There are various ways of classifying the many different processes. For our purposes, we separate them according to the aggregate state of the raw material. We will focus on the best-known and most common processes and therefore we will focus on the processes for manufacturing components from liquid and solid polymers.

Solidifying liquid polymers

Stereolithography (SLA) and Digital Light Processing (DLP) are two of the most common additive manufacturing processes that produce components from liquid polymers (resins). Both processes polymerize synthetic resins with the help of light. The SLA process uses a LASER as the power source, which is controlled with the aid of deflection mirrors. The DLP process, on the other hand, uses a projector or display to apply heat (energy) to cure the resin.  

Figure 1: A component is manufactured using the SLA and DLP processes. 

How does the application of the liquid polymer work?

First, liquid resin is filled into a container that is transparent from the bottom. The building platform onto which the future component is produced is moved down into the resin so that the gap between the transparent bottom of the container and the building platform is equivalent to the thickness of the first layer. In figure 1 the top of the building platform is submerged into resin.

The resin in the gap is then cured by the power source (LASER, or projector) on the building platform through the transparent bottom. Since the power source is located under the container, the layers are built up from below. This means that in order to produce further layers, the Z-axis must move upwards after curing to be able to cure the next layer. This process is repeated until the component has been manufactured. Since the component is manufactured from below and the Z-axis moves upwards layer by layer, both the component and the necessary support structures are "turned upside down" and hang from the top, i.e. SLA and DLP manufacture upside down! 

Advantages of SLA and DLP	Disadvantages of SLA and DLP
Low layer height -> high level of detail	Support structure necessary
High accuracy	Low construction volume
Smooth surface even without finishing	Cleaning of the component necessary
Use of synthetic resins with different mechanical properties	No reinforcement with fibers possible

Applying solid polymers

When manufacturing from the solid phase there are three ways to apply the polymer, as shown in Figure 2.  

Figure 2: Categorizing the manufacturing processes from the solid raw material state [1] 

Powder bed processes are some of the possible ways used to manufacture components from solid-state material. In these processes (SLS and 3DP), polymer powder is applied in thin layers to the building platform and sintered by an energy source (usually light) at the designated areas. Unlike the SLA and DLP processes, in the powder-based processes, the building platform (printer bed) is moved down for each new layer in order to apply new powder for the respective layer on top of the existing material.  

The only difference between the SLS and 3DP processes is that in the SLS process the powder is sintered directly by a LASER, and in the 3DP process a binder is used. 

Advantages of powder bed process (SLS und 3DP)	Disadvantages of powder bed process (SLS und 3DP)
Low layer height, high level of detail	Rough surface
High accuracy	Low construction volume
No support structure necessary	Cleaning of the component necessary
Use of different thermoplastics with different mechanical properties	No reinforcement by fibers possible

Another possibility for producing 3D printed components from solid polymers is the layer-by-layer bonding of polymer films (see figure 2 “Applying laminates”) that have already been cut to size. However, this process is little used in industry, so it will not be discussed here further.

The last option is to melt the solid polymer within a nozzle or print head and apply it sequentially to the building platform. The solid polymer is usually fed into the nozzle or print head as a filament (called FFF or FDM) or pellet (FGF). To be able to create the required layers, either the nozzle or print head must be moved upwards or the building platform alternatively moved downwards.

Advantages of FFF/FDM and FGF	Disadvantages of FFF/FDM and FGF
No cleaning of the component necessary	High layer height, low level of detail
Large construction volume	Rough surface
Use of different thermoplastics with different mechanical properties	Support structure necessary
Reinforcement by fibers possible

In summary:

• As with all additive processes, the construction space is limited.
• In all of today’s additive resin processes and powder bed processes, the reinforcement of the polymers with carbon or glass fibers is not possible; likewise, multiple different materials cannot be used for the production.
• Only processes that sequentially add material through a nozzle (or similar) can print with multiple different materials as well as be reinforced with short fibers or continuous fibers.
• If robots are used instead of stationary machines, the construction space can be significantly increased (if one or more additional axes are added, more construction space is required).  

By now, you should have some understanding of the most common ways of manufacturing components using polymer materials. We will showcase some advanced processes utilizing robots and 3D printing nozzles in our following articles. Our next article will be about the often-mentioned layers, how they are created by slicing, and how they are parameterized. 

Until then, stay safe and stay tuned. 

Title Image: Credits go to the Technical University of Munich.

[1] Berger Uwe, Hartmann Andreas und Schmid Dietmar, Additive Fertigungsverfahren RAPIDPROTOTYPING • RAPID TOOLING • RAPID MANUFACTURING [Buch] Verlag Europa-Lehrmittel, 2013.

 

• What is the maximum width of a gap between 2 stripes?
• What is the maximum width of a gap between 2 courses?
• How large is the distance between gaps of adjacent plies? (called Staggering)
• How far may a stripe deviate from its nominal position?
• How far may a stripe deviate from its nominal angle?
• How much may a stripe be steered? (curved) 

We will now look at each of these questions respectively and see why they are important and how we can measure the actual values. Please note, that the words “tape” and “stripe” are used interchangeably with a preference to “stripe”.

Gap width

Usually, there are different restrictions on the size of the gap between adjacent stripes and the width of the gap between adjacent courses. These restrictions also vary depending on the complexity of the part. Flat parts and parts of a “simple curvature” have tighter tolerances than parts that are complexly curved. 

Figure 1: Schema of design requirements of a CFRP component. When dealing with ¼in (6.35mm) prepregs, gaps between tapes are rarely allowed to exceed 1mm. Gaps between courses are rarely allowed to exceed 3mm. There are often additional constraints put on the total gap width within specified distances. Please note that these tolerances are not generally applicable to all components and are subject to change with respect to the application and surrounding conditions of the component.

Staggering 

How each ply must be staggered is usually defined in the laminate structure definition, where ply angles and boundaries are defined. We already covered staggering in a previous article. To summarize it very briefly, staggering is used to ensure the stability of laminates by shifting individual layers of the same orientation sideways to their material direction. 

