Acceptance of injection molds
The acceptance of injection molds begins with an inspection of their appearance and structure to ensure that the mold as a whole meets design requirements and operating specifications. This is the foundation of the acceptance process. First, check that the mold markings, including information such as the mold number, number of cavities, and weight, are clear. In one case, the mold’s weight was not marked, leading to the use of the wrong lifting device during hoisting, resulting in guide pin deformation. This requires special attention during acceptance. The mold surface should be free of visible scratches and rust, and the surface roughness of the cavity and core must meet the drawing requirements (typically Ra 0.8μm or less). A mirror-finished mold, tested with a surface roughness meter, was found to have a local roughness of Ra 1.6μm , requiring repolishing before passing acceptance. All joints must be secure and reliable, and bolt tightening torque must meet the standard (e.g., 35N · m for an M10 bolt ). In one case, the ejector plate bolts were loose, causing the ejector pin to tilt during a mold trial. This requires individual inspection during acceptance. The clearance between the guide pin and guide sleeve must be within the range of 0.01-0.03mm. Use a feeler gauge to check to ensure that there is no looseness. The guide pin clearance of a certain mold is as high as 0.05mm, resulting in inaccurate positioning when closing the mold. The guide sleeve needs to be replaced before acceptance.
Dimensional accuracy acceptance is a core indicator of mold acceptance. Precision measuring tools (such as coordinate measuring machines) are used to inspect key dimensions of the cavity and core, ensuring tolerances remain within design limits. For molds requiring a ±0.02mm dimensional accuracy for plastic parts, the cavity dimensional tolerance must be set to ±0.01mm. For example, the cavity depth of a mobile phone casing mold was designed to be 20mm, but measured at 20.005mm, which is within the ±0.01mm tolerance and meets the acceptance criteria. However, the pitch deviation of a gear mold reached 0.03mm, exceeding the ±0.015mm requirement and requiring repair and re-acceptance. Geometric tolerances (such as parallelism and perpendicularity) are equally important. For a flat plate mold, the cavity parallelism error was 0.02mm/m, meeting design requirements. However, another mold had a core perpendicularity error of 0.05mm, resulting in uneven part wall thickness and requiring adjustments before acceptance. Multi-cavity molds need to check the consistency of each cavity size, and the difference should be less than 0.02mm. The cavity size difference of an 8-cavity mold is as high as 0.03mm, resulting in a 1.5% deviation in the plastic part weight. It needs to be corrected to within 0.01mm before passing.
Mold trial production verification is a key step in mold acceptance. Actual injection molding production evaluates the mold’s molding stability and part quality, typically requiring 50-100 continuous production runs. The first molded part undergoes full-size inspection to ensure all dimensions meet the drawing requirements. For example, the critical aperture of one mold was designed to be 10mm, but measured at 9.98mm (with a tolerance of ±0.03mm), passing the test. However, the pitch deviation of a threaded part was 0.04mm, exceeding the ±0.02mm range and requiring adjustment of the thread core. During the mold trial, the mold’s smooth operation, including ejection, core pulling, and demolding mechanisms, must be observed. In one mold, the side core pulling mechanism experienced a jamming motion, which was attributed to iron filings in the guide runner. Cleaning restored smooth operation. The appearance of the molded part is 100% inspected to ensure there are no defects such as flash, missing material, or burnt material. During a mold trial, flash near the gate was observed, but after adjusting the gate dimensions, it was eliminated, increasing the appearance acceptance rate from 90% to 99.5%.
Mold performance parameters, including clamping force, injection pressure, and cooling efficiency, must be inspected and matched to design parameters. During mold closing, the mold parting surface should be tightly aligned. A 0.02mm feeler gauge cannot be inserted. In one mold, due to an uneven parting surface, the feeler gauge could not be inserted to a depth of 5mm, resulting in flash during the mold trial. The parting surface needed to be polished before inspection. The cooling system’s flow rate and pressure must meet design requirements. In one mold, the cooling water channel was designed for a flow rate of 3m³/h, but only 2.2m³/h was measured. Inspection revealed a clogged channel, which was cleared and restored to normal operation. Hot runner molds require nozzle temperature stability inspection. The deviation should be less than ±2°C. In one hot runner mold, nozzle temperature fluctuations reached ±5°C, resulting in partial burning of the plastic part. This issue was resolved by replacing the temperature control card. Ejector force should be uniform, with ejector mark depth less than 0.1mm. In one mold, uneven ejector force resulted in ejector marks of 0.2mm on the plastic part. Adjusting the ejector length corrected the problem.
Long-term stability verification is the final step in mold acceptance. Through continuous production of a certain number of molds (typically 500-1000), mold wear and part quality consistency are assessed. After 500 consecutive runs, a key cavity dimension of a mold showed wear of 0.005mm, which is within the acceptable range (<0.01mm). However, after 500 runs, a mold containing glass fiber material showed wear of 0.015mm, necessitating replacement of the wear-resistant insert. The defect rate of plastic parts must be monitored during long-term production and should be stabilized below 1%. A mold with an initial defect rate of 0.5% saw it rise to 3% after 500 runs. This was attributed to wear on the check ring, which returned to normal after replacement. Mold maintenance is also inspected, such as the ease of replacing wearing parts and the proper placement of lubrication points. One mold, due to difficulty removing the ejector plate, required maintenance of up to two hours, requiring structural improvements before approval. The final acceptance requires a complete report, including test data, mold trial records, and improvement items, which must be signed and confirmed by both parties. One company reduced the initial failure rate of molds from 15% to 5% by standardizing the acceptance process.
