Injection molding tiger stripes and solutions
Injection molded tiger stripes are alternating light and dark stripes resembling tiger fur that appear on the surface of a plastic part. They often appear on large surfaces or thick-walled areas of plastics such as ABS, PC/ABS, and HIPS, severely impacting the product’s appearance quality. This defect typically occurs along the direction of melt flow, with stripes spaced at varying intervals (0.5-5mm), creating a noticeable visual difference under light. For example, the appearance qualification rate of a black ABS television housing dropped from 90% to 65% due to tiger stripes, resulting in extensive rework. The essence of injection molded tiger stripes is differences in molecular orientation caused by shear rate fluctuations during melt flow, or density changes caused by temperature fluctuations. It is closely related to the plastic’s properties, process parameters, and mold structure. Addressing tiger stripes requires systematic measures to eliminate the stripe defects, including adjusting melt fluidity, stabilizing shear conditions, and optimizing mold cooling.
The melt flow characteristics of a plastic are an inherent factor in the formation of tiger stripes. Plastics with a low melt flow rate (MFR) or high viscosity are more prone to streaking. Plastics with an excessively low MFR (such as ABS with an MFR less than 15g/10min) experience high shear stress during flow, which can lead to unstable flow and the formation of tiger stripes. For example, a company using ABS with an MFR of 10g/10min to produce thick-walled plastic parts experienced severe tiger stripes. Switching to a grade with an MFR of 20g/10min significantly reduced the streaking. Uneven dispersion of toughening agents or fillers in the plastic can also exacerbate tiger stripes. For example, poor dispersion of the ABS phase in a PC/ABS alloy can lead to uneven melt flow. Therefore, it is important to select a well-mixed modified material or pre-mix it before processing. Furthermore, if the plastic’s thermal stability is insufficient, temperature fluctuations can cause changes in melt viscosity, resulting in streaking. It is recommended to select grades with good thermal stability, such as ABS with a high antioxidant content, to reduce viscosity fluctuations caused by degradation.
Improper process parameter settings are the primary cause of tiger stripes, particularly the control accuracy of injection speed, melt temperature, and mold temperature. Excessively high or unstable injection speeds can lead to dramatic fluctuations in the melt shear rate, resulting in periodic orientation differences and the formation of light and dark streaks. For example, an ABS mold part exhibited pronounced tiger stripes when injected at a constant high speed of 50 mm/s. However, the streaks disappeared after switching to a medium injection speed of 20-30 mm/s. Excessively low melt temperature increases viscosity and flow resistance, leading to an unstable flow front. For example, PC melt temperatures below 280°C are prone to fluctuating flow. Raising the temperature to 290-310°C while ensuring uniform temperature across the barrel (with a deviation of less than ±5°C) is crucial. Excessively low mold temperatures can cause the melt to solidify rapidly on the cavity surface, forming a hard crust. Continued flow within the mold causes shearing, leading to surface streaks. For example, ABS mold temperatures below 50°C are prone to tiger stripes. Raising the temperature to 60-70°C and ensuring uniform temperature across the barrel (with a deviation of less than ±3°C) can effectively improve this condition.
The impact of mold structure design on tiger stripes is primarily reflected in runner and gate design, cavity venting, and cooling system distribution. Improper gate placement can result in an excessively long or uneven melt flow path. For example, large plastic parts using a single gate are prone to streaking after the melt flows over 300mm. Switching to a multi-point gate (spacing ≤200mm) results in more stable flow. A gate that is too small increases the shear rate. For example, a side gate with a width of less than 5mm used in ABS plastic parts is prone to shear fluctuations. Increasing the gate to 8-10mm can reduce streaking. Poor cavity venting can obstruct the melt front and generate pressure fluctuations. For example, if venting grooves are not provided in the corners of deep-cavity plastic parts, gas compression can cause unstable melt flow. Venting grooves 0.01-0.02mm deep are required in the final fill area. Uneven distribution of the cooling system can lead to temperature differences on the cavity surface, forming stripes. For example, if there are not enough cooling water channels in the thick-walled area, the temperature will be too high, forming a temperature difference with the thin-walled area. It is necessary to ensure that the cooling water channels are 15-25mm away from the cavity surface and 30-50mm apart to ensure uniform temperature.
Addressing tiger stripe in injection molding requires a coordinated approach of raw material selection, process optimization, and mold improvement to gradually eliminate the stripe defect. Regarding raw materials, high MFR and thermally stable plastics are preferred, the proportion of recycled material is controlled (≤20%), and even dispersion of additives is ensured. Process adjustment is key. Adopt a medium-to-low injection speed (20-40 mm/s) to maintain a stable speed; increase the melt temperature to the upper limit of the processing range to ensure uniform barrel temperature; increase the mold temperature and optimize cooling to achieve a consistent cavity surface temperature. Regarding the mold, optimize gate location and size, using multi-point gates to shorten flow distance; add venting slots to ensure smooth gas discharge; and improve the cooling system by using conformal water channels or increasing the number of channels to improve temperature uniformity. Furthermore, regularly clean the barrel and runners to prevent carbon deposits that affect melt flow. For minor tiger stripe that has already occurred, flame treatment or sandblasting can be used to reduce the visual effect, but they cannot completely eliminate it. Through comprehensive optimization, the tiger stripe defect rate in ABS plastic parts for a home appliance manufacturer has dropped from 25% to below 2%, significantly improving the appearance quality.
