Woven preform/magnetic press process for thermoplastic honeycomb cores

Preimpregnated thermoplastic yarn is interwoven around spaced hex mandrels as weft members in a preformed ribbon, which may be folded into layers or cut into sheets for stacking in a honeycomb pattern in preparation for bonding. In block bonding, pressure and heat are applied to an entire fully stacked block to bond the interface facets together and form hex ducts around the mandrels, which must then be extracted, typically individually in a special pneumatic press process. In a preferred layer bonding process certain problems and limitations of block bonding are overcome by bonding each sheet as it is added to a stack buildup. An alternate layer bonding process is disclosed, which is facilitated by magnetic pressing, using electromagnets acting on ferrous mandrels to apply pressure evenly distributed along interfacial facets during thermal fusion bonding. The magnetically pressed layer bonding process is readily automated by disclosed apparatus. Heating and cooling are provided to the mandrels. The sheets used in this method may be either corrugated in a half hex pattern or interwoven with hex passive holding mandrels which may be pushed out and replaced by the machine's aluminum forming mandrels as each sheet is loaded onto the machine.

FIELD OF THE INVENTION 
This invention relates to composite structures as used in aircraft, and 
more particularly to a manufacturing process for processing preimpregnated 
thermoplastic fiber material into blocks of honeycomb ducted core material 
capable of sustained service at high temperature. 
BACKGROUND OF THE INVENTION 
Composite structure in aircraft typically utilize a tough skin surface 
supported by a lightweight core material. Development efforts to increase 
the strength/weight ratio of the core have resulted in cellular plastic 
structures such as rigid expanded foam of random cell pattern. Superior 
structural properties have been realized in cores formed in a geometric 
honeycomb pattern of hexagonal ducts, achieving very light weight due to 
the high percentage of air volume: 90% to 98%. Such a core, when 
sandwiched between two skins, forms a directional structure possessing a 
uniform crushing strength under compression. 
In known art, such cellular or ducted cores are commonly made from 
thermosetting resins, which are plastics which solidify when first heated 
under pressure, and which cannot be remelted or remolded, as opposed to 
thermoplastic resins, which are materials with linear macromolecular 
structure that will repeatedly soften when heated and harden when cooled. 
As utilization of such structures is expanded to include areas previously 
avoided due to structural demands and temperature, vibration and impact 
loading environments, new composite matrices are required. Thermosetting 
resins, commonly used, in most cases, lack the toughness and stability 
needed for these applications. 
New thermoplastic materials offer improved properties; for example 
composite skin-surfaced structures having honeycomb cores made from 
preimpregnated thermoplastic fiber material provide excellent impact 
strength and damage tolerance. However, by their nature, the new 
thermoplastic materials require new and unconventional processing methods. 
As opposed to conventional thermosetting processes where sticky and 
viscous fluids are saturated into reinforcing fiber forms to be cured by 
catalysis and heat, thermoplastics which have no cure cycle are hard and 
"boardy" initially, and have to be melted at high temperatures to be 
worked to the desired shapes. Thus completely different processing schemes 
are required for thermoplastics than those that have been developed for 
thermosets. This also holds true for the honeycomb core which is used to 
give light composite aircraft parts large moments of inertia to multiply 
stiffness and strength without proportional increases in weight. 
In known art, thermoset honeycomb material is made by a process that takes 
advantage of the flexibility of the reinforcing fabric before it is 
impregnated with resin. It is bonded and then expanded into hexagon 
honeycomb structure while it is soft, then wash coated with resin which is 
subsequently cured to give it its stiffness. 
In contrast, thermoplastics, utilized in the present invention for their 
superior ultimate properties, have no soft stage, and they are too viscous 
to be wash coated or by some other means saturated into the fabric after 
bonding the sheets together. The practical options for bonding 
thermoplastic core material together are further limited by the difficulty 
of making good adhesive bonds with thermoplastics. For these reasons, 
thermal fusion bonding of thermoplastic fiber material into a ducted 
honeycomb structural pattern has been selected as the method for producing 
strong lightweight core material in the present invention, which addresses 
new processing methods for realizing the full benefits of the superior 
ultimate properties of such structure. 
