Autoclave - A173

Autoclave
American Autoclave Co. industrial autoclave
Document Type	Article
Document Identifier	173
Themes	Thermal management Residual stress and dimensional control management Materials deposition and consolidation management
Relevant Class	Equipment
Tags	Convective heating Debulking Equipment Thermal transformation Consolidation Vacuum application Object
Factory Cells	Thermal transformation
Prerequisites	Convection Effect of equipment in a thermal management system Equipment Heat transfer Thermal transformation

Introduction

An autoclave is a piece of equipment that provides an environment of elevated temperature and pressure. It is typically used for thermal transformation and consolidation of advanced composite prepreg materials. An autoclave generally consists of a pressure vessel with a heating and cooling system integrated into it. The heating/cooling system consists of a blower, heating elements, and a controller to circulate pressurized gas throughout the vessel. The application of heat to the part functions according to the principles of convective heat transfer (forced convection), which is enhanced due to the higher density of pressurized gas being able to transfer heat more efficiently to the part. Autoclave processing is one of the most widely used processing method for high-performance composite material in the aerospace industry.

Autoclave depicting airflow

Scope

This page provides general information on autoclaves as devices for thermal transformation and consolidation. This includes: properties, use, and supplier information. If specific information regarding brands of autoclave is desired, it is best to contact a supplier. For information on suppliers, see the section on suppliers below.

Significance

An autoclave is a critical piece of equipment for processing prepreg composites because the high pressure it provides generally results in parts with better compaction, higher fiber volume fraction and less porosity. Along with ovens and hot presses, autoclaves are one of the primary pieces of equipment used in high-temperature composites processing. Autoclaves provide higher compaction pressure compared to ovens and are capable of processing larger components comparing to hot presses. Understanding the properties of such equipment allows for one to understand their function as part of a thermal, material deposition, and residual stress and dimensional control system. In addition, purchasing an autoclave is a significant investment, typically more expensive than an oven or hot press, which requires the customer is well-informed.

Prerequisites

Recommended documents to review before, or in parallel with this document:

• Thermal transformation
• Heat transfer
• Convection
• Effect of equipment in a thermal management system

Overview

An autoclave can be described as a heated pressure vessel or a pressurized oven. Once the uncured composite material (autoclaves generally work with prepreg material) is laid up and vacuum bagged, the autoclave applies a temperature, pressure and vacuum cycle/schedule (AKA a cure cycle) prescribed by the operator to cure the resin and solidify the composite material.

An autoclave typically consists of a gas circulation system, temperature control system (heating and cooling), a pressurization system and a vacuum system. For heating, the energy source can be either electricity or gas. For pressurization, the fluid is typically an inert gas to reduce the fire hazard. Nitrogen is the most commonly used. The heating and pressurization systems play important roles in the performance and cost of the autoclave. Typically for aerospace applications, uncured material/parts arrive at the autoclave already bagged and vacuumed. When parts are being cured in the autoclave, volatile gases come out of the material. Autoclaves are typically equipped with many vacuum ports to extract the volatiles and provide an isolated pocket for the positive pressure of the autoclave to be applied to.  

From the perspective of a composite part being cured in an autoclave, the thermal and pressure history the part experienced has direct and significant impact on the quality, longevity and dimensional stability of the finished products. Hence, it is crucial to get an autoclave of the correct size and with the correct performance specifications to achieve cost-effective processing. If the autoclave is under-specified then parts cannot be cured at the required rate, or perhaps at all. If the autoclave is over-specified, it will be uneconomical and inefficient to own and operate.

Due to the critical nature of autoclaves being pressure vessels, the "fail to safety" concept is implemented throughout their design. Industrial autoclaves are required to be manufactured according to the governing pressure vessel code for their jurisdiction of operation (such as the ASME boiler and pressure vessel code) and inspected and maintained regularly in accordance to various standards and jurisdictional requirements.

From the composite material engineering standpoint, it is essential to control and monitor parameters within the autoclave. Gas temperature uniformity, gas flow patterns, heat transfer coefficients and others should be characterized prior to the use of the autoclave to ensure the even cure/crystallization of the part. Lastly, temperature, pressure and vacuum history throughout the manufacturing cycle should be recorded and analyzed to link input system parameters to outcomes.

