Causes and Solutions of Pitting Defects in Mold Mirror Polishing
Mirror polishing of molds is a critical processing procedure for molds used to produce high-gloss plastic parts. The polishing finish directly determines the surface gloss and molding effect of plastic products. Mirror polishing requires the cavity surface of the mold to reach nano-level smoothness. Tiny pits will be directly copied onto the surface of injection molded products and cause mass appearance defects. Pitting is a frequent defect in polishing. Its causes cover mold steel quality, polishing auxiliary materials, operation technology, environmental cleanliness and many other links. Only by checking the root causes one by one and adopting targeted improvement measures can the standard of mirror polishing be stably achieved.
1. Substrate defects of mold steel
Internal impurities, sulfides, pores and looseness in steel are the basic inducements for polishing pitting. Non-metallic inclusions inside ordinary pre-hardened steel are not easy to find in rough polishing. When polishing proceeds to mirror level, pits will form on the mold surface after the inclusions fall off. If quenching and tempering treatment of steel is not in place, the internal structure has uneven hardness. Local soft spots will be preferentially ground during sandpaper polishing and form point depressions. Rust, sand holes and residual EDM white layer on the steel surface will be exposed after deep polishing and show dense pits. For substrate problems, high-purity mold steel such as S136 and STAVAX shall be preferred for mirror molds. Material certificates and metallurgical test reports shall be verified before raw materials are delivered. Sufficient tempering shall be carried out after machining to eliminate internal stress of steel and ensure uniform overall hardness. After EDM machining of the cavity, the white layer and deteriorated layer must be completely removed before polishing to avoid exposure of defects in the polishing stage. If the pitting comes from internal inclusions of steel, the slightly defective position can be properly face milled to remove defects. If the defect is serious, the steel needs to be replaced for reprocessing.
2. Contamination of polishing consumables and tools
Large hard particles contained in polishing sandpaper, diamond paste and polishing cotton are common inducements for surface pitting. Mixing of diamond paste with different grain sizes, residual coarse particles left in polishing cotton or mold cavity gaps, will scratch the mirror surface and form small point pits during fine polishing. Repeated use of polishing tools without separate storage will lead to coarse polishing debris mixing into fine polishing consumables and scratch the mirror surface directly. Improper selection of diamond paste with uneven particle size distribution will cause large-size abrasives to knock out pits during polishing. The improvement method is to strictly separate consumables and tools for rough polishing, fine polishing and mirror polishing. Polishing cotton, copper rods and bamboo rods shall be dedicated to individual processes and shall not be mixed. Before changing grain size, the mold cavity and fixture surface shall be thoroughly cleaned with dust-free cloth and special cleaning agent to remove residual abrasive. Select regular diamond paste with uniform particle size distribution. After each polishing process, fully clean the mold surface and confirm no residual particles before moving to the next level of polishing. Consumables shall be stored in sealed dust-proof packages to prevent dust from mixing into polishing paste.
3. Improper polishing operation technology
Excessive polishing pressure or overlong local polishing time will cause overheating on the mold surface, softening the surface layer and embedding abrasives into the steel surface to form pits. Single polishing direction and long-time fixed-point concentrated polishing will cause plastic deformation of local metal, which is easy to form point defects in subsequent polishing stages. Skipping polishing procedures, jumping directly from rough grinding to ultra-fine diamond paste polishing, cannot eliminate scratches and will produce dense pits. Unstable polishing operation and shaking of polishing tools will make abrasives hit the mold surface and form point depressions. Standardize the polishing operation process and replace sandpaper grain sizes step by step without skipping levels. Control the pressure during polishing, move the polishing tool at a uniform low pressure, and prohibit fixed-point polishing for a long time to prevent local temperature rise. Rotate the polishing direction by 90 degrees for cross grinding after each procedure to eliminate scratches left by the previous process. Maintain stable and uniform moving speed during polishing to avoid shaking of polishing tools. Further reduce polishing pressure in fine polishing stage to reduce the risk of surface metal deformation.
4. Insufficient control of polishing environment and cleaning
Dust, iron filings and sand grains floating in the polishing workshop will fall on the cavity surface. During polishing, impurities wrapped in abrasives squeeze the mold surface and form pits. Iron filings and grinding powder remaining in mold cavity gaps, screw holes and exhaust slots will be continuously released during polishing and scratch the mirror area. Grease and sweat from hands contacting the mold surface will cause local corrosion and form point corrosion pits. Polishing operations shall be carried out on a dust-free operation table. Before operation, fully blow and clean the mold cavity to remove debris in gaps and insert joints. Wear dust-free gloves throughout polishing and prohibit bare hands from touching the polished cavity surface. Keep the operation environment dust-free to reduce suspended particles in the air, and continuously clean the mold surface between processes.
5. Inadequate corrosion and maintenance after polishing
If the mold surface is not cleaned in time after polishing, the chemical components remaining in polishing paste will attach to the mirror surface for a long time and slowly corrode the steel to form point corrosion pits. Improper selection of cleaning solvent, strong corrosive cleaning agent erodes the mirror surface and forms point corrosion pits. If the polished mold is exposed to humid environment for a long time, tiny rust spots will appear on the mold surface, which look similar to pits. After polishing, clean all polished surfaces with neutral special cleaning agent to completely remove residual diamond paste. Select cleaning medium matching mold steel and avoid strong acid and alkali solvents. Blow dry the mold immediately after cleaning, spray anti-rust agent and store in dry environment to prevent oxidation and rust on the mirror surface.
Summary
The root causes of pitting defects in mold mirror polishing fall into five categories: steel substrate, polishing consumables, operation technology, environmental cleaning and post-corrosion maintenance. When dealing with pitting, the source of defects shall be judged first to distinguish substrate internal defects and pitting caused by external polishing scratches. For mirror mold projects, select high-purity steel in the early stage and remove EDM deteriorated layers. Polishing operations must be graded strictly, separate consumables, do well in dust-proof cleaning and standardize polishing pressure and techniques. Clean and apply anti-rust treatment in time after polishing. Full-process control can greatly reduce the bad rate of pitting, stabilize the quality of mold mirror surface and reduce the construction period loss caused by repeated rework.
