Mold Defect Analysis: Underfill and Flash
Mold defects are common in injection molding and can have a significant impact on the quality of a product. When parts do not fill out completely, it is known as underfilling, and when excess material extends past the mold edge, it is called flash. Both of these defects can occur due to a variety of reasons and can be difficult to identify and fix. This article will discuss the causes, identification, and solutions to these common injection molding issues.
Underfill
Underfill occurs when the plastic material does not completely fill the cavity of the mold. This can result in a significant decrease in material properties and strength. The most common causes of underfill include inadequate injection pressure, excessive mold temperature, and insufficient cooling time.
Inadequate injection pressure can be caused by a variety of problems, including insufficient clamping force and weak injection packing. Excessive mold temperature can also lead to underfill, as the plastic material will melt more quickly and not fill the mold in time before it begins to cool and set. Lastly, insufficient cooling time can also result in underfill as the material may not be given enough time to cool before being ejected from the mold.
Identifying and Troubleshooting Underfill
Underfill can be identified visually and by measuring the density of the finished part. High level of porousity or low weight indicate weak bonds between the material molecules, which is a telltale sign of underfill. Standard process parameters should be checked first to determine if they are in the proper range. If they are within the proper range, then the mold itself should be inspected for any issues such as worn-out parts or improper clearances. Additionally, the melt temperature should be measured during the injection process as this can also affect the amount of fill of the part.
Flash
Flash occurs when excess plastic material is forced out of the mold due to too much injection pressure, incorrect injection speed, or too little linear draft. Flash is problematic, as the excess material can interfere with the functions of the part or create a source of contamination, which can negatively affect the part’s performance. Additionally, flash can be difficult and time consuming to remove from parts.
Identifying and Troubleshooting Flash
Flash is easily identifiable by sight, as the excess material extends past the edges of the molded part. The root cause of flash can also be found by examining the mold itself. If the injection speed is too fast, then this can cause the material to be forced out of the mold before it has cooled and set, thus creating flash. Additionally, improper clearances or undersized molding gates can also contribute to flash, as the material may be forced out of the mold prematurely. Correcting the process parameters, such as injection speed, clamping force and linear draft, can help reduce or eliminate flash.
Conclusion
Underfill and flash are two common defects in injection molding. While these issues can be difficult to diagnose and fix, understanding their root causes and proper identification techniques can help reduce their impact and achieve better quality parts.
