Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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MOLDING THIN WALL PARTS IN A CLOSED MOLD
This invention relates to a method of molding thin wall part in a closed
mold wherein one or both of the mold parts are formed from a pre-formed
flexible
mold skin.
BACKGROUND OF THE INVENTION
The present matter relates to the molding of parts which are generally
but not necessarily of a composite nature in which a fibre reinforcement layer
is
infused with a resin to form a relatively thin wall part. In order for the
mold elements
to be formed cheaply, they are commonly formed as a pre-formed semi-flexible
mold
skin from a fibre reinforced resin material so that the mold skin has some
flexibility.
It is necessary therefore to reinforce the mold skin during the molding
process so
that it does not flex and thus distort the part.
The resin used is commonly, although not essentially, a thermo setting
resin so that it generates heat during the setting or curing process. It is
necessary
therefore to provide either a lengthy cooling time or to provide techniques
for
extraction of the heat or "exotherms" from the part as it is setting.
Parts of this nature have conventionally been manufactured in open
molding processes in which a resin is applied to a fibrous layer applied onto
a mold
surface. However this process takes place in the open environment thus
releasing
potentially toxic materials to the atmosphere. Environmental concerns and
regulations have therefore led to the use of techniques which enclose the part
from
both sides so that the materials emitted are contained in the molding process
while
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the resin is injected or otherwise transferred onto the mold cavity as is well
known to
one skilled in the art.
In U.S. Patent 6,623,672 (McCollum) issued September 23, 2003 is
disclosed a method of molding of the type generally described above in which
the
position of the flexible mold skin is located by applying behind the mold skin
a
pressure from a liquid bath. The top and bottom mold skins are similarly
supported
so that an equal pressure between the top and bottom liquid baths applies a
constant pressure throughout the resin within the part which is injected and
cured.
The liquid baths can be used to control process temperatures by flowing the
liquid
through suitable heat exchange systems. The pre-formed semi-flexible mold
skins
are attached as a closure member on an open face of the liquid bath using
peripheral seals.
This arrangement has achieved considerable commercial success
and is used to manufacture large numbers of parts such as boat holes.
In U.S. Patent 6,149,844 (Graham) issued November 21, 2000 is
disclosed a similar arrangement. This arrangement utilizes a flexible membrane
as
part of the system. Again the system has the same disadvantage as set forth
above.
In PCT published application W02004/054760 (Burgess) published
July 1, 2004 is disclosed a method which utilizes a similar pre-formed semi-
flexible
mold skin in which the mold skin is supported by a series of adjustable pins
each
having a pivotal head which engages the outside surface of the mold skin.
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The disclosures of the above three documents are incorporated
herein by reference or should be referred to for further information
concerning the
structures and processes relevant to this invention.
SUMMARY OF THE INVENTION
It is one object of the invention to provide an improved method for
molding parts in which the pre-formed semi-flexible mold skin is accurately
and
effectively constrained.
According to one aspect of the invention there is provided a method of
molding parts from a settable resin material comprising:
providing a first mold part and a second mold part which are arranged
when brought together to define a cavity therebetween within which the resin
can be
injected at molding pressures and caused to set for forming the part in a
molding
process;
at least one of the mold parts comprising a pre-formed mold skin
formed from a material which is flexible under the molding pressures;
providing a liquid in a chamber arranged to provide thermal contact
with the mold skin for changing the temperature thereof during the molding
process;
and providing a plurality of mechanical supporting elements for
providing a supporting engagement with the mold skin at a plurality of spaced
positions across the mold skin to prevent distortion of the mold skin during
the
molding, each mechanical supporting element including an engagement head
portion and an adjustable portion such that actuation of the adjustment
portions
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causes movement of the head portion relative to the mold skin to accommodate
different mold skins.
The parts to be molded are commonly composite parts which include
fiber reinforcement laid in the cavity but this is not essential to the
invention.
