Technology / product information

– Supporting cooling and pressure optimization through mold maintenance services –

In injection molding, sink marks are among the most common appearance defects, appearing as surface depressions in thick sections or behind ribs.
In addition to visual issues, sink marks may cause strength reduction and poor assemblability, making them a critical improvement topic in mass production.

■ What is a sink mark?

A sink mark is a phenomenon in which resin volume decreases due to cooling and shrinkage after injection, resulting in a visible depression on the product surface.
This defect tends to occur in thick-walled areas, where cooling is slower and internal shrinkage cannot be sufficiently compensated from the outside. Factors such as insufficient holding pressure and degraded cooling performance are closely related, involving both molding conditions and mold structure.

■ Typical parts and applications

Sink marks commonly occur in:

Electronic component housings

Connectors

Automotive electrical and electronic parts

Appearance-critical covers

Extra caution is required for parts with mixed thin and thick sections or multiple reinforcing ribs.

■ Benefits of improvement and key considerations

Effective sink mark countermeasures provide:

Improved surface appearance

Higher production yield

Optimized cycle time

Stabilized molding conditions

However, simply increasing holding pressure without redesigning the structure may cause other defects such as flash or increased mold load. A well-balanced approach is essential.

■ Technical approaches to improve quality

Suppressing sink marks requires both mold maintenance and molding condition optimization:

Optimizing holding pressure and time: ensuring sufficient compensation for shrinkage in thick sections

Reviewing wall thickness design: reducing unnecessary thickness and optimizing rib height and thickness

Enhancing cooling lines: adding or reworking cooling channels to improve cooling balance

Removing contamination and corrosion inside the mold: restoring cooling efficiency and minimizing temperature variation

In precision molds, cooling line degradation or clogging directly affects product quality, making regular maintenance indispensable.

■ Information required from customers

To achieve optimal improvement, please share the following:

Detailed photos or drawings of sink mark locations

Product geometry information (wall thickness distribution, rib layout)

Current molding conditions (holding pressure, cooling time, temperature settings)

Existing mold cooling line structure

Resin shrinkage rate and material grade

For molding machines under 100 tons, cooling capability has a significant impact on quality, and mold maintenance effects are particularly noticeable.

■ Features of SHINKO PRECISION VIETNAM’s mold maintenance services

SHINKO PRECISION VIETNAM comprehensively analyzes the three root causes of sink marks—cooling, pressure, and wall thickness—and proposes optimal mold improvement solutions.

Strong expertise in precision molds for machines under 100 tons

Design improvement proposals focused on extending mold life

Rapid support through a network spanning Japan, China, Taiwan, and Vietnam

Design consulting covering cooling line enhancement and rib geometry optimization

High-precision mold component manufacturing at overseas partner factories
(Japanese quality × Vietnam cost advantage)

Issues such as persistent sink marks, depressions only in thick sections, or unstable molding conditions can be significantly improved through mold maintenance.
Please feel free to contact us and share the molding phenomena and mold specifications for detailed consultation.