Vacuum Bagging for Prepregs - P172

 

Vacuum Bagging for Prepregs
Practice document
Document Type	Practice
Document Identifier	172
Themes	Materials deposition and consolidation management
Tags	Bagging Consolidation Consumables Debulking Material Porosity Preparation Prepreg Resin flow Vacuum application
Objective functions	Cost Maintain Rate Maintain Quality Maintain
MSTE workflow	Development
Prerequisites	Practice for Developing a Process Step Practice for Developing a Consolidation Step Vacuum Bagging

Overview

This document provides a step-by-step guide, including the equipment needed for a vacuum bagging process for Prepreg material. There are numerous online resources and videos available which also explain the vacuum bagging process. The purpose of this article is to provide a straightforward, generic how-to document for people with basic knowledge of composites, but minimal experience with vacuum bagging.

Introduction

Vacuum bagging is typically used to consolidate a prepreg, while removing trapped air and excess resin to create higher quality, more consistent parts. There are two bagging methods described in this practice document, one for a part that does not require much, if any bleeding of the resin during cure. The other method is for a part that requires some of the resin to be bled out of the part during cure, which has a more open pathway for entrapped air and resin to escape. These two methods are also more specific to the prepreg material system than the generic method showed in Vacuum Bagging.

Significance

Vacuum bags help to consolidate the plies of composite material by removing air, promoting better adhesion and reducing voids as well as removing other volatiles in the resin. A vacuum bag applies even pressure, facilitating resin flow and uniform resin distribution, for a uniform final part. It helps remove excess resin, optimizing the resin-to-fiber ratio. Additionally, it creates a controlled environment, preventing moisture and contaminants from affecting the composite.

Practice

See the figure below for illustrations of the two bagging methods mentioned.

Bagging Method 1

This method is for a material system that does not require bleeding of resin out of the material during the cure cycle.

1. Apply mould release to the tool surface. Non-porous release film can be used in place of mould release.
2. If the tool side surface is going to be bonded after cure, a layer of peel ply could be placed in the mould to provide a moulding surface.
3. Lay-up prepreg plies onto mould tool to form a laminate. This is the deposition step, see Practice for Developing a Deposition Step for more information.
4. Apply sealant/tacky tape around the perimeter of the mould. Leave backing paper on until the vacuum bag is applied.
5. Lay peel ply over laminate, covering the laminate fully.
6. Lay small off cuts of bleeder (1-2 cm wide) or designated bleeder strings on the edge of the peel ply. Ensure they are long enough to almost reach the sealant tape; this is to create a contact point between the strings and the breather.
7. Lay a layer of non-porous release film. Fully covering the laminate, but not fully covering the bleeder strings.
8. Lay a layer of breather inside the perimeter of the sealant tape, leaving approximately 1 cm gap between the tape and the breather, to fully cover the assembly. Ensure the bleeder strings are touching the breather.
9. Place vacuum ports in desired locations and add an additional square of breather underneath the vacuum port, see section on Prepreg.
10. Cut vacuum bag to desired size.
    1. Cut a large enough piece to allow it to conform to the part surface without bridging or stretching when vacuum is applied.
11. Lay vacuum bag over mould and sealant tape.
12. Starting at one point along the sealant tape, begin removing the backing tape, pressing the vacuum bag lightly onto the perimeter and Pleat tape to allow re-positioning of the bagging material as needed.
13. Once vacuum bag position is satisfactory, press bagging securely onto tape over entire perimeter and Pleats to complete the seal between the bag, tape and tool.
14. Make 1-2 small slits in the bag over the vacuum ports connect the outer portion of the vacuum port.
15. Connect the vacuum lines to their outer tubing.
16. Connect vacuum tubing to vacuum pump. Drawing a partial vacuum first will allow for adjusting of the vacuum bag so that there are no wrinkles over the part.

Bagging Method 2

This method is for a material system that requires more bleeding of resin out of the laminate during cure.

