Soft Tooling Selection and Amortization for Micro Runs
Soft tooling selection balances rapid lead times against progressive cavity wear, requiring direct NRE payment and scrap inflation modeling for micro runs.
Sink Marks
Meaning
Surface depression defect appearing in thick sections of a molded part occurs when the internal mass cools more slowly than the outer skin. Standard quality manuals classify sink marks as aesthetic or structural failures depending on their depth relative to the nominal wall thickness. Molten resin continues to contract after the exterior has solidified, pulling the surface inward.
This phenomenon stops being a concern when wall thicknesses remain uniform and cooling rates stay constant across the geometry.
Cooling Gradient
Differential temperature changes within a mold cavity determine the severity of surface irregularities. In the formation of sink marks, the lack of sufficient packing pressure during the solidification phase allows internal shrinkage to distort the exterior. High density polymers exhibit greater susceptibility to these variations.
Mold designers often use thin ribs to minimize the accumulation of heat in thick sections.
Design Rectification
Engineering standards published by the Standardization Administration of China provide guidelines for wall thickness ratios to prevent visual defects. Modification of the internal part geometry reduces the mass in areas prone to sink marks by introducing cores or hollow sections. Gating placement also plays a role in ensuring that the thickest sections receive the most pressure.
Revised cooling channel layouts help maintain a stable surface temperature during the cycle.
Inspection Criteria
Quality assurance protocols for consumer electronics dictate that surface depressions must not be visible under standard lighting conditions. Measurement of sink marks involves a depth gauge or a laser scanner to verify that the deviation falls within the limits set by GB/T 14486. If the part is intended for an internal assembly, the requirements are often relaxed.
Certification of the batch depends on the consistency of the finish across all cavities.