Jetting marks are common flow-related defects in injection molded products, appearing as snake-like, wavy streaks or irregular lines on the surface, which seriously affect the appearance quality of products and may even reduce structural performance. This defect is particularly prone to occur in thin-walled products, long-flow parts and products with small gate sizes, often accompanied by uneven surface gloss, weak weld lines and other problems. Understanding the root causes of jetting marks and implementing targeted solutions is essential to improve product qualification rate and ensure stable production. This article systematically analyzes the main factors leading to jetting marks and provides practical elimination measures for on-site injection molding operations.
1. Fundamental Causes of Jetting MarksThe core mechanism of jetting marks lies in the unstable filling state of the melt during injection molding. When high-speed melt is ejected from the gate into the mold cavity, if it does not fully contact the cavity wall and instead shoots forward in a continuous jet, the melt cools and solidifies in advance, while the subsequent melt continues to fill, forming irregular overlapping traces on the surface. This unstable flow is mainly affected by gate design, process parameters, material properties and mold temperature. Small gate size, high injection speed, low mold temperature and high melt viscosity are all common factors that trigger jetting marks.
2. Mold Structure Optimization Measures
Reasonable mold design is the fundamental solution to eliminate jetting marks. Enlarging the gate size or adopting fan-shaped, diaphragm-shaped and other large-area gate structures can effectively disperse the high-speed melt flow and promote laminar flow filling. The gate position should be set at the thick wall or non-appearance surface of the product to avoid direct impact of high-speed melt on the appearance surface. Adding a buffer well or runner expansion section at the front end of the gate can reduce the flow rate of the melt and stabilize the filling state. The mold cavity surface should be polished to reduce flow resistance, and the exhaust system should be optimized to ensure smooth discharge of air and volatiles during filling, avoiding secondary flow interference caused by trapped air.
3. Process Parameter Adjustment StrategiesAdjusting injection molding parameters is the most direct and flexible method to improve jetting marks. Reducing the injection speed, especially the initial stage injection speed, can slow down the melt flow rate and promote laminar flow filling. Setting multi-stage injection with low speed at the beginning and gradually increasing speed can effectively avoid jetting. Increasing the melt temperature and mold temperature can reduce the viscosity of the melt, improve the fluidity, and promote the melt to contact the cavity wall quickly to form a stable flow front. Appropriate adjustment of the holding pressure and holding time can reduce the residual stress caused by unstable filling and eliminate the appearance of secondary flow marks.
4. Material and Environmental Factors ControlDifferent plastic materials have different sensitivity to jetting marks. Materials with high viscosity, poor fluidity or high molecular weight are more prone to jetting, so the melt temperature and injection speed need to be adjusted appropriately according to the material characteristics. For filled materials such as glass fiber reinforced plastics, the shear thinning effect of the melt should be considered, and the initial injection speed should be controlled more strictly. Ensuring the drying quality of raw materials, especially hygroscopic materials such as PA and PC, can avoid the formation of gas flow interference caused by moisture decomposition, which indirectly triggers jetting marks.
5. Common Misunderstandings and Avoidance
In actual production, many operators simply increase the injection speed to solve the problem of short shots, which will exacerbate jetting marks. The correct idea is to combine mold structure optimization with process parameter adjustment, starting from stabilizing the filling flow state. Another common mistake is ignoring the impact of mold temperature on flow stability. Simply adjusting the injection speed without optimizing the mold temperature cannot fundamentally eliminate jetting marks. In addition, excessive back pressure will also increase the shear heat of the melt, leading to unstable flow and aggravating the defect.
SummaryJetting marks are typical flow-related defects in injection molding, and their elimination requires comprehensive consideration of mold structure, process parameters, material properties and other factors. By optimizing the gate design to stabilize the filling flow, adjusting the injection speed and temperature parameters to improve the melt fluidity, and strictly controlling the material quality and mold exhaust, the problem of jetting marks can be effectively solved. Reasonable mold design is the foundation, and flexible process adjustment is the key. Only by combining the two can the filling state be stabilized, the product appearance quality be improved, and stable mass production be realized.
