We would like to express our deepest gratitude to the authors of the article "Crystallization mechanism and mechanical properties of CF/PPS thermoplastic composites manufactured by laser-assisted automated fiber placement". The authors, Dacheng Zhao, Zhefu Li, Weiping Liu, Ting Li, Guangquan Yue, and Lijian Pan, have provided invaluable insights into the crystallization process of CF/PPS composites during the automated fiber placement process and its effect on the mechanical properties of the composites. Their work has significantly contributed to the following blog.

Problem

The key problem the article addresses is the challenge of controlling the performance and quality of thermoplastic composites manufactured by automated fiber placement (AFP) or tape laying (ATL). The unique processing characteristics of these methods, such as the short dwell time of temperature and pressure on layers and the exceptionally high cooling rate, make it difficult to achieve perfect consolidation and good crystallinity of the composites.

The article focuses explicitly on the crystallization mechanism of carbon fiber/polyphenylene sulfide (CF/PPS) composites during the AFP process and its effect on mechanical properties. Understanding this complicated crystallization process is crucial for controlling the performance of the composites.

Approach

We approach the problem by investigating the crystallization mechanism of carbon fiber/polyphenylene sulfide (CF/PPS) composites during the automated fiber placement (AFP) process and its effect on the mechanical properties.

The crystallization kinetics analysis indicated that the crystallinity of the CF/PPS composites was influenced by parameters including laser temperature, placement speed, and tool temperature, with the tool temperature being the main factor. The authors found that increasing the tool temperature was an effective method to achieve high crystallinity.

Table: Crystallinity of thermoplastic composites manufactured by AFP/ATP in the literatures.

When the tool temperature was in the range of the crystallization window, the composites could experience isothermal crystallization, which could further improve the crystallinity. With the increase of the tool temperature, the flexural strength and interlaminar shear strength were improved due to the enhanced self-adhesion of the matrix, while the Mode I fracture toughness was decreased because of the reduction of the matrix ductility.

Understanding the crystallization process

Understanding the crystallization process is crucial for controlling the performance of composites manufactured by automated fiber placement (AFP) for several reasons:

1. Crystallinity Affects Composite Properties: The properties of the composites are significantly affected by the crystallinity. For example, with the increase of the tool temperature, the flexural strength and interlaminar shear strength were improved due to the enhanced self-adhesion of the matrix, while the Mode I fracture toughness was decreased because of the reduction of the matrix ductility
2. Influence of Processing Parameters: The crystallinity of the composites is influenced by various processing parameters, including laser temperature, placement speed, and tool temperature. Understanding the crystallization process allows for the optimization of these parameters to achieve the desired crystallinity and, consequently, the desired composite properties
3. Isothermal Crystallization: When the tool temperature is in the range of the crystallization window, the composites could experience isothermal crystallization, which could further improve the crystallinity. Understanding this process is key to achieving high crystallinity
4. Crystallization Window: The crystallization window of CF/PPS composites was found to be 87–270°C. Beyond this temperature window, the crystallization ability of CF/PPS composites is blocked. Understanding this window is important for controlling the crystallization process during AFP

Findings: Key process factors

The crystallinity of carbon fiber/polyphenylene sulfide (CF/PPS) thermoplastic composites manufactured by automated fiber placement is influenced by several factors:

• Tool Temperature: The tool temperature is the main factor influencing the crystallinity of CF/PPS composites. As the tool temperature increases, the crystallinity of the composites also increases. For instance, when the tool temperature increased from 40°C to 120°C, the crystallinity of the composites manufactured at a laser temperature of 250°C increased from 17.6% to 34.9%, while the crystallinity of the composites prepared at a laser temperature of 380°C increased from 20.0% to 44.1%. This increase in crystallinity is attributed to the variation of the cooling rate at different tool temperatures and the occurrence of isothermal crystallization when the tool temperature is above 87°C
  

• Laser Temperature: The laser temperature also plays a role in the crystallinity of the composites. High laser temperature can keep the matrix in a better melting state, enhancing the movement ability of the molecular chains. This is conducive to the regular arrangement of the molecular chains and the improvement of its crystallinity
• Placement Speed: The placement speed impacts the crystallinity of the composites. Fast placement can raise the cooling rate around the molten pool of the polymer, which is not conducive to the improvement of the crystallinity of the composites. Therefore, the crystallinity decreases with the increase of the placement speed
• Compaction Force: While the document does not explicitly mention the impact of compaction force on crystallinity, it is listed as a parameter in the design of the laminates. This suggests that it may also play a role in the crystallization process.

A step-by-step guide to achieving better crystallinity

Here is a step-by-step guide for an Automated Fiber Placement (AFP) process operator to achieve better crystallinity in CF/PPS thermoplastic composites:

1. Control the Tool Temperature: The tool temperature is the main factor influencing the crystallinity of CF/PPS composites. As the tool temperature increases, the crystallinity of the composites also increases. For instance, when the tool temperature increased from 40°C to 120°C, the crystallinity of the composites increased significantly. Therefore, the operator should ensure the tool temperature is adequately controlled and possibly increased to achieve higher crystallinity.
2. Manage the Laser Temperature: High laser temperature can keep the matrix in a better melting state, enhancing the movement ability of the molecular chains. This is conducive to the regular arrangement of the molecular chains and the improvement of its crystallinity. Therefore, the operator should ensure the laser temperature is appropriately managed
3. Regulate the Placement Speed: The placement speed impacts the crystallinity of the composites. Fast placement can raise the cooling rate around the molten pool of the polymer, which is not conducive to the improvement of the crystallinity of the composites. Therefore, the crystallinity decreases with the increase of the placement speed. The operator should ensure the placement speed is regulated to achieve the desired crystallinity
4. Understand the Crystallization Window: The crystallization window of CF/PPS composites was found to be 87–270°C. Beyond this temperature window, the crystallization ability of CF/PPS composites is blocked. Understanding this window is important for controlling the crystallization process during AFP
5. Consider the Number of Plies: The crystallinity of the twenty-plies laminates was significantly improved compared with the four-plies laminates. This is because a few plies of the substrate underneath the laser heating area underwent repeated heating, leading to improved crystallinity

What's Next!

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