Composites offer tremendous possibilities for part fabrication once a
few basic concepts are understood. The key lies in understanding the
different materials available, their applications and the best ways to
This brochure is intended to be a general overview of composite
fabrication, with an emphasis placed on the fabrication of parts in molds.
The broad scope of this brochure limits the amount of detail included
about basic fundamentals and mold construction. Rayplex offers brochures
which delve into specific aspects of these areas in more detail.
The best place to start when learning about composites is an
understanding of the vocabulary used in this field. The following terms
are often used in describing the composite fabrication process:
Piece or Part: The finished product which you are making.
Plug: The actual item to be duplicated in fiberglass or other
composite materials, which is used ton construct the mold. The plug can be
the actual part or a custom-fabricated shape, made from virtually any type
Mold: The item from which the piece will be made. There are two
main types of molds, male and female. A male mold is identical to the item
being duplicated, and the piece is made over the mold. A female, or
cavity, mold is the reverse of the item to be duplicated, and the piece is
made inside the mold. The word can also be used to describe the composite
fabrication process: molding a part.
Laminate: A solid part constructed from a combination of resin and
reinforcing fabric. This term can also be used to describe the process of
laying up a part: laminating a part.
Gel Coat (of Surface Coat): The term gel coat is often used
generically to describe any resin-based surface coating, but the term
technically applies to polyester-based materials. The term surface coat
can be used to describe either epoxy of polyester materials. Surface coats
are specially formulated, thickened versions of resins which can be
applied to the surface of a mold or piece to serve as a cosmetic and
Release Agent or PVA: Any of a number of materials applied to the
mold surface before part fabrication, in order to aid in the release of
the piece from the mold. These could be waxes, oils or specialty release
coatings such as PVA.
Flange/Parting Dam: A temporary fixture attached to the plug when
building multiple-piece molds. This generally creates a surface for
materials to be molded against, perpendicular to the parting plane of
symmetry. The flange aids in clamping or bolting the mold sections
together, as well as serving as a mounting point during vacuum bagging
Once you know the "key words" of composites, the next step is
learning about the different resin and reinforcement options available
when working with composites. The first portion of this section deals with
the three main resins used for most composite structures, while part two
deals with the most common reinforcements.
Part 1: Resins
A composite structure consists of a thermosetting resin used in
conjunction with some type of reinforcement, such as woven fiberglass
cloth. The three main types of room-temperature-curing resins used in
composite fabrication are polyester, vinyl ester and epoxy resins.
Polyester resin is a general-purpose resin suitable for a wide variety
of applications. Methyl Ethyl Ketone Peroxide (MEKP) MUST BE USED AS A
CATALYST TO BEGIN THE CURING PROCESS. Catalyzation rates can be varied
with polyester resins, in order to adjust pot life and curing time to
compensate for factors such as environmental conditions. In thin
laminations or when gelcoat is sprayed as a top coat, the surface may
remain tacky and not cure properly if left exposed to the air. To get a
complete cure, thin laminations or top coats must contain either styrene
wax solution or have a coat of polyvinyl alcohol (PVA) solution sprayed
over them to seal out the air. With the former, the wax "floats"
to the surface as the resin cures, acting as a barrier to the air. Styrene
wax must be sanded off after curing, but PVA can be rinsed off with warm
Epoxy resins are not as forgiving in their measurement as polyester
resins, but epoxies provide greater part strength and dimensional
stability. They also adhere to other materials better than polyester
resins. Epoxy hardener ratios canít be varied, and adequate temperatures
(at least 70 degrees F.) must be maintained during the curing process.
Epoxy resin systems tend to cost more than polyester resins, but they are
a virtual necessity in some repair applications, such as with Sheet Molded
Compound (SMC). Epoxy resins are also highly recommended for use with
Kevlar and carbon fiber.
The third type of resin, vinyl ester, possesses qualities that fall
between polyester and epoxy resins for the most part. It excels above
both, however in the areas of corrosion resistance, temperature resistance
(itís good to 300 degrees F.), and toughness. Common uses include boat
hull repair, fuel tank construction and chemical storage tank linings.
Like polyester resin, it is catalyzed with MEKP, but vinyl ester has a
shorter three-month shelf life.
