Of all the advantages offered by composite materials, their ability to
be molded to complex shapes is perhaps the most popular. When a shape
needs to be reproduced numerous times, it is most efficient to build a
tool or mold within which the part can be fabricated. Molded parts emerge
perfectly shaped every time and require little post-finishing work.
Composite materials offer a cost effective way for anyone to make even
large production runs of identical plastic parts in molds they can produce
This brochure will describe the steps necessary to create accurate,
high-quality, low-distortion molds for producing composite parts. It is
intended to help novice through intermediate builders obtain successful
results with their first project. While many of the principles described
are the same as large scale industrial techniques, the suggestions offered
are meant to be used in small shops, garages, or workshops to help
individuals produce BIG results! For this reason, the examples listed are
intended to be scaled-down, helpful industrial hints.
Types of Molds
Males and female molds are the only two fundamental types of forms, but
they yield significantly different finished parts. The least time consuming and
cheapest method is the male or positive mold. This is a form which mimics
the final shape of the part, but the part is fabricated over its outer
surface. It is true that this type of mold is quicker to construct, but
each part produced will have a rough outer texture which requires
laborious finishing. The part will also "grow" during
lamination. Usually this is undesirable, but if the mold is intentionally
made slightly smaller the part will grow into the desired finished
dimensions. Male molds should be used when fewer than 5-10 parts are being
produced. Larger runs usually warrant the time and cost of female molds.
The remainder of this brochure will deal mostly with constructing female
molds, but male molds can be made using the same materials.
Female or cavity molds are generally more costly, but they offer
numerous advantages for medium to large production runs. Finishing time is
significantly reduced because every part emerges with a smooth outer
surface. Female molds also lend themselves to use with core materials
because the outer skin is always smooth, regardless of how inconsistently
the core is used inside the part. Either type of mold can be used for
vacuum bagging, but female molds are usually easier to seal while
achieving good surfacing characteristics. If more than 5-10 parts which
need smooth finishes are being produced, female molds are work the extra
Compression molds are sometimes made by using both a male and a female
form. These "matched" molds are excellent for producing
precision parts. The molds are loaded with reinforcement and resin before
they are closed and tightened. Excess resin is squeezed out, voids are
reduced, and parts emerge smooth on both sides. Compression molds can also
be modified for use with resin infusion of injection. The key is to think
about the intended use of the finished part and what type of mold will be
necessary to build it. If this is considered in advance, there is no limit
to the type of parts that can be produced.
Selection of Methods and Materials
Before beginning the construction of any mold, take the time to
consider the desired end results. The construction of the mold will be a
trade-off among the physical properties of the molds, cost of construction
and time involved to build the mold. What you want in a finished part will
have a direct bearing on these trade-offs. Careful consideration of these
factors will enable you to incur the least expense to get the desired
When selecting mold-making materials and the method of construction,
take into account such things as the length of the production run and the
desired quality of the surface finish on the part. The time and materials
put into the mold a the beginning will ultimately impact how many parts
you can make and the quality of those parts. Other things to consider
include technique specific modifications to the mold to aid in procedures
like vacuum bagging and compression molding. Larger flanges are worth
incorporating to make both these procedures easier. Locating pins along
the perimeter flange should also be planned for complex molds with
multiple pieces requiring precise alignment. Finally, consider how the
mold will be held while in use. An egg-crate structure will add both
support and manageability to molds with awkward shapes.
How you intend to release the mold from the plug and subsequent parts
from the mold will also impact the overall design and construction. The
first factor to
consider is the draft angle of the mold. This is the angle of the sides
of the mold compared to its base. A mold with zero draft has flat sides
perpendicular to the bottom. On a mold with positive draft, the sides are
wider at the top than they are at the base. Parts will easily pop out of
molds with positive draft. The sides of a mold with negative draft are
tighter at the top than at the bottom. For obvious reasons parts are
impossible to remove from a mold with negative draft. Shapes which have to
be molded with negative draft must be made in multiple-piece molds. Each
piece has positive draft for easy release, yet they all bolt together
forming the negative cavity.
The point where the mold pieces joint together is called the parting
plane. This is the imaginary line or plane that divides the negative draft
angle into two positive angles. Molds can be built with as many parting
planes as needed for complete separation.
The plane typically runs along the highest or widest crest of the part.