Commonly the mold skin is pre-formed from fiber reinforced plastics of
a relatively thin wall so as to be light weight and inexpensive to
manufacture, but
such mold skins are flexible under pressure so that they will distort if not
properly
supported. However other manufacturing materials and structures can be used
for
the mold skin.
The thickness of the mold skin may be constant over the whole area of
the mold but also it may vary by additional material, such as fiber
reinforcement,
being applied at certain areas. The head portions are thus brought up to the
exterior
of the mold skin at whatever position it is located relative to the inner
surface for
accurate support and location of the mold skin.
The mold skin may be formed on its exterior surface with specific
engagement points or pads defined on the mold skin for engaging specific head
points for accuracy. Thus for high accuracy, the mold skin may include a
specific
shape and defined points which match a computer controlled head and support
array. In this way the mold skin can be quickly and accurately located by
computer
control of the head location.
In less accurate systems required for cheaper parts where tolerances
can be higher, the adjustment of the heads on the adjustable supports can be
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carried out manually up to the exterior of the mold skin wherever that may be
depending on the thickness of the mold skin.
While the definitions and specific examples referred to herein mention
first and second mold elements it will be appreciated that more than two can
be used
5 in many examples depending on the shape and complexity of the mold.
The adjustment portions can have both fine and course movements so
that adjustment over several inches can be effected quickly by a first
adjustment
action followed by fine adjustment over small distances of the order of 0.0005
inch
for high accuracy parts and of the order of 0.005 inch for parts of less
required
accuracy.
The mold elements may also include not only reinforcing fibers but also
other elements of the part such as gel coats, fasteners, inserts all well
known to one
skilled in the art.
While the description herein refers to manufacture of a part, it will of
course be appreciated that more than one part can be formed on the same mold
element by arranging mold skins side by side for simultaneous side by side
manufacture.
The methods disclosed herein allow significant protection of the
environment by reducing emissions of potentially damaging materials formed in
the
molding process while allowing a system to be manufactures which is
potentially of
low cost for the low cost or smaller manufacturer. Alternatively the same
concepts
can be used for high accuracy high complexity machines using expensive
computer
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control.
6
The liquid which is contained in the chamber is generally water but
others can be used.
The liquid which is contained in the chamber can be maintained
stationary in the chamber or can be caused to flow through the chamber to
carry
heat to or from the part as required.
In one preferred arrangement, the liquid is arranged in the chamber so
as to be in directly contact with an outside surface of the mold skin.
In another preferred arrangement, there is provided a flexible
membrane between the liquid and an outside surface of the mold skin.
In one preferred arrangement, the mechanical supporting elements
each include at least a part extending through the chamber.
Alternatively the adjustment portion of the mechanical supporting
elements is mounted in the chamber.
Preferably the adjustment portion of the mechanical supporting
elements is mounted outside the chamber and each mechanical supporting element
has a portion extending into the chamber. Thus there may be provided a seal on
the
portion at the location where it enters the chamber or there is provided a
jacket on
the part of the mechanical supporting elements within the chamber.
In one optional arrangement, the head portion comprises a separate
element or plate or bar which is supported by two or more adjustment portions.
However the head may be defined simply by the end of the adjustment portion in
the
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form of a pin of shaft. Alternatively each pin or shaft may have its own head
mounted thereon as a pivotal member with a pad or surface in contact with the
exterior of the mold skin. Alternatively a combination of plates and bars and
heads
may be used designed to support different parts and shapes and angles of the
mold
skin and these elements may be supported by one or more pins as required
depending on their shape and the forces involved.
In one preferred arrangement, the chamber is closed and is defined by
a plurality of fixed walls leaving one side open and wherein the mold skin is
located
at the open side.
In another preferred arrangement, the mold skin sits on a flexible
membrane covering the liquid in the chamber such that the mold skin is removed
from the membrane leaving the membrane covering the chamber.
