Injection Molding Defects and Remedies
Injection molding is one of the most popular manufacturing processes used to produce plastic parts and components. Despite its popularity and its affordability, injection molding can be susceptible to defects. Here are some of the most common defects and remedies to help ensure the successful production of parts.
Flash
Flash is the undesired overflow of melted resin along the parting lines or other areas of the mold. This is the result of the pressures of the injection molding cycle forcing plastic to escape. Improper mold design, inadequate clamping forces, or incorrect injection speed can lead to flash. Cleaning of the mold, selecting the right material properties, and improving the injection molding process can help reduce flash.
Weld Lines
Weld lines appear as visible lines on the surface of the part and present two surfaces that were once connected but were later separated by the molding process. Improper filling can cause weld lines. To reduce weld lines, proper gate location and design should be considered. Additionally, reducing cycle times and part wall thickness can help decrease weld lines.
Short shots
Short shots occur when the plastic resin does not completely fill the entirety of the mold cavity. The result is an incomplete part. This defect can be caused by incorrect temperature and viscosity, lack of material feed, or an unevenly distributed gate. Ensuring the mold cavity is completely filled with resin should be the main priority. To do this, appropriate temperature, pressure, and melt viscosity should be set and maintained. Regular maintenance and checking the mold condition can also help prevent short shots.
Burn Marks
Burn marks appear as dark marks or discoloration on the surface of a part. These marks are indicative that the plastic resin had been in contact with hot surfaces for an extended amount of time. This can often occur when the melt temperature is too high. To remedy this, it is important to control and maintain the mold temperature; doing so will help to reduce the temperature of the injected plastic.
Sink Marks
Sink marks appear as indentations on the outer surface of the part. This can occur when more plastic shrinks to the inside of the part than the outside of it. This is caused by the difference in cooling rates. Setting the mold temperature lower, reducing the wall thickness, and increasing the molding time can help reduce or eliminate sink marks.
Voids
Voids are empty spaces or hollows inside the part. This defect is typically caused by air that cannot escape the mold before the cycle is complete. To reduce or eliminate voids in the parts, design a draft angle on the part walls and make sure that the resin is properly funneled throughout the part.
Conclusion
Injection molding is an incredibly powerful manufacturing process with numerous advantages to producing plastic parts at an affordable rate. Despite its advantages, injection molding can be susceptible to numerous defects. Understanding the common injection molding defects, such as flash, weld lines, short shots, burn marks, sink marks, and voids, can help reduce the occurrence of defects during the process.
