Splay Marks

Splay marks are circular or streak-like patterns on the surface of the part caused by moisture or other contaminants present in the material.


Splay marks are a common defect encountered in injection molding, characterized by streaks, discoloration, or surface imperfections on the molded parts. These marks occur due to the presence of trapped moisture or volatile substances in the molten material, which vaporize and create voids or bubbles during the injection molding process. Splay marks not only affect the aesthetics but can also compromise the structural integrity and functionality of the final product. Understanding the causes of splay marks and implementing effective preventive measures are essential for manufacturers striving to produce high-quality molded parts. In this article, we delve into the intricacies of splay marks, identifying their root causes, and providing insights into minimizing this injection molding defect.

Causes of Splay Marks

Splay marks are primarily caused by the presence of moisture or volatile substances in the molten material, leading to the formation of voids or bubbles. The following factors contribute to the occurrence of splay marks:

  1. Moisture Contamination: Moisture can enter the material during storage, handling, or processing, leading to splay marks. Improper material drying or inadequate drying time can result in residual moisture in the pellets, which vaporizes and causes surface imperfections.
  2. Material Hygroscopicity: Certain materials, such as nylon, ABS, or polycarbonate, have a high affinity for moisture absorption, making them more susceptible to splay marks. These hygroscopic materials require careful handling and storage to prevent moisture contamination.
  3. Inadequate Material Drying: Inadequate drying of the material before the injection molding process can lead to moisture being present in the molten material, resulting in splay marks on the molded parts.
  4. Excessive Melt Temperature: High melt temperatures can cause the trapped moisture or volatile substances to vaporize quickly, leading to the formation of bubbles and splay marks.
  5. Poor Venting: Insufficient venting in the mold can trap air or gases released from the material during the injection process. The accumulation of these gases can create pressure differentials and result in splay marks on the surface of the part.


To minimize the occurrence of splay marks and achieve visually appealing molded parts, manufacturers can adopt the following preventive measures:

  1. Material Handling and Storage: Handle and store hygroscopic materials properly, following recommended guidelines. Store materials in a dry environment and seal them tightly to prevent moisture absorption.
  2. Material Drying: Ensure thorough and proper drying of hygroscopic materials before the injection molding process. Follow manufacturer recommendations for drying time, temperature, and moisture content to eliminate moisture-related issues.
  3. Mold Venting: Optimize mold venting to allow for the escape of air, gases, and vaporized moisture during injection. Adequate venting prevents the accumulation of pressure and minimizes the formation of splay marks.
  4. Melt Temperature Control: Maintain the appropriate melt temperature range for the material being processed to minimize the rapid vaporization of moisture or volatile substances. Monitor and adjust melt temperature accordingly to prevent splay marks.
  5. Purging and Cleaning: Regularly purge and clean the injection molding machine and equipment to remove any residual moisture, contaminants, or volatile substances that can contribute to splay marks.


Splay marks pose a significant challenge in injection molding, impacting the quality and appearance of molded parts. By understanding the causes of splay marks and implementing preventive measures, manufacturers can effectively minimize their occurrence. Proper material handling and drying, optimized mold venting, precise melt temperature control, and regular purging and cleaning all contribute to mitigating splay marks. By addressing this defect, manufacturers can produce visually appealing, structurally sound, and high-quality molded parts,