To those troubled by sink marks during injection molding

～Supporting Optimization of Cooling and Pressure with Mold Maintenance Services～
Sink marks in injection molding are a particularly common type of appearance defect, appearing as depressions in thick-walled sections and behind ribs. In addition to affecting aesthetic quality, they can also cause reduced strength and impaired assemblability, making them a critical improvement theme in mass production environments.
■ What is a Sink Mark?
A sink mark is a phenomenon where the volume of the resin decreases during the cooling and shrinking process after injection, causing that part to appear sunken. They are particularly prone to occurring in thick-walled sections where cooling is slow, and internal shrinkage cannot be compensated for from the outside. Both molding conditions and mold structure, such as insufficient holding pressure and degraded cooling line performance, have an impact.
■ Target Parts and Applications
Sink marks are a defect that tends to be problematic in molded products requiring design aesthetics, precision, and durability, such as electronic component housings, connectors, automotive electrical components, and exterior covers. Particular attention is required for shapes where thin and thick sections coexist, and for parts with many reinforcing ribs.
■ Improvement Benefits and Precautions
Measures against sink marks offer benefits such as:
Improved aesthetic quality
Improved yield
Optimized cycle time
Stabilized molding conditions
On the other hand, simply increasing holding pressure without fundamentally reviewing the design carries the risk of causing other defects such as flash and increased mold load, making a balanced approach to improvement crucial.
■ Measures to Enhance Processing Precision
To suppress sink marks, mold-side maintenance and optimization of molding conditions are indispensable.
Optimization of holding time and pressure: Ensure sufficient holding pressure in thick-walled sections to compensate for resin shrinkage.
Review of wall thickness design: Reduce unnecessary thick-walled sections, optimize rib height and thickness.
Reinforcement of cooling lines: Improve cooling balance by adding or reworking cooling channels.
Removal of dirt and corrosion inside the mold: Restore cooling efficiency and suppress temperature unevenness.
For precision molds, degradation or clogging of cooling lines directly impacts production quality, making regular maintenance essential.
■ Key Points for Purchasers
When requesting sink mark improvement, sharing the following information will enable optimal improvement:
Detailed photos and drawings of sink mark locations
Product shape information such as wall thickness geometry and rib placement
Molding conditions (holding pressure, cooling time, temperature settings)
Current mold cooling line structure
Shrinkage rate and grade information of the resin used
Especially for molding machines of 100t or less, cooling capacity significantly affects product quality, and mold maintenance is particularly effective in this range.
■ Features of SHINKO PRECISION VIETNAM's Mold Maintenance Service
At SHINKO PRECISION VIETNAM, we comprehensively analyze the root causes of sink marks – "cooling, pressure, and wall thickness" – and propose optimal mold improvements.
Strong expertise in precision molds for machines 100t or less
Design improvement proposals aimed at extending mold lifespan
Prompt response through a collaborative network spanning Japan, China, Taiwan, and Vietnam
Design consulting including cooling line additions/repairs and rib shape improvements
High-precision mold components can be manufactured at overseas partner factories (Japanese quality × Vietnamese price)
Problems such as persistent sink marks, depressions only in thick-walled sections, or unstable molding conditions can be significantly improved through mold maintenance.
Please feel free to consult us regarding phenomena and mold specifications.