When first tackling mold building projects, it is helpful to draw the line
on the plug with a felt tipped pen. This permits trial-and-error sketching
until you are satisfied that it is located in the proper place. If further
lessons are necessary, look at the mold seams on plastic childrenís
toys. They are often quite exaggerated and rarely removed. Quite a bit can
be learned from their examples.
Large parts and molds have difficulty separating even once their edges
break free. Slight adhesion over broad areas and even static contribute to
the problem. Expect the worst, and plan ahead. Drill holes through the
mold and bond an air fitting to the back. Use Rayplex molding clay to fill
the hole during molding so resin does not contaminate the air line. Be
very careful compressed air is very dangerous. Part can crack or burst.
Use a low air pressure to assist in releasing. When the part is ready to
release, just hook up the compressed air and pop! Try to locate the
fittings in areas that will later be trimmed and removed from the part.
That way the marks from the clay wonít have to be sanded and polished
In selecting the actual resins and fabrics, approach it from the
standpoint of creating a mold or the lowest possible cost, given the
application. Nearly all composite materials can be utilized in mold
construction, but the part requirements often donít justify the expense
of more exotic materials. For many parts, a mold constructed with general
purpose polyester resin and 1.5 oz./sq. ft. fiberglass chopped strand mat
will produce satisfactory results. Use Isophthalic or Chlorendic resins
for ultra-low distortion. Mat offers quick build-up, along with uniform
strength and stiffness, in a minimum number of layers (typically 8-10
layers). Using a good tooling gelcoat sprayed at the proper thickness will
also aid greatly in achieving a top-notch mold surface. Use regular gel
when surface appearance is not important or when the number of parts to be
manufactured is low.
Some parts do require an extremely rigid mold for dimensional accuracy
or longevity. Epoxy surface coats and resins, which experience little to
no shrinkage, can then be justified. Mat cannot be used with the epoxies
because it is not compatible. That is just as well since the woven
fiberglass cloths are stronger anyway. Use carbon fiber for molds which
need the highest strength and rigidity.
One of the primary keys to success in mold construction is proper
preparation of the plug, which is the "original" used to create
the mold. Any imperfections in the plug surface will be transferred to the
mold, and then to future parts made from the mold. Recalling what we said
earlier about beginning with the end in mind, the plug needs to have a
finish AT LEAST as good as the parts you wish to produce.
The preferable surface finish for the plug would be a Class
"A" finish, which means it would be a polished, high-luster
finish free from any porosity or scratches. In order to achieve an
acceptable mold surface and along mold life, it is far more effective to
remove defects from the plug surface than attempting to remove defects
from the mold surface.
After the plug has been properly shaped and sanded, finish the plug
with a high quality surfacing primer, such as Rayplex Sanding Sealer #.
These materials can be readily sanded and polished to a Class
"A" finish. The brochure Plug Surface Preparation and Mold
Surface Maintenance goes into this process in greater detail.
Constructing the Mold
Once the plus has been prepared to a Class "A" finish,
construction of the female mold can commence. First, a mold release wax
& PVA will need to be applied to the plug. This is a critical step,
since it will allow you to separate the mold from the plug once the
materials used to construct the mold have cured. If the mold doesnít
release properly from the plug, the mold and the plug could be damaged or
destroyed, so follow these procedures carefully.
The two most common release agents employed are the traditional
combination of parting wax and PVA release film. When working with
wax and PVA, generally four coats of wax are applied, with an hour wait in
between the second and third coats of wax. After the final application has
dried and been buffed, the PVA can be sprayed onto the plug. For best
results, the PVA should be sprayed in three thin mist coats and allowed to
dry for 30-45 minutes. In that time it should cure into a thin smooth
film. 70 to 90 psi air pressure will help to fully atomize the PVA for a
Before starting the layup of the mold, parting flanges or dams must be
added to the plug along all the parting planes previously described. This
is the form which divides the mold segments during construction. This form
is removed once one side has been molded. Like the plug itself, these
parting flanges are constructed of the least expensive materials that will
support the curing fiberglass later. Rayplex Pattern Clay, masonite, waxed
posterboard, thin sheetmetal, and playing cards have all been known to
work. Typically, a "snake" of clay is rolled in the palm of the
hands and pressed onto the plug along the parting plane. When the symmetry
is simple, a silhouette cut-out can be made from masonite and attached
with the clay. It is easier to use the posterboard or playing cards on
plugs with complicated shapes. Scissors can quickly cut the contours
necessary before the dam is inserted into the clay. Use a mixing stick to
scrape away the excess clay on the side that will be molded first. More
elaborate fixtures can be constructed to do the same job, however this
method will provide repeatable results.
