U.S. patent application number 09/828293 was filed with the patent office on 2002-10-10 for mandrel-assisted resin transfer molding process employing resin outflow perimeter channel between male and female mold elements.
This patent application is currently assigned to Caldwell Design and Fabrication, L.L.C.. Invention is credited to Caldwell, James D. JR..
Application Number | 20020145218 09/828293 |
Document ID | / |
Family ID | 25251387 |
Filed Date | 2002-10-10 |
United States Patent
Application |
20020145218 |
Kind Code |
A1 |
Caldwell, James D. JR. |
October 10, 2002 |
Mandrel-assisted resin transfer molding process employing resin
outflow perimeter channel between male and female mold elements
Abstract
A mandrel-assisted resin transfer molding process and apparatus
therefor provides a generally continuous, narrow annular channel
between the perimeter of an inner male mold element and an outer
female mold element. This channel allows air, heat and vapor to
evenly escape and resin to outflow from the mold cavity everywhere
around the edge of the part being molded. A very thin resin band
forms in the channel is easily trimmed away, leaving the article
completely finished on both sides and having a well defined edge
that does not require shaping.
Inventors: |
Caldwell, James D. JR.;
(Greenville, NC) |
Correspondence
Address: |
CHRISTOPHER F. REGAN
Allen, Dyer, Doppelt,
Milbrath & Gilchrist, P.A.
P.O. Box 3791
Orlando
FL
32802-3791
US
|
Assignee: |
Caldwell Design and Fabrication,
L.L.C.
5485 Highway 17 North
Washington
NC
27889
|
Family ID: |
25251387 |
Appl. No.: |
09/828293 |
Filed: |
April 6, 2001 |
Current U.S.
Class: |
264/102 ;
264/257; 425/129.1; 425/389; 425/812 |
Current CPC
Class: |
B29C 70/48 20130101;
Y10S 425/812 20130101 |
Class at
Publication: |
264/102 ;
425/812; 425/389; 425/129.1; 264/257 |
International
Class: |
B29C 070/48 |
Claims
What is claimed
1. An apparatus for mandrel-assisted resin transfer molding an
article comprising: an outer female mold element having an interior
surface associated with a first surface of said article; and an
inner male mold element having an exterior surface associated with
a second surface of said article, and being sized to be placed
within said outer female mold element, so as to define a mold
assembly forming a mold cavity between said interior surface of
said outer female mold element and said exterior surface of said
inner male mold element, said mold cavity receiving a structural
preform to be impregnated with resin, said inner male mold element
having a perimeter sidewall that extends a vertical distance
alongside, but is spaced apart from, a mutually facing interior
sidewall of said outer female mold element, so as to form a
generally continuous narrow annular channel between an outer
perimeter edge of said inner male mold element and an inner
perimeter edge of said outer female mold element, said channel
extending to and being contiguous with said mold cavity, to allow
venting of air and expansion therethrough of resin that has been
introduced into said mold cavity and has impregnated said
structural preform therein.
2. The apparatus according to claim 1, further including a
plurality of indexing tabs affixed to said inner male mold element,
and being configured to engage said outer female mold element so as
to accurately dimensionally locate said inner male mold element
within the interior volume of said outer female mold element.
3. The apparatus according to claim 1, further including an
auxiliary closure engaging said mold assembly and coupled to a
vacuum, which is operative to augment the outflow of resin and
removal of air pockets from said mold cavity through said
channel.
4. The apparatus according to claim 3, further including a
plurality of indexing tabs affixed to said inner male mold element,
and being configured to engage said outer female mold element so as
to accurately dimensionally locate said inner male mold element
within the interior volume of said outer female mold element.
5. A method of manufacturing an article comprising the steps of:
(a) providing an outer female mold element having an interior
surface associated with a first surface of said article; (b)
providing an inner male mold element having an exterior surface
associated with a second surface of said article, and being sized
to be placed within an interior region of said outer female mold
element, so as to define a mold assembly forming a mold cavity
between said interior surface of said outer female mold element and
said exterior surface of said inner male mold element, said inner
male mold element having a perimeter sidewall that is adapted to
extend a vertical distance alongside, but spaced apart from, a
mutually facing interior sidewall of said outer female mold element
when said inner male mold is inserted into said outer female mold;
(c) placing a structural preform within said interior region of
said outer female mold element; (d) introducing resin into said
interior region of said female mold element and thereby wicking
fibers of said structural preform; (e) inserting said inner male
mold element within said interior region of said outer female mold
element, so as to form therewith said mold cavity of said mold
assembly, and also a generally continuous narrow annular channel
that is contiguous with said mold cavity, through which air is
vented and into which resin introduced in step (d) is allowed to
expand from said mold cavity; and (f) after curing of said resin,
removing said mold assembly to provide a resin transfer molded
article.