Object description

Gas Circulation system

The circulation system of an autoclave moves gas (commonly an inert gas such as nitrogen) within the autoclave to (ideally) provide uniform temperature distribution during autoclave operation. In the absence of uniform temperature distribution, knowledge of the actual temperature distribution and its effect on the part, or modification of the cure cycle to compensate is the next best set of information.

Typically, this circulation is achieved by a fan at the end of the autoclave that is opposite from the door. The fan moves the gas through the heating/cooling system to achieve a target temperature. Typically, the gas gets pushed towards the front of the autoclave while being heated/cooled. The curvature of the front door naturally redirects the gas and it flows to the center of the autoclave where the parts typically are, then back to the fan at the rear. The two most common gas flow configurations are shown in the figure below. The annulus duct guides gas along the outer perimeter of the autoclave towards the front door whereas the floor duct guides gas below the autoclave outer perimeter and internal floor. There are other gas circulation designs but the basic principle is similar.

Autoclave gas flow types annulus vs floor

A centrifugal blower, or fan, is typically used to circulate the gas. The motor that drives the fan can be either encapsulated within the autoclave pressure boundary, or be mounted externally. If encapsulated within the autoclave pressure boundary, the motor assembly needs to withstand the autoclave pressure and temperature during operation. A separate cooling system might be required for cooling the fan motor. The fan size and motor power should be specified to meet the required gas flow velocity at operating pressure. An example of fan sizing analysis can be found here: https://www.sciencedirect.com/science/article/pii/B9780128035818098994?via%3Dihub#f0050.

Achieving uniform HTC within the autoclave working volume is non-trivial, especially when the autoclave is loaded with tools and parts which can interfere with the gas flow. Different methods have been developed to improve the gas flow and the resulting local heat transfer coefficient. Flow adjustment panels/meshes attached to the exit of the blower where the gas enters the main autoclave working chamber (usually at the autoclave door) can improve the uniformity of the airflow as it is recirculated. Baffles installed on the side walls and roof can also be used to direct gas flow along the length of the autoclaves. It is ciritical that parts have a suitable HTC during cure and HTC may depend on the location within the autoclave. In practice, however, when a batch of parts (with different sizes, shapes and tools) are cured in the same autoclave run, the arrangement of the parts in the autoclave is often still done by experience. This can introduce variations in the convective boundary conditions that the parts experience from run to run, which ultimately affects part quality and consistency. Emerging research on flow simulation and machine learning is capable of optimizing the cure cycle in real time based on the different boundary conditions each part is experiencing such that every part will conform to the specifications.

Heating system

The autoclave heat source can be either electrical or natural gas. Consequently, autoclaves can be classified into electrical heated or steam heated. Electrical heating heats the autoclave using electrical coils while the steam heated autoclaves can be further characterized into wet steam and dry steam. Wet steam is created using a boiler, while dry steam is generated using a heat exchanger.

The choice of heating system is influenced by various factors such as temperature control accuracy, environmental impact, efficiency, maintenance and energy costs. Built-in control systems are typically used to manage the autoclave temperature to the desired cure cycle specifications. Depending on the specific control system and its accuracy, autoclave gas temperature can sometimes fluctuate around the prescribed temperature. When considering operational costs, it is important to evaluate the system as a whole. The operational costs are mainly driven by the heating cost and may exceed the capital costs over the operational life of an autoclave. An efficient heating system will not only lower the operational costs, but also reduce emissions and may affect the maximum heating and cooling rate.

Cooling system

Similar to the heating system, the autoclave fan pushes gas through the cooling coils/heat exchangers to decrease the gas temperature, thus cooling the autoclave. Water is typically used as the cooling fluid. To save water consumption, a closed-loop cooling system can be used to circulate water. A closed-loop system typically has a water holding tank and some means to remove heat such as a refrigeration system. This can be achieved by using open/closed evaporative towers, exchangers or chillers depending on the specific application. Key points to consider when choosing a cooling system are :

• Autoclave size and required cooling rates: large autoclaves and/or high cooling rates require more powerful/effective cooling systems
• Climate where the autoclave is located: when operating in cold (sub-zero) climates, glycol is commonly used in the water as anti-freeze.
• Total dissolved solids in cooling water: as water evaporates, lime and dissolved solids can build up in the water tank and piping over time. Maintenance such as flushing and refilling should be scheduled regularly
• Elevation of water towers: tower can be mounted on the roof of the building or on elevated stand structures

Pressurization system

Depending on the specific requirements, autoclaves can be pressurized by air or an inert gas such as nitrogen. In composites manufacturing, nitrogen is typically used to mitigate fire hazards in the autoclave. When nitrogen is used as the pressurization fluid, exhaust gas needs to be directed outside of the building/facility to avoid asphyxiation.

