Injection molded black stripes (black lines)
Injection molding black streaks (black lines) are dark streaks or linear defects that appear on the surface of plastic parts. They are usually distributed along the direction of melt flow, seriously affecting the appearance of the product and are particularly noticeable in light-colored or transparent plastic parts. This defect is common in the molding process of plastics such as ABS, PC/ABS alloys, and PMMA. For example, the scrap rate of a certain white ABS home appliance casing due to black streaks is as high as 20%, resulting in significant economic losses. The causes of injection molding black streaks are complex and are closely related to raw material contamination, melt degradation, mold structural defects, or improper process parameters. Systematic investigation is required to determine the root cause. The essence of black streaks is the presence of high-temperature decomposition products, impurity particles, or unmelted material in the melt, which are stretched during the molding process to form linear marks. Solving this problem requires multiple aspects, including raw material control, process optimization, and mold improvement, to effectively eliminate the defects.
Contamination or poor raw material properties are common causes of black streaks during injection molding, including insufficient raw material purity, contamination with foreign matter, or uneven dispersion of additives. Excessive proportions of recycled material can easily introduce impurities. For example, one company mixed 50% ABS recycled material with virgin material. Due to impurities such as dust and metal debris in the recycled material, noticeable black streaks appeared in the molded parts. Reducing the recycled material proportion to 20% and enhancing filtration virtually eliminated the black streak defect. If additives (such as antioxidants and lubricants) in the raw materials are unevenly dispersed, they can aggregate and decompose locally at high temperatures, forming black streaks. For example, when zinc stearate exceeds 0.5% in PP material, carbon deposits can form in the barrel, which then enters the mold cavity with the melt, creating black streaks. In addition, excessive moisture content in raw materials can lead to high-temperature hydrolysis, especially for hygroscopic plastics such as PC and PA. Water evaporates in the barrel and reacts with the melt to produce degradation products. For example, when the moisture content of PC exceeds 0.02%, black silver streaks will appear after molding, and the moisture content needs to be controlled by drying at 80-120°C for 4-6 hours.
Excessive melt degradation within the barrel or runner is the core factor causing black streaks, primarily due to excessive temperatures, prolonged residence time, or excessive shear. When the barrel temperature exceeds the plastic’s thermal stability limit, the molecular chains break and carbonize. For example, the optimal processing temperature for ABS is 220-240 °C. If it rises above 260 °C and the residence time exceeds 5 minutes, degradation will occur, producing black carbides that flow with the melt, forming black streaks. Excessive injection speed or a small runner diameter can cause intense shear, leading to a sudden localized temperature rise in the melt (shear heating) and thermal degradation. For example, a PC part produced using a high-speed injection speed of 60 mm/s and a runner diameter of only 4 mm , with a shear rate of 10⁵s⁻¹ , caused the melt temperature to locally exceed 350 °C, resulting in noticeable black streaks. Reducing the injection speed to 30 mm/s and enlarging the runner to 6 mm eliminated the black streaks. Too high holding pressure will also aggravate degradation. Excessive pressure will prolong the residence time of the melt in the flow channel. It is recommended that the holding pressure should not exceed 70% of the injection pressure, and the holding time should be controlled within 50% of the cooling time.
Mold structural design flaws can exacerbate black streaks, primarily manifesting in poor runner design, poor venting, or rough cavity surfaces. Right-angle turns, sudden contractions, or expansions within the runner can lead to poor melt flow, creating stagnant areas. Prolonged melt retention in stagnant areas can lead to degradation. For example, a mold with a right-angle transition between the main and branch runners caused stagnant flow in the corners, resulting in black carbon deposits. Changing to a rounded transition (R ≥ 3mm) eliminated the stagnant flow. An undersized gate increases shear forces. For example, a 1mm diameter gate used for ABS parts can cause overheating of the melt during shear flow, resulting in black streaks. Increasing the gate diameter to 2mm resolved the issue. When the cavity surface roughness Ra exceeds 1.6μm, melt flow resistance increases, making eddies and stagnation more likely to form in rough areas. It is recommended that the cavity be polished to Ra 0.8μm or less, especially for high-viscosity plastics (such as PC and PMMA), where polishing to Ra 0.4μm is recommended. In addition, poor mold venting can lead to local overheating and decomposition of the melt. For example, if there is no venting groove deep in the cavity, gas compression will generate high temperature, causing the melt to carbonize and produce black stripes. A venting groove with a depth of 0.01-0.02mm needs to be set in the last filling area.
Addressing black streaks in injection molding requires a comprehensive approach involving raw material control, process optimization, and mold improvements. Regarding raw materials, strictly control the proportion of recycled material (≤30%) and use a feeding device with magnetic separation and filtration to remove metallic impurities. Hygroscopic plastics must be thoroughly dried to ensure the moisture content meets the specified standard. Optimizing process parameters is key. Reduce barrel temperature by 5-10°C, shorten the melt residence time in the barrel (<5 minutes), and employ segmented injection (low-medium speed) to reduce shear. For easily degradable plastics, add 0.1%-0.3% antioxidant to inhibit degradation. Regarding molds, optimize runner design, employ streamlined transitions, enlarge gate and runner dimensions to reduce shear, improve cavity surface finish, and incorporate venting grooves. Regularly clean carbon deposits in the barrel and runners, performing thorough cleaning every 1,000 molds or when changing raw materials. Through these measures, one company's black streak defect rate for ABS parts has dropped from 15% to below 0.5%, significantly improving product quality and production efficiency.
