Allowance Allocation Standards for CNC Finishing of Precision Mold Cavities
Precision mold cavities are applied in molds for medical consumables, security waterproof housings, consumer electronics and optical components, with forming precision controlled within ±0.003mm~±0.008mm. Dimensional accuracy, surface profile tolerance and flatness of sealing surfaces directly determine the yield of injection-molded products. CNC machining allowance allocation is not fixed numerical values but formulated hierarchically combining steel heat treatment status, cavity morphology, subsequent polishing/EDM processes and mold size. The core goal is to maintain uniform finishing allowance across the entire cavity, avoid tool chatter, deformation and dimensional deviation induced by uneven cutting force, and balance machining efficiency and final surface quality of cavities. The complete allowance system consists of roughing allowance before heat treatment, finishing allowance after quenching, and differentiated allowances for special structures to form standardized control specifications.
1. Roughing and Semi-finishing Allowance Benchmarks Before Heat Treatment
For pre-hardened steel molds such as 718H and NAK80 without quenching and tempering, no deformation compensation allowance is required during the whole machining process. Large-diameter flying cutters are adopted for roughing to remove most materials rapidly, leaving unilateral side wall allowance of 0.25mm~0.4mm and bottom allowance of 0.1mm separately. Reduced bottom allowance avoids step marks generated during bottom finishing, and corner positions reserve 0.3mm allowance for subsequent corner cleaning with special slender cutters to prevent tool breakage caused by excessive residual allowance at corners. Semi-finishing acts as a transition process to correct shape errors from roughing, adjusting side wall finishing allowance uniformly to 0.12mm~0.18mm and bottom allowance to 0.05mm~0.08mm, ensuring allowance deviation of any position within the cavity does not exceed 0.03mm. Uniform allowance stabilizes cutting load of finishing cutters and reduces profile tolerance errors during curved surface machining. For mirror mold steel such as S136 and STAVAX requiring quenching hardening, heat treatment brings slight expansion and contraction deformation. Therefore, unilateral side wall allowance is expanded to 0.4mm~0.6mm and bottom allowance reserved as 0.15mm during roughing. After semi-finishing, side walls retain unilateral allowance of 0.15mm~0.2mm to reserve space for CNC hard milling deformation repair after heat treatment. Small mold inserts within 200mm adopt the lower limit 0.15mm, medium inserts of 200mm–500mm use 0.18mm, and large cavities exceeding 500mm reserve 0.2mm unilateral allowance to offset uneven deformation from quenching. Natural stress relief aging for 4–8 hours is mandatory after roughing before semi-finishing to prevent cavity deformation induced by internal stress release and damage to allowance uniformity.
2. Graded Finishing Allowance Standards After Quenching Heat Treatment
After steel quenching to HRC48~52 hardness, finishing processes are divided into three working conditions: direct CNC finish forming, finishing followed by polishing, and finishing matched with EDM trimming with strictly distinguished allowance values. For ordinary appearance cavities directly finished by high-speed CNC without electrical discharge machining, unilateral side wall finishing allowance is set as 0.04mm~0.07mm and bottom allowance as 0.02mm~0.04mm. High spindle speed and small cutting depth layered milling are adopted with single cutting depth limited to 0.02mm~0.05mm for smooth cutter stress and surface roughness below Ra0.8 after machining. For medical and optical mirror molds requiring polishing after CNC finishing, unilateral polishing allowance of 0.02mm~0.03mm is reserved during CNC finishing. All planes and curved surfaces of cavities are machined without milling textures, so polishing only removes tiny metal surface layers without altering dimensional precision of cavities and scrapping molds caused by over-polishing. For narrow deep ribs and inner corners smaller than R0.3 inaccessible to complete CNC corner cleaning, inner corners separately reserve unilateral allowance of 0.08mm~0.1mm for EDM processing, while other large curved areas follow the conventional finishing allowance of 0.04mm~0.07mm. Sealing surfaces on parting lines belong to precision matching areas, with finishing allowance tightened to unilateral 0.02mm for flatness within 0.005mm per 100mm after finishing, meeting flash-free sealing requirements of waterproof housings and medical shells. Tool radial runout during finishing must be controlled within 0.003mm, and temperature fluctuation of constant-temperature machining workshops is limited to ±1℃ to prevent dimensional changes caused by steel thermal expansion and contraction.
