Weld lines are the most common among many defects of injection molded products. Except for a few injection molded parts with very simple geometric shapes, weld lines occur on most injection molded parts (usually in the shape of a line or V-shaped groove), especially for large and complex products that require the use of multi gate molds and inserts.
The weld line not only affects the appearance quality of plastic parts, but also affects the mechanical properties of plastic parts, such as impact strength, tensile strength, elongation at break, etc. In addition, the weld line also has a serious impact on the product design and the life of the plastic parts. Therefore, it should be avoided or improved as much as possible.
The main causes of the weld line are: when the molten plastic meets the insert, hole, area with discontinuous flow rate or area with interrupted filling material flow in the mold cavity, multiple melts converge; When the gate injection filling occurs, the materials cannot be fully fused.
(1) Too low temperature
The shunting and converging properties of low temperature molten materials are poor, and the weld lines are easy to form. If the internal and external surfaces of plastic parts have welding fine lines at the same position, it is often due to poor welding caused by low material temperature. In this regard, the temperature of the barrel and nozzle can be appropriately increased or the injection cycle can be extended to increase the material temperature. At the same time, the amount of cooling water passing through the mold should be controlled, and the mold temperature should be appropriately increased.
(2) Mold defects
The structure parameters of the mold gating system have a great influence on the fusion of the flux, because the poor fusion is mainly caused by the shunt and confluence of the flux. Therefore, the gate type with less diversion shall be adopted as far as possible and the gate position shall be reasonably selected to avoid inconsistent filling rate and interruption of filling material flow. If possible, one point gate should be selected, because this gate does not produce multiple streams of material flow, and the molten materials will not converge from two directions, so it is easy to avoid weld lines.
(3) Poor mold exhaust
When the fusion line of the melted material coincides with the mold closing line or caulking, the air driven by multiple streams of material in the mold cavity can be discharged from the mold closing gap or caulking; However, when the welding line does not coincide with the mold closing line or caulking, and the vent hole is not set properly, the residual air in the mold cavity driven by the flow material cannot be discharged. The bubble is forced under high pressure, and the volume gradually decreases, and finally compressed into a point. Because the molecular dynamic energy of the compressed air is converted into heat energy under high pressure, the temperature at the molten material collecting point rises. When its temperature is equal to or slightly higher than the decomposition temperature of the raw material, Yellow dots will appear at the melting point. If the temperature is much higher than the decomposition temperature of raw materials, black dots will appear at the melting point.
(4) Improper use of release agent
Too much release agent or incorrect type will cause weld lines on the surface of plastic parts. In injection molding, a small amount of release agent is generally applied evenly only to the parts that are not easy to demould, such as threads. In principle, the amount of release agent should be reduced as much as possible.
Post time: Nov-04-2022