3-19. Therefore, during production, targeted measures should be taken to address different situations, such as drying the raw material, regularly cleaning the mold, changing the size and location of the mold cooling channels, controlling the flow rate of cooling water, increasing the insert temperature, using a larger diameter nozzle, and using a larger injection molding machine.
Figure 3-19 Weld line formed by the fusion of melts
Real-world examples
Large color difference on both sides of the weld line caused by excessively low mold temperature：
During injection molding, the filling area of different gates in the mold varies greatly, resulting in significant differences in the flow speed of the melt when they meet. If the mold temperature is too low, the leading edge of the slower-flowing melt will cool down excessively, causing too much cold glue in the solidified layer. This solidified layer may be compressed or pushed and pulled, creating hazy marks. Consequently, a large color difference will appear on both sides of the plastic part, as shown in the figure below.