3. Differentiated Allowance Control Rules for Special Cavity Structures
General allowance values cannot be applied to thin ribs, lifter forming positions, penetrating matching positions and deep cavities, which require independent allowance adjustment adapting to structural characteristics. For slender reinforcing ribs and waterproof sealing rib cavities thinner than 1mm, unilateral side wall finishing allowance is reduced to 0.02mm~0.04mm to lower cutting force and prevent rib vibration fracture and deformation, with rib bottom allowance set as 0.02mm processed by micro ball end mills with shallow cutting mode. For front and rear mold penetrating matching positions, front mold cavities are machined to finished dimensions with 0mm allowance, while corresponding rear mold positions reserve unilateral allowance of +0.01mm~+0.02mm for grinding and fitting during mold assembly, ensuring tight flash-free sealing without mold damage caused by mold clamping compression. For deep cavities deeper than 80mm, allowances are distributed segmentally from top to bottom: upper opening areas reserve unilateral 0.05mm~0.07mm allowance, while cavity bottoms adopt reduced unilateral allowance of 0.03mm~0.05mm due to weak cutter rigidity from long overhang lengths, avoiding taper errors (wide top and narrow bottom) caused by cutter deflection during bottom machining. Finishing unilateral allowance for sliding blocks, lifters and small inserts is unified as 0.03mm~0.05mm, and outer matching surfaces of inserts reserve 0.02mm grinding allowance for clearance adjustment during assembly. For wear-resistant mass-production molds requiring wear-resistant coating treatment, extra unilateral allowance of 0.008mm~0.012mm is reserved after CNC finishing for nitriding or DLC coating, so dimensions reach drawing tolerances exactly after coating treatment.
4. Allowance Verification and Abnormality Correction Rules
After semi-finishing, multiple points on cavities are measured by coordinate measuring machines to detect actual allowance values. Local semi-finishing trimming is required when allowance difference of any position exceeds 0.03mm to avoid excessive or insufficient partial allowance. Mold insert hardness and deformation data are retested before finishing; if quenching deformation exceeds 0.05mm, the overall finishing allowance is increased by 0.03mm to guarantee complete removal of deformed layers. Finishing never removes total allowance in a single pass, following layered finishing principles: the first layer removes 70% allowance, the second layer trims residual allowance, and a final light cut improves cavity profile precision significantly. Upper limit allowance values are adopted for soft pre-hardened steel machining, while lower limits are applied for high-hardness quenched steel and thin-wall structures to prevent plastic deformation of cavities induced by cutting extrusion.
Conclusion
The core of precision mold cavity CNC finishing allowance allocation is based on uniform allowance distribution, with graded values set according to steel heat treatment state, cavity structure and subsequent machining processes. Sufficient allowance is reserved before heat treatment to compensate quenching deformation, finishing allowance is tightened after quenching to guarantee dimensional precision, and special rib positions, penetrating matching areas and deep cavities reduce allowances separately to avoid vibration and deformation. The universal standard for pre-hardened steel cavities is 0.12mm~0.18mm semi-finishing allowance matched with 0.04mm~0.07mm finishing allowance, while mirror quenched molds reserve 0.02mm~0.03mm polishing allowance. This standard prevents texture roughness and deformation during finishing induced by excessive allowance, and avoids mold scrapping from insufficient allowance unable to eliminate quenching deformation. Strict implementation of this allowance allocation standard combined with stress relief aging, constant-temperature machining and low-runout cutter control stabilizes cavity profile tolerance within 0.008mm, adapting to precision mold processing for medical consumables, security waterproof housings and optical parts, reducing trimming workload of subsequent polishing and EDM processes while improving mold machining precision and delivery efficiency simultaneously.
