Consolidation of Thermoplastics - P170

 

Consolidation of Thermoplastics
Practice document
Document Type	Practice
Document Identifier	170
Themes	Materials deposition and consolidation management
Tags	Bagging Debulking Prepreg Resin flow Consolidation Material Porosity Thermoplastic polymers
Objective functions	Cost Maintain Rate Maintain Quality Maintain
MSTE workflow	Development
Prerequisites	Integrated Product Development Practice Practice for Developing a Process Step Thermoplastic polymers

Overview

The page covers a brief overview of consolidation processing of thermoplastics, including the mechanics behind the process and some of the equipment choices available for a consolidation process.

Introduction

Thermoplastic consolidation is a crucial part of creating a final product in order to dictate the final properties and shape of the part. Consolidation requires elevated temperature, applied pressure, and time, These parameters vary depending on the material type, form and shape, but the basics are the same across most thermoplastics. Temperature, pressure and time allow for the diffusion of polymer chains in order to create a cohesive part.

Significance

Consolidation of thermoplastics is a critical step in the fabrication of composite parts. This process ensures that the individual layers of thermoplastic materials, such as prepregs or films, are effectively bonded together to form a strong and cohesive structure. The consolidation process not only ensures the mechanical integrity of the final product but also enhances its thermal and chemical resistance properties.

Processing

Example of a consolidation and heating process for a thermoplastic.

Consolidation of thermoplastics consists of heating, applying pressure and cooling, refer to the image on the left for an example of a process cycle. Just like with other kinds of composite materials, the main things that affect how this process works are how long it takes (time), how hot it gets (temperature), and how much pressure is applied. Heating the thermoplastics can be done with a variety of different options like: infrared ovens, convection ovens, hot presses, and autoclaves. The time required to reach consolidation temperature depends on only the heating method, mass of material and mass of the tooling because there are no chemical reactions occurring.

The temperature needed for consolidation depends on the type of thermoplastic being used in the process. For thermoplastics that are amorphous, they need to be heated to above their Glass transition temperature (Tg). For semi-crystalline thermoplastics, they need to reach temperatures above their melting temperature to achieve consolidation. As a rule, if you're working with an amorphous thermoplastic, you should aim for a processing temperature 220°C above its glass transition temperature, and for a semi-crystalline one, it should be about 100°C or less above its melting point. But degradation can occur by heating most thermoplastics above 425°C.

The amount of time needed for consolidation at a certain temperature mostly depends on the form of the thermoplastic. For instance, if a well-consolidated hot-melt impregnated tape is being used, it can be consolidated successfully in very short periods, sometimes just minutes or even seconds, because the resin melts quickly and spreads through the material easily. However, materials like woven powder coted or comingled prepregs need more time for the resin to melt and spread through the fibers. Sometimes, a technique called film stacking is used, where layers of thermoplastic film and dry woven cloth are stacked and pressed together. In these cases, consolidation takes even longer because the highly viscous resin has to travel further through the layers to fully impregnate the fabric.

A typical processing cycle to ensure that the fibers are completely impregnated and the laminate is fully consolidated when using film-stacked layers would involve applying a pressure of 150 psi for 1 hour. Similar to the heating process, cooling rate after consolidation depends on the method used for processing and the mass of the tooling. However, it's important not to cool down semi-crystalline thermoplastics too rapidly (known as quenching) because doing so might prevent them from forming the desired semi-crystalline structure, which is crucial for optimal performance at high temperatures and resistance to solvents.

While cooling, it is necessary to maintain pressure until the temperature drops well below the glass transition temperature (Tg) of the resin. This helps prevent the formation of voids, delays elastic recovery in the fibres, and preserves the desired dimensions of the final product.

Lastly, the applied pressure during the consolidation process provides the driving force for the fibre to become impregnated with the resin and provide the shape of the part. It is important to note that the properties achieved by prepreg that has been hot-melt impregnated is generally higher than the properties seen by powder coated, comingled and film stack laminates. This is due to the fiber-to-matrix bond that is formed during the hot-melt impregnation process.

For more see Introduction to the processing of thermoplastic composites

Mechanics

Molecular movement through diffusion during the autohesion process.

