Causes of Sink Marks on Injection Molded Parts and Mold Modification Solutions
Sink marks are a very common cosmetic defect in injection molding, appearing as inward dents or wave-shaped depressions mainly on thick sections, rib roots, around boss columns and positions with sudden wall thickness change. They spoil surface appearance, and in severe cases interfere with assembly clearance and structural strength. The root cause of sink marks is the lack of sufficient molten material to compensate volume shrinkage during cooling and solidification. Once the gate freezes, the inner material keeps shrinking while no extra melt can be fed into the zone. Many workshop operators only adjust process parameters to relieve sink marks temporarily, which cannot solve the problem stably from the source. It is necessary to identify root causes and carry out corresponding mold modification.
Inherent shrinkage risks brought by product wall structure
Excessive local wall thickness is the primary cause of sink marks. The material in thick zones shrinks more during cooling. The outer layer forms a solid skin first, and the inner melt continues to contract without supplementary material, forming surface depressions. If the thickness of ribs and screw boss roots exceeds the recommended ratio relative to the main wall, heat accumulates in these areas and cooling speed falls behind surrounding thin walls. Random thickening for strength reinforcement or too small transition fillets will worsen shrinkage difference. Mold modification starts with product structure optimization if the customer approves. The thickness of ribs is controlled within 60% of the adjacent main wall. Hollow design is adopted for screw bosses to reduce local plastic volume, and transition fillets are enlarged to homogenize heat distribution.
Insufficient feeding and packing capacity of gating system
Undersized gates or gates placed far away from thick zones are typical reasons for failed packing compensation. The gate solidifies too early, so packing pressure cannot transfer continuously to thick sections to compensate shrinkage. Multi-cavity molds may have unbalanced runners, leading to large pressure loss for distal cavities and higher sink mark risks. Some cosmetic parts adopt tiny submarine gates or horn gates to hide vestiges, which also shorten the effective packing time. Mold modification measures include enlarging gate cross-section properly to extend gate freeze time, moving gates closer to thick bosses and ribs to shorten pressure transmission path, rebalancing runners for multi-cavity layouts. For cosmetic surfaces where gate relocation is not allowed, auxiliary gates or repositioned hot runner nozzles can be used to improve packing effect.
Uneven cooling system and local heat accumulation
Lack of cooling channels near thick features, too long distance between channels and molding surface or sparse channel layout trap heat and extend cooling cycle, raising shrinkage volume. Large temperature difference across one part causes asynchronous cooling and more obvious sink marks. Blocked channels or unstable mold temperature also amplify this defect. Mold modification adds baffled wells and insert cooling structures for hot zones. Channels are placed as close to plastic surface as possible. Independent cooling inserts are made for screw bosses and deep ribs to realize localized heat removal. Zoned mold temperature controllers are used to narrow temperature gap between different zones and synchronize cooling speed of the whole part.
Poor venting indirectly induces sink-like depressions
The connection between venting and sink marks is often overlooked. Trapped air blocks melt filling and packing pressure transfer. Air cushions inside thick zones prevent supplementary melt from entering, forming depressions similar to sink marks. Rib ends, melt weld lines and inner boss areas are high-risk positions for air trapping. Insufficient vent depth or missing vents worsen the situation. Mold modification adds vent inserts at weld lines and filling ends, setting proper vent clearance to balance venting function and flash control. Removing compressed air inside the cavity ensures packing pressure reaches defect zones smoothly.
Influence of ejection structure and inserts
Many small inserts are used for screw bosses and deep ribs. These inserts have poor thermal conductivity and accumulate heat continuously, resulting in recurring sink marks. Concentrated ejector pins near thick zones occupy space that could be used for cooling channels and weaken heat dissipation. Mold modification selects insert steel with better thermal conductivity and machines internal cooling passages inside inserts. Ejection layout is rearranged to avoid hot zones, reserving space for cooling channels under the premise of smooth demolding.
Boundary between process adjustment and mold modification
Raising packing pressure, extending packing time or lowering mold temperature can reduce sink marks temporarily, but these adjustments bring side effects including high residual stress, product warpage, longer cycle and excessive flash. They are only temporary solutions. If sink marks still exist after the process window reaches its limit, the root cause lies in mold structure, and mold modification is required instead of long-term process compromise. The treatment of sink marks follows the sequence of product structure optimization, gating improvement, cooling upgrade and venting adjustment. Eliminating thick plastic accumulation at the design stage, optimizing packing paths and heat dissipation, and setting reasonable vents widen the molding window significantly. Stable mass production can be realized with lower cosmetic reject rate.