OBJECTS AND SUMMARY OF THE INVENTION 
A primary object of this invention is to provide a method of processing 
pre-impregnated thermoplastic fiber to efficiently produce a ducted core 
material, preferably in a honeycomb pattern, that can be used at sustained 
high service temperatures without substantial loss of strength or 
degradation of mechanical properties. 
Further to the primary object it is an object to enable production of 
blocks of the ducted core material of substantially larger size, well 
beyond the limitations of known processing art. 
Another object is to provide a process yielding a continuous ribbon of 
thermoplastic fiber fabric with mandrels interwoven as weft members spaced 
apart so as to be stackable in a honeycomb pattern, by weaving 
pre-impregnated thermoplastic fiber yarn around the mandrels. 
Another object is to provide a layer bonding process enabling sheets of 
pre-impregnated preformed thermoplastic fibercloth to be thermal fusion 
bonded one sheet at a time onto a progressively built up stack so as to 
thus produce a block of ducted core material. 
A further object is to implement a layer bonding process with two rows of 
forming mandrels, which may be secured at one end to a mechanically 
operable attachment block by which the mandrels may be withdrawn from a 
stack following a layer bonding cycle, transferred and reinserted 
mechanically ready for a subsequent layer bonding cycle. 
A still further object is to provide means for compressing together and 
heating each interfacial facet between a most recently added sheet and the 
previously added sheet in a stack being built up, sufficient for thermal 
fusion bonding, without transmitting harmful pressure to any unsupported 
honeycomb ducts. 
Layer bonding is effected according to the present invention by the use of 
a magnetic attractive force applied at uniform intervals along the length 
of each mandrel in a fork-like row of ferrous forming mandrels to urge the 
material interfaces uniformly against each other during thermal fusion 
bonding, optionally supplemented by a cooperating adjacent row of 
non-ferrous forming mandrels. Such non-ferrous forming mandrels can be 
used if exact positioning of all ribbon layers is required. Uniform 
bonding heat is provided by electrical elements within the ferrous 
mandrels. 
This principle of magnetically pressed thermal fusion layer bonding is 
embodied in accordance with this invention in a machine which provides 
uniform interface pressure and heat, and mechanized transfer and 
manipulation of the mandrels and the workpiece to produce honeycomb ducted 
core material under automatic control. 
With regard to layer material preforming, as an alternative to corrugating 
ribbon material, a preferred embodiment of the present invention provides 
for weaving preimpregnated thermoplastic fiber yarn into a fabric ribbon 
with mandrels interwoven as weft members spaced so as to be stackable in a 
honeycomb pattern in preparation for thermal fusion bonding, thus 
providing, along with the versatility of being useable for both block 
bonding processes and magnetic press layer bonding, the advantages of 
eliminating roll forming and associated heating, labor saving in stacking 
by prepositioning mandrels, and strengthening the core structure with 
integrally woven plies. 
The manner of making and using the techniques and embodiments of the 
present invention to meet the above objects and to realize the advantages 
of the invention will be best understood from a study of the accompanying 
drawings and the following description thereof.

DETAILED DESCRIPTION OF THE INVENTION 
In the cross-sectional representation of FIG. 1, a flat ribbon of 
preimpregnated thermoplastic fiber 10 is shown being preformed between a 
pair of spring-pressurized toothed rollers 12 and 14 into a corrugated 
ribbon 16 in preparation for further processing to produce ducted core 
material. Heat is applied to the ribbon 10 from roller 12 which is heated 
by a heating unit 18 wrapped around its upper region. The corrugated 
ribbon 16 is led from the forming region in a downward inclination so as 
to cool it from the unheated lower roller 14 as it exits to thus set the 
material in the corrugated pattern. 
FIG. 2 shows corrugated sheets 20, cut to desired workpiece size from the 
corrugated ribbon (16 in FIG. 1), being stacked together with hex rod 
mandrels 22 of metal such as aluminum, which are laid side by side into 
the hex ducts of each sheet as stack 24 is built up layer by layer. The 
stack is built on a lower press platen 26 which provides conformal support 
to the bottom preformed sheet by half hex mandrels 28 placed on platen 26 
as shown or else by equivalent corrugations machined in the platen 26. 