Thermoplastic consolidation happens through a process known as autohesion. This means that when two surfaces meet, they need to come into intimate contact before the polymer chains can spread across the boundary and achieve full consolidation. Because thermoplastic prepregs don't flow easily and can have uneven thicknesses, the surfaces need to be physically pressed together and heated up to ensure they are close enough for the polymer chains to move between them effectively. This process creates the right conditions for the chains to bond at the interfaces between layers, see figure to the left.

To achieve intimate contact and autohesion, the material needs to be heated above its glass transition temperature (Tg) for amorphous materials and above its melting temperature (Tm) for semi-crystalline materials. Typically, using higher pressures and temperatures can shorten the time needed for consolidation. Autohesion involves a diffusion-controlled process where the polymer chains migrate across the interface and intertwine with nearby chains. As the contact time between the surfaces increases, more polymer chains become entangled, creating a stronger bond at the interfaces between layers.

For amorphous thermoplastics, consolidation generally takes more time due to the fact that the resin does not melt and maintains a higher viscosity. This can be somewhat mitigated by using higher pressures to force the movement of the resin. A thermoplastic must be pressed together to some extent before autohesion can occur, this is due to the fact that the time required for autohesion is directly proportional to the resin viscosity. The consolidation process is considered complete when the fiber bed is compressed enough to react to the pressures applied, see figure below.

Thickness change of a thermoplastic layup during consolidation and heating, showing different mechanisms during the process.

Equipment

There are many forms and types of equipment that can be used to consolidate thermoplastics. The following will highlight some of the common equipment and the main considerations when using them.

Platen press

This process typically uses a hand lay up of the thermoplastic prepreg sheets, which are preheated in an oven before being placed in a press with a die for the final shape of the part. Since it is a hand layup, a soldering iron is sometimes used to hold/tack the sheets together as they have no tack at room temperature. The platen press can be heated and then cooled down, maintaining pressure for a successful consolidation. If the time at pressure for a specific prepreg is very low, then the platens may not be heated and the consolidation process and cooling can happen at the same time. A similar process to this can be done with rolling belts, instead of platens, with a heating and cooling zone can be used for continuous processes.

see Hot press

Extrusion and Injection molding

Through the use of an Archimedes screw and different heating stages, this process applies pressure and heat to the thermoplastic as it is moved along the length of the machine. The screw gradually applies pressure to the material and can be designed in different ways to provide pressure at different rates and sections of the screw. The material is moved through heated sections to begin the autohesion process. For an extrusion process, the material needs to be fully consolidated before exiting the screw as that is the final stage before a cooling die. For an injection molding process, the material needs to be partially consolidated before entering the mold, but some pressure and consolidation effects can be applied in the mold as well.

also see Pultrusion of Thermoplastic Composites

Autoclave

For complex part configurations, autoclave consolidation is an option, however, it has drawbacks. It can be challenging to find autoclaves capable of reaching the high temperatures (about 340-400°C) and pressures (about 100-200 psi) needed for some advanced thermoplastics. Also, the tooling required for such high temperatures is expensive and often large, leading to slow heating and cooling rates. Matching the coefficient of thermal expansion between the tool and the part is crucial, typically using materials like monolithic graphite, cast ceramic, or Invar 42 for carbon fiber thermoplastics.

Bagging materials must withstand high temperatures and pressures, typically requiring high-temperature polyimide bagging materials, glass bleeder cloth, and silicone bag sealant. Polyimide materials like Kapton or Uplilex are brittle and more challenging to work with compared to nylon materials used for lower temperature thermosets. High-temperature silicone rubber sealants have minimal tack at room temperature, requiring clamped bars to help seal effectively.

A typical autoclave consolidation cycle for carbon/PEEK prepreg involves temperatures of 360-399°C and pressures of 50-100 psi for 5-30 minutes, but the actual cycle time to heat and cool large tools is usually 5-15 hours. Despite these drawbacks, autoclaves remain useful for fabricating thermoplastic composite parts that are too complex for other methods.

see Autoclave

Auto-placement

Auto-placement or in situ placement, involves immediately consolidating the material into the layer below through directed heat and pressure. This includes processes such as hot tape laying, filament winding and automated fibre placement. Two common ways in which heat is applied to the part is through conductive heating from blocks of heated metal, known as “shoes”, or through a focused laser beam aimed at the compaction point of the material into the substrate. Other options include, hot gas torches, infra red heaters and quartz lamps.