Part 2: Fabrics
There are many reinforcing fabrics available that can be used with the
resins discussed. The three types of reinforcing fabrics most commonly
used are fiberglass, Kevlar (Aramids) and carbon fiber (graphite). Each
possesses different qualities and advantages. All three are usually
available as tows or rovings, veil mats and woven fabrics. Additionally,
fiberglass is available as a chopped strand mat, which consists of short,
randomly oriented fibers held together by a binder.
Carbon fiber costs the most to purchase, but it offers exceptionally
high strength and stiffness, in combination with extremely light weight.
Kevlar also offers light weight, along with excellent abrasion resistance.
It is, however, difficult to cut and wet out with resin. For finishing
purposes, fabricators often use a surface layer of light weight fiberglass
cloth in Kevlar laminates, because Kevlar is virtually impossible to sand
Most general-purpose applications utilize fiberglass cloth. Although it
lacks the light weight and strength of carbon fiber or Kevlar, it is
considerably cheaper to purchase. Fiberglass cloth comes in a wide variety
of styles and weights, making it ideal for many applications.
High-strength weave styles are available, and these could be considered
cost effective alternatives to the more advance fabrics.
The first step in mold making consists of plug construction and/or
preparation. The plug may be constructed of nearly anything, as long as
its surface can be finished well enough to give a suitable mold surface.
As stated previously, the plug can either be an existing item or something
fabricated specifically for the mold-making process. Some of the materials
commonly used in plug construction include wood, plaster, metal and
polyurethane foam. The latter comes either as pre-formed sheets or as a
two-part "mix and pour" system that chemically reacts to form
the foam. The "mix and pour" foam will conform to the shape of
any cavity into which the ingredients are poured.
The surface of the plug must be finished at least as well as the
desired surface finish on the part to be produced. In most applications,
the preferred plug surface finish would be a perfectly smooth and polished
Class "A" finished. If a particular texture or pattern is
desired on the finished part, it can be incorporated into the plug
surface. A high-quality, sandable surfacing primer, such as Duratec
Surfacing Primer, works well as the finish coat on the plug. Incorporate
flanges and any necessary parting dams onto the plug at this point (see
"Special Mold Construction Considerations").
Before beginning construction of the mold, a release agent must be
applied to the plug. This is the most important step in the process,
because if the release agent fails to perform, the mold canít be removed
without damaging it and the plug. A little extra effort at this point is
better than hours spent trying to correct damage to the plug and mold. The
release agent can either be a combination of parting wax and PVA, or a
one-step release agent such as FibRelease. When using wax, apply
four coats, waiting one hour between the second and third coats. After the
final wax coat has been buffed, spray three thin mist coats of PVA and
allow it to dry for 30-45 minutes. FibRelease can be wiped or misted onto
the plug, and allowed to dry for 30 minutes. Be sure to apply the release
agent to the surface of any flanges and parting dams.
For most molds, polyester resin and 1.5 oz./sq.ft. chopped strand mat
yield satisfactory results. Mold strength and thickness can be built up
more rapidly by adding woven roving or tooling fabric. With polyester
molds, the first step in making the mold is the application of a tooling
gel coat, which is distinguishable by its bright orange colour. Prior to
its application, be sure to catalyze the gel coat at the proper ratio. For
best results, the tooling gel coat should be sprayed onto the plug with a
gel coat cup gun in three passes of seven to eight mils each, building to
a total thickness of 20-25 mils.
The surface coat should be stabilized with an initial layer of mat
within one-and-one-half to five hours, in order to prevent the gel coat
from shrinking or lifting off the plug surface. Apply a coat of resin to
the surface and lay the mat into the resin. Using a bristle brush, apply
resin to the mat, coaxing the mat into the various contours of the plug. A
dabbing motion is much more effective than a painting motion, as long
strokes tend to pull the mat around.
All trapped air pockets must be worked out so that the mat is tight
against the plug surface, and it must be uniformly saturated with resin.
Air bubbles and dry areas will appear milky against the tooling gel coat.