FIG. 3 is a pictorial representation of a novel alternative 
mandrel-interweave method of preforming the thermoplastic material. A 
weaving machine 30 receives as input hex mandrels 22, shown in end view 
loaded into a hopper 32, and preimpregnated thermoplastic fiber yarn 34 
supplied on spools 36. A mechanism of known loom technology in machine 30 
weaves the yarn 34 around mandrels 22 into a fabric ribbon 38 which 
carries the mandrels 22 interwoven as weft members in spaced ducts of the 
fabric as shown. 
FIG. 4 shows the fabric ribbon 38 with interwoven mandrels 22 being stacked 
on press platen 26 which provides conformal bottom support (as described 
in connection with FIG. 2) in a buildup of stack 24 in preparation for 
thermal fusion bonding. The ribbon 38 may be added to the stack 24 in 
continuous form by folding it back at each end as shown, or alternatively 
it may be cut into sheets of required workpiece size. 
FIGS. 5A-5C shows a basic block bonding process in which a block of 
material is thermal fusion bonded as a whole in a single bonding cycle. 
At step A, a full stack 24 of mandrels and preformed material, which may be 
corrugated as in FIGS. 1 and 2 or woven as in FIGS. 3 and 4, is built up 
in a press 40 between lower platen 26 and an upper platen 42, which 
conformally engages the top preformed sheet of the stack 24 as described 
for the lower platen 26 (in connection with FIG. 2). Downward pressure is 
applied to the stack 24 via upper platen 42 from a hydraulic cylinder 44 
which may be pressurized by a manual pump and monitored by a pressure 
gauge. 
At step B the press 40 is enclosed by heating units 46A and 46B and, with 
pressure applied to the stack from cylinder 44 sufficient to hold the 
sheet material interface facets in contact, hot air is directed onto the 
stack by jet arrays in heating units 46A and 46B to bring it up to bonding 
temperature, softening the material and fusion bonding it together at the 
interfaces while forming ducts around the mandrels. Then after heat is 
removed and the temperature drops enough for the material to set, pressure 
is removed. 
At step C, the cooled stack 24, removed from the bonding press, must have 
the mandrels removed. This is not a trivial task since the mandrels tend 
to be partially bonded despite the previous application of a release 
agent. A special long stroke pneumatic press 48 is fitted with a long thin 
drive pin 50, supported by a bracket 52 and a sleeve 54, actuated from 
pneumatic cylinder 56. Each mandrel is pressed out individually; this 
operation tends to be difficult and critical, requiring considerable skill 
and attention to avoid damaging the core duct walls, particularly from the 
mushroom deformation of the ends of the aluminum mandrels. To prevent 
buckling, the punch pin 50 may require support from both floating and 
stationary bushings. 
FIG. 6 shows a cross sectional representation of the key elements of a 
magnetically pressed layer process in which bonding pressure is applied in 
the form of magnetic attraction between the upward facets of a row of 
steel (or other ferrous metal) mandrels 58 in the hex ducts beneath the 
top two preformed sheets of the stack 24 as shown, and corresponding 
downward poles of electromagnets 60 embedded in a platen 62 configured on 
its lower side in a half hex corrugated pattern to mate with the preformed 
sheet at the top of stack 24. An optional interleaved row of aluminum 
mandrels 64 in the hex ducts of the top preformed sheet act in the normal 
manner to accurately form the duct during thermal fusion bonding, while 
having no magnetic effect. Required bonding heat is applied electrically 
to steel mandrels 58 by embedded 0.060" (1.52 mm) diameter nichrome wire 
elements 66 embedded near the upper hex facet. A cooling port 68 is also 
provided in each ferrous mandrel 58 below the heating element 66 to carry 
cooling fluid. To shorten the cycle time, additional nichrome elements 
and/or cooling ports may be provided in the platen 62 passing through the 
lower region of magnets 60, as indicated by dotted circles. Since pressure 
is applied uniformly along the length of mandrels 58 by equally spacing 
the magnets 60, and heat is applied uniformly along the length by elements 
66, this principle is readily applicable to larger core sizes. 
