Weld lines are one of the most common, serious, and difficult-to-manage defects in plastic parts.
This article will explore weld lines from multiple perspectives, including their formation mechanisms, hazards, and solutions, establishing a knowledge system and framework to help engineers fully understand weld lines:
1) Weld line prevention is paramount; don't wait until weld line defects occur before seeking solutions;
2) Even if weld line defects unfortunately occur, they can be addressed systematically, rather than simply blaming injection molding operator with "their skill level is too low; they can't even fix weld lines properly."

 

1.1 What is a Weld Line?
A weld line is a seam that forms when molten plastic in a mold cavity encounters insert holes, areas of discontinuous flow, or areas where flow of filling material is interrupted, causing multiple streams to converge and fail to completely fuse together.

 

1.2 Mechanism of Weld Line Formation
During injection molding, molten material flows within mold cavity. Generally, temperature of mold cavity wall is lower than melting point of plastic, so molten material begins to cool from the moment it enters mold cavity. A layer of molten material in contact with mold wall forms a stationary outer shell (condensed layer), while interior remains a relatively hot molten material (flow layer).

 

▲Red represents molten material, blue represents condensed layer, and red arrow indicates direction of heat transfer.
When two or more molten material flows converge, high-tension condensed layer surrounding flow front prevents flow from completely merging with other flow, affecting thorough and uniform mixing of two melt streams. This results in different microstructures at contact point between two melts.
Macroscopically, this manifests as a straight or curved, barely visible mark, resembling a noticeable seam; in severe cases, it can appear as a groove.
It is a three-dimensional region with a completely different morphology and mechanical properties formed by contact of two flowing plastic melts, unlike the rest of plastic.

 

▲ Formation of Weld Lines

 

▲ Weld line is clearly visible at junction of two molten materials.

 

▲ Weld line magnified 11 times.

 

▲ Weld line magnified 11 times.

 

▲ Glass fiber orientation at weld line.
1.3 Case Studies of Weld Lines

 

▲ Weld line occurs at a hole.

 

▲ Weld line is more obvious after adding metallic masterbatch to plastic.

 

▲ Various other weld line locations.

Weld lines are classified into cold weld lines and hot weld lines based on different angles of approach between two molten materials. Two types have different effects on appearance and strength of plastic parts.
2.1 Cold Weld Lines
Cold weld lines are mostly caused by multi-point injection.
During flow of plastic, temperature gradually decreases due to cooling effect of mold. Therefore, cold weld lines often occur near the end of filling, when plastic temperature is also relatively low, hence name "cold weld line."
Cold weld lines can be viewed as a seam formed by head-to-head impact of two molten material flows. Weld angle is close to 0 degrees, and plastic usually stops flowing immediately after weld. Therefore, cold weld lines have the weakest strength and are the most noticeable type of weld line.

 

▲Cold weld line
2.2 Hot weld lines
Hot weld lines occur when plastic flow encounters obstacles such as holes, openings, inserts, or cores, causing it to split. Split flows then rejoin after bypassing obstacle, forming a weld line in rejoining area.
Since two flows originate from same source and are split by an obstacle, their properties and temperatures are not significantly different. Furthermore, flow continues after weld, hence name "hot weld line."

 

▲ Hot Melt Weld Line
2.3 Distinguishing Between Cold Melt Weld Lines and Hot Melt Weld Lines
Accurate distinction between cold melt weld lines and hot melt weld lines depends on angle at which two melt flows meet. If angle is less than 135 degrees, it is a cold melt weld line; if angle is greater than 135 degrees, it is a hot melt weld line.

 

▲ Angle of Meeting

 

▲ Cold Melt Weld Lines and Hot Melt Weld Lines

Geometric factors causing weld lines include variations in wall thickness of plastic part, holes and inserts, and mold structures such as multiple gates, side gates, and inconsistent flow lengths.
3.1 Variations in Wall Thickness
When molten plastic fills a mold with varying wall thickness, resistance is low and flow rate is high at thicker walls, while resistance is high and flow rate is slow at thinner walls.
Due to this difference in flow rate, melts from different wall thicknesses converge at different flow rates, ultimately forming a weld line at convergence point.

