Vacuum Bagging - P168

 

Vacuum Bagging
Practice document
Document Type	Practice
Document Identifier	168
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	Integrated Product Development Practice Practice for Developing a Process Step Practice for Developing a Consolidation Step

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 bags are typically used to consolidate a laminate (prepreg, wet layup, vacuum infusion, etc.) by removing trapped air and excess resin to create higher quality, more consistent parts. 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.

For vacuum bagging of specific processes see:

• Vacuum Bagging for Prepregs
• Vacuum Bagging for Vacuum Infusion

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

Typical equipment needed for vacuum bagging is listed below with a brief description of their use. However, some items may not be needed depending on your material system and application. For example, some prepreg materials require a bleeder cloth, whilst other do not.

Equipment

• Vacuum Pump: Draws air out of the bag and creates the vacuum inside. Some of the key features to look for is the vacuum potential (how high of a vacuum it is able to pull), the displacement of air per unit time (air flow rate), and the horsepower of the pump as a measure of efficiency. In general, larger vacuum bags will require a pump with a higher displacement or more time to pull vacuum. Ideally any pump with the correct vacuum pressure potential should suffice, however, it is almost impossible to create a vacuum bag that is 100% air tight, so you will need a pump to keep up with the minor leaks that will occur.
• Vacuum lines: vacuum rated hoses that will be able to connect to the vacuum ports and the vacuum pump. If the part is being cured under vacuum in an oven or autoclave, the vacuum lines need to withstand the temperatures and pressures as well.
• Vacuum Port: The connection between the vacuum line and the inside of the bag. A typical vacuum port has two parts, one that sits inside the bag and another that sits on the outside, screwed tightly together over the vacuum bag.
• Vacuum gauge: A gauge that connects to the vacuum port in order to monitor the vacuum being pulled inside the vacuum bag. This will be used to check the integrity of your bag and if there are any leaks.

Consumable Bagging Material

• Vacuum bag: encase the consumables and laminate allowing vacuum or external pressure to consolidate the layup. Bagging films are generally made from nylon, Kapton or PVA. The different materials will give a different feel to the bagging film resulting in a variety of experiences working with them. The materials also have different temperature ratings, which is important when performing a high temperature cure. Some bagging films will be very compliant (stretchy), which will allow for better conformance to part shapes, however they may also move some of the reinforcement material around when vacuum is applied. It is not recommended to use a compliant bag when doing an infusion. There is also an option to use re-useable vacuum bags made from silicone. This is recommended for production parts of high volumes where the initial investment of a silicone bag will make sense .
• Breather: a thick cloth that can provide air and volatile passage to the entire part surface, allowing for uniform vacuum pressure. Breather can also be used to pad sharp corners or thermocouples to prevent puncturing the vacuum bag. The breather is often made from synthetic fibres or heavy fibreglass fabric. Breather cloth can come in a variety of thicknesses which will affect the handling and also its ability to transfer gases.
• Non-Porous release film: Creates a separation between the resin soaked laminate and the bagging material or the mold tool. Often made from Teflon FEP, this film can come in a variety of colors and temperature ratings. The film is also often partially compliant and sometimes very compliant. The film can vary in stiffness and handling feel depending on type and manufacturer, as well as the ease of releasing from different material systems after cure.
• Porous release film: Also known as perforated release film, it serves the same separating function as non-porous, however a grid structure of holes allows for resin to bleed through into the subsequent bagging material layers. This is often used in layups and processes where there will be excess resin that is required to be removed across the surface of the part.
• Bleeder: Used to absorb and retain excess resin if the material system is designed to bleed. Commonly made from a woven or non-woven, relatively thin synthetic or glass fibre material. This layer will also be able to act as a gas transport layer and must be in contact with the breather material. However, it must not come in direct contact with the laminate as it will become saturated and bonded into the laminate.
• Peel ply: A smooth woven fabric that does not bond to resin, even when saturated. The peel ply separates the laminate from other bagging materials while allowing resin to freely flow through, similar to the porous release film. The peel ply however, provides a textured surface ready for bonding, if either the mould side or the bag side of the part will be adhesively bonded after cure. Often made from nylon or polyester, the fabric is treated with a coating so that it does not stick to the laminate after cure.
• Vacuum bag sealant tape: a thick rubberized tape used to seal the vacuum bag onto the tool, allowing for vacuum to be drawn on the bag assembly. The tape has a paper backing that can be kept on during the bagging procedure to avoid sticking to any unnecessary materials. There are many different temperature ratings for these tapes, where they will begin to flow if used at too high temperatures. Generally speaking, higher temperature rating means higher cost.
• Dam: Provides the laminate with a vertical edge. Often made from rubber, with an adhesive on the bottom. If not used, the edge can taper from vacuum bagging. Dams are not necessary for very thin laminates.

Procedure

A step-by-step procedure for applying a vacuum bag is provided in this section. This procedure provides a simple form of the vacuum bag allowing for entrapped air and volatiles to be transported out of the laminate as well as providing a consolidation pressure to the layup.

Schematic of a generic vacuum bag method, applicable for most composite layups.

1. Apply mould release to the tool surface. Non-porous release film can be used in place of mould release.
2. Lay-up reinforcements 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 2cm past the perimeter of the laminate.
5. Lay a layer of non-porous release film. Fully covering the laminate, and allowing the peel ply to stick out all around like a frame.
6. 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 peel ply is touching the breather
7. Place vacuum ports in desired locations and add an additional square of breather underneath the vacuum port.
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, see Pleats for more detail.
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. Make 1-2 small slits in the bag over the vacuum ports connect the outer portion of the vacuum port
13. Connect the vacuum lines to their outer tubing
14. 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 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 (flow rate) for the application, and/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 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.

Pleats

Folded section of the bag sealant tape to be used as a pleat in the vacuum bag.

Additional piece of sealant tape to be used as a pleat in the vacuum bag.

A pleat is a folded piece of sealant tape that protrudes out vertically from the tape around the perimeter. Pleats are sometimes needed for complex geometry from a mold / tool, pleats provide excess vacuum bag material to fully conform with the geometry of the mold. Ensure enough bagging material has been cut to accommodate pleats. Too little material can cause bridging in corners and stretching of the bag, potentially leading to rips or tears. When cutting the piece of bagging material, include the material folds at the pleat locations to estimate an appropriate required area. It is better to err on the side of excess when cutting bagging material for pleats.

One method of creating pleats involves incorporating a folded section of sealant tape when initially laying the tape on the tool. This creates the extra surface area needed in the bag for creating a pleat. See the figure below as an example. Ensure the bottom of the fold is pressed securely against the tool to eliminate any gaps that may cause vacuum leaks. Another method of creating pleats involves incorporating the folded sealant tape on top of the previously laid perimeter tape, as shown in the figure below. This method has less chance of causing a vacuum leak as the sealant tape adheres well to itself for a strong airtight connection. This method also allows for pleats to be added as necessary.

Closing Remarks

The purpose of this article has been to provide instructions on applying a vacuum bag for either an infusion or prepreg process. There are many variations and local practices to how this process is done and you may need to consider alternative procedures and bagging materials to optimize your product.

For vacuum bagging of specific processes see:

• Vacuum Bagging for Prepregs
• Vacuum Bagging for Vacuum Infusion

Explore this area further

• Vacuum Bagging - P168
  • Vacuum Bagging for Prepregs - P172
  • Vacuum Bagging for Vacuum Infusion - P173

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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
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References

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.