Often, nitrogen generation systems are used to extract nitrogen from the air for use in the autoclave rather than purchasing nitrogen from a supplier. A typical nitrogen generation system consists of an air compressor, compressed air storage tank, nitrogen generator and a nitrogen storage tank. Ambient air first gets compressed by the compressor. The compressed air then flows through a series of filters to remove contaminants, water and oil from the compression phase (optional) before being stored in the storage tank. Compressed air is fed into the nitrogen generator, which separates the nitrogen from the compressed air. The separated nitrogen then flows into the nitrogen storage tank where it is held until needed. The nitrogen storage tanks are sized such that the free-air delivery (FAD) of the storage tank is around 2.5 times the FAD of the autoclave.

Autoclave nitrogen generation system

Vacuum system

Vacuum systems play a key role in the advanced composite manufacturing process. Upon arriving at the autoclave for curing, composite parts are typically already under vacuum, de-bulked and consolidated. The purpose of the autoclave vacuum system is to further remove entrapped air and voids in the composite part, remove volatiles during the resin curing process and contribute to part consolidation.

An autoclave vacuum system typically consists of a vacuum pump, catch can, vacuum reservoir (tank), buffer tank, measurement lines and suction lines. Rotary-vane, oil-seal vacuum pumps are typically used for composite manufacturing process because of the low cost, high vacuum level (29.8 in-Hg gauge at 60hz operation and sea-level) and high flow rate. Rotary-vane can produce flow rate up to 1600 m3/h . A rule of thumb for specifying a vacuum pump is that it should provide approximately 2 cfm to each of the autoclave’s source lines or 4.12 cfm (7 cubic meter per hour) per 1 square meter of bagged area.

As the name suggests, a rotary-vane oil-seal vacuum pump requires oil to ensure the seal and lubrication. The oil also helps cooling the pump by dissipating heat. A rotary-vane oil-seal vacuum pump may come with an exhaust filter, vibration isolators, and anti-suckback valves.

For most composite cure cycles, vacuum is required to be either fully on or off during the cure cycle. However, in some high-temperature polyimides and honeycomb composite processing cure cycles, partial vacuum is required. In those cases, a vacuum feedback and control system (typically pneumatic controlled) will be required. Other considerations when specifying a vacuum system for autoclave composite processing are:

• Ensure the vacuum system has adequate vacuum ports and vacuum line conductance
• Integration of catch cans to trap excess resin, solvents and volatiles
• Potential integration of gas flow or leak detection system to locate and assess vacuum leaks
• In event of leak or burst, the vacuum system should be capable of isolating the breached part

Advantages

• Good temperature/pressure control
• Capable of processing large components comparing to hot presses
• High fiber volume fraction and low porosity
• Good surface finish and dimensional control
• Capable of processing parts with complex geometry
• Capable of processing multiple parts simultaneously

• Higher heat transfer coefficient (HTC) compared to a typical oven; 60-200 W/m²K compared to 15-50 W/m²K, respectively, therefore allowing for better temperature control

Limitations

• High initial investment due to equipment and tooling costs. Typically higher than that of ovens and hot presses
• Not economically suitable for making a small number of parts
• Require inert gas supply
• Higher inspection/maintenance cost comparing to ovens and hot presses due to pressure cycles
• Higher energy consumption comparing to ovens and hot presses

Use

In the context of composite manufacturing, an (industrial) autoclave is typically used for the thermal transformation of composite materials. More specifically, curing or crystallization of thermoset or thermoplastic components, respectively. Due to the high capital and operating cost, autoclave use is usually associated with high performance composite material such as a thermoset pre-preg material systems for aerospace application.

In a production environment, an autoclave can either be dedicated for manufacturing a specific part or cure batches of multiple different parts. In the latter case, a batch of parts (with different sizes, shapes and tools) arrive at the autoclave to be loaded and cured in one autoclave run. All parts would theoretically experience the “same” cure cycle. However, in reality, the airflow in the autoclave may not be uniform. Compounding with the fact that the parts’ positions typically arranged by technicians based on experience, the cured parts may exhibit non-uniformity, inconsistency and out-of-spec properties.