6. The method according to claim 5, further including the step of:
(g) removing a band of cured resin formed along an edge of said
resin transfer molded article provided in step (e) as a result of
resin outflow from said mold cavity into said channel.
7. The method according to claim 5, wherein step (e) includes
locating said inner male mold element within said interior region
of said outer female mold element by means of a plurality of
indexing tabs that engage said inner male mold element and said
outer female mold element.
8. The method according to claim 5, wherein step (e) includes
coupling an auxiliary closure with said mold assembly and applying
thereto a vacuum, so as to augment outflow through said channel of
resin and removal of air pockets from said mold cavity.
9. The method according to claim 8, wherein step (e) includes
locating said inner male mold element within said interior region
of said outer female mold element by means of a plurality of
indexing tabs that engage said inner male mold element and said
outer female mold element.
Description
FIELD OF THE INVENTION
[0001] The present invention relates in general to resin transfer
molding (RTM) process and apparatus therefor, and is particularly
directed to a new and improved RTM architecture that employs a
generally continuous and relatively narrow, annular channel between
the perimeter of an inner male mold element and an outer female
mold element. This perimeter channel allows resin and air to
readily escape everywhere around the edge of the part being molded,
resulting in the molded part having a well defined edge of resin
material, so as to not require further shaping after removal from
the mold. The perimeter channel also allows the resin to expand
evenly around the part, facilitating heat dissipation, extending
the life of the mold, and providing higher part yield.
BACKGROUND OF THE INVENTION
[0002] Until recently, a variety of hardware manufacturing and
assembly enterprises, such as the boat manufacturing industry, have
made many of the parts employed in their products as partially
finished components--those which are finished on only one side (the
side visible to and engaged by the user). As market demands have
now driven many of these companies to produce many, if not all, of
their parts as completely finished components, these manufacturers
have turned to the use of resin transfer molding (RTM) processes of
the type which have been well established in the automotive and
aircraft industry. However, because of the relatively low volume
per part, these enterprises cannot afford the very substantial cost
of very robust (typically steel) molds used in RTM processes
employed in the automotive and aircraft industries. Instead, the
RTM systems that have been used for low margin, limited part
production, such as in the marine manufacturing industry, are
typically made from less robust materials, such as epoxy resins,
which suffer from poor part yield and are prone to frequent
failures.
[0003] A standard RTM process typically places a fiber preform of
the part to be molded within a mold cavity, whose geometric shape
is defined by the volumetric spacing between an outer, female mold
half, and an inner male mold half. The two mold halves or elements
are joined together and effectively sealed at their circumferential
edges, capturing a perimeter portion of a structural (fiber)
preform therebetween. Non-limiting examples of fibers used in fiber
preforms include fiberglass, graphite, carbon and Kevlar, and the
fibers are often braided or woven into a sheet form.
[0004] In order to mold a part, it is customary practice to inject
a liquid resin (such as an epoxy resin, polyester resin, and the
like) into the mold cavity, typically by a negative pressure
created by means of a vacuum pump. The liquid resin is usually
heated to mold temperature, i.e., catalyzed, in a heated pressure
tank and pressure-injected through a resin flow line coupled to the
mold cavity. The mold cavity has one or more bleed vents that are
ported to a collection reservoir to which the vacuum pump is
coupled. Transfer/injection of the liquid resin into the mold
cavity may be facilitated by introducing pressurized gas into the
pressure tank, which in turn causes the pressurized resin to flow
into the mold cavity. As the pressurized, heated resin is
introduced into the mold cavity, it thoroughly wicks or is absorbed
by the fiber preform.
[0005] At various times after pressure-injecting the heated resin
into the mold cavity, the mold operator will open the line coupled
between the collection reservoir and the mold cavity, in order to
bleed resin from the mold cavity and into the collection reservoir.
The collection reservoir may include a sight glass to enable the
operator to view released resin, and determine, based upon on his
experience and skill, whether it contains air bubbles. If air
bubbles are visible, indicating that the cavity has not been
completely filled with resin, the operator closes the line to the
collection reservoir, and continues to introduce additional
pressurized resin into the mold cavity. This resin bleeding and
inspection process is repeated, as necessary, until the operator is
satisfied that the mold cavity has been completely purged of air,
and filled with resin (which has saturated the fiber preform).