1. Apply mould release to the tool surface. Non-porous release film can be used in place of mould release.
2. If the tool side surface is going to be bonded after cure, a layer of peel ply could be placed in the mould to provide a moulding surface.
3. Lay-up reinforcements onto mould tool to form a laminate. This is the deposition step, see Practice for Developing a Deposition Step for more information.
4. Place the edge dam along the perimeter of the laminate.
5. Apply sealant tape around perimeter of the mould. Leave backing paper on until the vacuum bag is applied.
6. Lay peel ply over laminate, covering the laminate fully. This may not be necessary if the surface is not going to be bonded to later in the manufacturing process.
7. Lay a layer of porous release film over the laminate.
8. Lay bleeder cloth over the laminate extending past the edge dam to ensure it comes into contact with the breather.
9. Lay a layer of non-porous release film. Fully covering the laminate, but leaving the edges of the bleeder exposed.
10. Lay a layer of breather inside the perimeter of the sealant tape, leaving approximately 1 cm gap between the tape and the breather, to fully cover the assembly. Ensure the bleeder is touching the breather.
11. Place vacuum ports in desired locations and add an additional square of breather underneath the vacuum port, see section on Prepreg.
12. Cut vacuum bag to desired size.
    1. Cut a large enough piece to allow it to conform to the part surface without bridging or stretching when vacuum is applied.
13. Lay vacuum bag over mould and sealant tape.
14. Starting at one point along the sealant tape, begin removing the backing tape, pressing the vacuum bag lightly onto the perimeter and Pleat (slack in the vacuum bag to allow for changes in geometry) tape to allow re-positioning of the bagging material as needed.
15. Once vacuum bag position is satisfactory, press bagging securely onto tape over entire perimeter and Pleats to complete the seal between the bag, tape and tool.
16. Make 1-2 small slits in the bag over the vacuum ports connect the outer portion of the vacuum port.
17. Connect the vacuum lines to their outer tubing.
18. Connect vacuum tubing to vacuum pump. Drawing a partial vacuum first will allow for adjusting of the vacuum bag so that there are no wrinkles over the part.

Conduct a Leak Test

1. Turn on the pump to apply vacuum
2. Monitor the vacuum gauge and vacuum bag. If the desired level of vacuum is not achieved within a few minutes, the pump may not have enough capacity for the application, and/or there is a leak in the vacuum bag/seal.
3. Leaks – Large leaks can usually be heard as a small whistling sound of escaping air. Inspect the sealant perimeter, Pleats (slack in the vacuum bag to allow for changes in geometry) and other possible leak locations closely and listen for the sound of air. Smaller leaks may be harder to locate. Leaks typically occur at Pleats or the vacuum port connection, in addition to the sealant perimeter. Ensure the sealant tape is firmly affixed to vacuum bag and mould surface in all locations.
4. Once desired level of vacuum is reached, typically 25-30 inHg, disconnect the pump and monitor the vacuum gage. Various rules of thumb are used to determine if a leak test is successful. A common rule of thumb is the vacuum should not drop more than 2 inHg over 5 minutes, however this depends on a number of variables such as part size.

Vacuum Port Placement

Place vacuum port outside the perimeter of the part. When placing the bottom half of the vacuum port on the tool, place 1-2 additional pieces of breather material under the vacuum port that are in contact with the rest of the breather. This prevents potential scratching or damage of the tool by the port when vacuum is applied, prevents the port from sealing to the tool that could stop the flow of air out of the rest of the bag, and provides an airflow path between the port and the breather material. For more information see Prepreg. Depending on the size of the part, more vacuum ports may be required in order to create a consistent vacuum.

Topdown view of a vacuum bag to illustrate vacuum port positioning.

Closing Remarks

Prepregs typically have a very precise fibre volume ratio and thus do not require bleeding of the resin, however this depends on the specific material used. Also, prepregs are often used in autoclave processes, where the additional pressure in the autoclave can force resin out of the material system. This is why it is important to keep tight control over the pathways that resin can take out of the material system and into the bagging materials during cure and consolidation.

Related pages

Page type	Links
Introduction to Composites Articles
Foundational Knowledge Articles
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
Perspectives Articles

About	Help

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.

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.

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.