At this point, any locating keys or dowels for re-aligning the segments
of a multiple-piece mold should be added to the parting flange. If the
flange is made of clay, these key-ways can simply be imbedded in the soft
material. Masonite is also easy to add them to because it provides plenty
of support for holding the dowels. When using a paper or metal dam, simply
make a key from clay and stick it to the surface. The key-way will be
molded first, then remove the clay and the other mold section will be made
with the matching key. Regardless of the material used to make the
parting dam, spray or gently wipe on one final coat of release agent.
Once these steps have been completed, itís time to begin applying the
surface coat. The polyester tooling gel coat is available in black or
bright orange color. Although the surface coat can be applied with a
brush, a more uniform result will be achieved by spraying it. Gel coats
and other surface coat materials are too thick to be sprayed with normal
automotive spray equipment, so a special gravity-feed "cup" gun
must be utilized. Typically, cup guns accept disposable cups holding up to
a quart of material. After mixing in the proper amount of catalyst, youíre
ready to spray the surface coat. Once you start spraying, keep the
material flowing; donít start and stop a the end of each pass like when
spraying paint through regular siphon equipment. Exercise caution, though,
or too much material will build up too quickly. Using a gel coat thickness
gauge for testing, apply a uniform thickness of 20-25 mils over the plug.
This is best achieved in three passes of 7-8 mils each. DO NOT allow any
initial pass to tack-up before adding the next layer. All 20-25 mils must
cure as a single film for best results.
Once the surface coat has been applied, itís critical to stabilize it
with the first layer of reinforcement within 1.5 to 8 hours. This will
help prevent the surface coat from shrinking or lifting off the surface of
the plug. The first layer of reinforcement is also the most critical layer
in the mold to lay down without trapping air bubbles. All air pockets
dirctly beneath the surface coat are prone to cracking. When the chips
fall out after producing a part or two, the whole mold surface will become
cratered and need resurfacing.
With the stabilizing layer in place, the mold could sit in that
condition for days before being completed. The main advise there is to
avoid spraying your surface coat just before bed and expecting it to be
perfect when you return in the morning. You will be time and money ahead
if you wait and begin the process when the surface coat can be stabilized
within the 8 hour window of opportunity. This advice may seem overly
cautious to some, but it always works.
This also helps prevent heat distortion in polyester molds. After an
hour the gel coat is cool to the touch. One layer of 1.5 oz./sq.ft. mat
and resin will heat slightly while curing, but not enough to distort the
delicate surface coat. When the first reinforcement layer is cool to the
touch, it can be sanded in preparation for more mat. The remaining layers
can be added fairly quickly to this stabilized surface without much fear
of thermal distortion.
If chopped strand mat is used, cut the mat into manageable size chunks.
The frayed edges blend well with one another without trapping air like
sharp scissor-cut edges do. The flange areas will need some strips cut to
the proper width to butt into the corner of the parting dam to exclude
air. However, this is about the only area where they are needed. If coarse
woven fabrics are used, they will lay easier over severe contours when
they are cut off the roll on a 45 degree bias. Pre-cut much of the
reinforcement so 2-3 layers can be added at a time before the resin starts
to gel. Six layers are maximum number to be applied in a layup.
Using a natural bristle brush, pre-wet the surface with properly
catalyzed resin, then place the mat on the plug. The reinforcement will
soak up much of the resin, but white spots indicate more resin is needed.
Once again, begin by butting pre-cut strips into the angles where the
parting dam meets the plug. Then apply frayed patches on the main surface
overlapping nicely onto the flange. A mixture of Rayplex milled fibre and
catalysed resin into a putty can also be spread into those types of
corners to keep out air.
Roll the air out of the laminate at least every layer. Begin using a
fabric roller which will pop many bubbles within the mat. Next, switch to
a grooved saturation roller to compact the laminate. Be sure to use a
roller which contact the entire surface. There are many shapes to choose
from for this reason.