 

▲Wall Thickness Variation of Plastic Parts
3.2 Holes, Voids, and Inserts
When molten plastic passes through holes, voids, or inserts, it will inevitably split into two or more streams, creating weld lines at their confluence.

 

▲Hole, Void
3.3 Multiple Gates
Multiple gates entering plastic result in two or more streams, creating weld lines at their confluence.

 

▲Multiple Gates
3.4 Two-Sided Injection
Two gates entering from both sides cause weld lines at their confluence.

 

▲Two-Sided Injection
3.5 Inconsistent Flow Length
Inconsistent flow lengths of different streams result in weld lines at their confluence.

 

▲Inconsistent Flow Length

Weld lines severely affect appearance quality of plastic parts, reduce their strength, and cause stress concentration.
4.1 Impact on Appearance Quality
Weld lines significantly affect appearance quality of plastic parts, reducing surface smoothness and causing color differences in subsequent painting and electroplating processes.
Weld lines are particularly noticeable on transparent and semi-transparent plastic parts, impacting their appearance.

 

▲Weld lines on transparent plastic parts are very conspicuous.
Furthermore, because weld lines obstruct pressure flow, pressure-holding effect in that area is poor, potentially leading to shrinkage.
4.2 Impact on Plastic Part Strength
Weld lines also greatly affect mechanical properties of plastic parts, reducing their mechanical strength and posing safety hazards such as water leakage, air leakage, or breakage under load during normal use.
Due to wavefront fountain flow characteristics of plastic, plastic molecular chains at wavefront are parallel to wavefront. During welding, molecular chains become parallel to each other, reducing penetration and entanglement, thus weakening strength. This effect is particularly pronounced for plastics reinforced with glass fiber.
Figure below shows a simulated case of glass fiber orientation in a glass fiber reinforced plastic. As can be seen, glass fibers are almost parallel in weld line region.

 

▲ Glass fiber orientation at weld line
Table below lists decrease in tensile strength in weld line region for different plastics after addition of glass fiber. For most plastics, strength decreases by about 20% in weld line region, while for glass fiber reinforced plastics it decreases by more than 60-70%. Decrease in weld strength becomes more pronounced with increasing glass fiber content and aspect ratio.

Note: Tensile strength retention value = (Tensile strength at weld line / Tensile strength outside weld line) x 100%
If weld line occurs at the end or late stage of filling, lower plastic temperature reduces molecular chain mobility and diffusion, making condition worse. For thermosetting plastic parts, molten plastic is already close to later stage of cross-linking when it converges, resulting in poor welding and a more pronounced decrease in local strength in weld line area.
In addition, weld area is prone to trapping impurities, forming pinholes, which weakens strength.
4.3 Stress Concentration
A V-shaped notch is formed at confluence of two melt flows in weld line area. This near-crack structure easily causes stress concentration, resulting in poor mechanical properties and strength, and is also a potential crack zone.

 

▲ Weld line magnified 11 times -- V-shaped notch
Experiments revealed that molecular chain diffusion is less near mold wall than in the central area, resulting in lower strength. Size of this area varies depending on type of plastic and molding conditions.
For example, in PS, this weaker area is approximately 0.2~0.3mm. If weld line area is subjected to load or comes into contact with certain chemicals, it is very prone to cracking.
Before addressing weld line defects, we must understand types of weld lines, their formation mechanisms and geometric factors, and their adverse effects.
Only in this way can we propose targeted and systematic solutions.
Following two articles will focus on how to prevent and resolve weld line defects from five key aspects: plastic material selection, plastic part design, mold design, injection molding process parameters, and injection molding equipment.