Positional tolerances 

Figure 2: Schematic for the position tolerance around the nominal boundary. This value is usually below half the material width. For non-perpendicular angles (anything other than 0 or 90 in this figure), the boundary overlapping strategy is also defined. See our previous article about this subject.

Angular tolerances and steering of stripes

Due to the curvature of the layup surface, keeping a prepreg stripe straight is rarely possible on complex surfaces. Depending on the condition of the underlying surface, stripes twist or shift sideways. In the most complex cases, the result is that a stripe steers toward the direction of the center of the curvature. There are different strategies to alleviate the problem where the angle of the stripe deviates from the planned, nominal angle. In the next article, we will showcase where this problem comes from and which strategies may be applied.

The prepreg orientation tolerance is usually specified in the CFRP component’s design requirements by an angle of ±5°, for instance. The steering tolerance is usually specified by the limits of the prepreg material (its matrix material and composition and its width) and either comes from the material specification sheet or manual experimentation.

This article about design requirements went deep into specifications and tolerances. In next week’s article about orientations in 3D, we will show how a stripe can deviate from its nominal angle and why surfaces that are curved along multiple directions will lead to gaps in the finished part.

Until then, stay safe and stay tuned. 

What is the key difference to a “3D” laminate?

The key difference is that 3D laminates have no flat base plane. The underlying basis for programming in 3D is the CAD model of the mold surface. Mathematically speaking, the key difference between 2D and 3D laminates is the definition of this base surface. While we are dealing with planes in 2D, we are working with surfaces in 3D. The main difference is that surfaces may be curved in different directions, while planes are flat. This curvature component is the key factor for successfully manufacturing 3D components from CFRP. 

Figure 1: In this figure, you can see a simple, curved base surface. It has a constant profile along one direction (left to right). On this surface, a 45° ply has been placed.  

Due to the curvature of the surface and the desired angle, the tool must be constantly realigned when manufacturing a course:

What is the deal with curvature and CFRP prepregs?

There are multiple challenges that curvature poses when manufacturing with CFRP prepregs. Carbon fiber prepregs usually come in the form of long stripes. These stripes have similar properties as scotch tape. Have you ever tried to bend scotch tape? Or to have it stick to gift wrap paper that is not completely flat? Just like scotch tape, CFRP stripes will start to wrinkle when bent or will not stick to the surface when its structure is uneven. In the following articles, we will present some examples of challenges that arise when draping carbon fiber prepregs, since these are some of the trickiest challenges when manufacturing complex laminates on 3D surfaces.

To sum it up: 

See these visualizations for further illustrations:

When manufacturing flat plies, the orientation of the tool never has to change. Only its x and y position change for different z coordinates. Since the base plane is flat, the layup direction vector is constant.

When manufacturing 3D laminates, the orientation and position of the tool are constantly changing for each layer. Due to the curvature of the base surface, changing the z coordinate not only changes the position of the target point but also changes how the tool must be oriented (rotated) to keep the same layup direction as before.

By now you should understand where the increased complexity and requirements of 3D laminates and parts come from. Due to the curvature of the base surface, the manufacturing system needs to fulfill very high demands regarding the orientation of the tool. In some of the following articles, we will explain more about the effects of these by introducing you to steering, angle deviations, and bridging. But for the next articles, we will first introduce some basics about Design Requirements and how orientations in 3D are defined exactly.

Until then, stay safe and stay tuned. 

© SWMS Systemtechnik Ingenieurgesellschaft mbH

Figure 1: A screen capture of a layup process using NMBs tape laying machine as simulated by the TapeStation. On the left-hand side, a 3D model of the tape-laying machine is displayed. On the right-hand side, a simplified, 2D visualization of the process is shown. The yellow geometries in the 3D model are conveyors, which feed the material that is to be placed. From there on, the two smaller pick and place units take the prepreg stripes and move them onto the layup table. Once all stripes of a ply are placed, the bigger, rectangular pick and place unit transfers the ply to the second table, which adjusts its rotation depending on the angle of the ply. When two plies are placed on top of another this way, the welding units move. This can only be seen in the 2D visualization starting from 1:20 – the points that move towards the laminate are the welding units. These ultrasonic welding sonotrodes always fuse two stripes of two adjacent layers.  

Make sure to also take a look at the latest edition of the JEC COMPOSITES MAGAZINE, where NMB and SWMS published an article about the digital twin of the tape-laying machine.

Welding

Every stripe must have two welding positions to properly fuse it with the underlying stripes of the previous ply. An algorithm determines the optimal position of the welding spots by comparing the geometries of the stripes of both plies as certain requirements must be met; for example, that welding positions must have a certain distance to the edges of the stripes. The sonotrodes then fuse the stripes by heating the prepreg material via ultrasonic vibrations.

Digital Twins and Optimization

To correctly mimic the real machine’s behavior, the digital twin uses the same velocities and accelerations of the individual machine parts. This simulation may now be used to approximate the machine duration for the manufacturing operations of the laminates. We defined that the optimization algorithm of NMB’s tape laying machine has to optimize for one out of four distinct optimization goals. Minimizing the:

• Total cost,
• Material offcut,
• Manufacturing duration, or
• CO2-equivalent.

Depending on the primary optimization goal, the optimized laminate will be different in structure: The amount and widths of the stripes, as well as their cutting angles, will be adjusted in such a way that the selected goal is optimized. See this example: 

Figure 2: Different results for different optimization goals. Parameters that are considered for the optimization come from: the number of stripes, the cost of the material of the stripes and its associated CO2-equivalent per kg as well as the full manufacturing duration. Optimizing for CO2 for instance, heavily depends on the used material and its cost. Generally, the more expensive the material, the more the offcut is a factor. Therefore, when manufacturing with cheaper materials, the more cost-efficient the production time.

