Part manufacturers are challenged with juggling a multitude of requirements. They need to design and manufacture parts that perform well in the application, last a pre-determined lifespan without safety risks and optimize processing cycles to reduce time and cost.

Finding the right balance between these requirements has caused many manufacturers to avoid secondary processing steps, such as annealing, yet the benefits of annealing on part performance and lifespan can outweigh the extra effort. Annealing can result in a significant improvement on a parts performance.

In this blog we dive a little deeper into the annealing process, its benefits and attention points.

Understanding the annealing process

In the injection molding process, our high-temperature polyamide Stanyl demonstrates a rapid solidification. While this is beneficial for fast processing cycle times, it reaches its normal crystallinity level of 70% very fast, leaving the rest of the material to have a relatively low density (amorphous phase). The density of this amorphous phase can be increased using annealing.

Annealing is heat treatment of the injection molded part, decreasing moisture absorption while increasing mechanical properties. The heat treatment is irreversible and has been studied and modeled over a wide time-temperature range.

When a part is run through a standard or long-term annealing process at 230°C in nitrogen, modulus and strength of unfilled Stanyl increase significantly. This is especially evident above the glass transition temperature.

The process also reduces variability of parts over time (in operation), meaning that “not annealed” parts shrink more during operation and moisture uptake of the material decreases.

Figure 1: Effects of annealing on material properties above Tg

Also, annealing causes a substantial increase in the material’s heat deflection temperature (HDT), meaning that annealed parts can survive higher short-term temperature peaks under load. Additional advantages of annealing Stanyl include an improved salt spray resistance, also known as the Denso CaCl2 test.

A side effect of annealing can be a reduced elongation at break and impact behavior. Shrinkage upon annealing also occurs, which can be taken into account during the part design. Therefore, we often recommend long-term annealing in nitrogen over air.

Air annealing results in greater discoloration effects. For standard annealing, it is still possible to conduct the annealing process in air when some discoloration to the part is acceptable. For longer annealing times it is strongly advised to be conducted under nitrogen.

Annealing can be an interesting option to increase a part’s performance, reduce the moisture uptake and reduce the dimensional stability of the part. These things can increase the part’s lifespan.

Talk to us today to discuss if annealing is suitable to improve your part’s performance as well. Contact us for product inquiries or sample requests. 

Frank van der Burgt, PhD

Secondary technology expert at Envalior

Published on

06 June 2019


  • Blog
  • Material Processing
  • Stanyl
  • Processing & shaping
  • Cost reduction & simplification

Effect of annealing on material properties

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Frank van der Burgt, PhD

Secondary technology expert at Envalior

Frank is part of the advanced engineering group within Envalior, Research & Technology. He joined Envalior in 2006 and held positions in polymer characterization and research and development. Frank received his PhD in 2002 on the topic of polymer crystallization in the Chemical Technology department of the Eindhoven University of Technology.

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