Evaluation of Double-Vacuum-Bag Process For Composite Fabrication
T. H. Hou and B. J. Jensen
NASA Langley Research Center, Hampton, Virginia 23681
Abstract
A non-autoclave vacuum bag process using atmospheric pressure alone that eliminates the need for external pressure normally supplied by an autoclave or a press is an attractive method for composite fabrication. This type of process does not require large capital expenditures for tooling and processing equipment. In the molding cycle (temperature/pressure profile) for a given composite system, the vacuum application point has to be carefully selected to achieve the final consolidated laminate net shape and resin content without excessive resin squeeze-out. The traditional single-vacuum-bag (SVB) process is best suited for molding epoxy matrix based composites because of their superior flow and the absence of reaction by-products or other volatiles. Other classes of materials, such as polyimides and phenolics, generate water during cure. In addition, these materials are commonly synthesized as oligomers using solvents to facilitate processability. Volatiles (solvents and reaction by-products) management therefore becomes a critical issue.  SV
B molding, without additional pressure, normally fails to yield void-free quality composites for these classes of resin systems. A double-vacuum-bag (DVB) process for volatile management was envisioned, designed and built at the NASA Langley Research Center. This experimental DVB process affords superior volatiles management compared to the traditional SVB process.  Void-free composites are consistently fabricated as measured by C-scan and optical photomicroscopy for high performance polyimide and phenolic resins.
1. Introduction
A cure cycle (temperature and pressure profile) for manufacturing composite laminates with a reactive resin matrix such as a poly(amide acid)/N-methylpyrrolidinone (NMP) resin system or a solvent containing prepreg usually consists of a two-step ramp-and-hold temperature profile as shown in Figure 1. Temperature and hold duration in each step are unique for a given composite system. The low temperature ramp-and-hold step is called the B-stage. During the B-stage, prepregs are heated
___________________________________________
This paper is declared a work of the U.S. Government and is not subject to copyright protection in the United States.
and reaction by-products such as water from the resin’s chemical reactions are generated. Volatiles (i.e., solvent and reaction by-products) are free to escape because of the absence of pressure in this period. The resultant residual volatile content and residual fluidity of the matrix resin remaining inside the composite are determined by the B-stage conditions. After the B-stage period, a second temperature ramp-and-hold is followed. Pressure is applied during this high temperature ramp-and-hold period to afford laminate consolidation and to attain desired physical properties of the resin matrix and good mechanical properties for the composite. Once the consolidation pressure is applied, residual volatiles, if any, are locked in and unable to escape. In order to produce a void-free high quality laminate, the residual volatile content and processability of the resin matrix must be carefully controlled by a workable cure cycle, which is designed specifically for a given composite system.
Figure 1.  Schematic drawing of a two-step cure cycle profile.
An autoclave is traditionally used in the fabrication of composite materials. It offers enhanced processi
ng flexibility compared to other common processing equipment such as an oven or press. However, composite fabrication by autoclave is very costly in terms of capital investment, and limits the size of the parts that can be produced. To address this issue, recent NASA programs, such as the Next Generation Launch Vehicle (NGLV) program, have emphasized the development of out-of-autoclave processing techniques for high temperature resistant composites. Single-vacuum-bag (SVB) processing in an oven is an attractive alternative. SVB processing in an oven utilizes atmospheric
pressure created by the vacuum bag alone, and is cost effective for composite fabrication. In general, the SVB process is best suited for molding epoxy matrix based composites because of the absence of reaction by-products or other volatiles. However, the superior fluidity (low viscosity) of epoxy matrix may cause an excessive resin flash, which results in dry fibers and extensive void networks in the laminate [1]. Furthermore, SVB processing is often ineffective in composite molding when a reactive resin matrix with volatile by-products or solvent containing prepreg is present. The SVB assembly inherently hinders and/or retards the volatiles depletion mechanisms during fabrication because a compaction force (i.e., atmospheric pressure) is created and exerted onto the laminate during the volatile depleting B-stage period.
In the past two decades, a double vacuum bag procedure was devised at the Naval Air Warfare Center,
Warminster, PA to replace SVB for aircraft composite repair applications [2-8]. The double vacuum assembly consists of a rigid enclosure over a flexible vacuum bag. Prepreg plies are exposed to a vacuum but do not experience a consolidation force. When bonding the wet lay-up repair patch of epoxy matrix, the double vacuum procedure eliminates problems of excessive flash during the B-stage period [4-8]. Very limited studies and conflicting results were noted in the applications of high temperature bismaleimide based wet lay-up repair patches. The double vacuum procedure resulted in a lower interlaminar shear strength when compared to the SVB process, presumably due to the poor wetting out of the fibers and high void content [1].
