NASA CASE NO.

PRINT FIG.

NOTICE

The invention disclosed in this document resulted from research in aeronautical and

space activities performed under programs of the National Aeronautics and Space

Administration. The invention is owned by NASA and is, therefore, available for

licensing in accordance with the NASA Patent Licensing Regulation (14 Code of

Federal Regulations 1245.2).

To encourage commercial utilization of NASA-owned inventions, it is NASA policy to

grant licenses to commercial concerns. Although NASA encourages nonexclusive

licensing to promote competition and achieve the widest possible utilization, NASA

will provide the necessary incentive to the licensee to achieve early practical

application of the invention.

Address inquiries and all applications for license for this invention to NASA Patent

Counsel, Langley Research Center, Code 212, Hampton, Virginia 23681-0001.

Serial No.: 08/292,621

08/12/94 LaRC

(NASA-Case-LAR-152IT-I) METHOD FOR N95-24212

MOLDING STRUCTURAL PARTS UTILIZING

MOOIFIED SILICONE RUBBER Patent

Application (NASA. Langley Unclas

Research Center) 14 p

G3/27 0039292
NASA Case No. LAR 15217-1

METHOD FOR MOLDING STRUCTURAL PARTS UTILIZING

MODIFIED SILICONE RUBBER

AWARDS ABSTRACT

Control of the pressure at the cure temperature of polymer matrix composites is critical

to the fabrication of high quality composite structures. The application of pressure

during the cure stage can be performed in various ways. An autoclave may be used to

apply pressure, although the use of an autoclave can be expensive and complicated. A

less expensive and simpler method utilizes silicone rubber to apply pressure to the

preform material during the cure process. The thermal expansion of the silicone rubber

during the cure stage applies the pressure necessary to force the preform layup into

contact with the die. However, due to the high expansion rates of silicone rubber in

the "as received and cured" form, it is very difficult to maintain proper pressure

application levels at the cure temperatures of composite materials. The problem of not

managing the system's pressure generation frequently results in too much resin flow

out due to excessive pressure.

The present invention is an improved method for molding structural parts utilizing the

thermal expansion of silicone rubber. By introducing air bubbles into the silicone

rubber before curing the rubber, the bulk modulus of the rubber is significantly reduced.

The bulk modulus is the physical property of the rubber that defines the rate of

pressure applied by the rubber to preform material when the rubber is expanding.

During the preparation of the silicone rubber, dry silica powder is stirred into the liquid

mixture. By stirring in silica particles, discrete air voids are introduced into the silicone

rubber composition. The mixture is not deaerated prior to cure so that the air voids

remain entrained in the rubber. The resulting reduction of the bulk modulus allows for

precise control of the pressure being applied to the preform material by the expanding

silicone rubber during the molding process. The introduction of discrete air voids into

the silicone rubber system allows for a very manageable 20 psi change in pressure

applied to the preform material for every 10°F, as compared to 200 psi for every 10°F

when the silicone rubber is not modified. This modification to a standard silicone rubber

molding system results in a molding process that can be used to mold complex

structural parts without the use of an autoclave and still maintain accurate pressure

generation.
Novel aspects of the present invention include the addition of silica powder to standard 

castable silicone rubber before curing the rubber. This results in a lower bulk modulus 

which makes it possible to accurately control the pressure applied to preform materials 

during the molding process. 

Inventors: Erik S. Weisner 

Address: 4526 California Boulevard 

Santa Maria, CA 93455 

ss: 0 

Co-op Student 

Robert M. Baucom 

44 Cherbourg Drive 

New~ort News. VA 23606 

John J. Snoha 

28 Alexander Drive 

Initial Evaluator: Darrell R. Tenney 

Serial No.: 081292,621 

Filed: 0811 2/94 
LAR 15217-1 -1- PATENT APPLICATION

METHOD FOR MOLDING STRUCTURAL PARTS

UTILIZING MODIFIED SILICONE RUBBER

5

10

Ori.qin of the Invention

The invention described herein was made by employees of the United

States Government and may be manufactured and used by the government

for governmental purposes without the payment of any royalties thereon or

therefore.

Field of the Invention

15

This invention relates generally to molding and particularly to a

modified silicone rubber for applying precise pressure to preform materials at

the cure temperatures.

Description of the Related Art

Control of the pressure at the cure temperature of polymer matrix

20 composites is critical to the fabrication of high quality composite structures.

