Air marks (shadows) and their solutions
Air marks (or shadows) are common cosmetic defects in injection molding. They appear as cloud-like streaks or dark shadows on the surface of the product, aligned with the direction of melt flow. These streaks severely degrade the product’s visual quality, and are particularly noticeable in light-colored or transparent products. Air marks are closely related to the flow of gas within the melt and the contact between the melt and the cavity surface. They typically appear during the initial melt filling phase or during gas-assisted molding, primarily due to factors such as gas entrapment, uneven melt cooling, or mold surface contamination. If not promptly addressed, air marks can lead to substandard product appearance, increased rework rates, and increased production costs. Therefore, effective preventive and remedial measures addressing their causes are crucial.
Gas entrapment in the melt is a major cause of gas streaks, typically occurring when the melt is filling the cavity at high speed or when the mold is poorly vented. When the melt enters the mold cavity at excessively high speed, turbulence forms near the gate, entraining air or residual gas within the mold cavity. This compressed gas mixes with the melt, forming bubbles or cloud-like streaks on the part surface. For example, when injecting thin-walled PE parts, if the injection speed exceeds 100 mm/s, the melt will “jet” at the gate, entraining large amounts of air, which, upon cooling, results in noticeable gas streaks. Poor mold venting is also a significant factor in gas entrapment. Air trapped in the cavity cannot escape promptly and becomes trapped by the melt, forming air pockets. Excessive shear stress in the melt surrounding these pockets causes localized overheating, resulting in color differences from other areas, manifesting as shadows. Furthermore, volatiles in the raw materials (such as insufficiently dried water and decomposition products of additives) vaporize at high temperatures and mix with the melt, causing gas streaks. This is particularly true when processing hygroscopic plastics such as PA and PC, where incomplete drying significantly increases the incidence of gas streaks.
Improper control of melt and mold temperatures is a key process factor in causing gas lines. When the melt temperature is too low, the melt has poor fluidity, which can lead to stagnation when filling the cavity. Upon contact with the cavity surface, it rapidly cools, forming a solidified film. Subsequent pushes of the melt push this film forward, causing wrinkles and streaks similar to gas lines. For example, when the PP melt temperature is below 180°C, filling is slow, and the temperature difference between the solidified layer on the cavity surface and the melt inside is too large, easily resulting in noticeable flow lines. Excessively low mold temperatures exacerbate this phenomenon, especially near the gate of the cavity. The low temperature causes the melt to cool rapidly, preventing a close contact with the cavity surface, resulting in uneven light reflection and shadows. Conversely, when the melt temperature is too high, the raw materials are prone to thermal decomposition, producing gases and decomposition products. These gases, when mixed with the melt, form dark gas lines. For example, when the temperature of ABS exceeds 250°C, the gases produced by the decomposition of the rubber phase can easily cause black streaks on the surface.
Improper mold design can also lead to air marks, particularly due to the gate location, shape, and cavity surface quality. Improper gate placement, such as facing the cavity wall or too far from the cavity end, can cause the melt to impact and bend during filling, creating vortices that entrain air. For example, in box-shaped products, if the gate is placed in the center of the side, the melt will impact the opposite cavity wall and bend back, easily entraining air when mixing with the subsequent melt, resulting in air marks. An undersized gate can lead to excessive shear rates during melt flow, generating significant heat and causing localized melt decomposition. Furthermore, high-speed melt flow can easily entrain air pockets. Highly rough cavity surfaces, scratches, or oil stains can increase friction and create turbulence during melt flow. The rough surface can also trap air, forming air marks. Furthermore, right-angled corners of the cavity can generate vortices as the melt flows through, creating high-risk areas for air marks.
Solving the problem of air marks requires a comprehensive approach involving process parameter optimization, mold improvement, and raw material processing. Regarding process optimization, the injection speed should first be adjusted, employing a “slow-fast-slow” staged injection method. This involves low injection speed near the gate (to avoid jetting and air entrainment), high filling speed in the middle (to improve efficiency), and low speed holding pressure near fullness (to reduce shear). For example, when processing ABS products, reducing the initial injection speed from 80 mm/s to 40 mm/s can effectively reduce gate air marks. Secondly, appropriately increasing the melt and mold temperatures can improve melt fluidity and reduce the formation of a solidified layer. For example, raising the PC melt temperature to 280-300°C and controlling the mold temperature between 80-100°C can improve melt-cavity surface adhesion and eliminate shadowing. Furthermore, appropriately increasing the back pressure (for example, from 5 bar to 10 bar) can expel air from the barrel and reduce the amount of air entrained into the cavity.
Mold improvements require optimizing gate design and adjusting the gate location to areas with smooth melt flow, such as thicker parts or away from corners. Enlarging the gate size (e.g., increasing the diameter of a round gate from 1mm to 1.5mm) can reduce melt shear rates. Using fan gates or film gates instead of point gates can ensure even melt distribution. Mold venting capacity should be enhanced by adding venting grooves at the final melt fill point and corners, with a depth of 0.02-0.05mm (adjusted based on the plastic’s fluidity) to ensure smooth gas discharge. Polish the cavity surface to a Ra of less than 0.4μm, remove scratches and oil stains, and reduce melt flow resistance and gas entrapment. Regarding raw material processing, hygroscopic plastics (such as PA and PC) should be thoroughly dried. PA should be dried at 80-100°C for 4-6 hours, and PC at 120-140°C for 6-8 hours, to reduce the moisture content to below 0.02% and prevent the generation of volatiles. In addition, reduce the proportion of recycled materials in the raw materials, and the amount of recycled materials added should not exceed 20% to prevent excessive impurities and volatiles from causing gas marks.
For gas marks that appear during gas-assisted molding, the gas parameters need to be adjusted specifically. Injecting gas too early can lead to an unstable interface between the gas and the melt, forming cloud-like streaks. The gas injection time should be delayed until the melt is filled to more than 70%. Excessive gas pressure can cause the gas to break through the surface of the melt, forming bubbles and gas marks. The initial gas pressure needs to be lowered (for example, from 25MPa to 18MPa), and a step-by-step pressure reduction and maintenance method should be used. Check the clearance between the gas needle and the gate to ensure uniform gas diffusion and avoid gas marks caused by excessive local airflow. During the mold trial stage, tape can be applied to the cavity surface corresponding to the gas mark area of the product to observe the changes in the gas mark to determine the problem area, and then optimize the exhaust or adjust the process in a targeted manner. The above comprehensive measures can effectively eliminate gas mark defects and improve the appearance quality of the product.
