Mar 10,2026
Scientific Design of Mold Exhausting Systems: Systematic Solutions to the Problem of Breathlessness
Scientific Design of Mold Exhausting Systems: Systematic Solutions to the Problem of Breathlessness
Mold venting, Gas entrapment, Venting system, Vacuum venting, Parting line
In injection molding and press molding, mold exhaust is often overlooked but a crucial element. When high-speed molten fills the mold cavity, the air in the mold cavity and the volatiles that the material itself may release must be expelled in time and smoothly. If exhaust is not smooth, the compressed gas will generate high temperatures, causing the material to burn (burn marks); it will obstruct the flow of molten, causing insufficient filling or decreasing the strength of weld marks; it may even cause the mold molding surface to be pushed open due to too high air pressure, resulting in flying edges. Therefore, a scientifically designed exhaust system is a systematic engineering that addresses gas-related defects and ensures product quality.
The design of the exhaust system first requires a clear location for exhaust. In theory, gas will eventually gather in the last filled area of the molding cavity, that is, at the end of the molten fluid flow. Therefore, the exhaust chute should be prioritized to be opened on the fractal surface, directly facing the area at the end of the flow. In addition, the bottom of the deep cavity, the top of the reinforced tendon, the blind spots away from the pouring mouth, and the areas where multiple molten fluids converge to form weld marks are all “minefields” where gas can easily be trapped. For these locations where gas cannot be discharged through the fractal surface, special exhaust measures are needed, such as opening exhaust panels, using ventilated steel, or designing a vacuum system on the mold.
The size design of the exhaust chute is a precise science at the micrometer level. The depth of the exhaust chute must be precisely controlled: both deep enough to allow the gas to pass through quickly, and shallow enough to prevent molten matter from squeezing into the chute under high pressure to form flying edges. This critical depth depends on the viscosity of the material. For low-viscosity materials such as PA, POM, the exhaust chute depth is usually controlled at 0.01–0.02 mm; for medium-viscosity materials such as ABS, HIPS, the depth can be 0.02–0.03 mm; for high-viscosity materials such as PC, PMMA, the depth can reach 0.03–0.05 mm. The width of the exhaust channel is determined by the amount of exhaust, usually 5–10 mm. A deeper exhaust channel (0.3–0.5 mm) needs to be connected to the back of the channel, ultimately leading to the exterior of the mold, to ensure smooth escape of the gas.
For deep cavity and complex core exhaust, more advanced techniques are needed. When the core is thin and long and surrounded by molten matter, exhaust cannot be expelled through the fractal surface. At this time, a thimble or patch can be designed inside the core, utilizing the tiny coordination gap between the thimble and the mold as an exhaust channel. For closed areas where exhaust slots cannot be opened at all, ventilated steel (also known as porous welded metal) is an effective solution. The interior of ventilated steel has countless micrometre-scale connecting pores, allowing gas to pass through and block the molten matter. Embedding ventilated steel as patches into the trapped area can serve as a local exhaust. However, ventilated steel is less strong than cast steel and needs to be designed in areas with less pressure.
Vacuum exhaust systems are the ultimate solution to the problem of trapped air in complex, large-scale molds. For large, deep-cavity molds such as car bumpers and dashboards, natural exhaust alone is often not enough. Vacuum exhaust technology uses a vacuum pump to extract air from the cavity after molding and before injection, making the cavity near vacuum. This completely eliminates gas resistance, not only solving the problem of trapped air, but also reducing injection pressure, improving liquidity of molten fluid, and reducing internal stress. Vacuum systems require the design of dedicated seal slots (embedded with high-temperature seal strips) and vent channels on the molding surface to ensure isolation of the post-mold cavity from the outside world. The vent channels also require precise control of gaps to prevent molten fluid from entering.
The design of the exhaust system also needs to consider maintenance and cleaning. During the production process, tiny amounts of molten or decomposing material gradually accumulate in the exhaust vents, reducing exhaust efficiency. Therefore, the location of the exhaust vents should be designed in areas that are easy to clean, or additional cleaning channels should be opened on the mold. Regular cleaning and inspection of the exhaust system is necessary maintenance work to ensure its long-term effectiveness.
In short, the design of the mold exhaust system is a delicate engineering project that opens a “escape path” for gases at a micro-scale. It requires designers to deeply understand material flow behavior, thermodynamics, and micrometer-level spacing control, and through the combination of system schemes (forming surface exhaust, tile exhaust, ventilated steel, vacuum exhaust) to ensure that gases can be smoothly expelled from every shape cavity and every corner, thereby laying the foundation for achieving high-quality finished products.
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