Causes of weld lines in products during injection molding

During the injection molding process, when the molten plastic meets in the form of multiple strands in the cavity due to the holes in the insert, the areas where the flow rate is discontinuous, and the area where the filling material flow is interrupted, a linear weld seam is produced because it cannot be completely fused. In addition, when the gate injection molding occurs, a weld seam will also be formed, and the strength and other properties of the weld seam are poor. The main reasons are as follows:

1. Processing:
(1) The injection pressure and speed are too low, and the barrel temperature and mold temperature are too low, which causes the molten material entering the mold to cool prematurely and weld seams appear.
(2) When the injection pressure and speed are too high, injection will occur and weld seams will appear.
(3) The speed should be increased, and the back pressure should be increased to reduce the viscosity of the plastic and increase the density.
(4) The plastic should be dry well, and the recycled material should be used sparingly. If the amount of release agent is too much or the quality is not good, there will be weld seams.
(5) Reduce the clamping force to facilitate exhaust.

2. Mould:

(1) There are too many gates in the same cavity, and the gates should be reduced or set symmetrically, or set as close as possible to the weld seam.
(2) The exhaust at the weld seam is poor, and an exhaust system should be set up.
(3) The sprue is too large, the size of the gating system is improper, and the gate is opened to avoid the flow of the melt around the insert hole, or use as little insert as possible.
(4) If the wall thickness changes too much, or the wall thickness is too thin, the wall thickness of the parts should be uniform.
(5) If necessary, a fusion well should be opened at the fusion seam so that the fusion seam can be separated from the workpiece.

3. On the plastic side:

(1) Lubricants and stabilizers should be appropriately added to plastics with poor fluidity or heat sensitivity.
(2) Plastics contain a lot of impurities, and if necessary, replace plastics with good quality.

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