BACM Extended Abstract Anderson and Altan Final August 2014

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Bladder Assisted Composite Manufacturing(BACM): Challenges and Opportunities

CONFERENCE PAPER OCTOBER 2014

DOI: 10.13140/2.1.2139.6169

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2 AUTHORS:

J.P. Anderson

PPG Industries, Shelby, North Carolina

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M. Cengiz Altan

University of Oklahoma

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Available from: J.P. Anderson

Retrieved on: 06 November 2015
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Bladder Assisted Composite Manufacturing (BACM):

Challenges and Opportunities

J.P. Anderson and M.C. Altan

School of Aerospace and Mechanical Engineering University of Oklahoma, Norman, USA

Abstract.Bladder molding has been used to fabricate hollow composite components for more than two decades. In bladdermolding, the requisite consolidation pressure is applied from the inside of the composite laminate during cure via a flexible bladder.

In conventional bladder molding, the cure assembly is most often heated using an external heat source such as an oven or anautoclave. Due to the flexibility of applying pressure, the method is often the only viable process to fabricate composite componentswith continuous reinforcement and complicated geometry, i.e. non-axis symmetric and varying wall thicknesses. Despite theseadvantages, the technique is often overlooked because it requires the use of an oven or autoclave to cure the part, and the void

contents of the components produced may not satisfy the quality needed for structural applications. In this paper, we will introduceand discuss the important features of the recently developed Bladder Assisted Composite Manufacturing (BACM) technique.BACM eliminates the disadvantages of conventional bladder molding applications by heating the composite laminate internally

during cure and controlling cure pressure and resin outflow. As a result, the BACM technique was found capable of producingaerospace quality composite structures with fiber volume contents as high as 67% and void contents as low as 0.2%. In addition,

compared to the conventional bladder molding techniques which utilize external heating, the BACM process was observed toreduce the energy required to fabricate a composite component by over 50%. These results make the BACM technique a feasiblealternative to current fabrication methods for the production of composite components of varying geometrical complexity.

Moreover, with further development, the fabrication of large hollow composite structures such as aircraft wings and fuselages viaBACM is promising.

Keywords: BACM, Prepreg, Bladder MoldingPACS: 81.05.Qk, 81.05.Ni, 81.20.Hy

INTRODUCTION

The composite manufacturing industry has been presented with significant challenges in producing geometrically

complex and hollow components with high mechanical performance. To date, the most economically viable choice for the

fabrication of hollow components has been injection/compression molding using short fiber reinforced resin systems [1].

However, the components fabricated from these molding methods oftentimes have lower mechanical performance (e.g.,

stiffness and strength) when compared to those with continuous fiber reinforcement. For this reason, in industries wherestrength to weight ratio has high priority, such as in defense and aerospace, continuous fiber reinforced materials are often

used. Currently, pultrusion and filament winding are regularly used with great success to create hollow, structural

composite components with continuous fiber reinforcement. These process have been found capable of consistently

producing parts with good surface quality and a low number of defects. However, the parts compatible with the pultrusion

and filament winding fabrication techniques are generally axis-symmetric, exhibit limited geometric complexity, and have

uniform wall thicknesses. To overcome the shortcomings of filament winding and pultrusion, the composite industry has

from the mid 1980s considered/used variations of bladder molding for the fabrication of hollow composite parts which

can exhibit high geometric complexity and asymmetry (e.g., aircraft propeller blades, and wings with integral ribs/spars).

The bladder molding technique is a fabrication process in which a laminate layup is draped around an expandable or

inflatable bladder which is then pressurized within a closed mold. The bladders used are often made from either silicone

rubber, nylon, or rubber latex depending on a number of factors such as part geometry and cost [2]. Once the fabrication

assembly has been completed, it is heated using either an oven or autoclave to achieve part cure. Depending on the bladder

molding technique used, the consolidation pressure is either applied gradually, as with an expandable bladder, or

immediately with the use of an inflatable bladder. These variations of bladder molding have been used successfully toproduce continuously fiber reinforced hollow composite parts with void volume fractions less than 5% [3, 4].

EXPANDABLE BLADDERS

In this variation of the bladder molding process, a bladder is produced such that it is slightly undersized and is placed

in the interior of the laminate which has been laid against a female metallic mold. Upon the heating of the mold to

facilitate part cure, the relatively high coefficient of thermal expansion of the bladder relative to the mold material is

employed to cause a consolidation pressure to be applied to the laminate as shown in Figure 1. The primary disadvantages

of this bladder molding variation are: (i) an oven/autoclave or complex tooling with embedded heaters is required to

supply requisite cure temperature; (ii) maximum consolidation pressure is dictated by cure temperature.


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FIGURE 1.Use of expandable bladders for the manufacturing of hollow composite components [5].

