The laser-assisted tape/fiber placement (LATP/LAFP) process is an automated composite manufacturing technique often quoted for its potential ability to produce composite parts with an in-situ consolidation, which avoids an expensive time and energy-consuming step of post-consolidation. To be competitive in this process, it needs to ensure a certain level of quality of the fabricated parts.

The steps to produce a quality final product in this process are given in Figure 1. Each step is elaborated upon in an individual subchapter.

Selection, control, and quality of raw material UD thermoplastic prepreg

The material used in LATP/LAFP processes is a very important process parameter. The production process for obtaining composites from thermoplastic materials mainly depends on the viscosity of the resin and therefore impregnation is very important in the UD thermoplastic prepregs. Thermoplastic tapes are available in widths of up to 12 inches/305 mm and show as many as 30 defects in just 210 m ft. This is because the thermoplastic UD prepregs rely on a powder-based application process that is more difficult to control and can create resin-rich and dry areas. Such non-uniformity can lead to voids and problems in the LATP/AFP process.

The key point is that high-quality tapes, with low void content, enable fast, automated processing of high-quality composites. Tapes with high levels of voids will require longer consolidation cycles to produce high-quality parts. For the in-situ consolidated LATP/LAFP process the first main is the quality of the raw material, which can vary greatly from supplier to supplier and for each material matrix. As mentioned earlier in the production of thermoplastic tape can often occur defects of the tape itself (especially porosity) which requires a different temperature, greater consolidation, and melting time during the LATP/LAFP process.

Selection of process parameters in LATP/LAFP

In general, the LATP/LAFP process using laser heating involves many parameters that affect the results of the process. To obtain quality parts, the combined effect of the main process parameters must be studied and analyzed. Table 1 and Figure 3 show the factors that affect the output of the automated fiber/tape laying process using a laser heater. In the same table is entered a part where it is said which factor how much they influenced the final product and at what.

Table 1. Parameters that affect the quality of the final product of the process

How have the parameters been selected?

In research, some of these factors (Table 1) 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 4.

• Raw material (UD tape) – the influence of the characteristics of the raw material was previously explained.
• Mandrel – Mandrel temperature – Numerous studies have shown that heating the mandrel helps to release the stresses of the forces that occur in the cooling band so that there is no curvature of the flat parts.
• Machine accuracy –  Tool position – how precisely the roller of the LAFP/LATL head follows the laying and orientation path; deviation from the predicted orientation of the head can cause separation of the material from the background where the laying is performed (the robot accuracy). Each of them directly affects the quality of the final products. 
• Compaction roller – with its compact force its dimensions (diameter and width), and the different thicknesses of the siliconized layer of the roller can give different contact surfaces of the tape, and thus good or bad intimate contact between the layers. The development of intimate contact between the layers, 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 viscosity’s temperature dependence, the temperature increase facilitates the development of contact. In regions where intimate contact is achieved, interdiffusion of polymer chains occurs due to accidental thermal movement.

• Heat source (laser) – When the laser beam is positioned towards the composite, part of the initial intensity (I0) will be reflected (Ir), part absorbed (Ia), and part transmitted (It). The amount of beam that is reflected, absorbed, or transmitted depends on several factors such as the characteristics of the material or strain, the wavelength of the beam, and the distribution of the fibers.  Ideally, the surface of the fiber-reinforced strip is covered with a thin and ideally flat layer of 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 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 the absorption of the beam that helps to heat the material.
• The geometry of the object where the laying is performed and the most common irregularities (possible defects) that occur in the process of laying during the LAFP process are shown in Table 2. 
• Gap – The gap between courses is an inadmissible magnitude of the distance between two adjacent courses. The gap between tows is the inadmissible magnitude of the distance between two adjacent tows.
• Overlap is a defect in which two adjacent tows overlap.
• Missing tow is a space where in the reference state there should be a tow.