Typically, composite structures are formed by laying up preform material over

a mandrel or within a die and then subjecting the assembly to precise

temperature and pressure conditions.

The application of pressure during the cure stage can be performed in

25 various ways. An autoclave may be used to apply pressure, although the use

of an autoclave can be expensive and complicated. A less expensive and

simpler method utilizes silicone rubber to apply pressure to the preform

material during the cure process. The thermal expansion of the silicone

rubber during the cure stage applies the pressure necessary to force the

30 preform layup into contact with the die. Silicone rubber has also been used
LAR 15217-1 -2- PATENT APPLICATION

in combination with an autoclave to assist in applying pressure to selected

areas of a part in addition to the pressure supplied by the autoclave.

Prior art discloses the use of silicone rubber systems to apply pressure

during the molding of structural parts. For example, Arachi (U.S. Patent

5 No. 4,167,430) utilized the expansion of standard silicone rubber to fabricate

bonded composite structures, such as a missile fin. Due to the high

expansion rate of silicone rubber in the "as received and cured" form, it is

very difficult to maintain proper pressure application levels at the cure

temperatures of composite materials. The problem of not managing the

10 system's pressure generation frequently results in too much resin flow out

due to excessive pressure.

Prior inventions disclose various methods employed to control the

pressure of the expanding rubber. For example, Barraclough (U.S. Patent

No. 4,624,820) discloses an apparatus that employs hydraulic pressure, in

15 addition to the pressure from the silicone rubber, to apply pressure during the

initial stage of the curing process. As the silicone rubber expands, the

hydraulic pressure is reduced in order to maintain the desired pressure.

Kemp (U.S. Patent No. 4,889,668) discloses a method that does not employ

an external pressure source, but selectively heats and cools the rubber

20 system to produce the desired amount of pressure.

The object of this invention is to modify the silicone rubber system so

that the pressure exerted by the system can be controlled. This will allow for

the desired pressure to be applied to the preform part at the cure

temperature. This method allows for a relatively simple way to mold

25 structural parts without the need for an expensive autoclave to obtain

accurate pressure application.

Summary of the Invention

30 In order to control the pressure applied to the preform material by the
LAR 15217-1 -3- PATENT APPLICATION

expanding silicone rubber during the curing process, a modified silicone

rubber composition is prepared. This technology involves the introduction of

discrete air voids into the silicone rubber composition during the preparation

of the silicone rubber composition. The introduction of air voids into the

5 silicone rubber results in a significant reduction in the bulk modulus of the

rubber. The bulk modulus is the physical property of the rubber that defines

the rate of pressure applied by the rubber to the preform material during

thermal expansion.

The amount of pressure to be applied to the composite material during

10 the cure process dictates the volume of silicone rubber to be used in relation

to the volume of the rigid mold cavity. If it is calculated that a volume of

silicone rubber equal to the volume of the mold cavity would result in

excessive pressure application, the volume of the silicone rubber system is

reduced so that free expansion can occur before the expansion forces the

15 composite part into contact with the mold container. The necessary reduction

in volume is dependent upon the bulk modulus of the rubber as well as the

cure temperature and can be calculated using standard equations known to

those skilled in the art.

This modification to a standard silicone rubber molding system results

20 in a molding process that can be used to mold complex structural parts

without the use of an autoclave and still maintain accurate pressure

generation. As discussed below, the introduction of discrete air voids into

the silicone rubber system can result in the reduction of the thermal

expansion of a silicone rubber by a factor of ten. This allows for a very

25 manageable 20 psi change in pressure applied to the preform material by the

rubber for every 10°F, as compared to 200 psi for every 10°F when the

silicone rubber is not modified.

3O
LAR 15217-1 -4-

Brief Description of the Drawincl

PATENT APPLICATION

5

For a more complete understanding of the present invention, including

its primary object and benefits, reference should be made to the Description

of the Preferred Embodiment below. This Description should be read

together with the accompanying drawing, which is a plot of pressure versus

temperature for various preparations of silicone rubber.

10

Description of the Preferred Embodiment

Tests were conducted on a standard high-temperature castable

silicone rubber in order to measure how the bulk modulus of the rubber is

affected by altering the preparation of the rubber. The bulk modulus is the

physical property of the rubber that defines the rate of pressure applied by

15 the rubber to the preform material during thermal expansion. The silicone

rubber chosen for the tests was "Aircast 3700 _'', which is a high-temperature

castable silicone rubber manufactured by Airtech International, Inc. of

Carson, California.

