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Causes of Welding Seam in Plastic Parts

Due to meet the insert holes, the region where the flow speed is discontinuous and the region where the filling flow is interrupted, the molten plastics in the cavity meet in the form of multiple strands. At this time, there will be a linear welding seam because it can’t be completely fused. In addition, in the case of gate injection molding, a welding seam is formed, and the strength at the welding seam is poor. The main reasons are as follows:

Machining aspect

(1) The injection pressure and speed are too low, the temperature of barrel and mold temperature are too low so that the melt entering the mold is cooled prematurely and causing a welding seam.

(2) When the injection pressure and speed are too high, a spray will occur and be causing welding seam.

(3) Improving rotation speed and increasing back pressure to decrease the viscosity of plastic and the density will be increased.

(4) The plastic should be dried well, and the recycled material should be used less. If the amount of release agent use too much or the quality is not good, the welding seam will also appear.

(5) Reduce the clamping force to facilitate exhaust.

Mold aspect

(1) If there are too many gates in the same cavity, the gate should be reduced or symmetrical setting, or try to set close to the weld seam.

(2) Exhaust system should be set if the exhaust located at the weld seam is poor.

(3) If the gate is too large and the size of gating system isn't properly, the gate should be set to avoid that the melt flow around the insert hole or inserts should be used as little as possible.

(4) If the wall thickness changes too much, or the wall thickness is too thin, the wall thickness of the plastic part should be uniform.

(5) If necessary, a fusion well shall be set at the welding seam to disengage the welding seam from plastic part.

Plastic aspect

(1) Lubricants and stabilizers should be added to plastics with poor fluidity or heat sensitivity.

(2) Plastics contain a lot of impurities, and if necessary, it can be changed to the plastic with good quality.

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