[0006] Each repetition of the resin bleeding and inspection step is
time consuming, primarily due to the fact that, after the operator
bleeds resin into the collection reservoir, the pressure tank must
be pressurized again. Typically, this bleeding and inspection step
is repeated at least three or four times for each molded article,
making the resin transfer molding process a relatively lengthy and
expensive task. Resin is also wasted each time the operator is
required to bleed the resin into the collection reservoir to
inspect it for air bubbles. Because this occurs several times
during the molding of each article, the volume of wasted resin and
associated cost can be substantial.
[0007] Another drawback of such a conventional molding process is
the fact that it relies upon operator judgment to hopefully
correctly determine (based upon what the operator perceives is a
lack of air bubbles in the resin bled into the collection
reservoir) if and when the mold cavity and fiber preform have been
completely filled with resin. Even if the operator accurately
observes no air bubbles in resin that has been bled into the
collection reservoir, this is no assurance that the mold cavity is
free of voids. The operator can only rely on what the bleed port or
ports reveal; he has no picture of the entirety of the mold. This
inability of the operator to ensure that the mold is void-free
results in poor repeatability, and lower overall quality and yield
of the articles being produced. It also increases glass print
transfer into the finished surface of the part.
[0008] In addition, irrespective of the skill and experience of the
operator, the architecture and manner of assembly of the two mold
halve themselves can result in a less than satisfactory product.
The circumferential edge regions at which the male and female mold
halves are joined together customarily capture a perimeter portion
of a fiber preform placed between the two mold halves. As this can
cause bunching together or even pulling of the fibers of the
preform, it leads to inconsistencies in the thickness and density
of the preform material at the joined mold edges and also within
the mold cavity. This not only causes variations in the dimensions
of the molded article, but can create variations in the flow of
resin through the mold cavity, leading to air pockets, that are not
discovered until the part has been cured and removed from the
mold.
SUMMARY OF THE INVENTION
[0009] Pursuant to the invention, the above-discussed problems of
conventional RTM schemes are effectively obviated by a new and
improved mandrel-assisted RTM system which, rather than forming a
seal between its two mold halves, is configured as an unsealed
architecture, having a predesigned narrow, generally continuous
channel between the interior perimeter of the outer female mold
element and the exterior perimeter of the inner male mold element.
This annular channel not only facilitates the venting of air
everywhere around the edge of the part being molded, but allows for
expansion of the resin evenly around the part as it flows out into
the channel. This results in the part having a well defined edge
that does not require shaping after removal from the mold, and
serves to allow heat, air and vapor to escape. Heat dissipation
reduces glass print in finish, extending the life of the mold, and
providing higher part yield. The escape of vapor prevents softening
of the gel coat and mitigates against glass print and distortion.
Allowing air to escape prevents trapped bubbles from expanding
during exotherm and shrinking during cool down, that may otherwise
cause distortion of the part.
[0010] In accordance with a non-limiting but preferred embodiment,
the architecture of the improved molding apparatus of the present
invention, the channel results from spacing the vertical sidewall
of lip portion of the inner male mold element that terminates the
mold cavity a prescribed distance from a mutually facing portion of
the interior sidewall of the outer female mold element. In order to
accurately dimensionally locate the male mold element within the
interior volume of the female mold element, indexing elements may
be affixed a spaced apart locations around the top surface of the
lip portion of the male mold element and configured to engage the
top edge surface of the female mold element.
[0011] These indexing elements vertically locate the male mold
element relative to the top surface of the female mold element,
placing the male mold element into the female mold element at a
depth that precisely defines the geometry parameters of the mold
cavity. Once inserted, the male mold element may be retained in its
intended mold cavity-forming position by means of clamps and the
like at the various indexing elements. As an auxiliary measure, the
upper portion of the mold assembly may be engaged by a mandrel
coupled to a vacuum. The vacuum closure augments the outflow of
resin and removal of air pockets from the mold cavity through the
annular channel, as the male mold element is compressed against the
resin-impregnated preform.
[0012] As resin is drawn out from the mold cavity, it enters the
channel, forming a thin band of resin that is contiguous with the
resin in the mold cavity. This thin band creates resin weight
around the edge of the mold to assist in forcing air out of the
mold cavity and increasing part density. Once the resin has cured,
and the mold halves removed, the thin resin band is readily excised
from the part, leaving the article completely finished on both
sides, as desired.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIGS. 1-8 diagrammatically illustrate respective steps of
the mandrel-assisted RTM process embodying the present
invention;
[0014] FIGS. 9 and 10 are respective partial perspective and side
views of an RTM architecture associated with the process depicted
in FIGS. 1-8; and
[0015] FIG. 11 is a partial side view of a conventional RTM
architecture.