The perspective view in FIG. 7 represents a proposed manufacturing 
apparatus utilizing the magnetic press principle of FIG. 6. The workpiece, 
stack 24, is shown in an initial phase of buildup with only a few 
preformed sheets in place. An overhead plate 70 supports platen 62 in a 
short transverse track. Platen 62 carries a full array of magnets 60, of 
which a representative group is shown through the broken away portion of 
the drawing. 
The mandrels 58 and 64 are tapered and rounded at their free end so as to 
guide their entry into the ducts of stack 24, and they are secured at the 
other end to an attachment block 72 which is movable longitudinally, as 
indicated by arrows 74, by plunger 76 of actuator 78, with sufficient 
travel to withdraw the mandrels 58 and 64 from the stack 24, the movement 
being guided by rods 80 passing through guide block 82, affixed to 
carriage 84 which is movable laterally as indicated by arrows 86, under 
control of an automated transfer system. The range of this lateral 
movement, which is also applied to platen 62, is only a small amount equal 
to the offset between adjacent ducts in the honeycomb core. 
The lower platen 88, provided with a half hex groove pattern 90 on its 
upper side mating with the bottom preformed sheet in stack 24, may be 
raised or lowered as indicated by arrows 92 by a mechanism also under 
control of the automated transfer system. 
The machine functions by placing a first preformed sheet onto the steel 
mandrels 58 and then placing a second preformed sheet node-aligned on the 
first sheet and onto the aluminum mandrels 64. The lower platen 88 is 
elevated to lightly engage and support the first sheet. The upper platen 
62 located (as shown in FIG. 6, with the magnets 60 over the steel 
mandrels 58), and current is applied to the magnets 60, attracting the 
steel mandrels 58 so as to apply the required bonding pressure to the 
interface regions. The mandrels are then heated by the nichrome wire until 
bonding temperature is reached, then heating power is removed and mandrels 
58 are cooled by pumping coolant through the cooling ports. The mandrels 
58 and 64 are then withdrawn, lower platen 90 is lowered a half hex 
height, carriage 84 is shifted transversely to an alternate position which 
aligns the steel mandrels 58 with the ducts in the second (top) layer, 
platen 62 is shifted laterally equal to the offset between adjacent ducts, 
then actuator 78 inserts the steel mandrels 58, and a third preformed 
sheet is loaded on top of the second and onto the aluminum mandrels 64, 
and another bonding cycle is performed as before. The operational cycle is 
repeated until the required stack height has been built up in stack 24. 
Then, with the mandrels 58 and 64 retracted, the finished stack is removed 
toward the left in FIG. 7, ready to be cut to size for use as a core in 
composite structure. 
This method of extracting the mandrels 58 and 64 eliminates the high labor 
burden and the risk of mandrel end deformation and related core damage 
inherent in the press punch method of FIG. 5 step C. In the method of FIG. 
7, pulling the mandrels out under tension facilitates production of cores 
of greatly increased thickness. 
For illustrative purposes, the process described above in connection with 
FIG. 7 assumes the use of preformed sheets configured with half-hex 
corrugations as shown in FIG. 2. Alternatively, preformed material 
interwoven with mandrels as described in connection with FIG. 4 may be 
utilized. The woven ribbon 38 is cut into sheets of required length 
including interwoven mandrels 22, and then layer bonded during stacking in 
a manner similar to that described for corrugated material in connection 
with FIG. 7, except that as a new sheet is loaded onto mandrels 58, the 
interwoven mandrels 22 are pushed out of the stack and retrieved. 
The particular hex duct honeycomb pattern shown herein should not be 
considered as restrictive; the processes of this invention are generally 
applicable to ducted cores of various matrix patterns of which the 
honeycomb is representative. 
The invention may be embodied in other specific forms without departing 
from the spirit and essential characteristics thereof. The present 
embodiments are therefore to be considered in all respects as illustrative 
and not restrictive, the scope of the invention being indicated by the 
appended claims rather than by the foregoing description; and all 
variations, substitutions and changes which come within the meaning and 
range of equivalency of the claims are therefore intended to be embraced 
therein.