Most molds using chopped strand mat utilize about 6-10 layers. Heavier
fabrics such as woven roving or tooling fabric can be added after the
third layer of mat to more rapidly increase the build-up and strength of
the mold. Consider alternating the fabric weave patterns between 0/90 and
45/45 degrees so the strength remain uniform. Do not apply more than 3-4
layers at a time so that heat generation, or exotherm, is kept to a
Once all of the layers are in place and have properly cured, the
parting dam can be stripped off the back of the new flange and discarded.
Use clean rags to wipe away any excess clay that might remain on the
surface. Take care not to scratch the plug while doing this. Apply fresh
mold release agent to the newly exposed flange, as this will be the form
against which itís mate will be constructed. Once again, follow the
sequence described above from surface coat to final reinforcement until
all the segments of the mold have been built.
If egg crate support structures need to be added, now is the time. Most
are made from plywood or some other inexpensive flat stock. Make paper
templates of the mold contour where the panel is to attach so miscuts are
reduced. Cut the wood to shape so it fits the contour of the mold and any
other pieces of the framework itself. Do not allow the support structure
to touch the mould. This can cause printing in the mold. Use the resin and
reinforcement to bond it to the back of the mold. Join all other similar
pieces to the mold in the same way, and attach them to each other as
designed. Once cured, this will add even greater rigidity to the mold.
When all the portions are complete and cured, it is time to trim the
mold and drill any final clamping holes for bolts. Drill the holes first
so that if any part of the mold pre-releases while trimming everything
will still line up later. Trimming is actually best achieved with a saw.
Grit edge jig saw blades cut faster and with less effort than most air die
grinders. With the perimeter entirely trimmed, construction is complete.
Releasing the Mold from the Plug
It is time for the moment of truth Ė releasing the mold from the
plug! Release wedges can be used to help coax the mold off the plug. These
plastic wedges should always be used in place of screwdrivers and putty
knives because they will not chip the mold surface. Why perform all these
tedious tasks just to ruin it now? Insert the wedges around the perimeter
of the mold and gently tap them into place, progressing evenly around the
edges. Special air-injection release wedges, which attach to an air
compressor, can also be used for stubborn parts. The pillow of air that is
shot between the plug and the mold provides pressure where no wedge can
ever reach. Slowly the two should separate. If problems still persist,
light blows with a rubber mallet can send vibrations through the mold
causing separation. Donít get carried away. Heavy pounding can actually
fracture the mold surface causing star cracks in the gelcoat surface.
These combined hints will safely release the mold sections.
Preparing the Mold for Use
Once the mold is separated from the plug, clean and inspect its
surface. The residue of PVA mold release agent can be washed off with warm
water. Dry the surface, and look for any serious defects. Critical
problems will actually have to be ground out and resurfaced. Hinges, air
injection ports, and any other accessories should be attached at this time
if they are needed. If the directions were followed and nothing was
damaged during release, the surface should already be very smooth.
Typically, the mold release agents leave a slight texture behind, but this
can be quickly removed while achieving a class "A" surface.
Begin by wet sanding with 400 grit sandpaper, eventually moving to 600
grit, then 1000 grit paper. Rinse the bucket and the mold surface before
moving to the next grade of paper, so any remaining grit from the previous
sandpaper is removed. Once the sanding is completed, buff the mold surface
with an appropriate polishing compound. Rayplex recommends a two stage
polishing process for best results. For more detailed information on mold
surface preparation and maintenance, see our brochure Plug Surface
Preparation and Mold Surface Maintenance. Allow three days for the
mold to cure and age.
The final step before use is the application of the desired release
agent. This is just as straightforward as it has been during the mold
construction process. An unseasoned mold is often given an extra coat or
two of the wax and PVA just to be sure of its effectiveness. In fact, the
first few parts made in a mold are sometimes constructed then enough to be
broken out in the event of a major failure. However, if the mold was
designed correctly, there shouldnít be any problems.
Congratulations are in order for all those who made it this far. You
are now prepared to embark on projects which can truly open the doors to
new creations, even new careers. Following these simple instructions,
accurate, high-quality, distortion-free molds can be constructed for
producing composite parts. This information, combined with a bright idea,
offers the freedom to build structures many would consider impossible.
Then, if the idea takes off, produce as many as necessary to fill demand.
Hopefully, this brochure has instilled enough confidence and enthusiasm to
get you started on your first project. Even if you start small, you can
get BIG results.