In one preferred arrangement, the mold skin is removed with the part
and thus is freely rested or placed on its support. This allows the part and
the mold
skin to be removed prior to the complete curing of the material and to remain
supported while the curing is completed. In this way the main part of the
molding
system can be used to mold another part with another mold skin while the first
is
curing to speed up processing and increase rate of manufacture.
In another preferred arrangement, there is provided a backing wall
generally following the shape of the mold skin and wherein the liquid is
contained
between the mold skin and the backing wall.
In this arrangement, the liquid can be contained in a flexible bag or
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mattress which lies underneath or behind the mold skin. The bag is preferably
shaped to form a container for a constant thickness layer of the liquid in the
form of a
mattress. The bag is preferably self-contained so as to be portable. The bag
is
preferably closed so that it can be pressurized by the liquid to provide an
approximately constant pressure over its surface contacting the mold skin. The
bag
may have one or more outlets and inlets for passage of the temperature
controlling
fluid therethrough.
In this arrangement, the backing wall can be flexible and the
mechanical supporting elements are arranged to engage the backing wall so as
to
transfer forces to the mold skin through the liquid, which is preferably
contained in
the bag so that it can be pressurized, contained, manageable and portable. The
backing wall preferably approximately follows the shape of the outside surface
of the
mold skin so that the liquid layer is of constant thickness. However this may
be
approximate thus allowing the support to be manufactured cheaply. Also the
support may be deliberately shaped to depart from the shape of the outside
surface
of the mold skin so as to apply deliberately increased pressure on the bag and
the
liquid at hard to contact places on the mold skin to better maintain contact
by the
bag on the mold skin over its full extent.
The backing support may be the same size and overall shape as the
mold skin but may also be formed in separate parts where desirable for easier
manipulation. This also can be formed into a modular system in which
additional
supports and bags are used side by side to support a larger mold skin.
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In this arrangement, the backing wall may have holes therein through
which the mechanical supporting elements extend for applying forces to the
mold
skin.
In this arrangement, the mechanical supporting elements may directly
engage the outside surface of the mold skin through holes in the flexible bag.
In this arrangement, the flexible bag may have inner and outer walls
which have parts which are touched together at the mechanical supporting
elements
so that the mechanical supporting elements apply force directly to the outer
surface
of the mold skin, where the inner and outer walls are connected by bonding
heat
sealing at the touching parts.
The mold skin may be removed from the flexible bag with the part in
some cases. Thus the mold skin simply rests on the bag and can be lifted away
from the bag to support the part during cooling. Yet further the mold skin may
remain in place for the molding of the next part.
Alternatively in some cases where the heat transfer effect of the liquid
is required the mold skin and the flexible bag and the backing wall are all
removed
with the part.
In this arrangement, the backing wall may be formed from the same
material as the mold skin and is matched in shape to the mold skin.
BRIEF DESCRIPTION OF THE DRAWINGS
One embodiment of the invention will now be described in conjunction
with the accompanying drawings in which:
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Figure 1 is a cross sectional view showing a first embodiment of
molding system and method according to the present invention.
Figure 2 is a cross sectional view showing a second embodiment of
molding system and method according to the present invention.
5 Figure 3 is a cross sectional view showing a third embodiment of
molding system and method according to the present invention.
Figure 4 is a cross sectional view showing a fourth embodiment of
molding system and method according to the present invention.
In the drawings like characters of reference indicate corresponding
10 parts in the different figures.
DETAILED DESCRIPTION
In Figure 1 is shown a first embodiment of a molding system and
method according to the present invention. The system includes an upper mold
element 10 and a lower mold element 11 which can be moved to provide a cavity
12
therebetween into which resin is injected by a resin injection system 13 to
form the
part shaped and arranged in accordance with the shape of the mold elements.
The resin injection system 13 is well known to one skilled in the art so
that detailed description thereof is not necessary herein. Many examples of
prior
arrangements of this type are available in the prior art.
