**2.4 How the parameters have been selected? The effect of these parameters on the quality of the final product**

In research, some of these factors (**Table 2**) are taken as constants and some as variables. Most often, the selection of the variable parameters for the research within the tests and the analysis of their impact on the automated LATP/LAFP processes when laying the thermoplastic tape is given in **Figure 5**.

*Raw material (UD tape)* - The influence of the characteristics of the raw material was previously explained.

**Figure 5.** *Constants and variables for the LATP process.*

*Unidirectional Carbon Fiber Reinforced Thermoplastic Tape in Automated Tape Placement… DOI: http://dx.doi.org/10.5772/intechopen.101110*

*Mandrel* - In the process of passing this technology, important factors that affect the final product of the mandrel are as follows:

	- to determine the laying path choice of lane width, for complicated and small curves the lane width should be smaller,
	- selection of the angle of the laser depending on the shape of the mandrel, it is chosen how to position the laser so that we have even heating of the tape and the substrate

Compact roller - The dimensions of this roller (diameter, width and thickness of the siliconized layer) are important because they affect the intimate contact between the layers in the final product.

The development of intimate contact between the layers, which is a prerequisite for connection, consists of equalizing the unevenness of the tape and the laminate. The initial surface unevenness is deformed under the action of heat and pressure. The time required to achieve intimate contact depends on the unevenness of the surfaces, the applied pressure, and the viscosity of the matrix which depends of course on the temperature. Due to the temperature dependence of the viscosity, the increase in temperature facilitates the development of contact. In regions where intimate contact is achieved, interdiffusion of polymer chains occurs due to accidental thermal movement. The interdiffusion process is generally explained by the mobility of the polymer chains. The polymer matrix consists of intertwined chains that have limited movement. Their mobility, and thus the degree of diffusion, increases with increasing temperature. In the case of semicrystalline polymers, the presence of crystalline regions can severely inhibit the interdiffusion process. Based on this, it can be noticed that the interface is one of the important parameters, and therefore the thermal aspects when laying the tapes are very important.

When processing thermoplastic tapes, the connection is the result of intimate contact and autohesion (direct-*bonding* or self-*bonding*). *Autohesion* begins after the onset of intimate contact. Different connection models have been proposed in the literature depending on the type of process [18, 30–35]. Some processes require a long processing time to complete the layers. However, for LATP/LAFP processes, the temperature of the polymer rises to the melting point before the start of intimate contact [35]. Stokes-Griffin and Compston [34] later found that the bonding of carbon fiber-based thermoplastics and PEEK using LATP/LAFP takes place at temperatures below the melting point and above the glassing temperature. They found that PEEK polymers are very amorphous during consolidation and require short processing times with LAFP or LATP processes using extreme cooling rates (1000°C/s), and the polymer is heated above the melting point before starting intimate contact.

#### **Heat source (laser)**

Today, industries and research institutions do a lot of research on the application of lasers in automated fiber/tape laying procedures. Laser systems with high power

up to 10 kW are commercially available, and even more, if needed. Consider these variables when evaluating AFP diode laser sources:

**Beam intensity (I) Power:** The power of a diode laser module can be customized from just a few watts to 10s, even 100 s of kilowatts. For an AFP application, typically 1 kilowatt or less per tow is sufficient. The exact power depends mostly on the material and the processing speed.

**Beam profile (Y) Beam homogeneity:** The amount of heat, needs to be uniform across a designated target area.

**Beam size and shape** (**W**) **Size and weight:** A compact diode laser system allows for a more compact AFP head that can maneuver more intricate structures.

**Beam angle (β) Incoming tape angle (α):** When the laser beam is positioned toward the composite beam, part of the initial intensity (I0) will be reflected (Ir), partly absorbed (Ia), and partly transmitted (It). The amount of beam that is reflected, absorbed, or transmitted depends on a number of factors such as the characteristics of the material or strain, the wavelength of the beam, and the distribution of the fibers [4]. Ideally, the surface of the fiber-reinforced strip is covered with a thin and ideally flat layer of the thermoplastic matrix that aids in bonding. But in practice, such a layer rarely exists. The conical angle at which the radial rays fall on the material which is simultaneously under the pressure of the roller strikes the wedge-shaped cavity formed by the substrate and the thrust roller and causes different levels of reflection. The laser beam can be reflected from the thermoplastic matrix and also from the fibers on the surface of the tape. The part of the beam that is reflected from the surface should be as small as possible for laser heating to be effective. Any beam that is not reflected is transmitted or absorbed by the matrix and the fibers. It is actually the absorption of the beam that helps to heat the material.

#### **2.5 Process parameters**

Lee and Springer in their research [18] presented a model for the production of composites from thermoplastic composite tapes based on PEEK and carbon fiber using LATP/LAFP. They conclude that the process consists of three main steps—impregnation, consolidation, and crystallinity [18]. Impregnation is a parameter that cannot be controlled by automatic fiber/tape laying while consolidation and crystallinity can be controlled. Consolidation consists of two subprocesses—intimate contact and autohesion, that is direct connection or just connection (autohesion, healing). Autohesion is the formation of bonds between two surfaces of the identical polymer at elevated temperatures usually slightly above Tg. It is a new technique for precise bonding by self-bonding polymers without the need for adhesives. To achieve better final characteristics of the thermoplastic composite it is necessary to achieve a good degree of intimate contact and good self-connection.

Agrawal et al. [17] indicate that better on-site thermoplastic consolidation is achieved when a unified thermal model is applied by applying a wider beam with higher laser power. More authors [7, 38] also studied the effect of laser power, pressure, and deposition rate on the strength of the bond between layers. They found that the power of the laser had a dominant effect on the quality of the connection, and the force of the roller pressing had a minimal effect. Grove [36] in his research worked on the modeling of on-site consolidation processes and from experiments he concluded that the resulting poor crystallinity is due to the high cooling rate. Yousefpour and Ghasemi Nejhad [37] later proposed preheating the strip below the Tg glazing temperature to achieve better laminate consolidation.

*Unidirectional Carbon Fiber Reinforced Thermoplastic Tape in Automated Tape Placement… DOI: http://dx.doi.org/10.5772/intechopen.101110*
