Why do Injection Molds need vents? This is a very crucial question about injection molds. Fundamentally, vents in injection molds are needed to solve a simple but critical problem: when molten plastic is injected into the mold cavity at high speed, the air inside the cavity must have an "escape route." Without this route, the compressed air cannot escape, leading to a series of serious product quality problems.

Physics principle: Air always flows to some place.

Before Injection Molding, the mold cavity is filled with air. When molten plastic is injected under high pressure, this air needs to find an outlet. Plastic and air cannot coexist in the same place. Vent holes provide an outlet channel. Air is compressible, but its compressibility is limited. If the air cannot escape, the pressure inside the mold cavity will rise sharply. This trapped compressed air will hinder the flow of plastic.

What would happen if there were no vent?

Without a clear outlet, trapped air can cause several foreseeable problems:

Incomplete Injection: Due to excessive reaction force from residual air, the plastic cannot completely fill the mold cavity, resulting in incomplete part molding.

Burning Marks: When plastic flows in and Rapidly compresses air, the air temperature rises sharply (similar to the operation of a diesel engine). This can scorch the plastic, leaving black or brown marks on the part, usually appearing in the last filled area.

Voids: Air bubbles can become trapped inside the plastic part, forming internal voids, weakening the structure and affecting the appearance.

Poor Surface Finish: Trapped air affects the adhesion between the plastic and the mold surface, leading to surface defects.

Visible and Weak Weld Lines: When two streams of molten plastic merge, if gas is trapped at that point, it will hinder their complete fusion, resulting in very visible and weak weld lines, making the product prone to breakage at this point.

Whitening or Deformation During Ejection: Gas trapped in the cavity acts like a flexible "spring," hindering effective pressure transmission during the holding pressure stage, leading to uneven material density and increased internal stress, making the part prone to whitening or deformation during ejection.

How can the exhaust vents be designed?

Size: The depth of the vents must be sufficient to allow air (a low-viscosity fluid) to escape easily, but shallow enough to prevent the outflow of higher-viscosity molten plastic. The ideal depth depends largely on the type of plastic used. Typical depths range from 0.005 mm to 0.05 mm (0.0002 inches to 0.002 inches).

Width and Length: Wider and shorter vents allow air to escape more quickly. They require sufficient cross-sectional area.

Location: Vents must be placed at the end of the flow path and in any areas where air might be trapped, such as reinforcing ribs or bosses. Accurately determining these locations during the mold design phase is crucial. Simulation software can help, but experience also plays a significant role.

Maintenance of the vent:

Vent holes are very shallow channels that easily become clogged over time. Plastic residue, mold release agents, dust, or volatile deposits can accumulate, blocking the air venting channels. A clogged vent hole is essentially no vent at all. Regular mold cleaning and maintenance are crucial to keeping vent holes clear and functioning properly. Incorporating vent hole maintenance into your regular mold maintenance plan is the most effective way to ensure stable production and save on the high costs associated with downtime for cleaning and production waste.

Summary

Without vents, the injection molding process is virtually impossible. Even if molding is achieved, the product will inevitably be defective or scrap. The venting system is a crucial and indispensable part of mold design. One of the core skills of an excellent mold engineer is accurately predicting where gas might get trapped and designing an efficient venting system that doesn't produce flash.

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