An in-depth look at the in-situ consolidation in thermoplastic composites

What's covered

This article aims to provide

  • How thermoplastic AFP differentiates from the thermoset AFP

  • The promise of the thermoplastic materials

Specific details of how to improve thermoplastic placement quality

  • Learning principles and theories of in-situ consolidation

  • How to avoid thermal degradations while doing the thermoplastic placement


Introduction

An overview of in-situ consolidation (ISC) is provided in this article. As the ISC manufacturing process evolves, it is additive in nature, limited by the orthotropic nature of composite materials, and thermoplastic matrix behavior. Key topics covered are as follows

  1. Introduction to ISC and recent innovations

  2. Adhesion mechanisms and their influence on the process

  3. Crystallinity is the most influential parameter in determining mechanical properties.

  4. Degradation refers to the potentially irreversible changes in the polymer structure caused by the high temperatures required for the process.

The article is intended to help readers understand the key process parameters and enable them to achieve high product quality with a high deposition rate. This is achieved through an industrial process that is competitive with the current thermoset composite manufacturing process.


Jump directly to the right information

  1. Why do thermoplastic composites cost more than thermoset composites?

  2. Why thermoplastic composites are preferred over thermoset composites?

  3. What makes thermoplastic AFPs superior to thermoset AFPs

  4. What is the in-situ consolidation of thermoplastic composites?

  5. The effects of different layup conditions on porosity, layer joining, and processing speed

  6. Tooling temperature

  7. Compaction force

  8. Number of layers

  9. Duration of heating

  10. Tape quality

  11. Pre-heating the incoming tape

  12. High-speed effect

  13. Heated tooling size

  14. Roller cooling

  15. History of innovations in the in-situ consolidation process

  16. Other Key mechanisms

  17. Adhesion

  18. Intimate Contact by improved surface finish of tape

  19. Self-Adherence of Polymeric Chain

  20. Degree of bonding

  21. Crystallization

  22. Thermal Degradation

  23. Article Summary and references


Why do thermoplastic composites cost more than thermoset composites?

The development of thermoplastic polymeric matrix materials has not yet reached the same level as thermosets. This may be because of high process temperatures and lower productivity with thermoplastics. As opposed to thermoset composite materials, thermoplastic composite materials have no tackiness at room temperature and are solid at room temperature. Despite the fact that the matrix has already been polymerized, the viscosity values remain very high, making the process more complex and expensive than thermosets.


Why thermoplastic composites are preferred over thermoset composites?

Image Source: http://www.bbc.co.uk/guides/z9tysg8

  • Structures can be manufactured with high levels of integration, eliminating the need for subsequent assembly

  • Low risk of contamination or toxicity when handled by humans and practically limitless life without refrigeration

  • The ability of thermoplastics to be recycled and reused has become increasingly relevant as sustainability and environmental conservation become more critical.

What makes thermoplastic AFPs superior to thermoset AFPs

A thermoset AFP system with speeds of one meter per second is used to fabricate large structures. The material is partially cured, so it retains its tackiness, which makes it possible to join the different layers without any heating. During the second step, the completed layup needs to be placed in vacuum bags to be autoclave cured. The second step is highly manual and requires a huge autoclave i.e. pressurized oven, making the entire process extremely slow and expensive.


Thermoplastic AFP processes lack the tackiness between layers at room temperature, as opposed to Thermoset AFP, and require higher temperatures to melt the material (semi-crystalline) or to transition to glass (amorphous). As a result, it allows for the lay-up to be completed, while simultaneously ensuring high-quality bonding between layers, suppressing the need for subsequent curing stages in an oven or autoclave.


What is the in-situ consolidation of thermoplastic composites?

In situ consolidation refers to the placement of thermoplastic composite layers simultaneously while ensuring high-quality bonding between them. This process involves multiple physical parameters that take place simultaneously involving heating, cooling, pressure, placement speed, material FVF, tooling, and environmental temperature. If this process is to be applied in a profitable way, it must achieve high degrees of consolidation without having to repeat the re-consolidation process in an oven/autoclave twice.


The effects of different layup conditions on porosity, layer joining, and processing speed

Tooling temperature


When tool temperatures approach the melting temperature of the polymer used in the tape, the trend of decreasing void volume reverses. When the temperature is low, the tool usually acts as a heat sink, causing a high viscosity that prevents smooth movement of the chains. It is necessary to slow down the placement speed in this case due to the low degree of bonding. Alternatively, when the tool is heated to a high temperature (around 573 K for PEEK), effective bonding is achieved even close to 1.5 m/sec.


Compaction force

Compaction roller stress distribution
image source: https://cimcomp.ac.uk/wp-content/uploads/2017/05/Figure1.png

Bonding increases as contact forces rise, as is expected, and void content decreases as well. Its effect is more evident as the layup speed increases (the faster the layup, the greater the force). As a result of increasing force and increased speed, the porosity should decrease, whereas the degree of bonding decreases with speed despite increasing force.


Number of layers

Thick composites laminate

When the number of layers of a laminate is high, a double effect occurs; the lower ones receive more consolidation stages (reconsolidation), but the upper ones are more isolated from the tooling surface. As a result, they experience increased heating, resulting in larger pores.






