Other attributes
The vacuum bagging system consists of the airtight clamping envelope and a method for removing air from the envelope until the epoxy adhesive cures. This section discusses the
components of this system (Figure 2-1), which include both specialized equipment and
commonly available materials.
2.1 Vacuum pumps
The heart of a vacuum system is the vacuum pump. Powered vacuum pumps are mechanically similar to air compressors, but work in reverse so that air is drawn from the closed system and exhausted to the atmosphere. Vacuum pumps are designated by their vacuum
pressure potential or “Hg maximum” (Hg is the chemical symbol for mercury), their displacement in cubic feet per minute (CFM) and the horsepower required to drive the pump.
2.1.1 Vacuum pressure
The Hg maximum level is the maximum vacuum level (measured in inches of mercury) recommended for the pump. This vacuum level translates to the maximum amount of work effect or clamping pressure that can be generated. Two inches of mercury (2" Hg) equals
about one pound per square inch (1 psi) of air pressure. (Remember that 1 atmosphere =
29.92 inches Hg = 14.7 psi) If you are vacuum bagging a one square foot laminate, a 20"
Hg vacuum will yield 10 psi clamping force or a total of 1440 pounds of clamping force
over the entire laminate. If you are laminating a 4' × 8' panel, the same 20" Hg (10 psi) will
yield over 46,000 pounds of clamping force spread evenly over the entire panel.
2.1.2 Displacement
The volume of air a pump can move (rated in cubic feet per minute or CFM) is also an important consideration in the selection of a pump. If the vacuum system (the mold, bag,
plumbing and all seams and joints) were absolutely airtight, any size pump should be able to
eventually pull its rated Hg maximum vacuum regardless of the size of the system. However, creating a perfectly airtight vacuum bagging system is nearly impossible, especially as
the system gets larger or more complex. The greater the CFM rating, the closer the pump
can come to reaching its Hg maximum and maintaining an adequate clamping force against
the cumulative leaks in the system. A vacuum pump with a high CFM rating will also
achieve an effective clamping force more quickly. This is an important consideration if the
working life of the adhesive is limited or if the laminate will not hold its position until the
clamping force is applied.
2.1.3 Horsepower and performance
The horsepower requirement of the pump is an indication of how efficient the pump is and
is not in itself an indication of how well a pump is suited to vacuum bagging. When selecting a pump, use the “Hg maximum” and CFM ratings as a guide rather than horsepower.
Smaller pumps designed for specific applications may trade off either vacuum rating or air
displacement to suit a particular job. Generally, to get both higher “Hg maximum” and
CFM ratings, more horsepower is necessary. Pumps that are useful for moderate boat yard
vacuum bagging may range from 1/4 hp to 2 hp Pumps for large production operations may
be as big as 20 hp or 30 hp.
2.1.4 Pump selection
The size and shape of the mold and type and quantity of the material being laminated will
determine the minimum pump requirements. If you are laminating flat panels consisting of
a few layers of glass, flat veneers or a core material, 5" or 6" Hg (2.5–3 psi) vacuum pressure
will provide enough clamping pressure for a good bond between all of the layers. If the area
of the panel is limited to a few square feet, a 1 or 2 CFM pump will be adequate to maintain
that clamping pressure. As the panel area increases, the CFM requirement increases proportionately. A displacement of 3.5 CFM may be adequate for up to a 14' panel; for larger
jobs, a pump with a displacement of 10 CFM or more may be required. Poor seals in the
plumbing system or envelope, or material which allows air leakage, will require a larger capacity pump to maintain satisfactory vacuum pressure. The more airtight the system, the
smaller the pump you’ll need.
A higher “Hg maximum” rated pump will be required if you need more clamping pressure
to force laminations to conform to a more complex mold shape. Curved or compounded
mold shapes and/or laminations of many layers of stiff veneers or core materials may require at least a 20"–28" Hg vacuum to provide an adequate clamping force. Again, if the
panel size is limited to a few square feet, a 1 or 2 CFM pump with a high “Hg rating” will
work, if the envelope is airtight. However, a large panel or hull may take a minimum of 10
CFM pump to reach and maintain enough clamping force to press all of the laminate layers
to the mold shape and produce consistent glue lines throughout the laminate. Generally,
the best pump for a specific vacuum bagging operation will have the largest air moving capacity for the vacuum/clamping pressure required while operating at a reasonable
horsepower.
2.1.5 Pump types
Vacuum pump types include piston, rotary vane, turbine, diaphragm and venturi. They
may be of a positive or non-positive displacement type.
