Short shots in injection molding occur when the injected plastic material does not completely fill the mold cavity. There are several reasons for this issue:

• Insufficient Resin Capacity
• Insufficient Pressure in the Mold Cavity
• Insufficient Resin Flowability
• Poor Venting

To counter the issue of short shots in injection molding, the following measures can be taken:

• Lengthen the injection time to prevent resin backflow before the gate solidifies, making it difficult to fill the mold cavity.
• Increase the injection speed.
• Raise the mold temperature.
• Increase the resin temperature.
• Increase the injection pressure.
• Enlarge the gate size. The height of the gate should generally be equal to 1/2 to 1/3 of the product wall thickness.
• Position the gate at the thickest part of the product wall.
• Incorporate venting slots (with an average depth of 0.03mm and a width of 3-5mm) or venting rods, which are particularly important for smaller parts.
• Leave a certain distance between the screw and the injection nozzle for cushioning.
• Select materials with lower viscosity grades.
• Add lubricants to improve resin flow.

The root cause of flow marks in injection molding is fundamentally similar to that of short shots, although the extent differs. As a result, the solution essentially follows the same approach as mentioned earlier. Particularly for resins with low fluidity, such as polyoxymethylene, PMMA resin, polycarbonate, and PP resin, it is crucial to focus on properly increasing the gate size and adjusting the injection time accordingly.

Sink marks in injection molding arise from the same cause as short shots. The general solution involves achieving excessive filling, but it carries the risk of inducing stress. Therefore, it is important to emphasize the need for uniform wall thickness during the design phase. Additionally, reducing the wall thickness in areas with reinforcing ribs, convex columns, and similar features should be prioritized wherever possible.

To address flash (burrs) in injection molding, the focus should primarily be on improving the mold. In terms of molding conditions, efforts can be made to reduce fluidity. The following methods can be employed:

• Reduce injection pressure.
• Lower the resin temperature.
• Choose materials with higher viscosity grades.
• Decrease the mold temperature.
• Polish the mold surface where the flash occurs.
• Utilize harder mold steel materials.
• Increase the clamping force.
• Adjust the precise mating surfaces of the mold.
• Add mold support pillars to enhance rigidity.
• Determine the appropriate size of exhaust slots based on the specific material.

Weld lines in injection molding occur when the molten resin from different directions cools and fails to fully fuse at the joining point. Typically, weld lines primarily impact the appearance of the product and can affect processes like coating and plating. In severe cases, they can also compromise the strength of the product, particularly in fiber-reinforced resins. The following measures can be taken to improve this issue:

• Adjust the molding conditions to enhance fluidity. This can be achieved by increasing the resin temperature, mold temperature, injection pressure, and speed.
• Incorporate additional venting slots. Placing ejector pins at the locations where weld lines occur can also help with proper venting.
• Minimize the use of mold release agents.
• Implement overflow wells as locations for weld line formation. These can be cut off and removed after molding.
• If the issue only affects appearance, consider changing the gate position to alter the location of the weld lines. Alternatively, the affected areas can be treated with a matte finish or other decorative techniques to mask the weld lines.

The hygroscopic nature of the material primarily causes silver streaks in injection molding. Therefore, it is generally recommended to dry the material at a temperature that is 10-15°C lower than the resin’s thermal deformation temperature. For PMMA resin with higher requirements, drying at around 75°C for 4-6 hours is necessary. Particularly when using an automatic drying hopper, it is essential to select an appropriate capacity based on the molding cycle (production volume) and drying time. Additionally, the material residence time should not be excessively long, as this can result in silver streaks. It is not advisable to mix different types of materials, such as polystyrene and ABS resin, AS resin, or polypropylene and polystyrene.

Jetting in injection molding refers to the serpentine-like marks that occur along the flow direction, starting from the gate. It is caused by excessively high injection speeds of the resin from the gate. To address jetting, several possible measures can be taken. One option is to increase the cross-sectional area of the gate or reduce the injection speed.

Additionally, raising the mold temperature can help slow down the cooling rate of the resin in contact with the cavity surface. This can effectively prevent the formation of surface hardening skin during the initial filling stage and yield positive results in mitigating jetting.

Whitening primarily occurs in the ejection area of ABS resin products. Poor demolding is the main cause of this issue. Several methods can be employed to address whitening, including reducing injection pressure, increasing the demolding angle, adding more ejector pins or enlarging their surface area, and reducing the roughness value of the mold surface. However, it is important to note that using a mold release agent is also an option, but care must be taken to avoid adverse effects on subsequent processes such as hot stamping or painting.