We can see here what the different outcomes of the different optimization approaches are. Optimizing for manufacturing duration decreases the number of stripes so that as few stripes as possible need to be placed. Optimizing for offcut creates as many stripes as necessary to achieve the least amount of offcut. Optimizing for the total cost, on the other hand, tries to find the optimum between these both since machine cost is directly related to the manufacturing duration and material cost is directly related to the offcut.

Now that we have showcased the results of the optimization algorithm and how the machine manufactures laminates, this is where we wrap it up.

The project "OptiTape - Development of a machine-independent software for mapping a virtual process chain for the economical, resource-saving and mechanically optimal production of preforms based on unidirectional thermoplastic tapes" (funding code ZF4064612 PO8) was funded by the German Federal Ministry for Economic Affairs and Energy (BMWi).

In the last 3 articles, we gave you a short overview of the OptiTape research project, its goals, and outlined the most important features. We also briefly introduced how the NMB tape-laying machine works and why it is so efficient. If your interest has been sparked, you can get more information from the websites of NMB, SWMS, and REHAU.

In the next articles, we will dive deeper into complex subjects: the differences between 3D and 2D laminates, laminate angles in 3D, and path planning on complex, curved laminates.

Until then, stay safe and stay tuned. 

One of the main project goals was the implementation of an algorithm that holistically optimizes the production cost of the part. The optimization goals are either the material cost and offcut, machine cost and manufacturing duration, or the CO2 footprint. The project also included that the algorithm automatically determines the best fitting material widths for the given part to optimize the given criterion. In the AFP processes that we have already shown, varying material widths could not be realized since AFP end-effectors have fixed width material spools. Using wider tapes may however increase the efficiency of layup processes since wide areas may be covered quicker.

Today we will showcase how the tape laying machine works and how multiple material widths may be beneficial to the layup process. In the following weeks, we will also showcase the benefits of variable cutting angles, ultrasonic welding and demonstrate the digital twin.

© Neue Materialien Bayreuth GmbH

The basic principle of the machine works as follows: 

There are two different vacuum layup tables. Thermoplastic prepreg stripes are cut from spools and fed to the left and right side of the first layup table, where two pick-and-place units pick them up and place them in their corresponding position on the layup table. When all stripes of a ply are placed this way, a larger pick-and-place unit, dedicated to the whole ply, transfers the ply onto the second layup table, where ultrasonic welding units weld the different plies together. The second layup table can rotate so that a huge range of different angles may be realized. While the plies on the second table are being welded together, the placement of the stripes on the first table continues. As these processes are highly parallelizable, a very low process duration of 2 seconds per material stripe may be achieved. The resulting laminate may then be transferred to a consolidation unit to be processed further. 

© Neue Materialien Bayreuth GmbH

Figure 1: A top-down view onto NMB's Tape Laying machine.  

Why are multiple material widths beneficial?

Consider this: Using a pick and place process for tape laying means that every material stripe – regardless of its length and width – takes (almost) the same amount of time to cut, feed, and place. Following this logic, decreasing the number of stripes reduces the total amount of time needed to manufacture a laminate. If the laminate contour can be covered in the same way with fewer stripes, this method is clearly preferred.

See this example here:

In this article, we gave you a short overview of the contents of the OptiTape research project, its goals and outlined the most important features. We also briefly introduced how the NMB tape-laying machine works and why it is so efficient. Next week we will showcase why variable cutting angles are so effective at reducing offcuts and how they can be determined.

If your interest has been sparked, you can get more information from the websites of Neue Materialien Bayreuth GmbH, SWMS Systemtechnik Ingenieurgesellschaft mbH, and REHAU.

Until then, stay safe and stay tuned. 

The next segment in this series will cover the research project “OptiTape” and showcase its results. Afterward, we will dive deep into details about complex laminates: orientation in 3D, coverage strategies, and analyses of steering, bridging, and gaps. These phenomena are rarely an issue in simple, flat laminates but become very important (and complicated) in curved layup surfaces. We will shift from these relatively simple examples to complex laminates and how to solve challenges that appear when trying to program and manufacture them.

You can find a list of all articles on the CAESA homepage.

Until then, as always: stay safe and stay tuned.

Post-processing marks the final step of laminate programming. A post-processor takes the digital manufacturing information and transfers these into machine code, which the manufacturing machine can then run in order to manufacture the laminate.

Depending on the manufacturing system, the post-processor may utilize different, control-system-specific functions and instructions that are implemented on the machine. Since the software components of the control system can be configured and extended, different interfaces and functions are available for different setups. A good post-processor will utilize all these highly specific functionalities to optimize the layup program for the control system. This means removing unnecessary external calculations and calls to system functions.

An example: Let’s assume that the goal is to move the tool in a circle. One could go ahead and imitate a circular movement by approximating the circle at many positions (thus creating a polyline) and program the tool to move to each position. If the control system has implemented a function to move in a circle already, all these calls could be reduced to a single one. This would reduce many lines of code to one and reduce additional overhead. For this to work, however, the post-processor needs to be up to date with the latest changes in the control system.

Since all machines differ, it is possible to utilize different post-processors to export the same manufacturing program to different machines. This also means that the laminate only needs to be programmed once!

To showcase this, we have programmed an example ply and used three different post-processors to create the manufacturing programs for three different systems.

Figure 1: The example ply on a layup table. 

Figure 2: The full program of a course made of 16 tows. CONFIGURE_COURSES sets the start and end distance of the tow of the given spool. (The tow from spool 1 starts after 95.285mm and ends after 325.510mm with respect to the start of the tool path.). CONFIGURE_COURSES is defined in the control system. This function has been developed by the control system engineers and calculates internal parameters such as valve pressures, forces, indices, etc. These values might also be set from the external program, but by simplifying these to a single, simple function call, the probability of errors is minimized. SET_HEATER sets the heater’s power to 0% or 50% respectively. The G1 commands are movement instructions, which direct the end-effector to move to the location given by the X, Y, and Z position and the A, B, C angle.

