Weld line, also known as weld lines and weld seams, are formed when two melt fronts facing or parallel to each other meet in the process of mold filling. 

The weld line not only affects the appearance quality of the plastic part, but also affects the mechanical properties of the plastic part, such as impact strength, tensile strength, elongation at break, etc. to varying degrees. In general, the smaller the angle between the front end of the two confluent melts (the weld angle), the more significant the weld line will be, and the worse the quality of the product. When the welding angle reaches 120°~150°, the welding line disappears.

The welding line is a common defect of plastic parts, and its existence not only affects the white appearance quality of the product, but also has a great impact on the mechanical properties of the product, especially on the fiber reinforced materials, multiphase blended polymers, etc.
(1) Materials
The fluidity of the plastic is poor, and the melt front converges after a long time to produce obvious weld line.
(2) Mold
The flow channel is too thin, the cold material well is small, and the exhaust is poor;
The wall thickness of the product is too small or the difference is too large.
The gate cross-section and position are unreasonable, resulting in the wavefront convergence angle being too small;
The mold temperature is too low.
(3) Molding process.
The injection time is too short;
Injection pressure and injection speed are too low;
The back pressure setting is insufficient.
Poor exhaust due to excessive clamping force;
The temperature of the barrel and nozzle is set too low.
(4) Injection machine
Poor plasticization, uneven melt temperature;
Melt leakage occurs during injection and pressure holding, which reduces the mold filling pressure and material volume.

The weld line is essentially the union of two plastic flow fronts that are not completely melted. To eliminate and resolve weld lines, the viscosity of the plastic must be low enough, the temperature high enough and the pressure sufficient, and hold for enough time for the plastic to melt completely.

In the case of good exhaust, the speed of filling and shrinkage is accelerated, so that the pressure of the two plastic flow fronts can increase as soon as possible after the combination of the two plastic flow fronts, which helps to solve the problem of weld line.

(1) Materials
Lubricants and stabilizers should be added appropriately to plastics with poor fluidity or high heat sensitivity, and plastics with good fluidity or high heat resistance should be used if necessary;
Raw materials should be dried and liquid additives in formulations should be minimized.
(2) Mold
If the mold temperature is too low, the mold temperature should be appropriately increased or the local temperature at the weld joint should be purposefully increased;
Change the location, number and size of gates, change the thickness of the cavity wall and the design of the runner system to change the position of the weld line;
Opening, expanding, or unblocking exhaust channels, including the use of inserts, ejector slits, and venting.
(3) Molding process
Increase the injection pressure and holding pressure;
Set a reasonable injection speed: high speed can make the melt not have time to cool down to reach the confluence, low speed is conducive to the air discharge in the cavity;
Reduced clamping force to facilitate exhaust;
Set the temperature of the barrel and nozzle reasonably: the temperature is high, the viscosity of the plastic is small, the flow state is smooth, and the weld line becomes shallow; Low temperature, reducing the decomposition of gaseous substances;
Increase the speed of the screw to reduce the viscosity of the plastic; Increase the back pressure and increase the density of the plastic.