2.DVB Assembly and Processing Concept
A schematic drawing to illustrate the concept of a traditional SV
B in composite manufacturing is shown in Figure 2. Fiber reinforced reactive resin matrix prepregs (composite) are laid up between the caul and tool steel plates. The composite is then enclosed by a vacuum bag, which is sealed around the edges onto the tool plate. A vacuum port is built on the tool plate inside the bag. This assembly is typically installed in a forced air circulation oven and subjected to a cure cycle for composite molding.
Prior to the application of vacuum, the bag rests at an equilibrium balanced by the same atmospheric pressure (i.e., 14.7 Psi) from either side of the bag as shown in Figure 2(a). Under this circumstance, the composite is not subjected to any external compaction forces and remains bulky and loose. During the B-stage, the resin softens and becomes molten at elevated temperatures. Reaction by-products may be generated or volatile solvent is released and chemo-viscosity builds-up. In order to fully deplete the volatiles at lower temperatures, vacuum is pulled on the composite. However, because of the pressure differential, the vacuum causes the bag to collapse tightly onto the caul plate and compact the composite. At the same time, there is a thickness reduction due to softening of the resin
reaction tool
matrix and atmospheric pressure created by the vacuum as shown in Figure 2(b). Both the compressed fibrous architecture and increasingly viscous resin matrix inside the prepreg plies create narrower passages for volatiles to escape. In order to remedy this problem, in practice a prolonged B-stage
Figure 2.Oven SVB composite molding concept.
duration is often employed necessarily to achieve low residual volatile levels. However, this is not
always successful due to resulting poor residual matrix fluidity rendering the composite un-processable.B)A)
The SVB assembly and process, without the application of external pressure, are simply not flexible enough for high performance composite fabrication.
A schematic drawing illustrating the concept of the DV
B molding assembly for volatile management and composite manufacturing is shown in Figure 3. Fiber reinforced reactive resin matrix prepregs are laid up between the caul and the tool steel plates. They are then enclosed by a vacuum bag (designated as Inner Bag), which is sealed around the edges onto the tool plate. A vacuum port is built on the tool plate inside the Inner Bag and connected to a vacuum pump as with the SVB process. A second vacuum bag (designated as Outer Bag) is then assembled in the same fashion, with a vacuum port built on the tool plate, which is located between the Inner and Outer Bags and connected to a separate vacuum pump. Before assembling the Outer Bag, a perforated tool is first installed outside the perimeter of the Inner Bag. This tool has to be stiff enough to withstand the 14.7 Psi atmospheric pressure created by the vacuum. For the high temperature (371°
C curing) resin systems, a Kapton“ film of 0.002” to 0.003” in thickness is used as bagging material. This DVB assembly is then placed in a heating chamber and subjected to the proper cure cycle for composite manufacturing.
Figure 3.    Schematic DVB assembly.
During the B-stage (i.e., the low temperature ramp-and-hold period), full vacuum (30” Hg) is applied to
the Outer Bag, while a slightly lower vacuum level (i.e., 28” Hg) is set in the Inner Bag. The
Outer Bag is collapsed onto the stiff perforated tool due to the atmospheric pressure outside the bags. Because of the vacuum differential between the two bags, the Inner Bag is “ballooned” and presses against the perforated stiff tool leaving no compaction force, while still producing vacuum, on the composite. In the DVB arrangement, the composite lay-up assembly is not compacted by the atmospheric pressure via the Inner Bag, and remains loose. Volatiles are free to escape by the vacuum suction from the Inner Bag vacuum pump during the B-stage.
At the end of the B-stage, the Outer Bag is purged to atmosphere, while the Inner Bag vacuum is increased to 30” Hg. The Outer Bag becomes loose from the tool, and the Inner Bag collapses onto the caul plate with one atmospheric pressure. This pressure helps to consolidate the laminate during the high temperature ramp-and-hold period of the cure cycle.
The DVB process can be very flexible. For example, applying a partial vacuum (i.e., 25” Hg) to the Outer Bag, while pulling a full vacuum (i.e., 30” Hg) in the Inner Bag during the composite B-stage is also possible. In this case, The Outer Bag is collapsed onto the stiff perforated tool with a pressure

版权声明：本站内容均来自互联网，仅供演示用，请勿用于商业和其他非法用途。如果侵犯了您的权益请与我们联系QQ:729038198，我们将在24小时内删除。