Use a bristle roller to work air pockets out of the mat and a grooved
saturation roller to help compact the laminate. Watch for bridging
(lifting) of the fibers across sharp corners and in textured areas. Any
air bubbles remaining after the resin gels must be carefully cut out with
a sharp utility knife and a mat patch laminated in place.
Once the initial layer has cured, lightly sand it in preparation for
additional layers, following the same procedure as with the initial layer.
Most molds utilize eight to ten layers, but do not apply more than three
to four layers at a time to minimize heat generation (exotherm). After the
third layer of mat, a layer of woven roving or tooling fabric can be added
to more rapidly build thickness. In general, a mold should be a minimum of
twice the thickness of the part it is to produce.
Allow the completed mold to cure for at least 24 hours before
attempting removal. Any support structures should be laminated to the back
of the mold prior to releasing it from the plug. Release wedges can be
inserted around the perimeter of the mold, between the mold and the plug,
and gently driven into place in a progressive fashion. Air injection
wedges, which attach to an air compressor, can be used to coax stubborn
Once the mold is released, wash off any residue from the release agent
with warm water and inspect the surface. Any imperfections must be ground
out and repaired. You are then ready to begin prepping the mold for part
Before any part can be made in a new mold, it must be wet sanded and
polished to a Class "A" finish. Wet sand the mold in a
progressive manner, using 400, 600 and finally 1000 grit sandpaper. Be
sure to change the water in your bucket and rinse the mold surface when
changing to a finer paper, to insure none of the coarser grit remains. For
polishing, Rayplex recommends using a two-step polishing compound and a
high-speed buffer. The first stage removes the sanding scratches, while
the second polishes the surface to the desired finish.
After polishing the mold, apply a release agent to it, following the
procedures outlined for prepping the plug. A new mold is often given an
extra coat of the release agent as added insurance.
In the event a part doesnít release properly and damages the mold,
repair will be necessary. Any loose or damaged material must be removed by
sanding or grinding, and new tooling gel coat applied to that area. A coat
of PVA or wax paper placed over the repair will be necessary for proper
curing. Once cured, the repair can be sanded and buffed, as previously
SPECIAL MOLD CONSTRUCTION CONSIDERATIONS
Part 1: Multiple-Piece Molds
In some instances, the shape of the plug may require a multiple-piece
mold so that the mold can be removed from the plug and subsequent parts
removed from the mold. When making a multiple-piece mold, start by
constructing a temporary parting dam on the plug, along the desired
parting line. This dam may be constructed of masonite or a similar
material, and held in place with clay, as shown. A sharp corner without a
radius must be maintained on the portion to be molded first. Any locating
keys or dowels for realignment of the mold pieces should be added to the
parting dam. With multiple-piece molds, construct the entire mold before
releasing any part of the mold, in order to avoid realignment problems.
After the first portion of the mold cures, remove the temporary dam and
use the completed portion of the flange to form the parting dam for the
next half. Apply release agent to this surface before continuing the mold
Part 2: Alternate Construction Methods
If durability and dimensional stability are important factors in mold
construction, epoxy resin can be used in place of polyester resin. The
procedure for this is much the same as with polyester resin, except that
mat cannot be used with epoxy, as the binder that holds the mat together
is not compatible with epoxy resins. Start with two- or four-ounce fabric
to minimize print-through or the weave pattern. Then switch to a seven- to
10-ounce fabric. Be sure to place some layers on a 45-degree angle for
good stiffness. Epoxy surface coats should be brushed onto the plug for
best results. Because epoxies are less prone to shrinkage than polyester
materials, immediate application of a stabilizing reinforcement layer over
the surface coat isnít critical.
If exceptionally rigid molds are required, carbon fiber can be used in
place of fiberglass cloth. We recommend using epoxy resin with carbon
fiber, and a flexible rubber squeegee works best for distributing resin
through the fabric.
Molding Parts: Selecting Materials
Once the mold has been properly polished and coated with a release
agent, you can begin making parts! The first stage in the process of
molding parts is determining which resin and reinforcements will be used.
Having previously discussed the merits of the three main resins, we will
concentrate here on the specifics of reinforcement selection.