The part formed is generally a composite part including fibre layer
reinforcement and the fibre layers may be loaded onto the lower mold element
11
prior to closing of the mold. Again the techniques for applying the fibre
layers are
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well known to one skilled in the art.
11
The upper mold element 10 is movable on a lift system 14 which
allows it to be removed from the lower mold element to access the cavity 12
and to
access the finished part. Again the lift mechanism is available to one skilled
in the
art.
The lower mold element 11 is shown in detail herein and will be
described hereinafter. The upper mold element 10 is shown only schematically
and
this may be simply a fixed element with a bottom surface shaped to the
required
shape or it may more preferably be a symmetrical arrangement substantially
identical to the construction described in respect of element 11.
Suitable sealing arrangements are provided around the outside edges
between the mold elements so that the injection of the resin 13 allows the
resin to fill
the cavity 12, to expel any air or air bubbles and to compress the resin
within the
fibrous layers applied within the cavity 12.
The mold element 11 comprises a bath 15 formed from a base wall 16
and upstanding side walls 17 leaving an open upper mouth of the bath defined
by
peripheral edges 18 of the side walls 17. The baths can be filled with a
liquid 19
which is generally water and the water can be supplied into and out of the
bath
through conduits 20 and 21 to a heat exchange system for example of the type
disclosed in the above US patents. This allows the bath to be maintained at a
required temperature during the molding process which can be varied by
changing
the temperature of the incoming flowing liquid from the heat exchange system.
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The open mouth of the bath is closed by a flexible membrane 21 which
is attached to the peripheral edge 18. The membrane 21 can flex to the
required
position as required to accommodate the exterior shape of the mold skin which
is the
same as or approximates to the shape of the article to beg molded.
On the membrane 21 is supported a bottorn mold skin 22 which is pre-
formed and semi-flexible for example formed from fibreglass so as to define an
upper surface which provides the shaping of the part to be molded within the
cavity
12.
In the embodiment shown the mold skin is a simple generally channel
shaped element for example for forming the hull of a~ boat. However it will be
appreciated that other shapes which are more complex can be provided. The
shapes may also have a deeper recess at the center than the generally simple
channel shape as shown but of course this may require different shape and
arrangement of the bath and may require different shape and arrangement of the
membrane.
The mold skin being semi-flexible is therefore of a nature which will flex
under the pressures involved in the injection of the resin into the cavity 12.
The
mold skin is therefore supported by a plurality of mechanical supporting
elements
each indicated at 23. In the embodiment shown the mechanical support elements
23 include a head 24 and an adjustable portion 25 which can move the head
towards and away from the mold skin 22 by a linear actuator 26. This can
include
course and fine movements and the fine movements can be provided by rotation
of
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a screw either by rotating the pin or a nut carried on the pin.
In the embodiment shown the adjustable portion is in the form of a
shaft extending at right angles to the base 15. The shaft passes through a
seal 27 in
the base 15 so that a portion of the shaft is exterior of the base 15 and a
portion is
located within the liquid 19. The seal 27 prevents the escape of a liquid
while the
shaft can be moved upwardly and downwardly by the linear actuator 26. The
movement of the shaft can be effected by any suitable mechanism well known to
one skilled in the art for moving such a shaft along its axis, This can be
provided
simply by manual movement of the shaft and locking of the shaft via a sleeve
and
screw. More complex arrangements can be provided using linear actuator motors
where the motor is accurately driven by a numerically controlled machine so
that the
accurate location of the shaft can be determined. The head 24 is in the form
of a
plate having an upper surface 30 arranged to engage on a flat portion of the
outside
surface of the mold skin. The plate 30 is pivotal relative to the end of the
shaft on a
suitable pivot mounting 31 which is preferably universal in pivotal action so
that the
face of the head can be adjusted to the required orientation to engage the
outside
surface regardless of the angle of the outside surface.