Figure 3: The full program of a course made of 4 tows. This program is used for a robot system that uses a custom end-effector. First, the start and end position of the tapes are set to the layup tool's feeding units. Afterward, the tool path is defined by using the G01 commands and the corresponding X, Y, and Z coordinates and the A, B, and C angles for the orientation of the end-effector.

Figure 4: The full program of a course made of 3 tows. This program is used for a machine that can only manufacture flat laminates since it has no axis that moves the layup tool along the “up” direction. This makes the resulting programs as simple as just defining the required distances (feeding start, cutting, feeding end, etc.) along the tool path of the course. The control system of this machine does not have any additional functions for simplifying the programs like the first one. The parameters are set directly.

Note: The programs have been simplified to better illustrate the differences between the same commands for different machines.

This article will mark the end of the first segment of this series of articles. We introduced you to the very basics of the CFRP manufacturing components – what carbon fiber reinforced plastics are, how a layup tool works, and what effects their design has on laminates. We have furthermore given insight on different optimization methods and which parameters of a laminate are essential to evaluate them. We then showed you how CAM software utilizes the layup paths and simulates the machine that is to be used for manufacturing. Post-processing is the last step to transfer the laminate program to the machine.

The next articles will give insight into a specific project that SWMS has worked on: the project OptiTape. We will showcase the extended requirements and functionalities that were placed on the layup machine and how they have been implemented.

Afterward, we will progressively go into more detail on the specifics of CAM software and even layup algorithms and layup analyses of complex curved laminates. We hope to have sparked your interest and to see you again soon. We also highly appreciate any questions and feedback!

Until then, stay safe and stay tuned. 

Simulating manufacturing processes enhances productivity by extending the tool chain by offline capabilities that otherwise would require a lot of experimentation and testing on the live machine system. By giving the CAM programmer the possibility to verify that the machine is moving correctly, that the laminate is placed in the right position, that the axes are within their limits and that no collisions are detected, the actual time to check for these on the real system is greatly reduced. This however requires a perfect match between the real system and the digital twin.

The following questions may be answered confidently, when using a simulation to verify the laminate layup program offline:

Is the machine moving as intended? Is the movement direction correct, or is it inverted?

• This is especially important when optimizing the manufacturing time. See this example: when the layup direction is from top to bottom, the tool needs to move to the top after every course. When it is alternating its layup direction, the tool needs to rotate by 180° and can then lay down material on the “backward” movement.

Figure 1: A ply in which every course is to be laid from top to bottom. The diagonal yellow lines are the movement paths towards the start of the next course.

Figure 2: A ply in which every second course is to be laid from top to bottom. Therefore, the movement paths towards the next courses start are very short

Is the rotating table rotating clockwise or counterclockwise?

• When using a rotating layup table for laminate manufacturing, the positive rotational direction is usually defined by a clockwise rotation of the table around it’s z-axis. To verify that the simulation is equivalent to the exported program and to the real system, the simulation can be spectated to verify. This is a common error, that’s fixed easily and on which no time needs be wasted.

Are any axis limits reached? Which and when?

• By simulation the manufacturing environment and the layup system, checking against the individual axis limits (especially when using robot systems) becomes very easy. In our experience, it is one of the most important parts of offline CAM process simulations, since it can alleviate a lot of time-draining troubleshooting on the real machine.

Does the machine collide with anything or even itself?

• If there is any external geometry placed in the manufacturing area (such as another mold) it is of utmost importance to verify that the layup head is not colliding as not to destroy this sophisticated and delicate component. When working with robot systems that are packed with additional components (such as material spools, power cables etc.) there may also be additional collision geometry that needs to be avoided. Simulating the manufacturing process with the matching CAD models yields accurate results and successfully prevents any danger to the robot, machine, and environment.

Here are some video snippets from the CAESA® Composites TapeStation in a side-to-side comparison to the real manufacturing machines. You can find the full videos, showcasing a well implemented simulation that can reproduce the real system 1:1 on our YouTube channel: Opti AFP and IFW Kiteboard.

In next week’s article we will explain the functionality and necessity of a post processor. This piece of software transfers the data from the CAM system to the manufacturing system. With next week’s article we will then slowly change subjects to a special project and manufacturing system that we want to showcase. We will take about OptiTape, a project where new functionalities have been explored and realized: multiple materials of different widths, non-perpendicular cutting angles and welding during the layup process.

Feel free to comment or message us at any point. We appreciate any constructive feedback!

Until then, stay safe and stay tuned. 

The control of internal valves (e.g., for feeding and cutting the material) in most cases happens in integrated programs of the layup tool. Controlling the pressure for material clamping, feeding, and cutting are usually determined during the design of the end-effector, and their trigger points can be derived from the layup programs. Controlling these parameters using external programs can be done but is often error prone.

Controlling the heater (e.g., a laser heating unit or an infrared light source) on the other hand, is often realized by explicitly setting the heating values in the NC program that controls the machine movement, since the CAM software has information about the underlying material and can therefore adjust the heating parameters accordingly. For example, if the material of the first ply is laid directly on the layup surface, the heater should be heated longer and hotter than when material is laid underneath. There are several parameters that control the heating unit; some of which are listed below. 

These five parameters control the most important trigger points and temperatures for infrared and laser heating units. Values for the heating power are given in % of its maximum capacity. To see a heating unit in action, check out this video (also availabe on our youtube channel).

In next week’s article we will show you how CAM software can be used to simulate the layup process and verify the correct path the machine will take.

Until then, stay safe and stay tuned. 

In this article, we will highlight the most important parameters of the types of feeds and distances. The last article already hinted at some parameters and how they can be used to control different scenarios when manufacturing. If you are new to this series, we recommend starting with the article about end-effectors – the layup tools used to place CFRP material. From there you can easily catch up.