After choosing the type of reinforcement to be used, the biggest factor
becomes choosing the style (weave) and weight of fabric best suited to a
given application. The three main fabric styles are plain weave, twill
weave and satin weave. In addition, fiberglass is available as chopped
strand mat. Fabric weights are expressed in ounces per square yard, with
the exception of mat, which is expressed in ounces per square foot.
When fabrics are woven, the fibers are bundled into yarns running at 0
(warp yarn) and 90 (fill yarn) degrees. Plain weaves use an
"over-under" pattern, while in a satin weave one filling yarn
floats over three to seven warp threads before being stitched under
another warp thread, and twill weaves are a "2x2" pattern. Plain
weaves are the least expensive and are good general purpose fabrics, but
they do not offer the strength of satin and twill weave fabrics. Chopped
strand mat isnít as strong as the woven fabrics, but it is equally
strong in all directions.
The lighter the fabric weight, the easier it will drape over contours
and the less resin it will take to wet it out. Lightweight fabrics are
mostly commonly used for surfacing and radio-control (R/C) hobby
applications. Medium weight fabrics are the most commonly used in repair
and fabrication work. The heaviest fabrics are generally used for rapid
thickness build-up, such as in boat hulls and mold making.
Fabrics are sold by the running yard, generally in widths of 38, 50 and
60 inches, although not every fabric will be available in all of those
widths. For a given project, choose a width that most closely approximates
the width of the part to be made. The idea is to use as few separate
pieces of fabric as possible per layer. The amount of resin needed will
depend upon the weight of the fabric selected. Fabric to resin ratios for
the most woven fiberglass and Kevlar are about 50:50, while carbon fiber
is 60:40. Fiberglass mat will require about twice as much resin as woven
fiberglass for proper saturation.
Extra strength can be built into parts by means of sandwich core
construction. This process involves utilizing a core material, such as end
grain balsa wood, polyurethane foam, vinyl foam, or honeycomb, between two
laminate skins. Some core materials come in a variety of thicknesses,
depending on the needs of a particular application. The strength and
stiffness of a part can be increased significantly, with very little extra
weight added to the part.
Molding Parts: The Fabrication Process
With the fabric and resin selected, you are ready to begin molding the
part. As stated previously, when using a mold for the first time, add an
extra coating of release agent to insure a proper release. While the
release agent is drying, take the time to cut the reinforcement to the
proper size and number of pieces and stack the pile near your work area.
If using mat, tear it into workable sized pieces instead of cutting it.
The frayed edges of the pieces will intermix as they are placed in the
mold, giving a stronger bond than when two cut edges are butted together.
With woven fabrics, determine where the partís strength needs to be the
greatest and orient the fibers accordingly. With plain weave fabrics a
more uniform strength can be achieved by alternating the fiber orientation
between 0/90 degrees and 45/45 degrees.
The pat fabrication process is similar to the steps followed in making
the mold. When working with a female mold, start by applying the
appropriate surface coat to the mold surface. This step isnít absolutely
necessary when fabricating parts, but a much better cosmetic appearance
for the finished part will be achieved if it is used. Applying the first
layer of resin and fabric directly to the mold surface can result in
surface irregularities, pinholes, and print-through of the fabric weave
pattern if a heavier fabric is used. These blemishes can be corrected once
the part is removed from the mold, but is will require tedious sanding and
filling. Use of a lightweight fabric, such as two ounce or four ounce, as
the first layer can minimize these problems if a gel coat or surface coat
isnít used. As an alternative to gel coat, Duratec Surfacing Primer can
be sprayed into the mold, providing a durable surface finish.
Polyester gel coats come in either white or clear form, which can be
pigmented to a variety of colours. Clear gel coats reproduce colours very
accurately, while white gel coats yield pastel colours. Epoxy surface coat
is white in colour, and can also be pigmented.
When applying gel coat tot he mold, the best results will be achieved
by spraying unthinned gel coat with a cup gun, in much the same manner as
tooling gel coat is applied in mold construction. Slowly build up the gel
coat in three passes, to a thickness of 15-20 mils. A gel coat thickness
gauge is the best to make certain an even coat is being applied. Too much
or too little in some areas can cause wrinkling or distortion when the gel
coat cures. When using an epoxy surface coat, it should be brushed into
Adhering to the guidelines in the mold construction section of this
brochure, follow the gel coat with an initial stabilizing layer of
reinforcement. If you have pigmented the gel coat and want the same colour
throughout the part, the resin can also be pigmented to match.
