Ensuring Consistent Polymer Hardening

January 10, 2023

Autoclaves are useful instruments for applications across many industries, from aviation to medical research. These machines are capable of withstanding high temperatures and extreme pressure, making them excellent vessels for treating highly regulated items.

Learn what an autoclave is and how it is currently used for composite molding

An autoclave is a machine that uses pressurized steam to perform various processes, such as manufacturing and dye production. Initially called the steam digester, a modern version of that prototype is still in use today. Now, however, it is known as a pressure cooker.

While these devices have been around for hundreds of years, recent trends in autoclave technology are bringing them into present day applications. Many modern autoclaves come with computerized controls, enabling them to collect process data and send it to the cloud.

Today, the most well-known example of autoclave usage is in healthcare and life sciences, where the autoclave sterilizes instruments through prolonged exposure to high-temperature steam. The steam in an autoclave is hot enough to destroy proteins in the cell wall, effectively killing any microbes and spores that are present.

There are three key components to any autoclave process:

1. Time
2. Pressure
3. Heat

These three components also make autoclaves useful in industrial manufacturing processes, such as molding lightweight composite parts for space, aviation and other aerospace and aircraft applications.

Autoclave Technology for Composites

Manufacturers use autoclaves to process and cure composites. The high pressure and heat inside the autoclave are essential for producing a result with the desired fiber content.

Another reason manufacturers use autoclaves is their effectiveness for reducing residual porosity, also known as void content. Void content is the measure of empty spaces, or pores, in a product — the higher the void content, the weaker the item will be. The machine’s ability to dramatically reduce void content makes it useful for curing products that will experience extreme conditions in normal use.

Autoclave inerting, the process of introducing inert gases such as nitrogen, is critical for the safe production of composite parts. During the autoclaving process, carbon-based composites produce off-gases — an inert gas can safely remove these off-gases from the furnace and assist in heat transfer, reducing the risk of fires.

What Is Autoclave Molding?

Autoclave molding is a type of closed molding manufacturing. Closed molding processes involve curing raw materials inside a two-sided mold or a deoxygenated vacuum bag. These processes can produce enormous volumes of material, making them most suitable for large facilities.

Using an autoclave for manufacturing ensures consistent polymer hardening, making it an excellent tool for composite manufacturing. The mold for this process is double-sided:

• The lower half is a rigid mold.
• The upper half is usually either a flexible silicone membrane or a polymer film.

The Autoclave Molding Process

After applying a release gel to the mold’s surface, the process begins by placing fiber textiles into the mold. These textiles are prepregs — composite materials containing a high-strength fiber that has been pre-impregnated with resin.

Once the molds have been filled, you vacuum the air out of the mold cavity and place the whole assembly first in a vacuum bag — this step enables equal distribution of pressure across the workpiece. An absorbent material, or bleeder, inside the bag facilitates easier air removal and absorbs any excess resin that may bleed out during the curing process. After removing the air from the bag, the container goes into the autoclave.

Then, the curing process begins. The temperature and pressure inside the autoclave rise, hardening the material and removing pores. Once the product has cured for the specified period, you remove it from the autoclave and place it in an oven to improve its durability. Then, after trimming and inspection, your composite is ready for use.

Traditionally, industrial autoclaves used air for molding processes. However, because oxygen is highly flammable at extreme temperatures, using an inert gas is crucial. Nitrogen is currently the preferred gas for autoclave molding because it is safer, more cost-effective, and ensures a more consistent result than oxygen.

The Advantages of Autoclave Molding

When it comes to autoclave molding advantages and disadvantages, the positives far outweigh the negatives. The benefits of autoclave molding include:

• Stronger composites: The high temperature and pressure used in autoclave molding significantly increase composite strength to weight ratio. Autoclaved composites are also more resistant to heat and chemical damage, making them suitable for applications in demanding sectors, such as the military, aerospace, and marine industries.
• Higher quality products: Autoclave molding creates highly uniform products with minimal defects and significantly less waste than other techniques.
• Reduced production costs: Although investing in autoclaves for composite molding is a large upfront cost, the process of autoclave molding is a quick process, making it more cost-effective than many other manufacturing methods. For example, injection molding one part can take hours or even days, but autoclave molding is complete in under an hour.
• Quicker prototype production: The autoclave molding process allows you to create functional prototypes quickly, enabling shorter testing cycles and speeding up product development.
• Lower weight: Typically, composite manufacturers add extra bulk to compensate for high void content. In industries where additional weight can be dangerous, manufacturers must find a way to create lighter products that possess high strength. Autoclave molding is an optimal process for this purpose.
• Increased control: Using autoclaves provides you with highly precise control over the curing process, enabling you to efficiently manufacture complex shapes. More advanced autoclaves also have programmable cure profiles for highly controllable, repeatable processes.

If you operate in an industry that uses lightweight composites, autoclave molding may be the most efficient process for you. Sourcing an inert gas such as nitrogen is essential for safe composite manufacturing.

Autoclave molding is an efficient process for producing the high-quality parts necessary to meet the strict demands of the modern aviation and aerospace industries. You can trust Linde to supply the gases and technical support you need for efficient autoclave molding processes. We will conduct a thorough evaluation to determine the temperature, pressure, and gas usage necessary for optimal results.

Contact us today for more information about Linde's autoclave offerings

When you work with us, you get more than just gases and equipment — you get our commitment to making the world more productive every day through excellent products and customer service.