Duration of heating

Image source: https://www.heraeus.com/media/media/hng/media_hng/products_and_solutions/arc_and_flash_lamps/Humm3_table_advantages_EN.jpg

The level of porosity is increased by longer heating durations. In contrast, bonding is better, and the process can be carried out faster. Heating length and heating time are directly related since they depend on the speed of lamination.


Tape quality

Tape surface roughness measured with laser line scanner
Image Source: https://d2n4wb9orp1vta.cloudfront.net/cms/brand/CW/2019-CW/cw-designandtesting-0519-fig1-web.jpg

Improvements in the taped material quality could improve the thermoplastic AFP layup

Effect of additive approach: As a result of the sequential increase in layers of material, the layup process also has a penalizing effect. This leaves the material in an unfavorable state from the standpoint of stress. When the laminate cools evenly, as it does in an autoclave, rather than being consolidated layer-by-layer, the residual stresses are different. Depending on the residual stress gradients, microcracks may develop in the resin when annealing over Tg.


Pre-heating the incoming tape

In order to achieve a progressive drop in temperature when the tape and substrate come into contact, resulting in more progressive cooling and therefore reducing residual stresses.


High-speed effect

A higher fiber placement speed increases residual stresses because there are more point zones experiencing higher thermal gradients.


Heated tooling size

Residual stress is also affected by the size of the heated surface. The smaller the heated surface, the sharper the heating-cooling profiles. Better results are observed when the heated surface of the substrate is larger than that of the contribution.


Roller cooling

To achieve a higher surface finish, it is preferred to cool the roller with internal fluidic flow

Source: THERMOPLASTIC IN SITU FIBER PLACEMENT FOR FUTURE SOLID ROCKET MOTOR CASINGS MANUFACTURING

History of innovations in the in-situ consolidation process

It was first patented by DuPont in 1986 (layup method and equipment patent),

A patent issued by Northrop Corporation, an American aircraft manufacturer, on this topic was issued in 1991. It proposes to use a heated roller and a cooling system to accomplish this.

In 2002, Boeing also protected its work on the heating method and heating control system used in composite material layup, without specifying the type.

Using a technology called co-consolidation, Airbus filed a patent in 2003 to protect the development of a layup with integration

Following this, many machine manufacturers protected the following features

  • Heating control in the thermoplastic layup process

  • Layup equipment, layup material, and manufacturing technology for the incorporation of lightning protection systems.

  • The proposal for the use of a flexible compaction system consisting of segments that apply pressure independently

  • The compaction roller segments that make up the roller are in turn cylinders with an interior made up of elements with curvatures that give it increased flexibility.

Other Key mechanisms

Adhesion


Adhesion model over time
Image source: A Model of the Manufacturing Process of Thermoplastic Matrix Composites

It is necessary to describe the adhesion mechanism between the layers of composite materials in a manner that takes into account two factors: On the one hand, it is necessary to eliminate surface roughness in the incoming material in order to allow the polymer chains to move, and, on the other hand, a mechanism that controls this movement of chains.

Intimate Contact by improved surface finish of tape

As a result of the inherent irregularities in the surfaces of thermoplastic pre-impregnated composite materials, gaps occur between two surfaces that are brought into contact without applying pressure or temperature. Since thermoplastic resins have high viscosities, they are limited in their ability to cover these areas by natural flow, so pressing both surfaces is necessary (application of pressure) to force their contact.

Self-Adherence of Polymeric Chain

As two polymeric surfaces come into contact at temperatures above their glass transition, a diffusion of polymer chains occurs at the interface, often referred to as healing, diffusion, or self-adhesion. In order to develop a resistant joint, chains must diffuse correctly so that the interface becomes indistinguishable.

Degree of bonding

a. Deformation of each layer in the actual process b. Pressure affected length of various layers
Image source: Multi-pass layup process for thermoplastic composites using robotic fiber placement

After obtaining the degree of intimate contact and diffusion, it is possible to combine their results to determine the degree to which the bonds are generally formed. Since the contact time is limited, diffusion has a small effect on bonding. Temperature is largely determined by speed, heat source, and material properties.

Crystallization

Crystallization is one of the key factors in the development of certain mechanical or structural properties. During crystallization amorphous material nucleates and grows spherulitic. Transcrystallization is a process of crystal growth affected by restricted space, which is representative of fiber-reinforced composite crystallization behaviors.


 Schematic diagram of the difference between TC layer and cylindritic crystallization
image Source: On transcrystallinity in semi-crystalline polymer composites

As a result of the high adhesion between resin and fiber, the highest value is achieved at lower cooling speeds. It is possible for the mechanical properties of a material to vary because of the correct or incorrect development of the transcrystalline layer during cooling

Thermal Degradation

Thermal degradation is the irreversible loss of physical, mechanical, or electrical properties of a polymer due to the action of heat or high temperatures.

Hydroquinone and benzoquinone are also formed at 750°C and less stable radicalsand diradicals can be formed at this temperature.
Image source: Mechanism of thermal decomposition of poly(ether ether ketone) (PEEK) from a review of decomposition studies

As polymeric materials decay, their long molecules break down into smaller segments that can be volatile. As soon as the fragments are generated, their evaporation ability increases; the rest of the molecules remain in the condensed phase (liquid or solid) and continue to decompose until the fragments are of the appropriate size.


Article Summary

The in-situ consolidation process for manufacturing thermoplastic composites is a delicate balance between a lot of parameters, as summarized below

  • High temperatures are required for local melting and bonding of the substrate and incom