DETAILED DESCRIPTION
[0016] Before describing in detail the new and improved
mandrel-assisted resin transfer molding method and apparatus
therefor in accordance with the present invention, it should be
observed that the invention resides primarily in a prescribed
geometric relationship between mutually adjacent sidewall surfaces
of each of the interior male mold element and the outer female mold
element. As a consequence, the configurations of the components of
the mandrel-assisted resin transfer molding architecture of the
invention and the manner in which they are positioned relative to
one another have, for the most part, been illustrated in the
drawings in a readily understandable diagrammatic pictorial format,
which shows only those specific details that are pertinent to the
present invention, so as not to obscure the disclosure with details
which will be readily apparent to those skilled in the art having
the benefit of the description herein.
[0017] Attention is now directed to FIGS. 1-8, which are
diagrammatic illustrations of the molding of an article in
accordance with successive steps of the RTM process embodying the
present invention while FIG. 9 is a diagrammatic partial
perspective view of an RTM architecture associated with the process
depicted in FIGS. 1-8.
[0018] As illustrated in FIG. 1, the process employs an outer
female mold element 10, shown as having a generally U-shaped
cross-section for purposes of providing a reduced complexity
exemplary embodiment. As a non-limiting example, each of the male
and female mold elements may be made of a hard and rigid synthetic
material, such as polyester resin, and the like. This greatly
reduces the cost of the mold and is in sharp contrast to
conventional RTM processes of the type used in the aircraft and
automotive industries, in which the mold cavity is subjected to
relatively high resin-injection pressures. In the present
invention, such pressures are not encountered as the annular
channel around the mold cavity provides for air, heat and resin
outflow pressure relief.
[0019] The female mold element 10 has an interior sidewall surface
11 associated with a first surface of the article to be molded, an
outer surface 12, and a top edge surface 13. As pointed out briefly
above, and as will become apparent from the description to follow,
the outer female mold element 10 is sized such that the interior
sidewall surface 11 is sufficiently deep to accommodate a
prescribed `vertical indexing` of the position of a male mold
element that is placed within the female mold element to form a
mold cavity therebetween.
[0020] FIG. 2 shows the insertion of a structural preform 20
adjacent to and generally conforming with that portion of the
interior surface 11 of the female mold element that is to define
the mold cavity in association with an inserted male mold element.
As described above, the structural preform may comprise any of a
variety of fibers used in conventional preforms, including, but not
limited to fiberglass, graphite, carbon and Kevlar, and the fibers
may be braided or woven into a sheet form.
[0021] Next, as shown in FIG. 3, a conventional liquid resin
material 30 is introduced (for example by pouring or spraying) into
the interior of the female mold element 10, and begins wicking the
fibers of the inserted preform 20. The volume of liquid resin
introduced into the female mold element 10 should be at least equal
and preferably larger than the volume of the mold cavity. With the
presence of the structural preform 20, this will ensure that the
resin will completely saturate the fibers of the structural preform
in the course of insertion and compression of the male mold
element, and expand outwardly therefrom into the channel between
the mold elements.
[0022] FIG. 4 shows the insertion of an inner male mold element 40
into the interior space of the female mold element 10 containing
the structural preform 20 and the liquid resin 30. The inner male
mold element 40 has an exterior surface 41 associated with a second
finished surface of the article to be molded, and is shaped and
sized to form a mold cavity 42 between its exterior surface 41 and
the interior surface 11 of the outer female mold element 10. In
addition, a lip portion 43 of the inner male mold element 40 that
terminates the mold cavity 42 has a perimeter sidewall 44 that
extends a prescribed vertical distance alongside, but is spaced
apart by a prescribed gap or separation 46 from, a mutually facing
portion 15 of the interior sidewall 11 of the outer female mold
element 10.
[0023] As a non-limiting example, the perimeter sidewall 44 of the
lip portion 43 of the inner male mold 40 may have a vertical height
or thickness on the order of one-half inch, while the gap 46 may be
on the order of one-sixteenth of an inch. This separation between
the two mold halves creates a generally continuous annular channel
47 therebetween. While it is preferred that the channel extend
continuously around or cover one hundred percent of the perimeter
of the mold cavity, the invention may be accomplished with a less
than one-hundred percent continuous channel (namely, have a
generally continuous coverage, e.g., on the order of fifty-one
hundred percent). The generally continuous annular channel serves
to vent air, and allows outward exothermic expansion of the resin
30 that has been poured into the structural preform 20, in the
course of impingement of the male mold element upon the resin and
the compression of the male mold element toward the female mold
element to define the geometry of the mold cavity.