A generic cure cycle for a given material system is usually provided by the material manufacturer (manufacturer recommended cure cycle, MRCC). However, in practice, MRCCs may be fine-tuned for specific applications that account for parameters such as thickness gradient in a laminate, large tooling thermal mass or be used to reduce peak exotherm temperature, etc.

A temperature cure cycle can have multiple stages that consist of temperature ramps and holds. As the temperature heats up from ambient temperature, resin viscosity starts to decrease. The first stage typically aims to allow resin to flow, trapped air and volatiles to escape via vacuum and achieve better consolidation. As the temperature continues to increase, intermediate stages can be used to reduce peak exotherm temperature. The final stage helps to attain the target degree of cure. When varying the temperature ramp rates and hold length of a cure cycle, the resin cure rate, fiber-bed consolidation, resin viscosity (affecting resin flow and fiber wetting) and residual stress should be carefully and systematically considered such that the final cured part complies with specifications.

While pressure uniformity in an autoclave is relatively easy to achieve, temperature uniformity is much more difficult. Variables that can affect the part temperature uniformity within an autoclave include:

• Material: material type; usage of core material; exotherm; etc.
• Shape: part size; part thickness; part geometry; etc.
• Tooling: tool material (thermal mass); tool substructure; tool geometry (size, shape, thickness length etc.); tool orientation within the autoclave; tool position with respected to other tools and parts being cured at the same time; etc.
• Equipment: autoclave intrinsic airflow pattern; pressure; sensors/wires/tubing blockage of the airflow; etc.

Please see thermal management for more detail.

Due to the large number of variables, designers must specify the cure cycle for each unique combination of autoclave and part(s). Rather than the trial and error approach, process simulation is often used to reduce development cost and time. Thermal management within an autoclave plays an extremely important role in manufacturing good composite parts. See an example of Boeing specification D6-49327 Qualification of Autoclaves for Metal Bonding and Curing Composite Structure, which outlines the procedure for specifying and measuring temperature uniformity.

Applications

Typical sectors or products that use autoclaves include:

• Aerospace
• High performance automotive
• Sporting goods

How to Measure

Temperature

Measuring the air temperature in the processing environment is a critical step to controlling the cure or crystallization process. For ovens and autoclave, temperature is most commonly measured with thermocouples. Selecting the appropriate thermocouple types is dependent on several criteria, such as the operating temperature range, working environment, thermocouple response rate and accuracy. For autoclaves and ovens used in composite processing J-type or K-type are commonly used.

A typical J-type thermocouple has an operating temperature of 0° to 750°C (32° to 1382°F) whereas a K-type thermocouple can operate at -200° to 1250°C (-328° to 2282°F) . Oxidizing or reducing atmosphere environment as well as the state of the media (liquid, solid, or gas) can affect the thermocouple performance. Response time is also an important parameter associated with thermocouples. Response time is a function of wire gage size and whether or not the thermocouple tip is exposed (exposed junction) or encased in a sheath (grounded or ungrounded junction). Higher gage size increases response time while encasing the thermocouple tip in a sheath reduces response time. Therefore, it is important that all thermocouples used together share a common wire gage and tip configuration such that they respond to temperature changes at the same rate. Thermocouple specifications can vary from manufacturer to manufacturer. Designers should refer to manufacturer's data when selecting a thermocouple.

Gas flow

Gas flow measurements are critical to understand the convective heat transfer capability of an autoclave. Gas flow measurements are also important inputs to validate numerical results obtained with computational fluid dynamics simulations (CFD).

Anemometers are the simplest instruments to measure the gas velocity within an oven or an autoclave. They are typically hand held devices that include a display unit (handheld part) and a measurement probe that is directly wired to the display unit. During measurement, the display unit can be placed outside the autoclave with the autoclave door shut (but not sealed, due to the connecting wire). The display unit may also be left inside the autoclave within its operating specifications while an optional data logger is used to record the gas velocity. The measurement probe can be clamped on a stand. Two types of anemometers are commonly available: wind vane and hot wire anemometers. Wind vane anemometers are directional, unlike single filament hot wire anemometers, and can be used to understand the gas flow circulation. Streamers can also be used to obtain a visual appraisal of the gas flow direction. Wind vane anemometers can come with different sized probes with the larger probes capable of measuring lower gas speeds. Hot wire anemometers have been found to have minimal value in autoclaves because the highly turbulent environment creates large measurement variations (Ghariban (1992)).