On the left hand side of the figure is shown three such mechanical
supporting elements each of which comprises an individual head supported on an
individual shaft.
On the right hand side there are shown alternative arrangements
wherein a first mechanical supporting element generally indicated at 32
includes a
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head 33 which is supported by two or more shafts 34 and 35. The head 33 thus
bridges the shafts 34 and 35 and is pivotally connected to each at a
respective pivot
mounting 36. This allows the angle of the head 39 to be adjusted relative to
the
horizontal so as to accommodate differences of angle of the surface of the
mold skin
which is engaged by the head 33.
At mechanical support element 38 there is provided a further
arrangement in which a head 39 is carried on one or more shafts 40. The head
39
in this arrangement is more complex in that it is shaped to accommodate
different
shapes of the outside surface of the mold skin other than simply flat planar
portions.
In the embodiment shown the shape is defined by an apex between two surfaces.
However other arrangements can be supported using a shaped head of this type
including recessed portions, channels, ribs and the like.
In one mode of operation of the arrangement shown in Figure 1, after
injection of the resin 13 into the fibrous layer, the components are left in
position
during the curing action of a resin utilizing the bath liquid 19 for
controlling the
temperature and the exotherms from the curing process.
In an alternative arrangement, after the resin has been injected and an
initial curing action has occurred, the top mold 10 can be lifted away
exposing the
part sitting on the semi-flexible mold skin 22. The mold skin and the part can
then
be lifted from the membrane since the mold skin may simply sit loosely on the
membrane and is free to be removed therefrom allowing the part and the mold
skin
to be lifted away to a support cradle where the mold skin is supported for
completing
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the curing action till the part can be removed from the mold skin. In this
mode of
operation, a further mold skin identical to the first can be inserted onto the
support
elements and the membrane so that further molding can occur while the first
part is
completing its curing action. In this way the rate of molding can be
significantly
5 increased by having a series of partly cured parts in stations around the
single
molding system with each mold skin being returned to the molding system when
its
part is fully cured.
In some cases the mold skin is fastened to the frame and thus remains
attached thereto until changed for a new part.
10 It will be appreciated that when a new part is to be molded, new mold
skins can be supplied and inserted one at a time onto the flexible membrane
with the
support elements being adjusted to the required position to report that
particular
mold skin. This can be carried out manually or can be carried out utilizing a
numerical (PLC) controlled machine to actuate the movement of the adjustment
15 portions so that the heads are moved to the required locations. In certain
circumstances different head scan be selected.
Thus the shape of the semi-flexible mold skin is maintained accurately
by the support thereof by the heads of the mechanical supporting elements. The
number and arrangement of the heads is selected in dependence upon the shape
and design of the mold skin to ensure accurate support of the mold skin. It
will be
appreciated that different shapes of mold skin will have different forces in
different
directions and thus will require different positioning of the supports. In
practice,
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therefore, an array of such supporting elements can be provided and can be
selected with different heads to provide different supporting effects as
required.
Thus the pivot mounting 31 of each head can allow the head to be removed and
replaced. Turning now to Figure 2 there is shown an arrangement similar to
that of
Figure 1 in that it includes the bath 15A supporting a semi-flexible pre-
formed mold
skin 22A. In this arrangement there is no flexible membrane and the mold skin
22A
is itself attached to the upper periphery 18.
A further modification shown in Figure 2 utilizes a frame arrangement
50 which supports the actuators 26A for the adjustable portions 25A carrying
the
heads 24A. The frame 50 is mounted within the bath and is enclosed within an
enclosure 51 so that the actuators 26A are protected from the water 19A. In
this
arrangement the adjustment portion 25A or the shafts are contained within
flexible
sleeves 53 which exclude the water 19A from contacting the shaft 25A.
It will be appreciated that the arrangement utilizing the mold skin
directly on the surface of the bath can be used with the support arrangement
for the
actuators where they are exterior as shown in Figure 1.