The term “feed” comes from machining technologies and describes the movement across the part, while “speed” is used to describe the rotational speed difference between the cutting tool and the component. In CFRP manufacturing, feeds describe the movement speed (velocity) at which the layup tool traverses across the layup surface. The most common units are given in mm/min or m/s. “Distances” in this context refers to the distances between trigger points along the movement path. They are usually given in mm or in. When reaching a trigger point, the feed may change, or certain pressures or temperatures may be adjusted.

To describe a layup path, at least 6 different feeds [m/s] are necessary:

• Approach Speed: The velocity that is used to reach the beginning of the touch-down movement operation.
• Touch down Speed: The velocity with which the first position on the layup surface is to be reached. This velocity stays constant until the first layup position is reached.
  
• Initial Speed: The velocity used when one or more material stripes start to be placed. This is either at the beginning of a course or when replacing after a gap present in the course.
• Layup Speed: Is used during material placement from all slots.
• Cutting Speed: This speed is used when one or more tows are cut. It is lower than the layup speed, so the cutting units can safely cut the material.
  
• Rapid Traversal Speed: This is used when the end-effector moves from the end of one course to the beginning of the next course.
  

These trigger points need to be defined by certain distances [mm]:

• Approach: A vector that describes the movement component used to approach the position where the layup surface is first touched by the compaction roller, as defined by the “Roll in” position.
• Departure: A vector that describes the retreating position after a course is laid, as defined by the “Roll out” position.
  
• Rapid: The distance towards the layup surface that the tool must maintain when traversing from the end of one course to the start of the next.
  
• Roll in: Describes the additional distance before the layup of one course, which describes how long the compaction roller needs to keep surface contact, so that the material is safely and correctly placed at a given position.
  
• Roll out: Same as Roll in but after the layup has occurred. This is used to ensure that the ends of all material stripes are in their correct position. If retracted too early, they may fall on top of each other or shift sideways.
  

Let’s look at an example from last week’s article:

Figure 1: A course on a curved surface. 

The blue arrows (1, 10) denote the start and end vectors of the whole movement path. They surround an orange line segment, whose length is defined by the rapid distance. Starting from right to left, the following red line (2) denotes the approach vector, which defines the distance and velocity used to reach the „Roll in“ position (3). This is the first surface contact point. The light blue line (4) denotes the area in which the „Initial speed“ is used since this is the first area in which tapes will be placed. The long blue line in the center (5) tells us that the „layup speed“ is used until reaching point (6), where the „Initial speed“ is used again. Take a look at the bottom left side – an additional, short tow is now also being laid. Since a new tow is to be laid, the „Initial speed“ needs to be used. Shortly thereafter, all tows need to be cut as the course ends (7). The light gray area denotes the „Cutting area“, which surrounds the actual cutting position. The remaining layup is done using the layup speed (8). Just as we used the approach vector on the right side, the „roll out“ (also 8) and departure vector (9) are now being used to safely lift off the surface (10). The rapid traversal speed is used in (1 and 10) and to move towards the subsequent course, omitted in this example

In this article, we went in-depth into the process settings and how they can be used to influence the movement path of the layup operations. Next week’s article will cover pressures and temperatures – what they are used for and how they can be optimally used.

Until then, stay safe and stay tuned.

There are different types of parameters:

• Feeds:
• The velocity of the layup head.
• Distances:
• The (horizontal or vertical) distance between trigger points.
• Pressures
• Certain components of the layup head, like the feeding unit, require a nominal pressure that can be set.
• Temperatures
• The heating unit’s power may be adjusted depending on the area that is to be heated. 

Figure 1: The process settings, taken from the CAESA® Composites TapeStation software. The light gray arrow on the bottom denotes the movement of the tool during manufacturing. The blue line in the background shows the path taken, while the dark gray, thinner line is a representation of the CFRP material that is to be placed. Using the distances, the path can be adjusted as needed. 

The distances are used to span certain regions. For instance: In the following figure, there are two distances of 25.00mm. These are around a position, where a tow is cut. 25mm before the tow is cut, the layup head slows down from 0.25m/s to 0.15m/s (highlighted in blue). 25mm after the cut, the layup head accelerates back to 0.25m/s.

Figure 2: The relevant process settings for the feeds while a tow is being cut. 

Using the schematic, the programmer may adjust the heater’s energy level before the first tow is placed: basically, how much the tooling surface is to be heated before any material is placed. It is also possible to control, how much the layup tool must be lifted when retreating to the beginning of the next course.

These process settings are then combined with the information about the tows and courses. They then form the TCP-path (Tool Center Point) that the layup tool is taking during the manufacturing process. In the following figure, such a path is depicted. The blue arrows denote the first touch-down position and the last take-off position. 

Axis Values

The CAM software exports explicit axis angles and distances for the position and orientation of the system (for instance an angle for each joint of the robot system).

This is generally an option that is not to be recommended, as any alteration to the real system (physical deformation, changes to the control system) will create a difference between the real and the digital system, which is very hard to compensate. 

Machine Coordinates

The CAM software exports the position and orientation of the layup program relative to the base coordinate system of the machine. This coordinate system is measured and available in the CAD model of the layup machine or environment.

Product Coordinates

The CAM software exports the position and orientation of the layup program relative to a coordinate system that is predefined at a position on the layup table. This coordinate system is measured and usually available in the CAD model of the machine. 

Usually, two steps are involved in positioning the part correctly:

1. For a broad positioning, the referencing coordinate system from the CAD data is taken and the programmed laminate is positioned in such a way, that its origin matches the referencing coordinate system.

2. For a finer positioning, two sets of positions are taken:

• 3 positions in the CAD model are defined
• The tool is positioned in such a way, that the 3 points are hit by the tool-center-point (a dedicated measuring tool is used)
• By comparing the target coordinates with the actual coordinates, the offset in the position and orientation can be calculated and compensated
• This compensation can either be calculated in the machine’s control system or the CAM software 

The result of these operations is an optimal match between the digital and the real manufacturing environments. With an optimal match, the digital manufacturing process can mimic the real process as close as possible. Only then, collisions and axis limits can be verified offline correctly.

