**2.6 Trial test with DOE**

More and more factors have an influence on effectiveness and efficiency in industrial processes and systems. To find the optimum in control of the processes there are often a lot of experiments to realize—practical and theoretical ones. Design of experiments involve designing a set of experiments, in which all relevant factors are varied systematically (**Figure 6**). When the results of these experiments are analyzed, they help to identify optimal conditions.

In the previous chapter, we talked about some input parameters in LATP/LAFP process, and here in this chapter will talk about the three main output parameters that are determined after each experiment (trial test with DOE) in this process and that are—voids and crystallinity ILSS (**Figures 4** and **5**).

**Voids:** There are two types of voids that are inherent in the on-site consolidation process—intralaminar and interlaminar. The intralaminar cavity occurs during the impregnation of the tape, while, the interlaminar cavity is mainly the result of the process of laying the tape. The intralaminar cavity is embedded in the tape.

**Crystallinity:** Crystallinity also affects the mechanical properties of the final product and depends on the thermal cycle and cooling rate of the thermoplastic. In most LATP/LAFP applications, the cooling rate is as high as 1000°C per minute. Parts made of thermoplastic prepress tape based on PEEK and carbon fiber have a degree of crystallinity from 20 to 35% [18]. The two subprocesses—intimate contact and autohesia are a function of temperature, pressure, and consolidation time (passing speed), and these are the three most important parameters in any process. At a given pressure, a higher temperature and longer time are required to achieve the optimal degree of intimate contact and self-connection. Crystallinity affects the mechanical properties of thermoplastic composites. Higher crystallinity increases strength and rigidity [20]. On the other hand, the lower the degree of crystallinity increases the impact resistance and breaking strength [21–25]. The degree of crystallinity depends on the history of thermal processes. Low cooling rates result in a higher degree of crystallinity and vice versa. Hence, the degree of crystallinity achieved through on-site consolidation of the laying strip is limited due to the extremely high cooling rates. Kumar et al. [26] measured the degree of crystallinity of PEEK as well as the growth rate of spherolites (supermolecular forms of semicrystalline polymers) for samples heated to a melting point of 380°C and 420°C, then cooled to a crystallization temperature of 300°C or 320°C at a speed of about 3°C/s [26]. They concluded that the degree of crystallinity and the size of the crystals were higher for samples cooled by higher melting temperatures. In other words, at a constant cooling rate, the size of the polymer spherolites depends on the maximum processing temperature. Sonmez and Hahn [27] found that the degree of crystallinity of thermoplastic laminate obtained from one-way carbon fiber-based strips and REEK is between 25 and 35% in a single-site consolidation process. Similar degrees of crystallinity have been obtained in other studies indicating that PEEK is not sensitive to the cooling rates involved in the LATP/LAFP process [28, 29].

**Figure 6.** *Model of design of experiments.*

**Figure 7.**

*Preliminary quality verification for final flat parts: Low porosity + Hight crystallinity + good bonding = good mechanical properties (tensile, flexural strength, ILSS, NOL test).*

**Mechanical properties:** The most common mechanical tests performed to determine the process parameters are 3pbt and ILSS. These tests make it easier to see if the interlayers are well bonded during this process. If necessary, additional mechanical tests are performed. The ILSS survey is a well-established and fast calculation to be observed as a process (input parameters affect the final product).
