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2025

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

The core cause of sink marks and flash on injection molded parts is the collaborative deviation between mold design and CNC machining, and the matching degree of the two directly determines product quality. Sink marks are closely related to cavity design (runner and gate settings) and CNC machining precision, which can be effectively improved by high-precision machining combined with cavity expansion. Flash stems from insufficient sealing of parting surface design and insufficient machining fit, which can be solved by stepped sealing design and high-precision grinding. Establishing a "design-machining" collaborative system, connecting in advance to reserve allowance, adopting high-precision machining for key parts and conducting full inspection afterwards, can significantly reduce defects, improve efficiency and control costs.

1. Sink marks and flash have an incidence rate of over 30% in injection molding, affecting appearance, assembly precision and increasing rework costs;

2. Most enterprises only focus on adjusting injection parameters, ignoring the core cause: collaborative deviation between mold design and CNC machining;

3. The rationality of mold design relies on CNC machining precision for implementation, and their matching degree determines product quality and production stability;

4. "Injection molding defect solution", "mold CNC machining precision control" are popular Google search topics.

1. Sink Marks: Collaborative Impact of Cavity Design & CNC Machining Precision

1. Essence of sink marks: Insufficient melt cooling shrinkage and feeding, directly related to cavity design and CNC machining precision;

2. Design requirements: Set runner diameter and gate type according to material characteristics (e.g., shrinkage difference between PP and ABS); runner for high-shrinkage PP ≥6mm, fan gate preferred;

3. Machining key: Cavity size deviation ＞0.05mm aggravates sink marks; high-precision CNC milling (±0.005mm) improves it;

4. Optimization plan: High-precision machining + cavity expansion design (reserve 0.2-0.5mm), sink mark rate can drop from over 30% to below 5%, suitable for high-precision parts like household appliance casings.

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

1. Cause of flash: Melt overflows from mold gaps under injection pressure, core is insufficient fit between parting surface design and CNC machining;

2. Design stage: Parting surface adopts stepped sealing structure, reasonably set clamping force reserve coefficient;

3. Machining stage: Parting surface flatness deviation ＞0.01mm or Ra＞1.6μm easily causes gaps; CNC grinding can control flatness ≤0.003mm;

4. Case: An auto parts factory had 25% flash defect rate due to 0.015mm parting surface deviation; optimized to below 1%;

5. Auxiliary guarantee: CNC machining precision of guide parts (guide pillars, bushes) must meet standards to avoid parting surface misalignment.

3. Collaborative Optimization Path: Reduce Cost, Improve Efficiency & Avoid Defects

1. Core idea: Establish "design-machining" collaborative optimization system to avoid defects, reduce costs and improve efficiency from the root;

2. Design stage: Connect with machining team in advance, reserve allowance according to equipment precision (0.1-0.2mm for 5-axis CNC cavity);

3. Machining process: 5-axis CNC for key parts (cavity, parting surface); guide pillars/bushes adopt grinding + honing, fit gap 0.002-0.005mm;

4. Inspection mechanism: 100% post-machining inspection with coordinate measuring machine, adjust parameters in time;

5. Effect: An electronic factory reduced defect rate from 35% to below 10%, reduced debugging time and improved efficiency by 20%.

Key words:

injection molding defect solution,mold CNC machining precision,injection part sink mark,injection part flash,5-axis CNC mold machining

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