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2025

Sink Marks and Flash on Injection Molded Parts? Correlation Analysis Between Mold Design and CNC Machining

The core of injection molded parts' sink marks and flash defects is closely related to mold design and CNC machining. Sink marks are related to cavity design and CNC machining precision, while flash stems from insufficient parting surface design and machining fit. Collaborative optimization of reserving machining allowance in design, high-precision CNC machining of key parts and post-machining inspection can greatly reduce the defect rate and improve the qualification rate of injection molded parts.

Sink marks and flash are high-frequency defects in injection molding production. Most people ignore their core cause—the matching deviation between mold design and CNC machining. "Injection molding defect solution" and "mold CNC machining precision" have become popular Google search topics. In fact, key design points such as mold cavity size and parting surface precision rely on CNC machining to achieve precise implementation, and the coordination between the two directly determines the qualification rate of injection molded parts.

1. Sink Mark Defects: Dual Impact of Cavity Design and CNC Machining Precision

Sink marks result from insufficient melt cooling shrinkage and feeding, which are closely related to mold design and CNC machining: in the design stage, it is necessary to reasonably set the gate position and runner size; if the runner is too narrow, even if accurately formed during CNC machining, it will hinder melt flow; in the machining stage, CNC milling deviation of cavity size (such as more than 0.05mm smaller than the design value) will aggravate shrinkage extrusion and cause sink marks. By means of high-precision CNC machining (tolerance ±0.005mm) combined with cavity expansion design, the sink mark rate can be reduced by 80%.

2. Flash Defects: Key Role of Parting Surface Design and CNC Machining Fit

Flash is caused by melt overflow from mold gaps, with the core problem lying in parting surface design and machining precision: in the design stage, the parting surface not adopting a stepped sealing structure is likely to leave hidden dangers for machining deviations; in the machining stage, flatness deviation of the parting surface exceeding 0.01mm during CNC machining, or excessive fit gap between guide pillars and bushes, will lead to gaps during mold clamping. Using CNC grinding to ensure the flatness of the parting surface ≤0.003mm, and optimizing the parting surface sealing design, can completely solve the flash problem.

3. Collaborative Optimization: Defect Avoidance Path from Design to Machining

First, reserve CNC machining allowance in the design stage to avoid dimensional deviation caused by excessive machining removal; second, adopt 5-axis CNC machining for key parts (such as cavity, parting surface) to improve the forming precision of complex surfaces; third, verify the size through coordinate measuring machine inspection after machining to ensure consistency with design parameters. Collaborative optimization of design and machining can reduce the defect rate of injection molded parts by more than 60%

Key words:

injection molding defect solution,mold CNC machining precision,sink marks on injection parts,flash on injection parts,5-axis CNC mold machining

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