INFLATABLE BLADDERS

In the bladder molding process which uses an inflatable bladder, a bladder is produced whose geometry is near the net

shape of the inside of the part to be produced. During the set-up, dry or impregnated (i.e., prepreg) fabric/fibers are draped

around an inflatable bladder. After the laminate has been placed, the mold is closed, the bladder is inflated to the desired

consolidation pressure, and the mold is heated to enable part cure as shown in Figure 2. The advantages of using an

inflatable bladder instead of an expandable bladder are: (i) parts can be fabricated using prepreg or dry fibers impregnated

using resin transfer molding (RTM); (ii) consolidation pressure can be varied independently of cure temperature [6].

However, the disadvantage of requiring an oven/autoclave or complex tooling with embedded heaters to supply requisitecure temperature remains.

FIGURE 2.Manufacturing of hollow composite components using inflatable bladders.

BLADDER ASSISTED COMPOSITE MANUFACTURING (BACM)Of the two bladder molding processes described, the inflatable bladder variation is the most attractive for commercial

applications due to its versatility, i.e. ability to fabricate parts using ei ther prepreg or dry fibers impregnated via RTM.

However, the requirement to supply the heat for part cure by oven/autoclave or tooling with embedded heaters have offset

these advantages and has limited the use of bladder molding commercially. Because of this fundamental disadvantage, the

Bladder Assisted Composite Manufacturing (BACM) technique was developed to eliminate the need of an autoclave,

oven, or complex tooling with embedded heaters. The BACM process is a variation of the inflatable bladder process in

which the heating source required for part cure has been moved inside the uncured hollow layup as shown in Figure 3.

Thus, the temperature within the layup is controlled by circulating and when/if necessary venting the air within the bladder

which provides required consolidation pressure to the layup. A direct benefit of internal heating of the part/mold, is that


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the fabrication assembly is no longer required to be conductive as the heat for part cure is transferred by conduction

through the bladder. Thus, the use of cost-effective materials which are thermally insulative in conjunction with the

limited use of traditional mold materials is a possibility. In a recent research by Anderson and Altan [3], a thin aluminum

shell mold with a plaster casing was used to fabricate composite cylinders from prepreg. The energy required to produce

the parts was reduced by 50% when compared to the traditional inflatable bladder molding process. Moreover, the fiber

volume fraction and mechanical properties of the parts produced were shown to be equivalent to laminates manufactured

by hot-press molding [2]. In fact, the BACM process can be optimized by controlling resin outflow by the use of o-ring

seals and bleeder cloths. It was reported that hollow composite parts can be produced with void and fiber volume fractionsas low as 0.2% and high as 67%, respectively. Thus, the BACM fabrication technique, when optimized, may be a suitable

process for the fabrication of primary aerospace structures [7].

FIGURE 3.Bladder Assisted Composite Manufacturing (BACM) fabrication assembly and process schematic [3].

FUTURE CHALLENGES AND OPPORTUNITIES

Several challenges must be overcome in order for the BACM process to become recognized as a commercially viable

fabrication technique. Design, research, and development efforts are required to overcome the challenges in mold design,selecting the bladder material, and controlling the pressure and temperature of the heated air inside the bladder. In this

pursuit, it is imperative that the following issues be addressed:

From an industry perspective, the ability to simulate the effect of varying process parameters on a component is

advantageous. Thus, the BACM process must be modeled to allow for (i) a thorough understanding of the

technique; (ii) process parameters to be optimized to obtain the desired part geometry, fiber volume fraction and

void volume fraction.

Work should be done to allow for the production of very complex parts, i.e. parts with integrated structural

stiffeners. Hence, allowing for the number of bonded joints in a given composite structure to be reduced.

The BACM process tooling must be scaled up to allow for the production of large composite components.

Provided the above issues are resolved, there is a tremendous opportunity for industry to reduce total fabrication costs of

large parts with complex geometry such as aircraft wings and fuselages via BACM.

REFERENCES

1. M.G. Bader, Composites Part A: Applied Science and Manufacturing, 33, 913-934 (2002).

2. D. Rebard, "Bladder Molding With Latex in the Recreational Industry- Lessons Learned" in 49th International SAMPE

Symposium and Exhibition, edited by SAMPE,Long Beach, CA,2004, pp. 79-82.

3. J.P. Anderson and M.C. Altan,Journal of Engineering Materials and Technology, 134, 0445011-0445017 (2012).

4. A. Salomi, et al.,Advances in Polymer Technology, 26, 21-32 (2007).

5. U. Lehmann and W. Michaeli, Composites Part A: Applied Science and Manufacturing, 29, 803-810 (1998).

6. H. Ghiasi, et al.,Applied Composite Materials, 17, 159-173 (2010).

7. J.P. Anderson, "Manufacturing and Microstructural Modeling of Geometrically Complex Composite Components Producedby Bladder Assisted Composite Manufacturing (BACM)", Ph.D. Thesis, University of Oklahoma, 2013.

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BACM Extended Abstract Anderson and Altan Final August 2014