Injection Molding Black Lines (Black Stripes) and Solutions
Injection molding black streaks are a common surface defect on plastic parts. These streaks appear as black lines or spots along the plastic’s flow direction, affecting not only the part’s appearance but also its mechanical properties. In applications requiring high aesthetic standards, such as automotive interiors and appliance housings, black streaks can lead to product scrappage. The causes of black streaks are complex, involving multiple factors, including raw materials, equipment, processes, and molds. These factors require systematic analysis and targeted solutions. For example, a company’s polypropylene cover developed noticeable black streaks after molding. Investigation revealed that impurities had been mixed into the raw materials, and that material was accumulated in the barrel, causing the plastic to decompose at high temperatures and produce a black substance.
Raw material issues are common causes of black streaks during injection molding, primarily including insufficient raw material purity, inappropriate additives, and moisture. Impurities such as dust, metal debris, and other plastic particles can be trapped by the hot, molten plastic during the molding process, resulting in black streaks. Additives such as antioxidants and lubricants, if added in excess or incompatible with the plastic, can decompose and carbonize at high temperatures, producing black streaks. Hygroscopic plastics such as polyamide and polycarbonate, if not fully dried, can vaporize moisture at high temperatures, reacting with the plastic to form a black substance. For example, a company used recycled polyethylene to produce plastic barrels. Due to the high level of impurities in the recycled material, the barrels were riddled with black streaks, forcing the entire batch to be downgraded. Solutions to address raw material issues include selecting high-purity virgin material and strictly controlling the proportion of recycled material (typically no more than 30%); adding additives in the specified proportion to ensure compatibility with the plastic; and thoroughly drying hygroscopic plastics to keep the moisture content below 0.02%.
Equipment factors significantly influence the formation of black streaks during injection molding, primarily involving the condition of components such as the barrel, screw, and nozzle. A damaged heating coil or malfunctioning temperature control system within the barrel can cause localized overheating, leading to overheating and decomposition of the plastic and resulting in black streaks. Excessive clearance between the screw and barrel can cause the plastic to reside excessively within the barrel, leading to degradation. Blockages or dead spots in the nozzle flow path can cause the plastic to stagnate and decompose there, resulting in a black substance that enters the mold cavity along with the plastic. For example, a damaged heating coil at the front of the barrel of an injection molding machine caused localized temperatures exceeding the plastic’s decomposition temperature, resulting in continuous black streaks on the surface of the produced parts. Solutions for equipment issues include: regularly inspecting the heating coil and temperature control system to ensure that the barrel temperature is within ±5°C of the set value; checking the screw-barrel clearance and replacing the screw or barrel if it exceeds 0.3mm; and regularly cleaning the nozzle and flow path to remove accumulated material and foreign matter to ensure smooth flow.
Improper process parameter settings, mainly including parameters such as temperature, pressure, and speed, are also a common cause of black lines in injection molding. Excessively high barrel temperatures can cause the plastic to overheat and decompose, while too low temperatures can lead to uneven plasticization of the plastic, with incompletely melted particles forming black lines in the mold cavity. Excessive injection pressure or speed can cause severe shearing of the plastic within the flow channel, generating a large amount of heat and causing localized overheating and decomposition. Excessive holding time or high back pressure can increase the residence time of the plastic within the barrel, causing it to degrade. For example, when a company was producing polyvinyl chloride pipe fittings, in order to increase the filling speed, it set the injection speed too high (120 mm/s), causing severe shearing of the plastic within the flow channel and the appearance of black lines. Methods to solve process parameter problems include: reasonably setting the barrel temperature according to the characteristics of the plastic, such as the barrel temperature of polyethylene is usually 180-220℃, and that of polyvinyl chloride is 160-190℃; optimizing the injection pressure and speed, and adopting a multi-stage injection process to avoid excessive shearing of the plastic; reasonably setting the holding time and back pressure, and the back pressure is generally controlled between 5-15MPa. The holding time should be appropriate to ensure that the plastic part is full and there are no sink marks.
Improper mold design can also lead to black streaks during injection molding, primarily involving structures such as runners, gates, and venting systems. A runner cross-section that is too small or too long increases flow resistance and shear forces on the plastic, leading to localized overheating and decomposition. Improper gate size can cause plastic to jet at the gate, resulting in black streaks. A poor venting system prevents air from escaping the cavity, mixing with the plastic and forming bubbles. These bubbles then burst and form black streaks. For example, a mold with an undersized gate can cause plastic to enter the cavity at high speed, generating significant shear heat that decomposes the plastic and results in radial black streaks on the surface of the part. Solutions to these mold issues include optimizing runner design, increasing runner cross-sections, shortening runner lengths, and using circular or trapezoidal cross-sections. Designing gate dimensions appropriately based on part size and plastic properties to avoid jetting is crucial. Adding venting slots, typically 0.02-0.05mm deep and 5-10mm wide, ensures smooth air evacuation from the cavity. These combined measures can effectively address the black streak problem and improve the appearance quality and yield of molded parts.
