For those experiencing issues with weld lines during injection molding

射出成形時のウェルドラインでお困りの方へ～金型メンテナンスサービスで流動バランスを最適化～
In injection molding, "weld lines (fusion lines)" are typical appearance and strength defects that occur where resin flows merge. They are particularly problematic in micro-products and aesthetic parts, impacting functional aspects such as reduced strength, crack formation, and appearance inconsistencies. Therefore, they represent a critical improvement theme in mass production environments.
■ What are Weld Lines
Weld lines are phenomena that appear as linear marks when resin flowing from multiple directions within a mold merges but solidifies without complete fusion due to insufficient temperature and pressure. Since multiple factors such as resin flow stagnation, low temperature, and gas entrapment are involved, analysis from both mold structure and molding conditions is necessary.
■ Target Parts and Applications
Weld lines are defects prone to occur frequently in electronic component housings, connectors, aesthetic covers, and products with slender or complex shapes. Particular caution is required for parts where thin-walled and thick-walled sections coexist, and for molds using multiple gates.
■ Improvement Benefits and Precautions
By improving weld lines, the following benefits can be expected:
Improved appearance quality
Suppression of strength reduction and crack risk
Stabilization of molding conditions and cycle improvement
Increased yield
However, merely increasing material temperature or mold temperature will not lead to fundamental improvement and may induce other defects (burrs, short shots, burn marks). Therefore, optimizing the mold and balancing molding conditions are essential.
■ Measures to Enhance Processing Accuracy
For weld line countermeasures, reviewing the mold structure and optimizing the balance of molding conditions are crucial.
Optimization of material temperature and mold temperature: Ensures resin flowability and improves fusion at the merge points.
Review of gate position: Designs the resin flow path and moves merge points to locations with less aesthetic impact.
Improvement of material flowability: Stabilizes resin condition through grade changes and drying management.
Improvement of venting: Prevents gas entrapment and suppresses fusion defects.
Especially in precision molds, even slight temperature variations or disturbances in resin flow can induce defects, making the quality of mold maintenance a significant factor in molding results.
■ Key Points for Purchasers
When requesting weld line improvement, sharing the following information will lead to the most suitable proposal:
Weld line position, depth, and appearance level
Resin type, drying conditions, and flowability
Current gate position and runner design
Mold temperature, material temperature, and holding pressure conditions
Required product strength and aesthetic requirements
It is also important to note that for molding machines under 100t, the impact of flow resistance is significant, and the effects of mold structure improvement tend to be clearly visible.
■ Features of SHINKO PRECISION VIETNAM's Mold Maintenance Service
At SHINKO PRECISION VIETNAM, we analyze the root causes of weld lines from three perspectives—mold, molding, and material—and implement improvements that include recurrence prevention.
Processing technology specializing in precision molds under 100t
Design improvement proposals based on extending mold lifespan
Support system through a collaborative network in Japan, China, Taiwan, and Vietnam
Short-lead-time manufacturing of high-precision mold components by partner factories (Japanese quality × Vietnamese price)
Mold design consulting, including gate position optimization and flow analysis
Weld lines that don't improve, persistent appearance inconsistencies, inability to secure strength... Such problems can be significantly improved through mold maintenance and design optimization.
If you share the phenomena and product specifications, we will propose the optimal improvements.