Injection molded breathable steel exhaust
During the injection molding process, the smooth escape of gas from the mold cavity is crucial for ensuring part quality. However, traditional venting methods often have limitations when dealing with complex parts. This is why injection molding venting technology using breathable steel has emerged. Breathable steel is a specialized steel with a uniform microporous structure, typically ranging from 5-20μm in diameter. This allows gas to pass while preventing the plastic melt from penetrating, resulting in efficient venting. This material revolutionizes the traditional venting model, which relies on venting grooves. It is particularly suitable for applications where venting grooves are difficult to implement, such as deep cavities, thin walls, and complex cores. For example, when producing mobile phone cases with fine lines, breathable steel inserts can quickly vent gas from the mold cavity, preventing defects such as material shortages and burning caused by gas entrapment, and increasing product qualification rates to over 98%. Furthermore, breathable steel venting reduces melt filling resistance and injection pressure by 10%-15%, saving energy and extending mold life.
The venting mechanism of breathable steel is based on its unique porous structure and gas permeability. When plastic melt is injected into the mold cavity, air, volatiles, and other gases within the cavity are squeezed out by the melt and, under pressure, flow toward areas of lower pressure. Conventional venting slots only provide venting at specific locations, such as the parting surface. However, cores or cavity inserts made of breathable steel can vent gases from all areas of contact with the melt. Gas is discharged out of the mold through microporous channels within the breathable steel, creating a “comprehensive venting” effect. Experimental data shows that breathable steel vents gases 3-5 times faster than traditional venting slots, reducing the gas pressure within the mold cavity from 5 MPa to below 0.1 MPa in just 0.5 seconds. Furthermore, the microporous structure of breathable steel is self-cleaning. Any plastic that penetrates the micropores will be ejected with the part after mold opening, preventing blockage and ensuring long-term, stable venting. This venting method is particularly suitable for engineering plastics with poor flow properties and high molding temperatures, such as PEEK and PI. It effectively reduces localized high temperatures caused by gas compression and prevents material degradation.
The design of breathable steel in molds requires adherence to certain principles and specifications. First, breathable steel inserts should be placed in areas where gas is most likely to accumulate, such as the final melt fill point, the bottom of deep cavities, and the base of ribs. For example, when producing automotive door handles, breathable steel inserts should be installed at the corner of the handle end, a high-risk area for gas accumulation. The thickness of the breathable steel should be determined based on exhaust requirements, generally ranging from 5-15mm. Too thin will compromise structural strength, while too thick will increase gas flow resistance. The mating surface between the insert and the mold body must be sealed, typically with an O-ring groove in the contact surface to prevent melt from escaping the gap. Furthermore, the breathable steel insert must be connected to the mold’s exhaust duct, with a diameter greater than 8mm to ensure that exhaust gas can quickly exit the mold. For large molds, multiple breathable steel inserts can be used for combined exhaust, with each insert responsible for a specific area, forming a coordinated exhaust system.
The selection and maintenance of breathable steel are crucial to effective ventilation. The appropriate breathable steel grade should be chosen based on the material properties of the plastic part and molding conditions. For high-temperature molding plastics (such as PA66 with glass fiber, molding temperatures above 280°C), high-temperature-resistant breathable steel should be selected, such as Japan’s Shinto PORCERAX II, which can operate at temperatures above 300°C. For highly corrosive plastics (such as PVC), corrosion-resistant breathable steel, such as Sweden’s SSAB HPM50, is required. The micropores of breathable steel are easily clogged by plastic decomposition products or impurities, requiring regular maintenance, generally every 5,000-10,000 molds. Cleaning methods include removing the breathable steel inserts and cleaning them in an ultrasonic cleaner with a specialized cleaning agent for 30 minutes to remove any residue within the micropores. For severe blockage, high-temperature calcination at 400-500°C for 2 hours can be used to carbonize and dislodge any plastic residue within the micropores. After maintenance, breathable steel needs to be tested for air permeability to ensure that its exhaust performance is restored.
The advantages and limitations of breathable steel venting technology require an objective understanding for proper application. Its primary advantages include: high venting efficiency, which solves complex structural venting issues unsuitable for traditional venting methods; reducing surface defects such as silver streaks, burns, and missing material, thereby improving product quality; lowering injection pressure and clamping force, saving energy; and simplifying mold structure, eliminating venting groove processing, and reducing mold manufacturing costs. However, breathable steel also has certain limitations, including its high material cost, which is 3-5 times that of ordinary mold steel; its low mechanical strength, with a hardness generally between HRC30-40, making it unsuitable for molded parts requiring high surface wear resistance; and the potential for melt penetration in extremely low-viscosity plastics (such as PE and PP), leading to micropore blockage. Therefore, a cost-benefit analysis is necessary when applying breathable steel venting technology. For high-end precision plastic parts or products that are difficult to vent with traditional methods, the improved quality and reduced scrap rate more than offset the increased material cost, making it a worthy advanced venting solution.