Table 2. Defects during the LATP/LAFP process

Lay-up uniformity features	Placement features	Topical features
Gaps	Start and end placement position	Splices
Overlaps	Ply edge angularity Ply orientation	Wrinkles, twists, folds
Missing tows	The curvature of the center line of a tow	Bridging, crowning
	Splitting of a tow	Foreign Object Detection (FOD)

• Early add/late cut is early applying of a tow at the beginning of a course or delayed cutting of a tow at the end of a course. Late add / early cut is late applying of tow at the beginning of a course or early cutting of a tow at the end of a course. 
• Splitting of a tow is a defect that occurs when a tow is ripped, and it is laid up with a width smaller than the reference width.
• Fuzzballs – a tangle of fibers dropping on the laid-up part, which accumulate on some parts of the machine during AFP as a result of abrasion of fiber material.
• Bridging and crowning – defects that occur as a lack of contact between the two and the surface
• Wrinkle, twist, upfold – a defect indicating different forms of deformations of a tow.

Figure 5 shows some of the possible errors of the laid tapes, the possible errors of start and end placement position, and possible defects during the LAFP process

Trial test with DOE

In the previous chapter we talked about some input parameters in the 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, crystallinity and mechanical properties (3pbt, ILSS…) (see Figure 3, 4).

 Voids two types of voids are inherent in the on-site consolidation process: intralaminar and interlaminar. An intra-laminar void occurs during the impregnation of the tape and this void is embedded in the tape. While the inter-laminar cavity is mainly the result of the process of laying 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.  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.

 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.

Achieving complete consolidation by applying robotic processes requires the realization of additional research related to the optimization of many parameters for obtaining composite parts of different sizes and shapes. The interrelationship between the process parameters, the properties of the material, and the strength of the bond between the interlaminar layers has been investigated and shown in more references. 

Key points for LATP/LAFP

• Robotic tape-laying processes are very suitable for the efficient production of carbon fiber-reinforced parts, especially for use in aerospace and other industries.
• The application of laser heating has some advantages over alternative heat sources, such as hot gas. The two most important advantages of laser heating application are the high energy density (accumulated energy per unit system per unit volume [J / m3]) and the short response time. The first allows the application of higher speeds when laying lanes, while the second allows laying lanes of complicated geometries, including large variations in laying speed. However, the application of laser also has its drawbacks. The cost of the equipment is high compared to conventional heat sources, and also the laser always requires the equipment to be housed in a protected environment. In addition, applying laser heating requires a thorough understanding of the interaction of light with fiber-reinforced thermoplastic tapes.
• When the speed of laying the thermoplastic tapes is high, then the heat that enters from the laser source to the entrance to the material is negligible and insignificant. Therefore, the researchers concluded that a good model of heating the thermoplastic material should be made in correlation with the angle at which the radial rays fall on it and the laser placement speed to connect the layers successfully.
• Experiments from the papers have shown that excellent interlaminar quality. Thus good properties of the final parts can be obtained when the laying of the tape is at low speeds, higher compaction force, and laser beam in the case when it is primarily aimed at the tape.

REFERENCES

In drafting this blog, we extend our sincerest gratitude to the R&D team from the Institute for Advanced Composites and Robotics from Macedonia for their pioneering work detailed in the studies:

Risteska, S. (2021, September 30). Unidirectional Carbon Fiber Reinforced Thermoplastic Tape in Automated Tape Placement Process
https://www.intechopen.com/chapters/79552

Risteska S., Samak S., Samak V. (2021, December 30). The Factors That Affect the Expansion of the Tape for It to Avoid Side Effects in the Production of Composites in Online LATP Technology
https://www.mdpi.com/2504-477X/5/10/284

Samak S.., Risteska S., Dukovski V., Trajkoski S. (2020, October 12). Some Experimental Investigation of Products from Thermoplastic Composite Materials Manufactured with Robot and LAFP
https://www.ijert.org/some-experimental-investigation-of-products-from-thermoplastic-composite-materials-manufactured-with-robot-and-lafp

Their research offers invaluable insights into innovative technology methods for thermoplastic composites. We thank them for their dedication and for laying the groundwork that could lead to more eco-efficient manufacturing processes in the aerospace and automotive industries.