In the next article, we will display, how the visualization of the tool paths of the layup tool can be used to verify the correct manufacturing process.

Until then, stay safe and stay tuned. 

The material usage analysis displays various information about the material used for the part. It shows the size of the area that is to be filled with material and how much area is actually covered. From this, the amount of excess material – waste – is calculated and presented to the programmer.  

Figure 1: The material usage quota analysis. For every ply, group of plies, and the whole laminate this information is accessible. Material that is not needed to fill the surface is considered waste. For instance, every bit of material that is only needed to meet the minimum tow length is considered waste.  

The following figure shows, how much material is unwound per spool. Using this table, the manufacturer knows exactly, how much material needs to be available on each spool and can prepare the spools accordingly. 

Figure 2: This table shows the material usage per slot of the layup head. When shifting the course grouping by using the “seed point offset” or the number of tows per course, the amount of material per slot and spool changes. 

The simulation of the layup process is used to estimate the time, the machine needs to run to manufacture the part. Since machine time is often expressed in time/hour, a comparison of material cost vs. machine time can be made easily using these metrics. How such a simulation can be used to check for errors and collisions will be shown in an extra article later in this series. 

Here are some examples with various parameters sets.  

Figure 1: In this figure, the tows have been grouped into courses in such a way, that the last course consists only of three tows. If this part is manufactured in series, the corresponding spools would run low faster than the others. Because the machine cannot continue manufacturing while spools are being swapped out, it makes the most sense, to swap as many spools as possible at the same time. To balance the material amounts between the different spools, the courses may be rearranged accordingly. Meaning that for the other plies, the grouping should complement the existing material usage to balance the total amount of material per spool across the laminate. The blue arrow denotes the centerline of the last course, in which only the bottom-most tow is placed. 

Using the so-called “Seed point offset” shifts the starting point of the grouping by a certain number of tows. In our example, the first course contains 16 tows. If the first course were to contain only 8 tows, the last course then was to contain 9, which would shift the last, single tow to another slot. (Slot number 9, instead of slot number 1) This is equivalent to a seed point offset of 8. Since this does not create any additional courses, this parameter is used to optimize the material usage per spool and to avoid collisions at the start and end of the ply. One could also look at the available material before manufacturing and rearrange the courses in such a way that all spools are used corresponding to the material stored. 

Figure 2: The same tows as in Figure 1, but with a seed-point offset of 8. The first course at the top only contains 8 tows, while the last one contains 9. The blue arrow denotes the centerline of the last course, in which 9 tows are being placed. 

Another parameter that can be used in conjunction is the number of tows that are to be used in a course. Instead of always using the maximum number of spools at the same time, you could also only use the first half. Or the last half. Let us say that on the first 12 spools CFRP material is stored and on the last 4 spools GFRP material for the outer plies is stored. Only the first and last ply shall be made from GFRP – this would then mean, that the first and last ply only use the last 4 spools for manufacturing, while the other plies use the first 12. When you are trying to establish a multi-material process, this parameter controls how to realize it. 

• Time: Reduction in manufacturing time
• Cost: Optimization of material usage and machine time
• Quality: Minimization of the chance that defects occur

Using the Time/Cost/Quality triple constraint analogy helps us understand that only two of these three criteria can be met at the same time and that there always is a trade-off when selecting them. If a laminate needs to be manufactured quickly and at low cost, its quality is likely to suffer, and defects may occur more frequently. Trying to find the optimal balance is rarely the best outcome since these three criteria are rarely of the same importance. Understanding these trade-offs early helps to communicate that CFRP laminates are dependent on a variety of parameters and processes.

Staggering, minimum tow length, boundary coverage, minimum gap length – each parameter contributes to this triple constraint of time/cost/quality. All parameters of the manufacturing process do: the selection of boundaries, process parameters such as feeds and heating parameters all influence the balance between the manufacturing of the highest quality parts at the lowest possible cost and in the shortest time.

The CAM programmer’s job is to find the right balance between these three constraints and the corresponding parameter set to implement them. Most parameters are defined for the whole part or the whole ply. As further optimization of the programming is done, these may be overridden in courses or even for single tows. An individualized, optimal laminate is then ready to be manufactured.

Next, we will look at how we can change the manufacturing process of the laminate using the end-effector chosen. Since an end-effector can lay multiple tows in parallel, we can also modify which of these “material slots” are to be used.

Until then, stay safe and stay tuned. 

Two more factors directly relate to the minimum gap length; the first being manufacturing time. The manufacturing time decreases when small gaps can just be covered by material since the end-effector does not need to slow down to cut, therefore decreasing the overall duration of the process. Secondly, some end-effector systems also require a minimum gap length since they cannot possibly cut and feed very short material lengths reliably. 

The following figure illustrates the functionality of the parameter as it is implemented in the CAESA® Composites TapeStation:

Figure 1: The blue circles denote a gap of 25mm in length. The gray rectangles represent CFRP prepreg material. In 1) the gap is larger than the minimum gap length and therefore will not be covered with material. In 2) The gap is shorter than the minimum gap length and therefore covered. 

Figure 2: A minimum gap length of 0mm: All gaps shorter than 0mm are covered. Therefore, all gaps are in the ply are left as is.

In this minimalistic example, you can see how this minimum tow length affects the coverage of boundaries.  

Figure 1: In this visualization, the square boundary is fully covered by tapes. More precisely: This is the optimal 45° layout for this geometry, using the 100% boundary coverage setting and ¼’’ prepreg tapes. No other coverage layout uses less material to cover the boundary.

Due to geometrical constraints of AFP layup heads, the tows laid need to be of a certain length – the minimum tow length. For almost all laminates this leads to excess material being placed where short parts of the boundary need to be covered. This might be considered one of the disadvantages of AFP processes but must be viewed in comparison to the many benefits of an automatable and efficient layup process. Alternative manufacturing processes might utilize pick-and-place technology to further reduce the waste produced. These often come with different disadvantages, such as limited part size and increased manufacturing durations.