This unique method of consolidating thermoplastics utilizes a very short heating and cooling cycle at very high rates, where consolidation can occur at less than 0.5 seconds. This avoids material degradation issues, which are known to occur when thermoplastics are held at high temperatures for extended periods of time. The downside of this technology is that the short consolidation time leaves little time for diffusion of molecular chains to occur. This can lead to voids in the ply interfaces, resulting in reduced interlaminar shear strengths.

Related pages

Page type	Links
Introduction to Composites Articles
Foundational Knowledge Articles	Glass transition temperature (Tg) - A210 Thermal phase transitions of polymers - A102 Thermoplastic polymers - A161 Viscosity (resin) - A203
Foundational Knowledge Method Documents
Foundational Knowledge Worked Examples
Systems Knowledge Articles
Systems Knowledge Method Documents
Systems Knowledge Worked Examples
Systems Catalogue Articles
Systems Catalogue Objects – Material
Systems Catalogue Objects – Shape
Systems Catalogue Objects – Tooling and consumables
Systems Catalogue Objects – Equipment
Practice Documents	Consolidation of Thermoplastics - P170 Debulking - P167 Practice for Developing a Consolidation Step - P158 Practice for Developing a Deposition Step - P157 Vacuum Bagging - P168 Vacuum Bagging for Prepregs - P172 Vacuum Bagging for Vacuum Infusion - P173
Case Studies	Use of Right Sized Simulation to Aid Decision Making in Thermoplastic Composite Manufacturing Systems - C114
Perspectives Articles	Introduction to the processing of thermoplastic composites - A322 Introduction to the welding of thermoplastic composites - A323 Pultrusion of Thermoplastic Composites - A334

References

About	Help

Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.

A class of polymer, some common examples include polypropylene and polyethylene.

They soften and melt upon heating (i.e. potentially recyclable), high viscosity when melted, therefore difficult to saturate fibres. Usually needs a lot of pressure and heat to process.

Literally "without structure", randomly coiled molecular polymer chains.

A portion of the molecular polymer chains tend to ‘fold up’ into densely packed regions called crystals.

The glass transition temperature (T_g) is the temperature region where the polymer transitions from a hard, glassy material to a soft, rubbery material. It is one of the most important properties of any amorphous polymer.

For polymer matrix composites (PMCs), resin refers to the matrix; the continuous material phase that binds the reinforcement together, maintains shape, and transfers load. Resins are divided into two main groups: thermosets and thermoplastics.

Pre-impregnated (prepreg) material refers to fibre that is already combined with resin. It is the most common material form used in aerospace.

During prepreg production, (e.g. fibres are run through a resin bath), prepreg is heated and partially cured to B Stage (< 5 % degree of cure). Thermoset prepregs (e.g. epoxy prepreg) have to be kept in a freezer at around -20 °C. At room temperature, the epoxy starts to cure.

The continuous material phase that binds the reinforcement together, maintains shape, transfers load, protects the reinforcement from environment and damage, and provides the composite support in compression.

Desirable characteristics:

• Moisture/chemical resistance
• Low density
• Processability

In composites processing, viscosity is an indicator of how easily the resin matrix will mix with the reinforcement and how well it will stay in place during processing. The lower the viscosity, the more easily resin flows. Resin viscosity ranges considerably across chemistries and formulations.

By scientific definition, viscosity is a measure of a material’s resistance to deformation. For liquids, it is in response to imposed shear stresses.

A single unit of fibre reinforcement that cannot be separated further without breaking it. It often has a circular cross section that is formed by the dies used to create it or its precursor material. Many filaments are manufactured in parallel to produce a strand or tow. The number of filaments in a strand or tow is often references by how many thousands of filaments comprise the tow. e.g. an 18k tow is made up of ~18000 filaments.