Causes and Solutions for Weld Lines in Injection Molded Products

Introduction to Weld Lines

Weld lines not only impact the appearance quality of plastic components but also affect their mechanical properties, such as impact strength, tensile strength, and elongation at break to varying degrees.

Additionally, weld lines have a significant impact on product design and the lifespan of plastic components. Therefore, efforts should be made to avoid or improve them as much as possible.

Formation of Weld Lines: Weld lines are formed when molten plastic encounters inserts, voids, regions with inconsistent flow velocity, or areas where the molding material flow is interrupted within the mold cavity. Multiple molten streams converge under such circumstances.

When gate injection molding occurs, the material may not fully fuse.

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Here, we share the specific causes and corresponding solutions for the formation of weld lines.

Causes and Solutions for Weld Line

1. Low Temperature

Poor flow convergence of low-temperature molten material can easily lead to weld line formation.

If weld lines appear on both the inner and outer surfaces of the plastic part in the same area, it is often due to poor welding caused by low material temperature.

In this case, it is advisable to increase the temperature of the material cylinder and nozzle or extend the injection cycle to promote a rise in material temperature.

At the same time, the flow rate of cooling water inside the mold should be controlled, and the mold temperature should be increased moderately.

Generally, the strength of the weld line on plastic parts is weaker. If localized heating is applied to the corresponding area in the mold where the weld seam occurs, raising the local temperature of the molded part can often enhance the strength of the plastic weld seam.

If, due to specific requirements, a low-temperature molding process must be adopted. It’s possible to increase the injection speed and injection pressure appropriately to improve the coalescence performance of the melt. Alternatively, a small amount of lubricant can be added to the raw material formulation to enhance the flowability of the melt.

2. Mold Defects

The structural parameters of the mold’s casting system have a significant impact on the fusion condition of the molten material, as poor fusion mainly occurs at the juncture of molten material.

Therefore, it is advisable to use a casting gate form with minimal branching and select gate positions reasonably to avoid inconsistent mold filling rates and interruptions in material flow.

Where possible, a point gate should be chosen because this type of gate does not produce multiple material flows, preventing the molten material from converging from two directions and making it easier to avoid weld lines.

If there are too many or too small gates in the mold’s casting system, improper positioning of multiple gates, or excessive spacing between the gates and the fusion point of the material flow. And if the inlet of the main runner and the cross-sectional area of the branch runner are too small, it can lead to poor fusion, resulting in noticeable weld lines on the surface of the plastic part.

To address this, it is recommended to minimize the number of gates, set gate positions sensibly, increase the gate’s cross-sectional area, introduce auxiliary runners, and enlarge the diameters of the main and branch runners.

To prevent the occurrence of weld lines due to the injection of low-temperature molten material into the mold cavity, it is advisable to increase the mold temperature and simultaneously incorporate cooling channels within the mold.

Furthermore, the occurrence of weld lines in plastic parts is often attributed to the formation of flash due to high-pressure mold filling. Moreover, after the generation of this type of flash, weld lines do not result in shrinkage holes. Therefore, such flash is often not treated as a troubleshooting issue. Instead, a shallow groove is created at the location on the mold where flash is generated, transferring the weld lines on the plastic part to the additional flash wing.

After the plastic part is formed, the small wing can be removed. This is also a commonly used method when troubleshooting weld lines.

3. Poor Mold Venting

When the melt weld line coincides with the mold parting line or the insert joint, air expelled by the pressure of the multiple molten flows inside the cavity can escape through the mold parting gap or insert joint.

However, when the weld line does not align with the parting line or insert joint, and if the vent holes are improperly positioned, the trapped residual air within the cavity, driven by the molten material, cannot escape.

Subsequently, bubbles are forcefully compressed under high pressure, gradually reducing in size and ultimately compressed into a point. As the kinetic energy of the compressed air molecules transforms into heat energy under high pressure, the temperature at the weld line increases.

When this temperature equals or slightly exceeds the decomposition temperature of the raw material, yellow dots appear at the weld point.

If the temperature is much higher than the decomposition temperature of the raw material, black dots appear at the weld point.

In general, these types of spots that appear near the weld lines on the surface of plastic parts tend to reoccur in the same location. Besides, their appearance is consistently associated with the weld points.

During the operational process, these spots should not be mistakenly identified as impurity spots. The primary reason for the occurrence of such spots is poor mold venting. They are the formation of carbonized points after the high-temperature decomposition of the melt.


After the occurrence of such a fault, the first step is to check whether the mold vent holes are blocked by solidified material from the melt or other objects, and if there are any foreign bodies at the gate.

If carbonized points persist after clearing any blockages, additional vent holes should be added at the melt convergence points in the mold.

It is also possible to accelerate the convergence of the melt by repositioning the gate or appropriately reducing the clamping force and increasing the venting clearance.

In terms of process operations, measures such as lowering material and mold temperatures, reducing high-pressure injection time, and lowering injection pressure can also be taken.

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4. Improper Use of Mold Release Agent

Excessive use or the selection of an incorrect type of mold release agent can cause the surface of plastic parts to exhibit weld lines.

In injection molding, mold release agent should generally be uniformly applied in small amounts only to areas that are difficult to demold, such as threads. In principle, the use of mold release agent should be minimized.

The selection of various mold release agents must be determined based on molding conditions, the shape of the plastic parts, and the type of material used.

For example, pure zinc stearate can be used for various plastics except polyamide and transparent plastics. But when mixed with oil, it can be used for polyamide and transparent plastics.

Another example is a solution of silicone oil in xylene, which can be used for various plastics and can be used for an extended period after a single application. However, it requires heating and drying after application, making the process somewhat complex.

5. Unreasonable Design of Plastic Structure

If the wall thickness of a plastic part is designed too thin, with a significant thickness difference or too many inserts, it can lead to poor weld lines.

Thin-walled parts, during molding, tend to solidify too quickly, resulting in defects. Additionally, the molten material consistently converges at the thin walls during the filling process, creating weld lines. Once weld lines appear on thin walls, it can reduce the strength of the plastic part, affecting its performance.

Therefore, in the design of the plastic part’s structural form, it is essential to ensure that the thinnest part of the plastic piece is greater than the minimum allowable wall thickness during molding. Furthermore, the use of inserts should be minimized, and the wall thickness should be made as consistent as possible.

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6. Other Causes

  • When the moisture content or volatile substance content of the used raw material is too much.
  • Unclean oil stains in the mold.
  • Presence of cold material in the mold cavity or poor distribution of fiber fillers in the molten material.
  • Unreasonable design of the mold cooling system.
  • Too fast solidification of the molten material.
  • Low temperature of inserts.
  • Nozzle holes are too small.
  • Insufficient plasticizing capacity of the injection molding machine.
  • Excessive pressure loss in the material barrel of the injection molding machine.

All of these factors can lead to varying degrees of weld line defects.

To address these issues during the operation process, specific measures should be taken for each situation, including:

Pre-drying raw materials, regularly cleaning molds, adjusting the mold cooling water channel settings, controlling the flow of cooling water, increasing insert temperatures, using nozzles with larger apertures, and switching to larger-sized injection molding machines.


From the above analysis, we know that the occurrence of weld lines can be avoided.

During the production process, attention should be paid to the aspects mentioned above to make the products more aesthetically pleasing.

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