Note: Descriptions are shown in the official language in which they were submitted.
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WO 93/20g94 P~/US93/0190~
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METHOD OF FORMING A PREFORM USIN6 A PO~DER
BINDER
TECHNICAL FIE~D
This invention relates generally to directed
fiber preforming and, more particularly, to an improved
process for forming a glass fiber preform.
BAC~GRO~nJD OF THE INV~NTION
Glass fibers have been commonly incorp~rated
in thermoplastic molded objects and other cured plastics
for added strength and durability. The glass fiber is
introduced in a mold where resinous plastic is then
injected such that the glass fibers become imbedded into
the final formed object. It has been found advantageous
to form a preform of the final object out of glass
fibers and place the preform into the mold. The glass
fibers are often chopped and blown onto a preform
screen. Tmmediately following the fiber placement, a
liquid binder agent is sprayed on and allowed to cure
which sets the fibers in place.
The use of a liquid binder coats substantially
the entire chopped glass fiber which can prevent matrix
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resin in the molded part from fully wetting-out the
fibers in the preform. This lack of penetration of the
matrix resin into the fiberglass strand within the
preform for the finished molded part restricts its
mechanical strength and integrity.
What is needed is a method for adding a binder
to the preform fibers that prevents substantial coating
of the fibers so that the preform is more supple during
handling thus improving its integrity and such that the
later added matrix resin in the subsequent molding
process more fully wets ~ut the fibers of the preform.
SUMMARY OF T~E INVENTION
In accordance with one aspect of the inven-
tion, chopped fibrous ma~erial, such as chopped glass
fibers, are deposited on a preform screen. The fibers
may be retained on the screen by a vacuum draw there-
through. A solid particulate binder material, commonly
ref rred to as a powder, is deposited onto the deposited
chopped fibers. The particulate binder are retained on
the deposited glass fibers. The particles of binder ~ay
be retained by static electrical charges on either the
binder particles or glass fibers or both.
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The binder particles can be softened while on
the deposited fibers such that the fibers are adhered to
the fibers independently of the electro-static charge.
The softening of the binder particles also allows fibers
to adhere to other fibers via the binder particles. The
softening of the binder particles may be by applying
heat to a binder made from a thermoplastic polyester
material such that upon softening it becomes tacky and
sticks to the adjacent glass fibers. The particles are
then re-solidified to form a.cured preform. The preform
is then removed from the preform screen.
BRIEF DE8C~IPTION OF THE DRA~ING8
Reference is now made to accompanying drawings
in which:
Figure 1 is side elevational view of a direct
fiber apparatus used in a method according to the
invention: and
Figure 2 is an enlarged and schematic view of
the glass fibers and particulate binder deposited onto
the preform screen shown in figure 1.
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DETAILED DESCR~TION OF ~HE PREFERRED EMBODI~ENT8
Referring now to figure 1, a preform screen
assembly 10 includes a mounting table 12, a foraminous
preform screen 14 and a suction fan 15. The screen 14
has a plurality of holes 27 and its contour conforms to
one surface of the desired preform shape. The screen 14
is mounted about its periphery to the table 12 which in
turn has appropriate duct work 13 for housing the
suction fan 15. The suction fan when actuated draws a
vacuum through the screen 14. A nozzle control system
generally indicated as 20 is pointed toward the preform
screen 14.
The system includes a blower nozzle 18 for
deposition of blown glass fibers 19 and at least one
nozzle 21 for the spraying of dry binder powder 25. The
glass fibers used may be chopped from a commercially
available roving such as one sold under the brand name
PPG-5542. The fiber nozzle 18 is the distal end of a
flexible tube 26 that has i~s other end 27 mounted onto
a rigid delivery tube 28 communicating from a chop-
per/blower assembly 30. The tubes 26 and 28 and chop-
per/blower assembly 30 are mounted on a gantry 32 that
can move the nozzles left, right, up, and down relative
to the screen 14.
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The chopped glass fibers 19 from nozzle 18 are
directed onto the preform screen 14. The chopped glass
fibers may be chopped between 1" and 4" in length
depending on the application. Simultaneously or immedi-
ately after the deposition of the fibers, the powderbinder is blown onto the preform screen 14. The binder
may be Reichhold's Atlac 32-626 and can be blown by a
Volstatic Coatings LTD~s Solidspray-90 spray station in
combination with their SPG-1351 spray gun onto the
preform screen at a rate of approximately 2.14 g/s.
Other particulate thermoplastic polyester binders may be
alternately used. The particulate sizes of the powder
binder may range from mesh size 35 to 200 which corre-
sponds to .00122l' to .0021".
'5 The binder powder is retained on the glass
fibers through static electrical charges. The glass
fibers are retained on the preform screen by the use of
a vacuum draw through the screen produced by suction fan
15. As shown in figure 2, the particles and qlass
fibers form a porous mat 50 with many interstices 52
between the fibers and particles. The amount of binder
by weight is about 2-6~ of the glass fibers.
After the appropriate amount of glass fiber
and powder binder is applied onto the preform screen 1~,
heat is applied to the preform 40. In one case, heat
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lamps 43 may be positioned about the preform screen 14
to direct heat onto the preform 40. The screen assembly
10 may be moved on a multi-station line such that the
heat is applied at a different station than the station
5 that deposits the glass fibers. The heat may be applied
by heat lamps 43, heated air, or other conventional
heating methods. The heat softens the particles of
the binder such that each particle becomes tacky and
sticks to any glass fibers with which it is in contact.
10After sufficient softening, the binder and
glass fibers on the preform screen 14 are cooled such
that the binder is set, i.e. cured, to form a one-piece
preform 40. The preform is then allowed to cool by use
~ ~ of chilled air or deactuation of heat lamps 43. As
-~ 15 shown in figure 2, the glass fibers are not coated with
binder and as such, preform release from the screen is
significantly easier compared to preforms made with
sprayed liquid binders. Furthermore, preform inteqrit~
is improved. The strength derived from adhesion with
particles exceeds that from coated fibers touching each
other.
Furthermore, coated fibers prevent matrix
resin in the molded part from fully wettinq-out the
fibrous strands which are composed of thousands of
filaments. By not coating the fibrous strands, nothing
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interferes with wet-out. Each fiber can be penetrated
by the matrix resin to fully wet-out all the filaments
in any subsequent molding process.
Other variations and modifications are possi-
ble without departing ~rom the scope and spirit of the
invention as defined in the appended claims.