Hot wire anemometer

Wind vane anemometer

Key considerations During Use

While utilizing the autoclave, the following are some of the key aspects to focus on to increase the quality of the outcomes:

Handling

When running a cure cycle, operators must follow operating procedure. A typical procedure can include the following steps:

• Check nitrogen storage/generation system
• Check cooling system
• Turn off all unused vacuum lines
• Ensure no unintended items are in the autoclave
• Vacuum leak test on parts
• Check autoclave ventilation system
• Program autoclave cure cycle according to specification
• Ensure thermocouples are functioning
• Check parts positioning for optimum gas flow

Cleaning

Since parts are usually contained within vacuum bags, cleaning the autoclave between runs is unnecessary. However, operators should make sure no unintended items are left within the autoclave and the workspace surrounding the autoclave should be kept clean.

Preventative Maintenance

Servicing and calibration should be done according to the manufacturer recommendations. All maintenance should be carried out by experienced individuals and/or contractors from the manufacturer.

Suppliers

Product suppliers

Common providers of this equipment include:

• Aeroform Composites
• American Autoclave Company
• Advanced Precision Fabricators, Inc.
• ASC Process Systems
• Akarmak
• Bondtech Corporation
• Essential Innovations Inc.
• Fluitron, Inc.
• Magnabosco s.r.l
• Maroso s.r.l
• Melco Steel Inc.
• OLMAR S.A.
• Jiangsu Olymspan Thermal Energy Equipment Co., Ltd
• Precision Custom Components, LLC
• Rentec nv
• Taricco Corporation
• Terruzzi Fercalx Group
• Thermal Equipment Corporation
• Ural Center Komplekt Ltd.
• Walton Process Technologies.
• Wisconsin Oven
• WSF Industries, Inc.
• World Autoclave Solutions LLC

Expert support providers

• ASC Process Systems
• Process Combustion Systems
• Flametech Combustion Services

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.

Volume fraction of either matrix or fibres with respect to total composite volume (matrix + fibre).

The act of combining reinforcement fibre, resin to shape on a mould (tool).

There are three main methods to combine resin and fibre to deposit onto a tool (i.e. material deposition):

• Wet Processes: Mixing liquid resin and dry fibre directly in the mould, e.g. Wet Compression Moulding process for parts in BMW 7 series
• Liquid Composite Moulding (LCM) processes: Infusing dry fabric with resin using a pressure gradient, e.g. High Pressure Resin Transfer Moulding process (HP RTM) for parts in BMW i3
• Prepreg Processes: Laying up using prepreg where resin and fibre are already combined, e.g. Automated Tape Laying (ATL) process for some of the parts in Boeing 787.

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.

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.

Sizing or fibre sizing refers to a coating that is applied to fibre during manufacturing. Highly proprietary (formulation and process).

Sizing serves two functions:

• Protects and aids fibre during processing
• Aids in the bonding of fibre and matrix

Any manufacturing and/or decision making activity that occurs during any stage of the development design cycle (e.g. conceptual design to production).

In the context of Knowledge in Practice, practice refers to the systematic use of science based knowledge to reduce composites manufacturing risk, cost, and development time.

Thermosets are a class of polymer that undergo polymerization and crosslinking during curing with the aid of a hardening agent and heating or promoter. Initially they behave like a viscous fluid. During curing, they change from viscous fluid to rubbery gel (viscoelastic material) and finally glassy solid.

If heated after curing, initially they become soft and rubbery at high temperatures. If further heated, they do not melt but decompose (burn)

Comes in two parts: part A (resin) and B (hardener). When mixed, curing reaction starts and is not reversible.

Examples include epoxy or polyester.

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.

Manufacturer's Recommended Cure Cycle (MRCC).

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.

Degree of cure (DOC) is an indication of how far the chemical curing reaction (crosslinking process) has advanced in a thermoset resin.

DOC is defined with a number between 0 and 1 (or 0% and 100%) where 100% is a fully cured resin. It does not have to fully reach 100% for the resin to become solid or the part to be used. In some aerospace applications, resins are only cured to about 90%. Higher the degree of cure, higher the mechanical properties.