Vacuum Bagging for Vacuum Infusion - P173

 

Vacuum Bagging for Vacuum Infusion
Practice document
Document Type	Practice
Document Identifier	173
Themes	Materials deposition and consolidation management
Tags	Bagging Consolidation Consumables Debulking Material Preparation 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 typical vacuum bagging process. There are numerous online resources and videos available which also explain the vacuum bagging process. The purpose of this article is to provide a straight forward, generic how-to document for people with basic knowledge of composites, but minimal experience with vacuum bagging.

Introduction

Vacuum bagging can be used to consolidate a prepreg or typical wet layup part, removing trapped air and excess resin to create higher quality, more consistent parts. Vacuum bagging is also used in resin infusion processes. A number of common materials and steps are required for creating the vacuum bag for an infusion process. Most of the bagging steps are the same for both processes. This article will outline some of the differences as well as the differences in the processes themselves.

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

A step by step procedure on how to apply the vacuum bag for a simple infusion process. Refer to the schematic and example images below. Also see Vacuum assisted resin transfer moulding (VARTM)/resin infusion (VARI) for more detail and videos of the process.

Schematic of a vacuum infusion vacuum bag.

1. Lay non-porous release film on the tool surface. If the tool has been treated with mold release, you do not need to use this release film.
2. Lay-up reinforcement onto mould tool to form a laminate. This is the deposition step, see Practice for Developing a Deposition Step
3. Apply sealant tape around perimeter of the mould. Leave backing paper on until the vacuum bag is applied.
4. Lay peel ply over laminate, covering the laminate fully and extending at least 2 cm past the perimeter of the laminate. As well as another 5 cm to where the vacuum will be pulled from.
5. Add a layer of flow medium, either partly over your laminate, covering fully, or none at all. Use of and implementation strategy for flow medium depend on your laminate material and the resin used in the infusion process.
6. Place spiral (cut) hoses, or other/preferred style of inlet tubing where the resin will enter the layup and at the last place resin will saturate, where vacuum will be drawn. Tape may be used to secure them in place.
7. Connect hoses to be used for vacuum and resin input to the spiral hoses. Add a collar of sealant tape where the hoses will cross the sealant tape border.
8. 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
9. Lay vacuum bag over mould and sealant tape
10. 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
11. 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
12. Connect the vacuum hose to a catch pot and close of the resin inlet hose.
13. Connect the catch pot to the vacuum pump. Drawing a partial vacuum first will allow for adjusting of the vacuum bag so that there are no wrinkles over the part. Also adjust the spiral hoses to ensure they stay in position when full vacuum is drawn.
14. Draw vacuum.

Example of a vacuum infusion for a flat panel. Note that resin will flow from left to right and that there is no flow medium used in this example.

Conduct a Leak Test

A layup free of leaks is of utmost importance. If a leak is present then air is likely to be drawn into the laminate during resin flow and/or cure, resulting in porosity.

1. Apply vacuum to the laminate
2. Monitor the vacuum gauge and vacuum bag. Vacuum infusion processes are typically run at full vacuum (29.92 in-Hg) but less is acceptable depending on the application/process. If the desired level of vacuum is not achieved within a few minutes, the pump may not pull enough vacuum for the application, or there is a leak in the vacuum bag
3. Leaks – Large leaks can usually be heard as a small whistling sound of escaping air. Inspect the sealant perimeter, pleats 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 the desired level of vacuum is reached, typically 25-30 in-Hg, disconnect the pump and monitor the vacuum gage. Vacuum should not drop more than 2 in-Hg over 5 minutes.

Schematic showing the entire system of a vacuum assisted resin infusion process.

Deciding Resin Feed and Vacuum Locations

Feed lines should be strategically placed to allow even resin flow through the part, ideally only reaching the vacuum port location(s) at the end of the infusion, once the part has been fully wet-out. It is typical to have multiple feed lines distributing resin to various starting points in the part. Vacuum port and resin feed location may require a few test trials be run to determine the ideal location, especially on large parts, or parts with complex geometry. Simulation may also be used. For more information see Vacuum assisted resin transfer moulding (VARTM)/resin infusion (VARI).

• Concentric Flow – When resin feed line is located around the perimeter of the part, the vacuum line is placed in the center and the resulting flow front is called concentric.
• Eccentric Flow – When a resin feed line is located in the center of the part, the vacuum line is placed on the perimeter of the part and the resulting flow front is called eccentric

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.

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.

Flow front refers to the leading edge of the resin as it is flowing into and through the fibre preform.