Ken Mintz Composite Information pg.7 ==
Page 1 Introduction=================================
Page 2 A Short Description of Fiber Types and Properties=====
Page 3 Advantages/Disadvantages of Composites===========
Page 4 The Matrix===================.==============
Page 5 Setting Up The Shop===========================
Page 6 Preparing to Layup and More Tools=.==============
Page 8 Hot Box Pictures and Dimensions.======.==========
163 Home Page====================================
Composites Corner No. 7 - The Core and Its Preparation
This issue should be fairly short since there is not much to say in detail about the core material that I haven't
The core is the form around which the composite shell is layed. Its functions are to provide shape and
compression resistance. The core material is very weak by itself but when contained and confined by
the resin-fiber shell the combination or "composite" produces the extremely strong smoothly contoured
complex structures that composites have become know for. The plans and kit designers determine
what type of core materials are to be used in their designs. These are chosen based on the character-
istics that the structure must have. For example, the core material that is usually used around composite
fuel tanks must be impervious to any fuel that may leak into it. Urathane foam is used as the core material
in my Varieze's fuel tanks and fuselage for this reason. Urathane will not dissolve in avgas whereas styrene
foam will and very quickly. My EZ's wing cores are made of a fire retardant styrene foam because they will
most likely never be exposed to avgas and styrene can be quickly cut to shape by use of a heated wire.
Urathane must be saw or knife cut and sanded to the proper shape and takes much longer to prepare for
that reason. Urathane does not cut well with a hot wire and produces several toxic gases when this is
attempted. Don't burn this stuff for the same reason!
Core materials like the foams mentioned above come in varying densities (grams/cubic centimeter,
pounds/cubic foot, etc). The density is chosen mostly in consideration of the compression strength required
though stiffness is a consideration as well. Stiffness is handeled mostly by the resin-fiber shell but the core
can contribute some in this area. High density cores are more resistant to impact damage than light density
cores whatever the material type but the trade-off is that they are correspondingly heavier.
The core materials that most homebuilders encounter are the two discussed above - urathane and styrene.
Balsa wood is seen occasionally in my experience as are some other types of plastics and woods but less
often than urathane and styrene. Whatever the material if you wish to produce a composite plane with the
lightest weight one must start with the core.
In general the smoother the core surface is and the truer its shape is to the design dimensions the less filler
material will be needed on the final surface and the lighter it will be. Usually foam cores are rough cut close
to the final dimensions and then sanded to them. Use sanding boards and blocks on flat surfaces. Make
these tools long and rigid enough to bridge across the core so that level surfaces result. Hand sanding broad
flat surfaces will inevitably produce shallow depressions and rises in the surface that are not usually visible
until the shell is applied. Smoothing out the surface then requires the use of weight gaining filler materials.
Use templates and contour gages whenever possible to achieve the exact core shape called for. Make
these yourself when necessary. Often the plans makes will supply dimensions for such aids but you can
make your own with some ingenuity. Concave surfaces can often be made using scrap pieces of the core
material itself to "dish out" the area. The scrap and the core wear each other down in the process.
Use the contour gage if possible to gage the final dimensions of the area being dished out.
Styrene foam does not sand well in my experience. It has a tendency to tear out in chunks. I have sanded
this material using a fast sander with coarse grit sandpaper but the result is not the best. The material is springy
and likes to compress under the sander and the rise back up when the sander passes making it harder to sand
to the required dimensions. It can also soften under the heat of friction produced by the sander. This stuff is
usually hot wire cut to shape using an electrically heated inconel wire of around .025 inch to .032 inch diameter
guided by templates. The instructions for making the templates and positioning them on your block of styrene
foam come from your plans in most cases. Follow these instructions carefully or you can end up with some
very stange looking parts that cannot be used.
Hot wiring presents some challenges best learned by experience. If one cuts too fast the center of the hot wire
will dive into the core producing a severe dip in its middle. If one cuts too slow then the radiant heat of the
hot wire will melt channels into the core producing a washboard surface. Both of these flaws will reqire a lot of
filler to correct especially on wings. If the core is too damaged then one must try again and this costs extra to
buy more material. In other words, practice first on scrap if possible. Also there is no easy way to hot wire
without an assistant on the other end of the hot wire cutter. Other problems include controlling the temperature
of the hot wire and keeping it tight as it heats up. There are various ways of dealing with both these problems
which I may go into later.
In closing, do take the time to produce a smooth accurate core. Don't short the effort on this component if you
want that smooth wave and ripple free surface that composites are famous for along with a much lighter stucture
than you will get trying to make up for the irregular surface with fillers later on. Wings can be especially adversely
affected by poor cores. Subtle differences in shape can make a set of wings fly very diffenently from each other
resulting in the very least in trim problems. Trim difficulties must be overcome by drag inducing trim tabs making
for a slower plane. It is already slower due to increased weight from the filler used. Get the picture?
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