TM 1-1500-204-23-11
(e) Machinability. Small cell size (1/8 inch)
(3) Types of Foam Core. There are four types
repair core sections are easier to machine than larger
of foam core used in composite repair: Infusible core,
cell sizes as the large cell size material tends to roll
cure in-place core, pre-cured core, and X-Cor and
over during machining.
K-Cor.
b. Foam Cores. Foam cores include a wide vari-
(a) Infusible Core. Recent industry interest
ety of liquid plastic materials that are illed with
in the resin infusion process has ushered in a new
chemically released or mechanically mixed gas bub-
variety of core materials. These cores are made
bles to produce rigid forms.
Several types of
primarily from polyester and are available in several
foam core are available including: Polyvinyl Chloride
geometries. Infusible cores are formed with channels
(PVC), Polyurethane, Polyimide, and Polymethacrylim-
or holes through which resin can low and are available
ide (PMI). Foam cores are sometimes used in place
in several thicknesses and with many cell (or hole)
of honeycomb core. Foam core is made by dispersing
sizes.
gas or hollow spheres in the case of syntactic foams,
into a solid plastic material. There are two basic
(b) Cure In Place Core. The cured in place
types of foam core materials: closed cell and open
type is either injected, troweled or in frozen sheet
cell, both available in different densities. Closed cell
form. The injected foam is either premixed or a two
foam has cells that are predominantly non-intercon-
part mixture which is injected into place. The troweled
necting; whereas, open cell foams have cells that are
type is typically a two part mixture. The sheet form
interconnecting. Closed cell foams tend to be heavier
is similar in consistency to taffy. All forms expand in
than open cell, though they do not absorb liquids
place upon curing. The cure in place types of foams
and moisture as readily. Consult your system speciic
typically require refrigerated storage for at least one
technical manual for speciic types and applications
of the mixing components. The pre-cured foams are
used.
supplied in either rough shape or standard blocks that
can be machined to shape as required. Many repair
(1) Foam cores are often used in sandwich
scenarios use injected foam to ill a repair area. After
panel applications in cases where mark off on the
cure, the expanded foam is trimmed to match the part
exposed skin is not desirable (unlike honeycomb core)
contour. In addition, syntactic two part epoxy foams
and in sandwich applications with thin edges as
can be mixed and applied in areas as iller. Some
the foam can be formed and handled easier than
of these products have the capability to be cured in
honeycomb. Closed cell foams are typically used in
5 to 10 minutes and then sanded to contour.
applications requiring buoyancy, insulation, and are
excellent iller materials.
(c) Pre-cured Core. Pre-cured foam comes
in sheets similar to honeycomb that can be cut and
(2) The type used for a speciic application
shaped to it each repair. A pre-cured piece of foam
depends upon temperature, cost, structural, manufac-
can be placed in the repair area, similar to the core
turing, environmental, and electrical property consid-
plug method used to repair honeycomb core, and
erations. Typically, foam cores are less expensive
bonded in place using an adhesive. In these repair
than honeycomb cores; however, in similar structural
methods, an overlay patch is typically placed over the
applications they are heavier. The mechanical prop-
repaired core area and bonded on with an adhesive.
erties of foam cores vary in an approximately linear
fashion with material density. Foams tend to have
(d) X-Cor and K-Cor. X-Cor and K-Cor-
lower mechanical properties than honeycomb for a
are variants on pre-cured foam cores. The carrier is
given weight. Closed cell foams do not absorb mois-
a PMI foam that has had pultruded iber pins, typically
ture and are an excellent choice for applications in
carbon, inserted at prescribed angles and locations.
wet or humid environments. For example, epoxies are
In X-Cor the pins stick out from the surface of the
typically less expensive but have lower temperature
foam and interlock with the composite face sheets.
capabilities than other types. Cyanate ester based
Since interlocking is essential to using X-Cor, this
foams have excellent resistance to moisture intrusion
material is only used in manufacturing. Structures with
and have higher temperature capabilities, but they
X-Cor weigh less compared to honeycomb since the
are also much more costly than other types. Most
adhesive used to bond the honeycomb in manufactur-
of the foam cores available today are environmen-
ing is not required. K-Cor is a denser version with
tally friendly in terms of by products produced during
a heaver PMI foam carrier, thicker pins, more pins,
cure, handling wastes produced, and excess product.
and the pins have been folded over during manu-
Rohacell is light, rigid, polymethacrylimide (PMI) foam.
facture. K-Cor has better structural properties than
Its high compression modulus makes it ideal for use
X-Cor but is also considerably heavier. K-Cor is
as a core in high strength composite structures.
used when high out of plane loads are applied to
sandwich structure, such as from a lare dispenser.
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