Precision Machining Case of Micro-hole Plastic Molds for Environmental Protection Equipment in China

Micro-hole filter components are key consumables for waste gas and wastewater treatment equipment, and integrated injection molding has obvious advantages in aperture consistency compared with post-processing drilling. This case focuses on a PP filter disc mold for industrial waste gas pretreatment in China. The filter disc has a diameter of 120 mm, covered with thousands of 0.4 mm through holes. Stable aperture, burr-free hole edges and flatness are required, with the target yield above 95% and mold service life of 500,000 shots. The concentrated micro core pins bring multiple difficulties in machining, exhaust, cooling and ejection, so modular insert structure is adopted to solve the problem. During the early DFM review, the integrated cavity scheme is abandoned. 

The micro-hole area is split into four independent replaceable inserts. If individual micro pins are worn or broken, only the corresponding insert needs replacement instead of reworking the whole cavity, greatly reducing later maintenance cost. The main cavity uses S136H pre-hardened stainless steel with good rust resistance to adapt to the humid production environment of filter products. Tungsten carbide is selected for micro core pins to reduce abrasion and keep the aperture stable. A balanced three-point side gate is arranged on the outer ring of the filter disc, and moldflow simulation ensures the melt arrival time difference in all micro-hole areas is controlled within 0.05 seconds.

Precision machining process for micro inserts and micro core pins

Conventional CNC cannot process 0.4mm ultra-fine core pins. The process adopts high-speed CNC for roughing, matched with micro EDM for fine machining. After the insert base is processed, micro core pins are machined one by one with low pulse energy to reduce electrode loss. The coaxiality of each core pin is controlled within ±0.005mm, and the aperture tolerance is within ±0.01mm. Manual polishing is forbidden on the micro-hole area to avoid pin deflection and hole deformation. Only chemical polishing is used to remove carbon deposits from discharge processing. All inserts are inspected in a constant-temperature room with optical testing equipment before assembly.

Design of venting, cooling and ejection systems

Dense micro-hole zones easily trap air, which leads to burning marks and incomplete filling. An annular vent slot is set on the parting surface around the micro-hole area, and the matching gap between the core pin and insert is used for auxiliary exhaust. A conformal annular cooling channel is arranged below the insert, which keeps the temperature difference of the micro-hole area within 5℃ and prevents warpage caused by uneven cooling. A full-circle sleeve ejection structure is adopted to evenly disperse ejection force, and the micro core pin is set with 0.5° draft angle to reduce the risk of hole tearing during demolding.

Trial mold correction and mass production maintenance

Three typical defects appear in the first trial: incomplete filling of partial micro holes, burrs at hole edges and slight warpage. The filling problem is solved by adjusting multi-stage injection speed and cleaning vent carbon deposits. The burr is eliminated by reducing the fitting clearance between the core pin and insert, and the warpage is improved by adjusting the cooling flow and optimizing the wall thickness structure. After multiple rounds of adjustment, all indexes meet customer standards. In mass production, the workshop keeps dust-proof management, and the exhaust gaps are regularly purged with dry air. A sampling inspection mechanism is established to check the aperture every 10,000 shots, and spare inserts are replaced in advance once wear is found. This modular micro-hole mold scheme can be copied to water treatment filter sheets, dust removal filter screens and other similar environmental protection filter products.