A weld line, also known as a water pinch line, is a visible line or weak area formed when two or more molten plastic streams meet during injection molding process. This occurs due to cooling of front material and inability of molecular chains to fully entangle and fuse.
Core cause is low temperature of melt front and insufficient fusion pressure, which leads to insufficient diffusion between molecular chains, thus forming a "weak boundary" at interface.
Occurrence of weld marks on exterior surfaces can be predicted early in product design.

 

Weld mark angle and depth: Weld marks are visible if angle is less than 75°.

A weld line is more than just a single line; it can cause a range of defects:

 

Countermeasures: By changing glue thickness to change convergence angle of weld line, weld line of appearance surface was successfully improved.
Appearance Defects
Visible Lines: Noticeable grooves or color differences appear on product surface, affecting aesthetics, especially on high-gloss or light-colored products.
Gloss Inconsistency: Gloss of weld line differs from that of surrounding area.
Mechanical Property Defects (This is the most serious issue)
Reduced Strength: Tensile strength and impact strength of weld line area are significantly lower than those of bulk material. When subjected to stress, product is very likely to break at weld line.
Increased Brittleness: Weak molecular chain bonding in this area results in brittleness and poor impact resistance.
Sealing Defects
For products requiring airtightness or watertightness (such as housings and containers), weld lines can become leak paths.
Chemical Defects
During secondary processes such as electroplating and painting, weld lines are prone to problems such as poor coating adhesion and cracking of paint film.

Factors that contribute to severity of weld line defects can be categorized into three main categories: materials, molds, and processes.
1. Material Factors
Flowability: Poor material flowability (high viscosity) causes melt front to cool more quickly, making fusion difficult. Materials such as PC and PPO are prone to forming noticeable weld lines.
Additives and Fillers:
Excessive release agents and lubricants can migrate to melt front, hindering fusion. Reinforcing fillers such as glass fiber (GF) are oriented parallel to weld line at fusion point, rather than across it, like "zipper teeth not interlocking," significantly reducing strength. Certain pigments can also affect fusion.
Crystallinity: Crystalline plastics (such as PP, POM, and PA) exhibit varying degrees of crystallinity at the weld line, leading to a more pronounced performance degradation.
2. Mold Factors
Gating System Design:
Number and Location of Gates: Excessive or improperly positioned gates can cause multiple melt streams to converge at unfavorable locations.
Undersized Runners/Gates: This results in significant pressure loss, resulting in insufficient pressure reaching fusion zone.
Venting System: Weld line is often the last area to be filled and the most challenging to vent. Trapped air can compress and generate high temperatures, burning material and hindering melt fusion.
Mold Temperature: Low mold temperature is primary mold cause of weld line defects. A cold mold wall rapidly cools melt front.
Product Structure: Areas with holes, pores, and uneven wall thicknesses inevitably form weld lines.
3. Process Parameter Factors
Melt Temperature: Too low a temperature results in poor material flow and poor fusion.
Injection Speed: Too slow an injection speed results in excessive cooling of melt front during flow. Holding Pressure and Time: Insufficient holding pressure or a short holding time prevents weld line from being formed from being compacted and promotes molecular chain diffusion.
V/P Switching Point: Switching from injection to holding pressure too early or too late will affect effective pressure transmission to fusion zone.

Solving weld line issues is a systematic optimization process that should be followed in following order:
Mold Optimization (Preferred and Most Effective)
Increasing Mold Temperature: This is the most direct and effective approach. A higher mold temperature maintains melt front temperature, buying time for molecular chain fusion.
Improving Venting: Adding venting grooves or using inserts at weld line location ensures smooth gas discharge.
Adjusting Gate Location/Number: Optimizing gate design through mold flow analysis (such as Moldflow) can relocate weld line to a non-visual, non-stressed area.
Increasing Runner and Gate Size: Reduces flow resistance and ensures effective pressure transmission.
Adjusting Process Parameters
Increasing Melt Temperature: Within material's permitted range, appropriately raise material temperature to enhance fluidity.
Increasing injection speed: This allows for faster filling and reduces thickness of cooling layer at melt front. However, be aware of potential jetting or trapped air.
Increasing holding pressure and extending holding time: This ensures adequate shrinkage compensation in fusion zone.
Using multi-stage injection: Use high speed and high pressure when passing through weld line, and return to normal speed in other areas.
Material Selection: Choose a grade with better fluidity.
For products requiring high strength, avoid using materials with high glass fiber content, or consider using mineral fillers that have less impact on weld line strength.

Case 1: Electrical Enclosure (Appearance and Strength Requirements)

 

Surface varnish cannot cover weld lines

 

Option 1 - One-point side entry (worse)

 

Solution 2 - Two-point glue injection (improved)

 

Surface weld lines;
Breakage during assembly.

 

Runner size configuration and product thickness distribution diagram

 

Short-shot distribution diagram of melt flow wavefront
Melt weld line occurs in a weak area of product structure, which can easily lead to fracture during assembly.

Fusion line and wind accumulation location distribution

 

Melt Temperature Distribution Diagram
When mold cavity is fully filled, material temperature in yellow area drops to around 250℃, which coincides with weld line (where temperature difference is significant).

 

Melt temperature distribution diagram when fully filled

 

Pressure drop distribution diagram

 

This product required an injection pressure of 63 MPa, but cavity pressure near end of flow (where weld line occurs) was only 28 MPa.
Flow channel size configuration and product thickness distribution diagram

 

Short shot distribution diagram of melt flow wave front

 

Melt temperature distribution diagram

 

Melt temperature distribution diagram when fully filled

 

Pressure drop distribution diagram

 

Occurrence of weld marks on exterior surface can be predicted in the early stages of product design.

 

Weld line angle and depth: Weld lines are visible if angle of convergence is less than 75°.
Countermeasures: By changing glue thickness to change convergence angle of weld line, weld line of appearance surface was successfully improved.

 

 

Base wall thickness: 2mm, thin wall section: 1mm
Causes of Short Shots and Weld Lines

 

Melt always flows in direction where it flows most easily (thicker walls).
What is optimal wall thickness?

 

We can use AMI to determine optimal adhesive thickness!
Summary: Methods for Improving Weld Lines
√ Offset weld line by diverting or blocking flow
√ Change location or size of inlet
√ Improve appearance by increasing melt and mold temperatures
√ Improve convergence angle of weld line by adjusting wall thickness
√ Add a flash pack to guide weld line

 

To address weld line issues, a systematic approach is essential:
Prevention is better than cure: During product and mold design stages, use mold flow analysis to predict and optimize weld line location.
Mold is fundamental: Good mold design (gating, venting, and cooling) is foundation for resolving weld line issues.
Process is key: Using existing molds, systematically adjusting parameters such as mold temperature, material temperature, injection speed, and pressure can maximize weld line quality.
Materials are foundation: It is crucial to understand characteristics of materials used and choose appropriate grade.