How Chinese Precision Injection Molds Achieve ±0.002mm Machining Accuracy

The dimensional requirements for optical, medical and micro-transmission plastic parts keep rising. For some precision mold cavities, inserts and fitting positions, stable machining tolerance of ±0.002 mm is required. This accuracy level exceeds the conventional capacity of ordinary CNC machining. It cannot be achieved by a single machine tool alone. A closed-loop control system covering constant-temperature workshop, unified datum system, equipment selection, electrode management, measurement verification and assembly protection must be established. Tiny errors from any link will lead to out-of-tolerance dimensions.

Constant-temperature workshop and environmental control

Thermal expansion and contraction of steel caused by temperature change is the main source of micron-level errors. The linear expansion coefficient of mold steel is approximately 11.5×10⁻⁶/℃. For a 100 mm steel block, every 1 ℃ temperature fluctuation can trigger deformation over 0.001 mm. To steadily reach ±0.002 mm precision, machining, EDM and inspection zones need constant-temperature, constant-humidity and dust-free workshops. Temperature is controlled at 20±0.5℃, humidity kept between 40% and 60%. Direct air flow and sunlight on workpieces and machine spindles should be avoided. The workshop foundation is designed for vibration isolation, separated from high-vibration equipment such as stamping presses and conventional milling machines. Workpieces must stay inside the constant-temperature area long enough to balance temperature before clamping, eliminating temperature difference between the workpiece and ambient environment.

Unified datum system and fixture scheme

Confusing datums are a common risk in ultra-precision machining. All mold components share the same set of datum holes and positioning surfaces. Datum features are finished in one clamping setup, and all subsequent processes reuse these datums to reduce positioning deviation caused by repeated disassembly. Fixtures are made of high-rigidity, low-deformation stainless steel or cemented carbide. Fixtures go through stress relief in advance. Clamping force needs proper control. Excessive clamping force will generate elastic deformation on thin inserts, and spring-back after unclamping results in dimension drift. Vacuum chucks or micro-force fixtures are adopted for micro thin inserts to fix parts reliably while minimizing clamping stress.

Selection and maintenance of ultra-precision machining equipment

For ±0.002 mm precision machining, key finishing processes apply ultra-precision wire EDM, mirror EDM and ultra-precision CNC. Ultra-precision CNC machines require spindle runout and repeat positioning accuracy within 0.001 mm. Lead screws and guide rails are compensated regularly. Submerged wire EDM with pure water is used for narrow slots, precision holes and fitted inserts. Mirror EDM handles sharp corners and fine textures inside cavities. Machine tools follow a strict maintenance schedule. Pitch error, backlash and spindle level are calibrated periodically. Filter elements and lubricating oil are replaced on schedule to protect guide rails from scratches.

Electrode fabrication and wear compensation

EDM is essential for forming precision cavities. Electrode tolerance is controlled within ±0.001 mm, higher than the target cavity accuracy. High-density tungsten copper is preferred for lower erosion during finishing. Roughing and finishing electrodes are separated and inspected fully by CMM or optical comparator before use. Finishing adopts low-energy, narrow pulse width and low current parameters to reduce re-cast layer and electrode loss. Wear compensation is preset and adjusted dynamically. All electrodes for multi-cavity molds are inspected in the same batch to guarantee cavity consistency.

Staged machining and stress relief

Residual stress remains inside steel after forging and roughing. If finishing is carried out directly, slow stress release will cause gradual deformation. The process is divided into roughing, semi-finishing, stress relief and final finishing. Roughing leaves 0.08~0.12 mm stock, followed by low-temperature aging or cryogenic treatment. Semi-finishing reserves 0.01~0.03 mm allowance before ultra-precision machining. Pre-hardened stainless steel is widely selected for high-precision inserts to minimize post-heat-treatment deformation.

In-process inspection and final verification

Vernier calipers and ordinary micrometers are not qualified for this grade of measurement. Coordinate measuring machines, high-precision optical comparators and laser height gauges are placed inside vibration-isolated constant-temperature inspection rooms and calibrated by third-party metrology institutes regularly. Key dimensions are checked after each finishing operation instead of waiting until all components are fully processed. Workpieces are cooled and thermally balanced before measurement. Inspection records are archived for traceability.

Assembly protection and precision maintenance in mass production

Qualified machined parts are only half of the whole work. Assembly is completed in dust-free constant-temperature stations with precision fasteners and dowels. Forcing inserts into position is forbidden to prevent cavity deformation. Concentricity and parting surface fit are rechecked after mold closing. During mass production, guide components wear after repeated opening and closing. Wear-resistant coatings are applied on guide pins and sleeves, and vulnerable parts are replaced on schedule to maintain long-term dimensional stability.