Technology / product information

For those facing weld line issues in injection molding

– Optimizing flow balance through mold maintenance services –

In injection molding, weld lines are typical defects affecting both appearance and mechanical strength, occurring at locations where multiple resin flow fronts meet.
They are especially critical in micro parts and appearance-critical components, as they can lead to strength reduction, cracking, and surface unevenness, directly impacting product performance. Therefore, weld line improvement is a key issue in mass production.

■ What is a weld line?

A weld line is a phenomenon in which resin flows from multiple directions meet inside the mold but fail to fully fuse due to insufficient temperature or pressure, leaving a line-shaped mark after solidification.
Since it is influenced by resin flow behavior, temperature, and trapped gas, comprehensive analysis from both mold structure and molding conditions is required.

■ Typical parts and applications
Weld lines are commonly found in:
Electronic component housings
Connectors
Appearance covers
Long or complex-shaped products

Extra attention is required for:
Parts with mixed thin and thick sections
Molds using multiple gates

■ Benefits of improvement and key considerations
Improving weld lines can provide:
Enhanced surface appearance
Reduced risk of strength loss and cracking
Stabilized molding conditions and improved cycle time
Higher production yield

However, simply increasing material or mold temperature does not result in fundamental improvement and may cause other defects such as flash, short shots, or burn marks. A balanced optimization of mold design and molding conditions is essential.

■ Technical approaches to improve quality
Effective weld line countermeasures focus on mold structure review and flow balance optimization:
Optimizing resin and mold temperatures: ensuring sufficient flowability and fusion at meeting points
Revising gate positions: controlling resin flow paths and relocating weld lines to less visible areas
Improving material flowability: stabilizing resin condition through grade selection and drying control
Improving venting: preventing gas accumulation that interferes with resin fusion

In precision molds, even minor temperature imbalance or flow disturbance can trigger defects, making high-quality mold maintenance a critical factor in molding performance.

■ Information required from customers
To propose optimal solutions, please share the following information:
Location, depth, and appearance level of weld lines
Resin type, drying conditions, and flow characteristics
Current gate position and runner design
Mold temperature, resin temperature, and holding pressure conditions
Required mechanical strength and appearance standards

For molding machines under 100 tons, flow resistance has a greater impact, and mold-side improvements tend to show clear effectiveness.

■ Features of SHINKO PRECISION VIETNAM’s mold maintenance services
SHINKO PRECISION VIETNAM analyzes the root causes of weld lines from three perspectives: mold, molding conditions, and material, and implements improvements aimed at preventing recurrence.

Strong expertise in precision molds for machines under 100 tons
Design improvement proposals focused on extending mold life
Support network across Japan, China, Taiwan, and Vietnam
Short lead-time production of high-precision mold components by partner factories
(Japanese quality × Vietnam cost advantage)

Mold design consulting including gate optimization and flow analysis

If weld lines persist, surface unevenness remains, or required strength cannot be achieved, these issues can be significantly improved through mold maintenance and design optimization.