In injection molding, gate air marks on PC parts have long been a major headache for many manufacturers. They not only affect the product’s appearance but can also weaken its structural performance. Gate air marks typically appear as cloud-like or silvery streaks near the gate. In severe cases, they can even form visible air bubbles. Addressing this issue requires a thorough analysis of its root causes, encompassing a comprehensive investigation across multiple dimensions, including raw material characteristics, equipment parameter settings, and mold design.
From the perspective of raw material properties, the characteristics of PC material itself are one of the key factors causing gate air marks. PC material has a high melt viscosity and relatively poor fluidity, requiring high temperatures and pressures during the injection molding process to successfully fill the mold cavity. When the molten PC material enters the mold cavity through the gate, if the melt temperature is uneven or the raw material contains trace amounts of moisture, air marks are likely to form near the gate. Especially when the raw material is not fully dried, the moisture will vaporize under high temperature and pressure, forming bubbles. These bubbles are stretched during the melt flow, ultimately showing up as obvious air marks near the gate. In addition, PC material is shear-sensitive. Excessively high shear rates can cause local overheating and decomposition of the melt, generating gas, further exacerbating the air mark problem.
Improper equipment parameter settings are also a key cause of gate air marks on PC parts. The matching of the injection molding machine’s parameters, such as injection speed, pressure, hold time, and barrel temperature, directly affects the melt’s flow state. If the injection speed is too fast, the melt will experience severe shearing as it passes through the gate, causing a sudden local temperature rise, triggering melt decomposition and generating gas. If the injection speed is too slow, the melt may cool too quickly during the filling process, forming cold material near the gate and causing air marks. Furthermore, improper barrel temperature settings can also cause problems. Too low a temperature increases melt viscosity, hindering flow and making it easier for air to be entrained. Too high a temperature causes PC material decomposition, generating volatile gases. These gases cannot be discharged in a timely manner, forming air marks near the gate.
The rationality of mold design has a significant impact on the occurrence of gate air marks in PC parts. The shape, size, and position of the gate are key factors in mold design. If the gate size is too small, the melt will be subjected to strong shear during passage, causing localized temperature increases and decomposition. On the other hand, if the gate size is too large, the melt filling rate may be slowed down, cooling prematurely, and forming air marks. Inappropriate gate placement can also cause problems. For example, placing the gate in a thin-walled area can easily cause turbulence during melt filling, entraining air and forming air marks. An improperly designed mold exhaust system is also a major factor. If the exhaust is not smooth, the air in the cavity cannot be discharged in time, and will be compressed by the melt, forming bubbles, which will eventually appear as air marks near the gate.
In addition to the aforementioned factors, operational management during the production process can also contribute to the occurrence of sprue gas lines in PC parts. For example, an improper clearance between the injection molding machine’s nozzle and the mold’s sprue bushing can cause melt leakage during injection, entraining air and forming gas lines. Improper storage and handling of raw materials can also affect product quality. If PC materials absorb excessive moisture during storage and are not adequately dried before processing, moisture will be generated during the injection molding process, leading to gas lines. Furthermore, if residues in the barrel and nozzle are not promptly cleaned during production, these residues will decompose at high temperatures, generating gases that can also cause gas lines.
In summary, the occurrence of gate air marks in PC parts is the result of a complex interplay of factors, encompassing raw material properties, equipment parameter settings, mold design, and production operations management. Addressing this challenge requires optimization across multiple dimensions, such as selecting appropriate raw materials and thoroughly drying them, properly setting injection molding machine parameters, optimizing mold design to ensure good venting and gate structure, and strengthening operational management during production. Only through comprehensive and systematic analysis and improvement can the occurrence of gate air marks in PC parts be effectively reduced and product quality improved.
