Anti-scratch Machining Process for PSU Transparent Plastic Molds

Polysulfone (PSU) is a high-temperature resistant transparent special engineering plastic with high melt viscosity. Any tiny scratches, machining lines or polishing defects on mold cavities will be replicated directly on molded transparent parts, resulting in haze, bright streaks and poor light transmittance, further causing mass rejection. Compared with ordinary plastic molds, transparent PSU molds require a full-process anti-scratch control system covering raw material preparation, CNC machining, EDM, grinding and polishing, assembly and trial molding to avoid surface damage. The standardized anti-scratch machining workflow is sorted out as follows.

I. Basic Control: Material Preparation and Heat Treatment for Anti-scratch Performance

S136, STAVAX and other high-purity martensitic stainless steels are preferred for PSU transparent molds with few internal inclusions and stable polishing performance. Hardness after heat treatment is controlled within HRC 48–52. Raw steel plates receive full visual inspection after warehousing. Sections with collision marks, scratches and rust must be cut off and cannot be directly used for cavity processing. Wire cutting is adopted for blanking; flame cutting is forbidden. Rubber mats are used for isolation during transfer. Steel plates cannot be stacked directly. Soft slings are selected for hoisting to avoid steel wire contacting molding areas. Heating rate is controlled during heat treatment to reduce internal stress and prevent surface peeling and fine cracks in subsequent machining. Oxide scale is removed by sandblasting after heat treatment, followed by stress relief tempering. Quality inspectors confirm no surface cracks or pits on cavity blanks before transferring to machining procedures. Hard tools are prohibited from knocking molding blanks in the whole preparation stage to avoid hidden damage from the very beginning.

II. Anti-scratch Specifications for CNC Roughing and Finishing

Uniform finishing allowance is reserved during rough machining, with unilateral allowance of 0.3–0.5 mm for cavities to prevent surface pulling caused by excessive local allowance and tool extrusion. Brand-new carbide end mills are adopted. Tools with chipped edges or worn tips are replaced immediately; dull cutting tools tend to form tool marks and tear metal surfaces. Equal-height machining strategies are prioritized in finishing to reduce lateral force. Stable feed speed lowers vibration-induced micro vibration lines. High-pressure clean air and dust-free brushes are used to clean workpieces after machining; metal brushes are banned for cavity sweeping. Finished workpieces are placed on silicone mats. The worktable is cleared of iron scraps in advance, because tiny metal debris can form indentations when pressed onto workpieces. Semi-finished mold inserts are transported independently in dust-proof storage boxes. Molding surfaces cannot contact fixtures or metal tables directly. Clamping positions avoid final molding areas, as clamped scars cannot be used for transparent appearance surfaces afterwards.

III. Anti-scratch Control Points for EDM Electrical Discharge Machining

Large-area EDM should be minimized for PSU transparent mold cavities. Main surfaces are completed by CNC machining, and EDM is only applied for deep narrow ribs and dead corners. High-purity copper electrodes are precisely finished without tool marks. Medium-low current and moderate pulse width are selected for machining parameters to control discharge energy and reduce risks of carbon deposition, pits and electric arc burns. Work fluid filters are cleaned regularly to maintain fluid cleanliness. Suspended metal particles in liquid will cause point scratches on cavities during discharge. Soft copper brushes and anhydrous alcohol are used for gentle cleaning instead of hard scrapers to remove carbon residues after EDM. Spark lines remain on surfaces after electrical processing and must be eliminated by sequential grinding. Direct fine polishing without intermediate grinding procedures will leave permanent micro scratches.

IV. Sequential Grinding and Mirror Polishing: Core Anti-scratch Procedures

Polishing is the most critical anti-scratch procedure for PSU transparent molds and must be completed in an independent dust-free polishing station. Dust suspended in air creates irregular scratches during polishing. Grinding strictly follows progressive grit upgrade rules; skipping grit grades is prohibited. Standard sequence: 400# oil stone → 800# →1200# →2000# oil stone, followed by 3000#, 5000#, 8000#, 10000# diamond polishing paste. Cavities and polishing tools must be fully cleaned with anhydrous alcohol after each grit grade to eliminate coarse abrasive particles mixed in fine polishing procedures, which will generate long difficult-to-remove scratches. Polishing direction follows the ejection direction of products. Cross reciprocating grinding is forbidden to avoid mesh texture. Soft polishing accessories fit curved surfaces and fillets. Excessive local pressure causes orange peel and local indentation. Bamboo sticks wrapped with polishing cloth are used for narrow slots and deep ribs; hard tools cannot contact cavities directly. Final mirror surface roughness reaches Ra ≤0.025 μm (SPI A2 grade). Multi-angle strong light inspection is required to confirm no scratches, pits or lines before assembly.

V. Anti-scratch Management during Fitter Matching and Assembly

All inserts, sliding blocks and lifter molding surfaces are covered by transparent protective film before fitting. Grinding debris must be cleared timely. Chips trapped on parting surfaces will scratch cavities during mold closing. Operators wear dust-free gloves during assembly; direct bare-hand contact with mirror cavities is prohibited. Vacuum suction cups or soft clamping tools are adopted to transport mold cores; pliers and wrenches cannot grip molding surfaces. A small amount of high-temperature clean grease is applied to guide pins, ejector pins and sliding pairs. Excess overflow grease contaminates cavities and forms stubborn carbonized stains that pull transparent parts during ejection. Low-pressure gradual mold closing is adopted for trial assembly. Production stops immediately for cleaning if foreign bodies are detected, and forced clamping is forbidden. Cavities are fully cleaned again after assembly with one-way wiping by dust-free cloth; back-and-forth rubbing on mirror surfaces is not allowed.

VI. Protection Specifications during Trial Molding, Transfer and Storage

Transparent protective film is fully attached to molds during transfer and storage without residual glue. Cavities are purged with clean compressed air before the first trial molding to remove dust and metal debris. Low temperature and low pressure molding is adopted initially to reduce high-speed melt erosion on cavities. Soft tools such as copper shovels and bamboo slices are used to clear residual runners and carbon deposits; hard tools cannot touch cavities during trial runs. Neutral anti-rust agent is sprayed on cavities during production intervals to avoid tiny rust spots on stainless steel mirror surfaces. Rust protrusions continuously scratch transparent molded products. Cavities are fully cleaned and covered by protective film before long-term shutdown and stored in dry environments.

Conclusion

Any scratch on PSU transparent mold surfaces will damage optical quality of transparent products. Anti-scratch control cannot rely solely on repairing defects in final polishing, and must run through steel preparation, CNC machining, EDM, grinding polishing, assembly and trial molding protection. Core management ideas include reducing mechanical damage in early stages, controlling particle pollution and vibration lines during machining, complying with sequential grinding specifications in polishing, and isolating molding surfaces during assembly and transfer. Strict implementation of the standardized process can minimize scratches, lines and pits on mold cavities, stabilize yield of transparent PSU products, and avoid prolonged re-polishing, cost and schedule loss. It meets mass-production demands of high-end products such as medical windows and high-temperature resistant transparent housings.