Weld line, the most common of many defects in injection moulding products. Except for a few very simple injection molded parts, they occur on most injection molded parts (typically in the form of a line or V-groove), especially large complex items that require use of multi-gate molds and inserts.
Weld line not only affects appearance quality of plastic parts, but also affects mechanical properties of plastic parts to varying degrees, such as impact strength, tensile strength, elongation at break, etc. In addition, welding marks also have a serious impact on the design of product and life of plastic parts, and therefore it should be avoided or improved as much as possible.
Main reason for weld line is that molten plastic encounters in the cavity when insert, hole, flow rate is inconsistent, or area where filling flow is interrupted, confluence of multiple melts occurs; when gate injection is filled, materials cannot be completely blended.

 

Splitting and converging performance of low-temperature melt is poor, and it is easy to form weld lines. If inner and outer surfaces of plastic parts are welded at the same place, welding is often caused by material temperature being too low. In this regard, temperature of barrel and nozzle can be appropriately increased or injection cycle can be lengthened to promote temperature rise. At the same time, throughput of cooling water in the mold should be controlled, and mold temperature should be appropriately increased.
Under normal circumstances, strength of weldment of plastic parts is poor. If local heating of corresponding part of weld in the mold is performed to increase local temperature of welded part of molded part, strength of welded part of plastic parts can be improved.
If a low-temperature molding process is necessary due to special needs, injection speed and injection pressure can be appropriately increased to improve confluence of melt. It is also possible to appropriately add a small amount of lubricant to raw material formulation to improve  flow properties of melt.

Structural parameters of mold casting system have a great influence on welding condition of flow material, because welding failure is mainly caused by splitting and converging of molten material. Therefore, gate form with less shunt should be used as much as possible and gate position should be reasonably selected to avoid inconsistent filling rate and interruption of filling flow. Where possible, a one-point gate should be used. Since gate does not produce multiple streams, melt does not merge from both directions, and it is easy to avoid weld marks.
In the casting system of mold, gates are too many or too small, multi-gate is not positioned correctly or spacing between gate and flow joint is too large, main channel inlet portion of casting system and runner cross section of sub-runner are too small. which causes flow resistance of material to be too large, poor welding and a relatively obvious weld line on the surface of plastic part. In this regard, number of gates should be reduced as much as possible, gate position should be reasonably set, gate section should be enlarged, auxiliary flow path should be set, and  diameter of main runner and sub-runner should be enlarged.
In order to prevent low temperature melt from being injected into cavity to produce weld line, a cold hole should be placed in the mold while increasing mold temperature.
In addition, production part of weldment of plastic parts often produces flash edges due to high pressure filling, and weld line does not cause shrinkage after generation of flash, so such flash is often not used as a fault, but a shallow groove is formed in the part of mold where flash is generated, and weld mark on plastic parts is transferred to additional flying winglet to be molded. Winglet is removed after molding, which is also a common method to eliminate failure of weld line.

When weld line of melt coincides with mold line or caulking of mold, air in the cavity can be discharged from mold gap or caulking. However, when weld line does not coincides with mold line or caulking, vent hole is not set properly, residual air in the cavity that is rushed by flow can not be discharged, bubble is strongly pressed under high pressure, and volume gradually becomes smaller. Finally, it is compressed into a point. Since molecular kinetic energy of compressed air is converted into thermal energy under high pressure, temperature at melting point of melt rises. When temperature is equal to or slightly higher than decomposition temperature of raw material, a yellow spot appears at fusion point. If temperature is much higher than decomposition temperature of raw material, a black spot appears at fusion joint.
Under normal circumstances, such spots appearing near weld line on the surface of plastic parts are always repeated at same position, and appearing parts always appear regularly at meeting point. During operation, do not mistake such spots as impurity spots. Main cause of such spots is poor mold exhaust, which is carbonization point formed after molten material is pyrolyzed.
After such a failure, first check whether vent hole of mold is blocked by solidified material or other objects of melt, and there is no foreign matter at the gate. If carbonization point still occurs after obstruction is removed, vent hole should be added at mold feed point. It is also possible to accelerate confluence by repositioning gate, appropriately reducing clamping force and increasing exhaust gap. In terms of process operation, auxiliary measures such as lowering material temperature and mold temperature, shortening injection time of high pressure, and reducing injection pressure can also be adopted.

If amount of release agent is too large or selected type is incorrect, it will cause welding marks on the surface of plastic part. In injection molding, a small amount of release agent is generally applied only to a portion which is not easily demolded such as a thread. In principle, amount of release agent should be minimized.
Selection of various release agents must be determined according to molding conditions, shape of plastic parts and variety of raw materials. For example, pure zinc stearate can be used in a variety of plastics other than polyamides and clear plastics, but can be used in polyamides and clear plastics when mixed with oils. Another example is silicone oil toluene solution which can be used for various plastics, and can be used for a long time after being brushed, but it needs to be heated and dried after being applied, usage is complicated.

If wall thickness of plastic parts is too thin and insert is too much, it will cause poor welding. When thin-walled parts are formed, since melt solidifies too quickly, defects are easily generated, melts always meet at thin wall to form weld lines during filling process. Once thin-walled weld lines are formed, strength of plastic parts is lowered, which affects performance. Therefore, when designing plastic part structure, it should be ensured that the thinnest part of plastic parts must be larger than minimum wall thickness allowed during molding. In addition, use of insert should be minimized and wall thickness should be as uniform as possible.

 

When moisture or volatile content of raw materials used is too high, oil stains in the mold are not cleaned, cold filler in cavity or fibrous filler in melt is poorly distributed, mold cooling system is unreasonably designed, melt solidifies too quickly, insert temperature is too low, nozzle hole is too small, plasticizing capacity of injection molding machine is not enough, and pressure loss in barrel of injection molding machine is too large, which may result in poor welding of different degrees. In this regard, in the course of operation, raw material pre-drying should be taken separately for different situations, mold should be cleaned regularly, mold cooling water channel setting should be changed, flow rate of cooling water should be controlled, insert temperature should be increased, nozzle with larger aperture should be replaced, larger-size injection molding machines should be replaced.