how to improve the weld line

how to improve the weld line

1、 Formation mechanism of weld line

The weld mark (called water mark) is the joint mark formed when the molten plastic flowing out of the gate at different positions of the mold converges. The formation of weld line is caused by the inconsistency of temperature, pressure and speed when the molten material from each gate passes through the complex cavity to the convergence.

The joint formed by melt confluence can be divided into weld line and weld line. The performance of weld line is obviously better than weld line. Generally speaking, when the confluence angle is greater than 135 degrees, a fusion line is formed, and when it is less than 135 degrees, a fusion mark is formed.

The performance of fusion line is obviously better than that of fusion mark. The confluence angle has an important impact on the performance of fusion joint, because it affects the sufficiency of molecular chain fusion, entanglement and diffusion after fusion. The greater the confluence angle, the better the performance of fusion joint.

Figure: definition of weld lines and fusion lines

2、 Manifestation of weld line

Weld line is the weak link of injection molded parts, which not only affects the appearance of products, but also easy to produce stress concentration and affect the overall strength of products.

3、 Causes of weld lines and Countermeasures for improvement

The quality of weld line is directly or indirectly related to factors such as man-machine, material, method and ring. The key is to grasp the main influencing factors and determine the correct improvement measures.

4.material Poor liquidity

Improve the stagnation of mold gating system, expand gate, runner and injection port, and improve the fluidity of materials.

5.Too much reinforcement added

The higher the glass fiber content, the lower the weld line strength. When the length diameter ratio of glass fiber increases, the strength of weld line decreases.

6.product structure

The wall thickness is too thin or the wall thickness difference is too large  .Optimize product wall thickness

The wave front convergence angle is too small.

When the wave front convergence angle is less than 135 °, a fusion line is formed, and when it is greater than 135 °, a fusion line is formed. Compared with the fusion line, the fusion line has less mutual diffusion of molecules on both sides and poor quality. When the contact angle is between 120 ° and 150 °, the surface trace of the fusion line gradually disappears. The increase of intersection angle can be achieved by adjusting the thickness of products, changing the position and number of gates, changing the position and size of runner, etc.
7.Mold temperature too low

Increase the mold temperature and reduce the coolant flow in the mold

8.Improper position of runner and gate or too small or too long

The length of gate is generally less than 1mm. Good at this, easy to cause problems. The use of gate insert makes it easy to modify the gate size. The gate shall be tested from an early age, and the increment shall be 10%. For example, if 0.50mm is too small, try 0.55mm next time. Adjust the gate position so that the weld mark moves to the invisible position.

9.Poor mold exhaust

Check whether the cold material hole is set or whether its position is correct, and add an exhaust groove or exhaust hole at the under injection position. For crystalline materials, the clearance of exhaust slot is 0.025mm; for non crystalline materials, the clearance of exhaust slot is 0.035mm.

10.Injection speed too low  .Increase injection speed

11.Holding pressure too low .Increase the pressure holding and switch the pressure holding as soon as possible

12.Material temperature too low. Appropriately increase the temperature of barrel and nozzle, prolong the injection cycle and reduce the flux of coolant in the mold

13.Insufficient flow of molten plastic .Increase injection flow or gate size

14.Material pipe temperature is too low

When the temperature of the material pipe is too low, the temperature is too low, the joint is poor, and the lines are obvious when the melt line is formed by the melt front. Increase the material temperature so that the temperature is moderate and the line is not obvious when the melt front forms the melt line. When the fusion line is formed, the difference of two wave front temperature, the temperature of each wave front and the pressure after the formation of the fusion line determine the quality of the fusion line. The lower the temperature, the greater the temperature difference (above 10 ° C) and the lower the pressure, the worse the quality.

15.Insufficient back pressure

Increasing the back pressure can improve the quality of welding line.

16.Low jet pressure or speed

Increasing the jet pressure or jet speed can naturally improve. Jet pressure and jet velocity are related, and it is not appropriate to increase them at the same time. Before adjustment, it is not clear whether the obvious cause of the fusion line is jet pressure or jet speed. We should choose an adjustment, observe its aftereffect, and then decide the next action. The increment of each ejection pressure or velocity adjustment shall be 10%. After each adjustment, it takes about 10 times of glue injection to reach a stable state