Let us take a look at how the coverage layout changes when the minimum tow length is taken into consideration: 

Figure 2: In this visualization, the tapes have been extended to match the minimum tow length of the end-effector. In this case, the minimum tow length is 85mm. To fully cover the top left and bottom right corners, large amounts of excess material need to be placed. They are then trimmed afterward and therefore considered waste.

In this example, given the parameters chosen, the excess material cannot be reduced any further. In some scenarios, the direction in which the tows are extended might matter significantly. In the following illustrations, we show you some possible optimizations.

Figure 3: This figure shows the default case: tows are extended symmetrically to match the minimum tow length. The tows between the hole and the outside boundary need to be extended. The difference between that distance and the minimum tow length is split and added to either side of the tow.

Figure 4: This figure shows the tows when they are extended to match the minimum tow length by extending them towards their starting point on the right.

Figure 5: This figure shows the tows when they are extended to match the minimum tow length by extending them towards their endpoint on the left.

Another very useful optimization is an extension towards the center. If you compare the figures above, you will easily be able to imagine how this works. (This exercise is trivial and therefore left to the reader.) In some scenarios, this might make one additional step of trimming the excess material around the outside boundary obsolete. Conversely, always extending to the outside of the boundary might make trimming of the inside holes obsolete. This of course depends on the follow-up processes.

In this article, we showcased why AFP processes often create excess material by default. We also highlighted how the excess material can be positioned most effectively to reduce its downsides. The next article will be about the minimal gap length, which is a parameter that controls how big the gaps in the laminate are allowed to be.

Until then, stay safe and stay tuned. 

Some processes use ultrasonic cutting tools to efficiently trim excess CFRP material after layup. An oscillating blade is moved along the EEOP to quickly cut out a laminate, which has the same* contour (*as close as possible) as the desired part. The boundary needs to be fully covered (and therefore overlap the boundary) by material for this process to work. If there is too little material, the blade might not properly cut the material but instead, tear little pieces apart and delaminate parts of the laminate. This means we need to make sure that for this kind of process a full coverage of the boundary is guaranteed. The same principle holds for most press systems. Because the laminate will be pressed into another shape, it is beneficial when some excess material is available, so it is ensured the EEOP is fully covered afterwards.

We will now compare three different boundary coverage types: 0%, 50%, and 100%. You will quickly see which is best suited for the ultrasonic cutting process!

Depending on the kind of part that is being programmed, it might also make sense to use the boundary as a delimiter, meaning that no tow is supposed to reach outside the boundary. This might be the case for tightly nested parts or if certain holes in a boundary should not be covered. You might then use the 0% boundary coverage strategy.

In our experience, the default case of the boundary coverage parameter is 100%. However, when manufacturing CFRP laminates, the following processes need to be kept in mind, as hardly any of the processes in this complex field of manufacturing can be viewed completely independently of the others.

We hope that this article helped you learn when and why to use different boundary overlap strategies. The next article will be about how the minimal tow length inevitably leads to excess material being placed and how this excess material may be minimized.

Until then, stay safe and stay tuned. 

Background: To accurately calculate the mechanical parameters of CFRP laminates - like the moduli, strength, the coefficients of thermal expansion, and stress responses of the material - the Classical Lamination Theory (CLT) is used. The CLT yields accurate results only when laminates are designed “symmetric and balanced” and “quasi-isotropic”. This means that the order of the plies of the laminate is of symmetric structure and that the most common angles are covered. Usually, these are 0°, 45°, 90°, 135°. Note, however, that a -45° ply is equivalent to 135°. Quasi-isotropic laminates lead to the most predictable parts and are therefore easier to design with.

Figure 1: Two symmetrically designed, quasi-isotropic laminates. The left laminate uses two central plies (135°). The right laminate only uses one central ply. The “vertical offset” describes the offset in “up”-direction, which is perpendicular to the layup table surface. The lateral offset describes the sideward offset of the ply. It is given as a fraction of a tow-width. Ply.5 of the left laminate is therefore shifted by 1/8’’ inch (0.5 * ¼’’). Ply.4 of the right laminate is shifted by (0.33 * ¼’’).

Using the same orientation multiple times can lead to defects in the laminate. Layup tools usually have a (albeit very narrow) gap between their tows by design to prevent them from sticking to each other’s sides during layup. If these gaps are not covered from above, certain regions between the tows may never be directly covered by prepreg material, which leads to air being enclosed inside the laminate. These air pockets are considered weak spots in a laminate and must be avoided at all costs. This is where the staggering comes into play. Plies are shifted sideward relative to the other plies of the same orientation. This leads to the effect that the gaps are immediately covered by the matrix of the following ply. The matrix will melt and expand during the consolidation and the autoclave process to fully fill these gaps. The staggering is usually given as a fraction of the tow width, meaning that a lateral offset of 0.5 is equal to 0.5 times the used material width. The following illustrations visualize the effect of the staggering on the coverage of the boundary.

Figure 2: Lateral offsets of 0 and 0.5. The plies are shifted by exactly half a tow width, which also means that the gaps of one ply are directly above/below the center line of the tows of the other ply.

Figure 3: Lateral offsets of 0, 0.33 and 0.66. The plies are shifted by a third and two thirds of the tow width. This means that their gaps are not directly above each other but rather spread apart and distributed evenly.

A good analogy is a brick wall:

Figure 4: In this figure you can see a sketch of two walls. The wall on the left may look tidier, but it lacks stability. Since the lighter and weaker mortar is stacked, the wall’s strength is greatly reduced in this position. In an even distribution of these weak spots, the walls average stability is much higher.

In this final visualization you can see how the tows of staggered plies are placed. This visualization is taken from the CAESA® Composites TapeStation, which we use to program and emulate laminate manufacturing processes.

