Vacuum Bag Composite Curing Technology: A Comprehensive Overview

Posted: Jun 10, 2024

As industries using advanced technologies continue to grow, so does the need for high-strength, lightweight materials. Manufacturers of products used in space exploration, high-efficiency vehicles, aircraft manufacturing, and wind power constantly pursue opportunities to reduce weight without sacrificing strength and durability. Composite materials are among the best candidates for this light-weighting trend. Composites were once primarily used in the aerospace industry but are increasingly used in more industries looking to reduce product weight.

Composite products are made using a woven base material such as fiberglass, carbon fiber, or Kevlar (aramid) fiber and impregnating it with a resin-like epoxy, polyester, polyurethane, or other plastic in a liquid state. The woven material base is known as a "prepreg" since it has not been impregnated with a resin. Once resin is added to the prepreg, the material is cured under heat and pressure to create a solid, reinforced plastic. 

What is vacuum bag curing?

One of the most common methods of manufacturing composite materials is vacuum bagging, which uses atmospheric pressure to hold the composite material tightly in a mold and heat to cure the material. Vacuum bag curing begins with placing a fabric prepreg made of carbon or other fiber into a mold and manually forcing epoxy resin into the weave with a roller or other tools. Multiple prepreg mats are then layered onto the first, with the epoxy embedded into and between each layer. A plastic bag is placed over the mold and connected to a vacuum system to draw the air out, which forces the bag tightly onto the impregnated material, to shape the material to the mold. The mold is then heated and cured in an industrial oven while the vacuum is maintained. 

The Vacuum Bag Curing Process

  • Preparing the mold: The mold is cleaned and prepared for the layup.
  • Laying up the prepreg: The prepreg mats are cut to size and layered in the mold.
  • Applying the resin: The resin is applied to the prepreg mats to saturate the fibers.
  • Bagging the laminate: The saturated mats (laminate) is covered with a vacuum bag and sealed onto the mold.
  • Generating a vacuum: A vacuum pump removes air from the bag through holes in the mold, applying pressure to the laminate and compacting it against the mold.
  • Curing the laminate: The mold and laminate are heated in an industrial oven according to the resin's curing specifications.
  • Cooling down the laminate: After the heating process, the oven cools down slowly to prevent internal stresses in the cured part.
  • Demolding: Once cool, the vacuum bag is removed, and the finished part is extracted from the mold.
  • Trimming the part: The part is trimmed to its final size.
  • Post curing: The part may require post curing, which is a second heating process, depending on the resin formulation 

Key Oven Features for Vacuum Bag Composite Curing

The design of industrial ovens used for curing composite materials includes specific features and options to meet the requirements of the process. 

Air supply system: The oven must have a high-volume air supply system designed with an airflow arrangement, usually combination, to provide uniform temperature throughout the oven so there are no dead spots that remain unheated. Airflow calculations should be performed to determine the necessary heat input and air circulation volume. 

Control System
Advanced controls system

Control and data acquisition system: The control system should be able to monitor and control the temperature, vacuum level, and other process parameters. This system can not only perform closed-loop control of the heating and cooling processes but can also be designed to gather and record inputs from a multitude of thermocouples sensing the oven air temperature and part temperature, as well as transducers sensing the vacuum level at each vacuum port. 

Thermal overdrive: Composite curing control systems often feature thermal overdrive, also known as a controlled heat head. With thermal overdrive, the oven air temperature is temporarily set higher than the part cure temperature to accelerate the heating rate. The control system is programmed to do this automatically and uses thermocouples buried inside the composite material during layup to monitor the parts to avoid overheating. As the part temperature approaches its required cure temperature, the control system automatically reduces the oven heat output and the oven air temperature to prevent overheating the parts.

Data acquisition: The control system can also record all part temperatures and the vacuum at each port. These data sets are associated with a discrete batch identification number and can be automatically sent to a network for storage. This provides traceability of the temperature and vacuum for the processed parts. The system documents any alarms and other anomalies for each batch to help with troubleshooting and continuous improvement. 

