Causes and Solutions of Welding Line of injection molding

Posted on : Nov 2 , 2022 By  GREFEE

Among the defects in injection molding products, welding lines are the common ones, in addition to the injection molded parts with very simple geometry, welding lines can be found in many injection molding parts(normally is in the shape of one line or V-groove), especially for the large and complex products requiring the use of multi gate molds and inserts.  

Welding lines lowers the value of the outer appearance quality of parts, but also impacts the mechanical performance, such as impact strength, tensile strength and elongation at break, etc. besides, it also affects the service life of and the design of products. Therefore, welding lines should be avoided as much as possible.

Cause: the molten plastic meet the inserts, holes and areas in which the slow speed is not smooth in the cavity, or when filling the area where the material flow is interrupted, multiple strands melted together. The material cannot be melt completely when it comes to the gate injection mold filling.

Cause and solution:

1. Low temperature

The shunt and confluence performance of low temperature melts is poor, and easy to cause the welding lines. If the inner and outer surface of plastic parts both have the welding lines at the same place, this is usually caused by the low temperature. Regarding to this, increasing the temperature of barrel and nozzles or extending the injection cycle to raise the material temperature. In the meantime, the volume of cooling water in molds should be limited and increase the mold temperature appropriately.

Generally, the strength of welding lines is poor. If preheat the areas that have welding lines and increase the local temperate of the welding parts, the strength can be improved.

Due to the special needs, low temperature molding process must be adopted, which can be achieved by increasing the injection speed and injection pressure to improve the melting performance of melts. The flowability of melts can be improved through adding small volume of lubricants.

2. Defects of mold

The structure coefficients of the mold filling system has a huge impact to the welding conditions. As poor welding mainly is mainly caused by the diversion and confluence of melts, and thus gate form with less diversion and gate position shall be reasonably selected are important to avoid the interruption of filling material flow due to the inconsistent mold filling speed. Try to use the point-gate if it is possible because it will not produce multi-stream, and the melts will not meet from two directions, and it is easy to avoid welding marks.

If there are too many gates or the size of gates is too small in the mold filling system, multiple factors will lead to welding lines due to the large flow resistance of melts, such as the positioning of multiple gates is incorrect or the distance between the gate and the flow material fusion joint is too large, and the flow channel section at the inlet of the main flow channel and the branch flow channel of the gating system is too small. The way to address this is to reduce the number of the gate and set the position of gates appropriately, expand the cross area of gates, and set auxiliary flow channel, expand the diameter of main flow channel and branch flow channel.

To avoid the generation of welding marks when the low temperature melts enter into the mold cavity, the cold slug well should be set inside the molds while increasing the mold temperature.

In addition, the generation place of welding marks is usually caused by the high pressure mold filling, so the burrs are produced. What’s more, after burrs, the shrinkage will not be generated on the welding marks, and thus this kind of burrs usually are not regarded as obstacles. The way to address it is to  open a small shallow groove at the part of the die where the flash occurs, and transfer the welding marks on the additional flying wing. After the part is molded, remove the flying wing, which is one common method to remove the welding marks.

3. Poor venting

When the welding lines coincides with the clamping line or caulking of the mold, the air driven by multiple streams indie the mold cavities can be escaped from the mold clamping gap or caulking. However, when the welding lines cannot coincide with the clamping line or caulking, and the vent hole setting is inappropriate, the air driven by multiple streams indie the mold cavities cannot be escaped. The air bubbles are squeezed under the strong force, and the volume is gradually smaller, and then become a only spot. As the compressed air molecules cannot be transmitted into thermal energy under high pressure, resulting in the temperature of the melting at the fusion point raising. When the temperature is equal or slightly higher than the decomposition temperature of raw material, yellow spots will appear on the welding spots. If the temperature is far higher than the decomposition temperature of raw material, there will be back spots on the fusion point.

Generally, the spots are likely to appear on the same position over and over, and the position is always on the fusion point areas. During the operation process, this kind of spots should be regarded as impurity spots. The main reason for the generation of spots is that poor venting of mold, which is the carbonization point formed after the high-temperature decomposition of the molten material.

If this kind of problems occur, the venting holes of mold should be inspected first to see whether the the solidified product or other object of the molten material is blocked, or is there any foreign matter on the gate. If the blocking matter is cleared, the carbonization spots are still exist. Adding the venting holes at the material collecting point of the mold, or it can also be accelerated by repositioning the gate or appropriately reducing the closing force and increasing the exhaust gap. In terms of the process operation aspect, there are some helpful measures, such as lower the temperature of material and mold temperature, shortens the time of high pressure injection, reduce the injection pressure.

4. Improper use of release agent

If the volume of mold release agent is too much or the type is incorrect will both lead to the welding marks on the surface of plastic parts. During the injection molding, less volume of mold release agent only be applied on the threads and other parts not easy to de-mould. In principle, the amount of release agent shall be minimized.

The selection of different kinds of mold release agent should be determined by the molding conditions, shape of plastic parts and varieties of raw materials. For instance, the pure zinc stearate can be used in all kinds of plastics except polyamide and transparent plastics, and it is also applicable to the polyamide and transparent plastics after mixing with oil. The silicone oil toluene solution can be used for all kinds of plastics, and it can be used for a long time after brushing, but it needs to be heated and dried after brushing, which is complicated to use.

5. Unreasonable design of plastic structure

If the wall thickness is too thin or too thick, or with too many inserts, it will cause poor welding. When the thin walled parts are molded, as the melts solidified too fast, defects are likely occur. Besides, the melts will produce the welding marks at the confluence at thin wall during the mold filling. Once the welding marks are generated on thin-walled parts, the strength of plastic part will be decreased, affecting the using properties.

Therefore, when designing the shape structure of plastic parts, the thinnest part of plastic part should be larger than the minimum allowable wall thickness during molding. In addition, try to reduce the use of inserts and maintain the consistency of wall-thickness.

6. Other reasons

There are many factors will affect the welding to some extent, including the content of water and volatile substance is high in raw material, the grease is not cleaned, the cold material or fiber filling material is poorly distributed in the melts, the design of mold cooling system is poor, the melts solidified quickly, the temperature of inserts is too low, nozzles are too small, the plasticizing capacity of the injection molding machine is insufficient, and the pressure loss in the barrel of the injection molding machine is too large.

Regarding this, during the operation, some measures can be taken to deal with the problem, including pre-drying of raw materials, regular cleaning of molds, change of cooling water channel settings of molds, control of cooling water flow, increase of insert temperature, replacement of nozzles with larger apertures, and replacement of injection molding machines with larger specifications.

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