Turning now to Figure 3 there is shown a further alternative
arrangement in which the mold skin 22B is supported on a flexible bag or
bladder 60
having an upper sheet 61 sealed around its edges to a lower sheet 62 and
forming
therebetween a cavity 63 for receiving the liquid 19B. The bladder or bag 60
is
supported on a second mold skin 64 which is shaped identically to the mold
skin 22B
and formed from similar material so that it is another one of the mold skins
of the
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array of mold skins which can be used in the molding process. The mechanical
supports 23B are substantially the same as previously described and support
the
second mold skin 64. In this way the bladder 60 is contained within the two
mold
skins and acts to support accurately the mold skin 22B at its required
position since
there is a constant pressure through the bladder and the liquid 19B therein
and since
the outer mold skin exactly matches the shape of the mold skin 22B which acts
to
form the part.
The liquid 19B can be stationary or can be circulated as previously
described to control temperature at the molding process. The mold skin 22B can
remain in place during the complete molding process or can be removed partway
through the curing for separate support and completion of the curing action so
that
the system can be used for further molding action as previously described.
The embodiment shown in Figure 4 is similar to that of Figure 3 in that
it includes the mold skin 22C supported on a second outer mold skin 64C. The
system is again supported by the mechanical support elements 23C as previously
described. However in this arrangement the actuators 23C project through holes
70
in the lower mold skin 64C. In addition the bladder 60C is formed with points
71 in
which the bottom layer 62C is touched to the top layer 61 C. At these points
it can be
heat sealed or otherwise connected so that the bladder has dimples or quilt
points
across its area both in transverse or longitudinally spaced array. Thus the
pins or
support elements 23C, also arranged in an array longitudinally and
transversely
spaced project through the holes 70 and engage against the points 71 so that
the
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head of the pin provides a force pressing directly against the inner mold skin
22C
through the thicknesses of the top and bottom layer of the bladder which are
in
contact. The durometer of the bladder can be selected to allow compression of
the
bag material to an amount to control the compression of the resin in the
cavity for
control of the thickness and thus resin content of the final part. The outer
mold skin
64C can be supported by additional pins (not shown) or by suitable shaped
support
plates or other suitable supports which hold it in place accurately spaced
form the
main mold skin.
As the point 71 are spaced in an array, the liquid 19C covers the
majority of the area within the bladder and can either remain stationary and
can flow
as previously described. The support elements 23C can be provided with heads
as
previously described and can form nearly the upper end of a shaft or pin.
The arrangement disclosed in Figures 3 and 4 thus provides a three
layer sandwich of:
1 ) Mold skin
2) Water bed (for self contained exotherm management).
3) Shaped support of water bed by separate mold skin.
In a first method of operation, the mold skin is directly moved by set of
PLC structural pins. Then waterbed and waterbed support mold skin are moved by
a separate set of pins until they contact the mold skin. Pins slide through
holes in
the water bed layer (made in a regular grid layout so the pattern fits all
molds). The
water is temperature and flow controlled within the water bed layer.
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In a second method which is basically the same as but there are no
holes in water bed layer. Instead only the support skin layer is moved by
actuators
and the first set of pins is eliminated. The bridged pins may be best for
this.
The pins can be as noted above can move through holes in the
support skin but not through the water bed. Instead the pins flexibly push the
sides
of the water bed against the mold skin.
The following options can be included:
The pins can be single or bridged and can be in any number as
required.
Optional walls can be added to support skin to assist in containment of
water bag.
The water bag can be pressurized to assist in molding quality.
The water bag can be attached or detached from either or both mold
skins to assist in molding logistics.
The water can circulate in the water bed and can be hot or cold.
Since various modifications can be made in my invention as herein
above described, and many apparently widely different embodiments of same made
within the spirit and scope of the claims without departure from such spirit
and
scope, it is intended that all matter contained in the accompanying
specification shall
be interpreted as illustrative only and not in a limiting sense.