[0024] In order to accurately dimensionally locate the male mold
element 40 within the interior volume of the female mold element
10, a set of indexing elements or tabs, one of which is shown at 50
in FIG. 5, may be affixed a spaced apart locations around the top
surface 48 of the lip portion 43 of the male mold element 40, and
are shaped so as to engage the top edge surface 13 of the female
mold element 10. A respective indexing element 50 is sized so as to
locate the top surface 48 of the lip portion 43 of the male mold
element 40 a prescribed vertical offset 53 from the top surface 13
of the female mold element. This prescribed vertical offset is
preestablished to place the male mold element 40 into the female
mold element 10 at the requisite depth that precisely defines the
geometry parameters of the mold cavity 42. Once inserted, the male
mold element 40 may be retained in its intended mold cavity-forming
position by means of clamps and the like at the various indexing
elements 50.
[0025] As an auxiliary measure, shown in FIG. 6, the upper portion
of the mold assembly of FIG. 5 may be engaged by a closure mandrel
or frame 60, having a flexible cover 61 (such as a silicon/neoprene
cover, as a non-limiting example) , and coupled to a source of
vacuum 62. This vacuum closure augments the outflow of resin and
removal of air pockets from the mold cavity 42 through the annular
channel 47, as the male mold element 40 is compressed against the
resin impregnated preform 20. As resin is drawn out from the mold
cavity, it enters the annular channel 47, resulting in the
formation of a thin band or ring of resin 53 that is contiguous
with the resin filled mold cavity 42. This thin resin band 53
creates resin weight around the edge of the mold cavity 42, which
assists in forcing air out and increasing part density. It also
allows air to wick from inside the mold cavity 42 and escape around
the edge into the annular channel 47.
[0026] As shown in the partial side view of FIG. 10, the generally
continuous annular channel 47, described above, prevents excess
flange material of the molded part from forming along the top edge
surface 13 of the female mold element 10. This allows the molded
part to be readily extracted from the inner mold 10, when an
attempt is made to remove (rotate) the inner male mold element 40
from the molded part and out of the female mold element.
[0027] In constrast thereto, in a conventional mold, shown in the
partial side view of FIG. 11, it is not uncommon for fibers of the
structural preform 20 to extend onto the top edge surface 13 of the
female mold element 10. When the inner male mold 45 is clamped down
onto the female mold 10, these fibers become compressed or squeezed
into a tightly bunched matting 110 between a lip 112 of the inner
male mold 40 and the top edge surface 13 of the female mold element
10. As a consequence, the molded part can have excess flange
material 110 extending between the lip 112 of the inner male mold
40 and the top edge surface 13 of the female mold element 10,
causing the molded part to become effectively `locked` to the
outer, female mold 10. This impairs removal of the molded part,
when the inner male mold element 40 is removed.
[0028] FIG. 7 shows the molded article, after the resin has cured,
and the mold halves have been removed. In addition to precisely
conforming with the configuration of the mold cavity, the molded
article 70 has a very narrow (e.g., 62.5 mils wide) annular or
circumferential resin band 72 that has formed along its peripheral
edge 74, as a result of resin being drawn out from the mold cavity
42 and entering the narrow channel 47 contiguous therewith, as
described previously. Using a simple knife cut, this cured resin
band is readily excised from the part, as shown in FIG. 8, leaving
the article 80 completely finished on both sides.
[0029] As will be appreciated from the foregoing description, the
unsealed mold architecture of the mandrel-assisted RTM system of
the present invention effectively obviates the above-discussed
problems of conventional RTM schemes by its use of a narrow channel
between the interior perimeter of the outer female mold element and
the exterior perimeter of the inner male mold element. This annular
channel facilitates venting air around the edge of the part being
molded, dissipates heat to reduce glass print, and allows for
expansion of the resin evenly around the part as it flows out into
the channel.
[0030] This results in the part having a well defined edge that
does not require shaping after removal from the mold, and serves to
allow heat, air and vapor to escape. The dissipation of heat
reduces glass print in finish, extends the life of the mold, and
provides higher part yield. The escape of vapor prevents softening
of the gel coat and mitigates against glass print and distortion.
Allowing air to escape prevents trapped bubbles from expanding
during exotherm and shrinking during cool down, that may otherwise
cause distortion of the part. Because the resin band that forms in
the channel is very thin it is readily trimmed away using a simple
knife cut, leaving the article completely finished on both sides
and having a well defined edge that does not require shaping.
[0031] While I have shown and described an embodiment in accordance
with the present invention, it is to be understood that the same is
not limited thereto but is susceptible to numerous changes and
modifications as known to a person skilled in the art. I therefore
do not wish to be limited to the details shown and described
herein, but intend to cover all such changes and modifications as
are obvious to one of ordinary skill in the art.
* * * * *