Vacuum Pump
Vacuum pump and piping

Vacuum pumps: The main feature of a composite cure vacuum system is the vacuum pump. The pump must have the capability to draw the necessary vacuum and provide sufficient displacement to meet the requirements of the shape and complexity of the part being molded. The maximum flow rate (at the start of the cycle) and minimum flow rate (at full vacuum) both need to be considered during the selection process. If one of these ratings is ignored, the system can either take too long to draw down (while under minimal vacuum) or fail to maintain the required vacuum level after drawdown (at maximum vacuum) due to system leakage.

Common Composite Curing Options

  • Vertical Lift Door
    Vertical lift door with viewing windows
    Vertical Lift Door: This allows the space directly in front of the oven to be used for loading and unloading, reducing floor space requirements.
  • Interior lights and windows: Windows in the oven doors or walls and interior lighting allow operators to see inside during the curing process. Sometimes the layup is performed inside the oven, making this feature a necessity. 
  • Thermocouple jack panels: A thermocouple jack panel provides a convenient connection of the thermocouples used to monitor part temperatures to the recording system.
  • Safety features: Composite curing ovens frequently include options to enhance the operator's safety, such as door safety latches, trapped personnel pull cords, temperature overshoot protection and emergency shut-off switches.
  • AMS2750 Mil Specification Compliance: AMS2750 is a pyrometry specification that governs temperature uniformity tolerance, the location of test TCs, the calibration of the TCs, and similar variables. To ensure compliance, the oven manufacturer should perform AMS2750 testing before the oven's shipment, with documentation given to the end user.

Benefits of Vacuum Bag Curing

  • Superior Part Quality: High-quality parts with exceptional strength-to-weight ratios.
  • Improved Surface Finish: Smooth, blemish-free surfaces that require minimal post-processing.
  • Dimensional Accuracy: The vacuum pressure ensures close adherence to the mold, delivering precise and consistent part dimensions.
  • Versatility: Works with a wide range of materials and mold shapes, making it a truly adaptable solution.

Where is Vacuum Bag Curing Used?

Products manufactured using vacuum bag curing are all around. From rockets and aircraft to cars and wind turbines, vacuum bag curing technology can be found in a multitude of applications across industries, such as:

  • Aerospace: Aircraft components, structural elements, engine parts
  • Marine: Boat hulls, rudders, sailboat masts
  • Automotive: Car bodies, interior panels, engine covers
  • Sports and Recreation: Skis, tennis rackets, surfboards
  • Renewable Energy: Wind turbine blades, solar panel frames
  • Construction: Building panels, structural beams, bridge components

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Choosing an Industrial Oven for Vacuum Bag Composite Curing

ChecklistBelow are some of the key features to keep in mind when choosing an oven for your composite curing process.

  • Oven size: The interior chamber should accommodate your largest mold and ensure adequate clearance for connecting the vacuum, bagging and layup (if performed inside the oven) and handling.
  • Airflow management: Effective air circulation using properly designing ductwork in the oven prevents hot spots and ensures uniform heat distribution, preventing your part from warping or cracking.
  • Temperature control: Different resins have different curing requirements. Choose an oven that can accommodate the specific temperature requirements of your chosen resin system.
  • Vacuum system capacity: Select a vacuum pump and piping system that can efficiently remove air from the bag and seamlessly integrate with your vacuum sealing system.
  • Control system: An advanced control and data acquisition system should be considered to control the heating and cooling processes along with gathering and recording data points.

Vacuum bag composite curing is a versatile and effective method for manufacturing high-quality composite parts. Selecting the right vacuum bag curing oven isn't just about checking boxes. Find a manufacturer with a proven track record in composite curing technology. They should be able to guide you through the oven selection process and recommend a design that meets your exacting specifications. A company that offers a premium 3 to 5 year warranty will also give you the confidence and peace of mind that the equipment is designed with quality components and manufacturing methods. 
 

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