Figure 5: A 3D visualization of 3 staggered 45° plies using values of 0, 0.33 and 0.66 tow widths. It is clear that the gaps between the tows are well distributed along the direction of the laminate thickness.

Staggering is one of the parameters needed to optimize the properties and the manufacturing of CFRP parts. The next article will be about another parameter, the so-called “boundary coverage type”. We hope your interest has been sparked and we hope to see you again soon!

Until then, stay safe and stay tuned.

Now we will show you how the plies, also called the ”stacking”, can be defined using our AFP process planning software CAESA® Composites TapeStation. This type of software allows for the offline process planning and programming of AFP processes and closes the gap between design and manufacturing.

The design requirements (see article 3: The AFP Process) define the order and orientation of the plies that the laminate is made of. They also define which exact areas a ply must cover to successfully strengthen the part according to the stress it must withstand. There are further restrictions regarding the maximum allowed gap between tows, how much a tow may deviate from its nominal orientation and many more. A detailed article about design requirements will follow later in this series.

Figure 1: A laminate in which 2 plies have been placed. The orientation of the lower ply is 45°. The emerging “sawblade” pattern is typical when covering straight boundaries with 45° tows.

Laminates. A laminate is built from many different plies that are placed on top of another. In our article about the principles of the AFP process, we visualized how the stacked plies look like. To program the manufacturing process of a laminate, each of these plies needs to be parameterizable. There are some basic parameters like the order, the orientation (which is just an angle for flat laminates), and the boundary (which is a flat contour from a DXF file). The more advanced parameters will be shown in the following weeks: staggering, extending to minimum tow length, boundary coverage types, and parameters for optimizing the material usage of the spools of the layup head.

Figure 2: A stacking that is made from 8 plies. The table shows the name of the ply, the layup angle, and the most common optimizations: vertical offset (“up”) and lateral offset (“sideways”). You can also see the prepreg material and the layup head with which it will be manufactured. These may be changed to a different tool. This enables multi-material or multi-tool processes!

Once a ply has been parameterized, the tows needed to fill the boundary in the desired angle may be calculated. These tows are immediately grouped into courses. Each course contains as many tows as the layup head can lay at the same time. For each of these courses, the layup path the head must take is calculated in the CAM software. Once these, along with the information on where to cut, are transferred to the machine, the layup process can begin.

Figure 3: For better illustration this geometry with a square and a circular hole has been used. You can see the calculated tows that need to be placed to cover the boundary (orange). The angle for the left ply is 0°, for the right ply 45°. In this example, a course groups 4 tows – the alternating darker and lighter hues indicate that the tows belong to different courses.

Once these tows and courses have been calculated, they are used to generate a machine specific layup program which can then be transferred to the control system of the layup tool. The manufacturing can now begin.

In the following articles, we will show the layer parameters in detail, how they influence the laminate, and how you can fine-tune your laminates. We will start with the staggering.

Until then, stay safe and stay tuned.

Note that the term “end-effector” usually refers to the last element in a kinematic chain, such as a robot system. Since this fiber placement head is supposed to be attached to a robot system, the terms “end-effector”, “tool” and “head” can be used synonymously.

Figure 1: A screenshot from the CAESA Composites TapeStation during the simulation of the layup process. The IFWs layup head is attached to a KUKA robot.

The IFW’s fiber placement head can process 4 ¼’’ (6.35mm) tows in parallel. The material storage is integrated into the layup head: 4 times 150m of material is stored on bobbins (spools) that are positioned on the top of the head. From these bobbins, the tows are guided towards an elastic compaction roller. The compaction roller presses the four parallel tows against the layup surface. When the end-effector then moves along the layup surface, a dedicated feeding unit unwinds the correct amount of material from the spools. The feeding unit reduces stress on the tow when it is laid on the mold. Without the feeding unit, the pressure applied between roller and surface and the rotation of the compaction roller would put a lot of tension onto the tows.

Since the material itself needs to be constantly kept cold to prevent the thermoset or thermoplastic matrix material from sticking, the surface it is placed upon needs to be heated. An infrared heating unit is used, which heats the area in front of the compaction roller. In this area there is either the mold surface or previously placed CFRP layers. When the surface is warm enough, the matrix of the applied material heats up on contact with the preheated layup surface and becomes sticky enough to stay in position.

In this video you can see the IFW layup head in action. The material spools are inside the cover. The heating unit starts heating the surface before material is placed. The feeding unit has fed material up to the compaction roller. Once the roller contacts the surface, the material will be fed from the feeding unit. Moving the end-effector while maintaining contact to the layup surface “pulls” the material from the feeding unit onto the surface.

Since all tows may start and end at different locations, the end-effector needs to be able to cut every tow individually. This allows for full flexibility in positioning and length of the tows. Just as the feeding unit enables separate feeding of all 4 tows, a cutting unit enables separate cutting. However, because the cutting unit cannot physically be placed in the same location as the compaction roller, it needs to be placed somewhere between the feeding unit and the point where the compaction roller makes contact to the surface. (If there is no feeding unit, the cutting unit needs to be placed between the bobbins and the contact point.) The closer it is placed toward the compaction roller, the better, as the amount of waste and the duration of the manufacturing process decreases, and the size of the tooling may be reduced. The length between the cutting unit and the compaction roller is called “minimum tow length” since no tow can physically be shorter. The IFWs layup head for instance has a short minimum tow length of 68mm.

Figure 2: A detailed description of the layup heads modules. Image taken from the article „Automated Fiber Placement Head for Manufacturing of Innovative Aerospace Stiffening Structures“ published by the IFW. [ScienceDirect]

Now that we have covered the fundamentals of the AFP process and the minimal requirements to manufacture CFRP laminates, we will go into detail about specific challenges that arise and how to prevent them.

Until then, stay safe and stay tuned.

Thanks to our colleagues at the Collaborative Research Department for Composite Technologies at CFK Nord for supporting us with their media content. Feel free to contact Dr.-Ing. Carsten Schmidt for further information or visit the LinkedIn page.