First, in accordance with the manufacturer's directions, the liquid

20 rubber was mixed with a curing agent at a ratio of 10:1. The mixture was then

held under a vacuum in order to remove air pockets in the rubber. The

rubber mixture was allowed to cure at room temperature for approximately 24

hours and then post cured at 300°F for 30 minutes.

Next, to determine the bulk modulus of the rubber, the rubber was

25 placed in a heat press with the platens in contact with the rubber. The bulk

modulus of the rubber was calculated using a dial micrometer to measure the

displacement of the lower platen as force was applied to the rubber. The

equation used was:

30 K=FL/A61
LAR 15217-1 -5- PATENT APPLICATION

Where K is the bulk modulus, F is the applied force, L is the length of the

rubber, A is the cross-sectional area, and 61is the change in length. The

bulk modulus of the composition was calculated to be 9.36 x 104psi.

5 The bulk modulus of Aircast 3700®that was determined by the above

experiment indicates that small increases in temperature would result in

significant rubber expansion as well as a significant increase in the pressure

applied to the preform material by the expansion of the rubber. Calibrations

indicate that the pressure application rate is approximately 200 psi change in

10 pressure for every 10°F increase in temperature. This drastic increase in

pressure during heating makes it difficult to accurately obtain a desired

pressure at a predetermined cure temperature.

In an effort to reduce the bulk modulus of the silicone rubber,

experiments were conducted wherein various materials including silica

15 powder, sand, and glass beads were individually added to Aircast 3700®

before curing. Although all of these additives were somewhat effective in

lowering the bulk modulus of the rubber, silica produced the most favorable

results. Quantities of silica powder ranging from about 2% to 8% of the

silicone rubber volume were effective in lowering the bulk modulus of the

20 rubber. The addition of silica powder in quantities greater than about 8% of

the silicone rubber volume rendered the mixture difficult to stir. A volume

equal to about 5% of the silicone rubber volume provided the optimum

results.

One experiment involved stirring in about 5% by volume of dry silica

25 powder, and then curing and deaerating the mixture. This preparation

resulted in a bulk modulus of 2.73 x 104psi which represents a substantial

reduction as compared to the 9.36 x 104psi of the silicone rubber that was

not doped with silica. The substantial reduction in bulk modulus in the

deaerated material containing the 5% silica was determined to be due to air

30 voids remaining in the rubber which could not be effectively removed during
LAR 15217-1 -6- PATENT APPLICATION

deaeration because of the presence of silica particles.

Another experiment with the silica doped rubber was conducted

wherein the deaeration step was eliminated prior to cure. The addition of the

silica particles creates a stream of discrete air voids in their wake during the

5 mixing. By eliminating the deaeration step, the air voids remain within the

cured rubber. This experiment resulted in a further reduction in the bulk

modulus to 0.928 x 104 psi. This new preparation according to the present

invention lowered the pressure application by the rubber during heating to a

very manageable 20 psi change for every 10°F increase in temperature. As

10 illustrated in Figure 1, the addition of the silica powder and the elimination of

the deaeration step results in significantly lower pressure application when

the rubber is heated. This small change in pressure per degree Fahrenheit

(20 psi / 10°F) makes it possible to conveniently produce composite parts for

a variety of applications.

15 The following is a non-limiting example of the use of the method

according to the present invention to produce graphite/epoxy twisted spar

tubes to be used in helicopter rotor blades. Each spar tube is hollow and 24

inches long with a 70-degree twist from one end to the other.

The pressure required for complete consolidation and cure of the spar

20 tubes was calculated to be approximately 125 psi. A modified silicone

rubber system was utilized to apply the necessary pressure to the composite

part during the molding process.

A silicone rubber composition was prepared in the form of the desired

core shape of the spar tubes. The silicone rubber used in this case was

25 Aircast 3700 ® doped with dry silica powder. In casting the silicone rubber, a

10:1 ratio of 200 grams of rubber to 20 grams of catalyst was used. Dry

silica powder was then added, the amount being about 5% by volume of the

mixture. The mixture was not deaerated as would normally be done in

accordance with the manufacturer's instructions.

