Chinese Injection Mold Sealing and Pressure Resistance Test Procedure for Waterproof Molds

Water leakage stands as one of the most frequent malfunctions during mass production in China’s injection molding industry. Seepage from cooling water channels triggers mold rusting, water stains on finished plastic products, circuit short circuits and unexpected production halts. Waterproof plastic molds demand extra pressure-resistant sealing capacity on cavities and parting surfaces. Formulated in accordance with Chinese standard GB/T 12554-2006 Specification for Plastic Injection Moulds, this testing system consists of two core modules: pressure testing for cooling channel sealing and pressure resistance testing for cavity parting surfaces. It covers three application scenarios including factory delivery inspection, re-inspection after maintenance and regular routine checks, matching the assembly, mold trial and maintenance workflow adopted by domestic mold factories. Both wet hydraulic pressure testing and dry air leakage detection are available to adapt to inspection conditions of small mold workshops and precision injection molding factories across China.

1. Pre-Test Preparation Steps

After full mold assembly, disconnect mold temperature machine connectors and external water pipes, then blow out metal shavings, anti-rust grease and sundries inside cooling channels with compressed air to keep pipelines unobstructed, as blocked pipelines will cause uneven pressurization and false test results. Inspect water channel sealing rings, plugs and sealing grooves at insert joints one by one. Nitrile rubber O-rings with temperature resistance are adopted, coated evenly with high-temperature lubricant during installation and kept 0.05mm higher than the sealing groove surface to eliminate hidden leakage risks caused by improperly pressed sealing rings. Select testing equipment according to mold specifications: manual hydraulic test pumps for ordinary daily plastic molds, digital airtight pressure detectors for automotive waterproof structural parts and precision electronic waterproof molds. Maintain test ambient temperature between 18℃ and 26℃ to avoid fluctuation of pressure gauge readings caused by temperature differences. Lay the mold horizontally on a flat inspection platform, fully close and lock moving and fixed halves with clamping force adjusted to the standard pressure used in formal mass production, so as to prevent pressure leakage on parting surfaces due to excessive mold opening clearance. Mark each independent circuit of cooling channels in advance for segmented leakage location later.

2. Hydraulic Pressure Testing for Cooling Channel Sealing (Universal Scheme for Domestic Mold Factories)

Block the outlet of a single water channel circuit, connect the pressure test pump to purified softened water mixed with trace water-based anti-rust agent, then open the pressure valve to boost pressure steadily at a rate of 0.1MPa per minute. Sudden pressure surge is forbidden to avoid cracking of water channel welding seams and falling off of plugs. For household appliance and daily commodity molds, pressurize to 0.4–0.6MPa and hold pressure for 30 minutes. For precision waterproof molds operating continuously under high temperature, perform testing at 1.5 times the rated working pressure up to 0.75–0.9MPa with a 45-minute pressure holding period. Inspect mold frame surfaces, insert joints, water plugs and ejector pin holes throughout the pressure holding phase for seepage, dripping or water stains, and record pressure gauge readings simultaneously. The mold passes the water channel sealing test if pressure drop stays below 0.005MPa without water seepage anywhere. In case of continuous pressure drop or local leakage, release pressure slowly, detach plugs of corresponding circuits and conduct segmented pressurization to locate leakage points. Disassemble the mold to replace sealing rings and re-seal water channels after marking leakage positions, then repeat full pressure testing until qualified. After testing, open both ends of water channels and blow away residual water completely with high-pressure dry air to prevent rust formation from retained water.

3. Pressure Resistance Test for Cavity Parting Surfaces (Exclusive Procedure for Waterproof Plastic Molds)

Molten plastic squeezes parting surfaces under high pressure during molding of waterproof products, which necessitates verification of sealing performance under pressure. Two detection methods are adopted: dry air test and water immersion test. The dry nitrogen test applies to high-gloss waterproof molds. Lock the mold tightly, inject dry nitrogen into the sealed cavity up to 0.35MPa and hold for 15 seconds; the parting surface is qualified when the overall leakage rate is lower than 50Pa/min. The immersion method suits general structural waterproof molds: submerge the lower half of closed molds in clean water, inflate the cavity to 0.3MPa and check for continuous bubbles rising from water, where bubbles represent sealing failure positions. For stacked waterproof molds and molds with multiple sliders, lock each layer separately for independent testing to eliminate mutual interference between multi-layer parting surfaces during leakage detection. Polish leaking parting surfaces with red lead coloring matching and conduct pressure re-test until no bubbles emerge and pressure remains stable. For hot runner waterproof molds, add a sealing test for hot nozzles after heating up to production temperature to rule out leakage induced by softened hot nozzle sealing rings under high temperature.

4. Rapid Air Leak Detection for On-Machine Molds During Routine Inspection

For molds mounted on injection machines that cannot be disassembled for immersion testing, dry air leakage inspection replaces hydraulic testing. Connect an airtight tester to water channel ports, inject dry clean compressed air up to 0.3MPa and hold pressure for 20 minutes to observe pressure attenuation. This method eliminates drainage and drying steps without interrupting production, widely used for daily inspection in injection molding clusters in Jiangsu, Zhejiang and Guangdong provinces across China. Air testing only serves as preliminary screening; once abnormal pressure is detected, the mold shall be taken offline for hydraulic testing to pinpoint tiny leakage points that may be missed by air inspection. Waterproof molds operating in humid plum rain seasons shall receive air leakage inspection every 15 working days to prevent mold rusting caused by water seepage.

5. Pressure Relief, Final Treatment and Filing Standards

Release pressure gradually through relief valves after testing, with the entire pressure relief process lasting no less than 3 minutes to prevent water hammer impact from instant pressure release that damages mold sealing structures. Blow dry water channels and cavities thoroughly for molds after hydraulic testing, then spray a thin layer of dry anti-rust spray on parting surfaces. Molds passing air testing can be connected to mold temperature machines and put into production directly. Fill in mold pressure inspection ledgers with mold serial number, test pressure, pressure holding duration, pressure drop value, inspection result and maintenance records. Precision waterproof molds shall retain paper test reports and inspection photos to realize full-lifecycle traceability of sealing performance. Unqualified molds are sent for maintenance and must pass two full re-tests before approval for mass production.

6. Differentiated Pressure Parameters for Various Waterproof Molds

For consumer electronics IP67 waterproof shell molds: water channel test pressure 0.6MPa with 30-minute holding time, cavity air pressure 0.35MPa. For automotive waterproof connector molds: water channel pressure 0.9MPa for 45 minutes, parting surface bearing pressure 0.4MPa. For simple daily waterproof plastic part molds: 0.4MPa water channel pressure and 25-minute holding time are acceptable. Domestic mold manufacturers can adjust parameters properly according to product waterproof grades and mold steel materials, while test pressure shall never exceed the bearing limit of water channel structures to avoid pipeline deformation and rupture.

Conclusion

Developed to fit the operational mode of batch production and frequent mold replacement adopted by Chinese injection mold manufacturers, this sealing and pressure resistance procedure takes hydraulic testing as the gold standard for factory delivery and air leakage detection as the online inspection method, combined with parting surface pressure testing to satisfy sealing requirements of molded waterproof plastic parts. It covers all links including preparation, segmented pressurization, leakage location, pressure relief and filing. Most mold leakage failures in mass production originate from omitted pressure holding tests before delivery, as simple water flow checks cannot discover hidden micro-leakages. Strict implementation of this standardized process prevents mold rusting and water stain defects on plastic parts, extends the service life of waterproof molds and reduces unexpected shutdowns caused by water leakage, meeting the cost reduction and efficiency improvement demands of China’s injection molding factories.