When laying up the reinforcement, try to utilize a single, uncut piece
of fabric for each layer. Unfortunately, this is not always possible.
Sometimes a part is too large to be covered by a single piece of fabric,
so two or more pieces must be used. When two separate pieces must be
joined together in a mold, it is best to overlap the pieces by one-half to
one inch, as opposed to butting the pieces together. Butt two pieces
together to form a seam only when maintaining constant thickness is
The contours and shapes of a part may also make it difficult to get
good adhesion using a single piece of fabric. Indentations and sharp
angles, in particular, present this kind of problem. Composites can be
formed into many shapes, but it is very difficult to achieve sharp angles
(90 degrees or sharper) with continuous pieces of fabric. The fabric will
tend to lift in these area, resulting in air bubbles and weak spots in the
laminate. If a sharp angle is required in a part, the best way to approach
it is by butting two cut pieces of fabric together at the turn. For added
strength at these butt joints, mix a small amount of resin with milled
glass fibers to form a structural putty filler. Apply this to the joint
before laying in the fabric. With indentations, it is better to cut a
smaller piece of fabric to fit the indentation, rather than trying to fore
a larger piece of fabric down into it.
As with mold construction, use rollers and squeegees to thoroughly
saturate the fabric, work air pockets out of the laminate and compact the
layers as much as possible. This will help avoid weak spots and
delamination problems in the finished part. As the layers of reinforcement
go into the mold, pay attention to the orientation of the fibers if using
woven cloth, alternating the orientation by layer to increase part
If a sandwich core construction is going to be utilized, determine
which type of core material best suits the application. Polyurethane foam
is very rigid and does not conform well to contours, whereas vinyl foam
can be heated and formed to a variety of shapes. Balsa, which generally
consists of small end grain blocks held together by a scrim fabric, can
conform to mild curves. Honeycomb core materials are very flexible and
will bend to a variety of shapes.
Several steps must be taken to prep core materials, in order to get a
strong piece. After cutting and shaping the core material to the contours
of the part, bevel the edges of the coreís perimeter to a 45-degree
angle to smooth fabric transition. Mix a portion of resin with glass
microspheres to a slurry consistency, and use this to fill any gaps, as
well as splice multiple pieces of core material together. Pretreat
open-celled foams and honeycomb cores with this slurry mix, in order to
fill the open cells with something lighter than pure resin. Once these
steps are completed, the core can be bonded in place.
When dealing with multiple-piece molds, almost always assemble the
pieces of the mold before laying up a part. Laying up the part and then
assembling the mold pieces will make it difficult to get a good bond
between the pieces and a smooth cosmetic finish. The exception to this
rule would be an enclosed item, such as a fuel tank, which would be
impossible to lay up if the mold was assembled in advance.
If a compression mold is being used, the other half of the mold can be
clamped to the first half once all of the reinforcing layers are in place.
If a compression mold is not being used, but a smooth surface is desired
on both sides of the part, a surface coat can be applied over the final
layer of reinforcement. When the laminate reaches the "leathery"
semi-cured stage, trim the edges with a sharp utility knife. Doing this
now will significantly reduce finishing time and dust generation down the
Once the part has cured, remove it from the mold in much the same
manner as the mold was removed from the plug. Any residue from the release
agent can be rinsed off the part, and it can be finished in whatever
manner is necessary. Finishing usually involves sanding down any seams and
sanding the edges of the part.
Inspect the mold for any damage or dulling of the mold surface. If
everything is fine, reapply the release agent when you are ready to build
the next part. If repairs or buffing are necessary, carry out those
operations as previously described.
By carefully following the guidelines in this and our other brochures,
you can produce molds and finished parts that meet or exceed your
expectations. If something does go wrong, nearly any damage or problems
can be repaired. Remember that working with composites is like any other
new skill you learn: the more you work at it and practice honing your
abilities, the better the results will be. Once you have mastered the
basics, and then refined those skills, nearly anything is possible.