Instructions for use of injection molded breathable steel
As a specialized mold material with a porous structure, injection molding breathable steel, with its excellent breathability, plays a crucial role in resolving defects in plastic parts (such as bubbles, scorch marks, and missing material) caused by poor mold venting. Its evenly distributed micron-sized pores enable rapid evacuation of gases from the mold cavity, making it particularly suitable for molding deep cavities, thin walls, and complex structures. However, the use of breathable steel requires strict adherence to relevant regulations. Failure to do so can lead to a decrease in breathability and even shorten the mold life due to improper maintenance or incorrect parameter settings. For example, after using breathable steel in a mold for an ABS mobile phone case, the venting efficiency decreased after three months due to lack of regular pore cleaning. Bubbles reappeared on the surface of the molded part, and normal function was restored only after ultrasonic cleaning.
The selection of breather steel should be determined based on a comprehensive consideration of the part material and mold structure. The steel’s porosity (typically 15%-30%) and air permeability are key parameters. For plastics with poor flow properties (such as PC and POM), a highly permeable steel with a porosity of 25%-30% should be selected to ensure rapid gas evacuation. For glass fiber-reinforced plastics, a steel with a porosity of 15%-20% should be selected to prevent glass fiber particles from clogging the pores. The thickness of the breather steel should be designed based on the cavity depth, generally ranging from 5-15mm. Too thin will reduce structural strength, while too thick may increase gas flow resistance. For example, when producing 3mm-thick thin-walled PP parts, using 10mm-thick breather steel with a porosity of 28% and a 0.01mm venting gap can reduce cavity venting time to less than 0.5 seconds. Furthermore, the breather steel should be installed in areas where the melt will be last to fill, such as corners of the part and at the base of ribs, where gas is likely to accumulate.
The processing and assembly process of breathable steel has a significant impact on its performance. During the processing, it is necessary to avoid using cutting fluids containing silicon to prevent silicon from clogging the pores; high-speed and low-feed parameters (such as a speed of 3000-5000r/min and a feed rate of 50-100mm/min) should be used during drilling and milling to reduce the entry of metal debris into the pores. Before assembly, the breathable steel needs to be ultrasonically cleaned with anhydrous ethanol for 30-60 minutes to remove residual oil and debris from processing; during installation, a sealing gasket (such as a heat-resistant silicone gasket) should be installed between the breathable steel and the mold base to prevent the melt from penetrating the pore channel. For example, when assembling the breathable steel component of the PC lampshade mold, a 0.1mm thick fluororubber sealing gasket is used, which is precisely fixed with a locating pin to successfully avoid pore blockage caused by melt leakage.
Breathable steel requires daily maintenance and cleaning during use. Before each production run, the surface of the breathable steel should be purged with compressed air (0.3-0.5 MPa) in reverse to remove any remaining plastic debris. After production, the breathable steel should be disassembled and cleaned in an ultrasonic cleaner (40 kHz frequency) with a neutral detergent for 20-30 minutes, then rinsed with deionized water and dried. For breathable steel used over a long period of time, deep maintenance is required every 1,000-2,000 molds. This involves soaking the steel in a 5%-10% citric acid solution for 2-4 hours to remove oxidized impurities trapped in the pores. For example, one appliance company, by establishing a maintenance process of “pre-production purging – post-production cleaning – monthly deep maintenance,” has extended the service life of its breathable steel to over 80,000 molds, far exceeding the industry average of 50,000 molds.
The use of breathable steel requires proper injection molding process parameters. The injection speed should not be too high (typically controlled at 50-80 mm/s) to prevent the melt from impacting the breathable steel at high speed, potentially clogging the pores. The melt temperature should be 5-10°C lower than that of conventional molds to reduce the adhesion of plastic degradation products to the inner walls of the pores. For plastics that easily produce volatiles (such as PVC and POM), the holding time should be shortened by 5-10 seconds to reduce the amount of volatiles deposited in the pores. Furthermore, the mold cooling water temperature should be controlled at 50-60°C to prevent condensation from clogging the pores due to excessively low temperatures. For example, when using breathable steel to produce PVC pipe fittings, reducing the melt temperature from 180°C to 170°C and the injection speed from 70 mm/s to 60 mm/s, combined with weekly deep cleaning, successfully resolved the frequent clogging of the breathable steel, increasing the qualified rate of plastic parts from 85% to 99%. With the development of 3D printing technology, breathable steel with customized pore structure is gradually being used. Its maintenance details need to be adjusted according to the pore shape to maximize the exhaust efficiency.
