Sourcing Criteria for CMM Inspection Points of Injection Molds in China's Plastic Molding Industry

In China’s domestic injection molding industry, high-precision plastic parts for new energy vehicles, medical devices, consumer electronics and optical products impose stricter dimensional tolerance requirements on injection molds. Coordinate Measuring Machine (CMM) inspection has become the core quality control procedure for mold factory acceptance, mold revision verification and regular wear inspection of mass-production molds. The selection of measurement points directly determines the authenticity and comprehensiveness of inspection data. Too few points will miss hidden dimensional deviations, while redundant points will greatly reduce the detection efficiency of domestic mold workshops. Combined with the processing standards, inspection habits and mass production demands of China’s injection mold manufacturers, this paper sorts out standardized, operable CMM inspection point sourcing criteria covering mold benchmarks, cavity curved surfaces, assembly matching structures, parting sealing surfaces and moving mechanical components.

I. Benchmark Priority Sourcing Criterion

All inspection work of domestic injection molds must establish coordinate systems based on processing benchmarks, assembly benchmarks and product design benchmarks, and benchmark points shall be collected first before measuring any molding features to avoid cumulative dimensional errors caused by coordinate offset.

For mold base benchmarks, take mold base reference corners and standard positioning pin holes as core measurement positions, with at least 3 points taken on each reference plane to calibrate the flatness of the mold base and lock the coordinate origin. For cavity benchmarks, collect points at the cavity center, positioning column center and main gate center corresponding to the origin of the 3D digital model, which are the mandatory key benchmark points for all inspection tasks. For positioning matching components such as guide sleeves and positioning cones, collect both circle center and end face points for each part to verify coaxiality and depth tolerance.

Domestic mold inspection specifications clearly prohibit skipping benchmark calibration to directly measure curved surfaces; complete benchmark data shall be archived synchronously for subsequent repeated comparison during mold revision and regular re-inspection.

II. Sourcing Criterion for Cavity Appearance Curved Surfaces

Most high-end plastic molds manufactured in China feature high-gloss transparent surfaces, curved appearance surfaces and thin-wall structures, whose contour deviation is the key inspection item of CMM. The point distribution follows the rules of uniform coverage and densified feature extremum.

For large flat appearance surfaces, lay out measurement points in a 50mm×50mm grid to fully capture flatness deviation; for arc transition, R-corner and product corner positions, densify points at the starting point, midpoint and end point of arcs, as these areas are prone to collapse and dimensional offset after EDM processing. For thin-wall products, collect paired points on inner and outer cavity walls to accurately calculate wall thickness uniformity. For optical PC transparent molds widely used in China’s electronics industry, double the density of curved surface points to comprehensively inspect tiny pits, depressions and contour errors on mirror surfaces.

All tool marks, discharge texture areas and hand-polished transition zones are set as key measurement positions to avoid missing appearance defects that will affect mass-produced plastic parts.

III. Sourcing Criterion for Assembly Matching Feature Points

Assembly structures including snaps, positioning columns, thread cavities and reinforcing ribs are the high-failure areas of plastic parts after mass production, which are the focus of inspection for domestic mold factories. Each matching feature must be measured with paired points on male and female inserts to judge actual assembly clearance.

For snap undercuts, collect points at the root, top and inclined matching surface of the undercut to detect undercut depth, draft angle and assembly gap; for thread cavities, take at least 3 points per circle to check thread medium diameter and profile contour. For long reinforcing ribs, arrange points evenly along the rib length to inspect rib thickness and verticality, preventing shrinkage deformation of plastic parts caused by thin ribs. For multi-cavity molds widely adopted by domestic mass-production manufacturers, the number and distribution of measurement points of each cavity are completely consistent, and the dimensional data of corresponding positions are compared to judge the consistency between cavities.

IV. Sourcing Criterion for Parting Surface and Sealing Area

Domestic high-pressure injection molds have strict requirements for parting surface fitting, and flash defects caused by uneven sealing surfaces are one of the main quality complaints of downstream customers. When selecting inspection points, fully cover the whole circle of parting surfaces, with one point set every 30mm, and densify points at four corners, slider joints and insert splicing gaps.

At each splicing position of inserts, collect more than 3 points to detect step difference of the sealing surface; separately measure the width and depth of glue sealing surfaces to check over-cutting or under-cutting during processing. The depth of exhaust grooves is a key controlled dimension for domestic nitrogen-assisted thick-wall molds, so the bottom and side walls of exhaust grooves must be sampled separately. For three-station rotary multi-color molds popular in China’s auto parts industry, lay out points with equal density on the parting surface of each station to compare the height difference between stations.

V. Sourcing Criterion for Moving Mechanism Components

Sliders, angle lifters, ejector pins and hot runner inserts are standard moving parts of domestic injection molds, which need to take into account both molding dimensional accuracy and movement matching clearance during inspection.

For sliders, collect points on the wear-resistant inclined surface and locking surface to detect inclined angle and matching gap; for angle lifters with deep undercuts, measure the molding surface and guide inclined surface in pairs to avoid scratch marks on plastic parts during demolding. Each ejector pin corresponds to two groups of measurement points of end face and hole position to check ejector pin height and coaxiality, eliminating ejection whitening defects. For hot runner molds used in precision electronic parts, sample points at the gate and hot nozzle to control gate size and concentricity, reducing shear-induced scorching of molten plastic.

In addition to molding surfaces, the guide and locking matching surfaces of all moving parts shall be added with measurement points to balance molding quality and smooth mold operation.

VI. Sourcing Criterion for Retention of Re-inspection Points

After the first CMM inspection of domestic new molds, all benchmark points, assembly key matching points, curved surface extremum points and sealing surface splicing points shall be marked and archived as fixed re-inspection points. After mold polishing, texturing and revision, the archived fixed points shall be adopted for re-inspection to ensure comparability of dimensional data before and after modification.

For mass-production molds in long-term service, regular wear inspection only needs to test archived key points instead of full-area sampling, which balances inspection accuracy and workshop detection efficiency. Random replacement of measurement points during each inspection is strictly forbidden, otherwise the dimensional change of the mold after long-term wear cannot be quantified and traced.

Conclusion

Combined with the actual production and inspection mode of China’s injection mold industry, the core logic of CMM inspection point sourcing is benchmark first, uniform coverage, densified feature points, paired comparison and fixed-point retention. Standardized point selection rules can effectively avoid missed inspection of key dimensional features, reduce batch defective plastic parts caused by mold dimensional deviation, and control CMM inspection time for domestic mold workshops. This set of criteria is applicable to whole-process quality control including new mold acceptance, post-revision size verification and regular wear re-inspection of mass-production molds, helping domestic mold manufacturers improve inspection standardization and overall product delivery quality.