30 In order to obtain the desired pressure application of 125 psi at the
5

LAR 15217-1 -7- PATENT APPLICATION

cure temperature of the graphite/epoxy composite (350 ° F), it was determined

that 92% of the mold cavity volume should be filled with cast rubber. That is,

8% void space for free expansion of the rubber prior to contact initiation

results in a final pressure of 125 psi. The amount of void space required for

free expansion before contact can be determined using the following

equations:

v2Rubber= vlRubb_r(1 + aRub_rAT)

10 _Vc Rubber- PV2Rubber / K

15

2O

Vl Rubber

V2 Rubber

ocRubber

AT

P

K

_,Vc Rubber

= volume of rubber at room temperature

= volume of rubber at final temperature

= the cubic coefficient of thermal expansion

= final temperature minus room temperature

= final pressure after full expansion of the rubber

= the bulk modulus of the rubber

= the decrement of rubber volume necessary to fit the

freely expanded rubber into the mold cavity at the

final temperature.

In order to prepare the necessary volume of the silicone rubber, the mold

cavity was filled with a uniform layer of tape so that only 92% of the cavity

25 was filled with rubber. The rubber mixture was allowed to cure at room

temperature for approximately 24 hours and then post cured at 300°F for 30

minutes.

The preform material was then laid up on the rubber core, the

assembly was placed in the rigid mold cavity, and the entire mold setup was

30 heated to the cure temperature of the composite preform material. The

thermal expansion of the silicone rubber material during heating caused the
LAR 15217-1 -8- PATENT APPLICATION

preform material to be forced into intimate contact with the mold surface. The

pressure exerted by the cast rubber core at the cure temperature of the

composite material was maintained for the duration of the two-hour cycle.

After the cure cycle was complete and cooling to ambient temperature

5 occurred, the rubber core shrank from the composite part back to its original

state. The rubber core was then removed from the fully cured composite spar

tube. Due to the accurate control of the pressure application during the

curing process, the tubes produced were of good quality manifested by the

uniform thickness along all surface walls of the tubes.

10 In addition to the molding method described above, the technique can

be modified to produce parts that are not hollow in structure. Instead of

positioning the silicone rubber composition at the core of the preform part, the

silicone rubber can be positioned within the mold cavity so that it expands

and applies pressure to the outer surfaces of the preform part. Furthermore,

15 the above description pertains to the molding of composite materials in which

the process requires precise pressure application at the cure temperature.

The identical molding process utilizing silica doped silicone rubber can be

employed to mold parts fabricated from various materials.

Another advantage of the modified silicone rubber is the ability to

20 debulk and cure preform materials in one step utilizing the thermal expansion

of the silicone rubber. Certain preform materials require significant debulking

before final curing because the preform material is many times thicker than

the desired thickness of the final part. For instance, powdered towpreg is

usually much thicker before debulking due to the bulk acquired when the tow

25 is impregnated with powder particles. Debulking is typically conducted by

applying pressure to the preform material thereby inducing the resin flow and

fiber movement necessary to eliminate voids and fill the intra- and inter-tow

spaces. The thermal expansion of the silicone rubber can provide the

pressure necessary to debulk as well as cure preform materials. Prior to

30 heating the mold container to the final cure temperature, the mold container
5

LAR 15217-1 -9- PATENT APPLICATION

can be maintained at a lower temperature to allow for the silicone rubber to

expand and apply the necessary pressure to the preform material so that

debulking occurs. Therefore, while the present invention has been disclosed

in connection with the preferred embodiment thereof, it should be understood

that there may be other embodiments which fall within the spirit and scope of

the invention as defined by the following claims.

What is claimed is:
LAR 15217-1

Abstract

PATENT APPLICATION

METHOD FOR MOLDING STRUCTURAL PARTS

UTILIZING MODIFIED SILICONE RUBBER

5 This invention improves upon a method for molding structural parts

from preform material. Preform material to be used for the part is provided.

A silicone rubber composition containing entrained air voids is prepared. The

silicone rubber and preform material assembly is situated within a rigid mold

cavity used to shape the preform material to the desired shape. The entire

10 assembly is heated in a standard heating device so that the thermal

expansion of the silicone rubber exerts the pressure necessary to force the

preform material into contact with the mold container. The introduction of

discrete air voids into the silicone rubber allows for accurately controlled

pressure application on the preform material at the cure temperature.

15
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