SPI Tooling Classifications in Micro Injection Molding

In the realm of micro injection molding, achieving precise and consistent results hinges upon various factors, with tooling being of paramount importance. Tooling classification is a critical aspect, guiding manufacturers in selecting the appropriate materials and design specifications for their molds. The Society of the Plastics Industry (SPI) classification system serves as a standard reference for categorizing injection molding tools, providing insights into their characteristics and capabilities. 

Introduction to SPI Tooling Classifications: 

The SPI tooling classification system categorizes injection molding tools based on their surface finish, draft angles, and other key features. This system, widely adopted in the industry, enables clear communication between manufacturers, mold designers, and molders, ensuring that everyone involved understands the requirements and expectations for tooling. 

Range of SPI Classifications: 

SPI classification spans from Class 101 to Class 105, each denoting specific criteria regarding tooling quality and performance. Let’s delve into the characteristics of each class: 

  • Class 101: This classification represents the highest level of precision, quality, and longevity in injection molding tooling. Class 101 molds are meticulously crafted with tight tolerances, exceptional surface finishes, and long mold life expectancy. They are typically used for high-volume production of intricate and demanding components, such as those found in medical devices and electronics. Their primary distinction from Class 102 is the inclusion of features enabling very fast cycle times and/or minimal waste material such as hot runner / gates. 
  • Class 102: Class 102 molds maintains the high standards and longevity of a Class 101 tool, but may have less efficient cold runners / gates, resulting in longer cycle times, runner waste, and secondary degating. They are suitable for applications requiring excellent surface finishes, dimensional accuracy, and longevity, but may not justify the costs that the optimization of a Class 101 tool provides. Class 102 molds can serve as an excellent low cavity stepping stone to a future Class 101 tool.
  • Class 103: Class 103 molds offer good quality and durability, making them suitable for moderate-volume production with less stringent requirements than Class 101 or 102 molds. These are helpful to think of as hybrid tools, bridging the space between low and high volume production. They utilize a mix of hardened and unhardened components, targeting high durability on the most critical components, while allowing for cost savings on the less critical. They provide reliable performance for a range of applications.
  • Class 104: Class 104 molds are designed for low to moderate-volume production of simpler parts. Construction is typically of unhardened steel, providing finishes and part quality of higher classifications, but with less longevity. They offer adequate quality and durability for less demanding applications.
  • Class 105: Class 105 molds are the most economical option, typically used for prototyping, short runs, or low-volume production. Typically utilizing unhardened steel or mold grade aluminum, these molds still offer the same potential for high quality parts, but with significantly shorter lifetime. They serve as excellent vehicles for testing or iterating part designs for injection molding or fulfilling development milestones prior to moving to more durable tooling. 


SPI classification serves as a valuable framework for understanding the quality and capabilities of injection molding tooling, including in the context of micro injection molding. By adhering to SPI standards and selecting the appropriate class of tooling for their specific needs, manufacturers can ensure the success of their micro molding projects, delivering precise and reliable components for a wide range of applications. 

In summary, SPI classification is not merely a set of numbers; it’s a guide that empowers manufacturers to make informed decisions about their tooling requirements, ultimately contributing to the success of their micro injection molding endeavors. 

SPI Classification and Micro Injection Molding: 

In micro injection molding, where precision and repeatability are important, SPI classification guides the selection and design of tooling to meet the unique requirements of small-scale, intricate components. Even in the micro world, the principles of SPI classification hold true, with Class 101 and Class 102 molds often being preferred for their ability to maintain tight tolerances and exceptional surface finishes, crucial for achieving the desired part quality.