Positive displacement vacuum pumps may be oil-lubricated or oil-less. Oil-lubricated
pumps can run at higher vacuum pressures, are more efficient and last longer than oil-less
pumps. Oil-less pumps, however, are cleaner, require less monitoring and maintenance,
and easily generate vacuums in a range useful for vacuum bagging. Of the several types of
positive displacement vacuum pumps useful for vacuum bagging, the reciprocating piston
type and the rotary vane type are most common. Piston pumps are able to generate higher
vacuums than rotary vane pumps, accompanied by higher noise levels and vibration. Rotary vane pumps may generate lower vacuums than piston pumps, but they offer several advantages over piston pumps. While their vacuum ratings are more than adequate for most
vacuum bagging, they are able to move more air for a given vacuum rating. In other words,
they can remove air from the system more quickly and can tolerate more leaks in the system
while maintaining a useful vacuum level. In addition, rotary vane pumps are generally more
compact, run more smoothly, require less power and cost less.
Non-positive displacement vacuum pumps have high CFM ratings, but generally at vacuum pressure levels too low for most vacuum bagging. A vacuum cleaner is an example of a
non-positive displacement or turbine type pump.
Air operated vacuum generators are simple, low cost venturi devices that generate a vacuum using air pressure supplied by standard air compressors. Their portability, relatively
low cost and the accessibility of compressors in many shops and homes make them ideal for
many smaller vacuum bagging projects. Single stage generators have a high vacuum rating,
but move a low volume of air, limiting the size of the vacuum bagging operation. The WEST
SYSTEM 885-1 Venturi Vacuum Generator develops over 20" Hg (10 psi) at 1 CFM. It is designed to run off conventional shop air compressors that deliver at least 60 psi at 2 CFM.
Larger two-stage pumps are comparable to mechanical pumps for most vacuum bagging
operations, but require a proportionately large compressor to run them.
Vacuum pumps have been manufactured for a wide variety of industrial applications. Used
pumps of various sizes and ratings may be found at a reasonable price. For small projects,
some builders have successfully used old milking machine pumps and even vacuum cleaner
pumps.
A variety of other materials are needed to complete the vacuum system and assist in the laminating process. The materials referred to in this manual are available from WEST SYSTEM
or readily accessible through hardware or automotive supply stores. Alternate materials
that function the same as those listed may be used.
2.2.1 Release fabric
Release fabric is a smooth woven fabric that will not bond to epoxy. It is used to separate
the breather and the laminate. Excess epoxy can wick through the release fabric and be
peeled off the laminate after the laminate cures. It will leave a smooth textured surface that,
in most cases, can be bonded to without additional preparation. Surfaces that will subject to
highly-loaded bonds should be sanded.
WEST SYSTEM 879 Release Fabric is a strong, finely woven polyester fabric, specially
treated so that epoxy will not bond to it. It is not recommended for post cure temperatures
over 120°F (49°C). A variety of release materials are produced specifically for vacuum bagging operations. They may be known as release fabric, peel ply or release film. Many are designed for use at higher temperatures or to control the amount of resin that can pass
through them.
2.2.2 Perforated film
A perforated plastic film may be used in conjunction with the release fabric. This film helps
hold the resin in the laminate when high vacuum pressure is used with slow curing resin systems or thin laminates. Perforated films are available in a variety of hole sizes and patterns
depending on the clamping pressure, and the resin’s open time and viscosity.
2.2.3 Breather material
A breather (or bleeder) cloth allows air from all parts of the envelope to be drawn to a port
or manifold by providing a slight air space between the bag and the laminate.
WEST SYSTEM 881 Breather Fabric is a 45" wide lightweight polyester blanket that provides air passage within the vacuum envelope and absorbs excess epoxy. A variety of other
materials can be used such as mosquito screen, burlap, fiberglass cloth or a bubble type
swimming pool cover.
2.2.4 Vacuum bag
The vacuum bag, in most cases, forms half of the airtight envelope around the laminate. If
you plan to use vacuum pressure of less than 5 psi (10 hg) at room temperatures, 6-mil polyethylene plastic can be used for the bag. Clear plastic is preferable to an opaque material to
allow easy inspection of the laminate as it cures. For higher pressure and temperature applications, specially manufactured vacuum bag material should be used. A wrinkled type film
is available from Film Technology, Inc. Its special texture is designed to channel air and
eliminate the need for breather fabric. WEST SYSTEM 882 Vacuum Bag Film is a 60" wide,
heat stabilized nylon film that can be used at temperatures up to 350°F (176°C) and high
vacuum pressures. The vacuum bag should always be larger than the mold and allow for the
depth of the mold. When a bag wider than the standard width is needed, a larger bag can be
created by splicing two or more pieces together with mastic sealant.
2.2.5 Mastic sealant
Mastic is used to provide a continuous airtight seal between the bag and the mold around
the perimeter of the mold. The mastic may also be used to seal the point where the manifold
enters the bag and to repair leaks in the bag or plumbing.
2.2.6 The plumbing system
The plumbing system provides an airtight passage from the vacuum envelope to the vacuum pump, allowing the pump to remove air from and reduce air pressure in the envelope.
A basic system consists of flexible hose or rigid pipe, a trap, and a port that connects the
pipe to the envelope. A more versatile system includes a control valve and a vacuum throttle
valve that allow you to control the envelope vacuum pressure at the envelope. A system is
often split to provide several ports on large laminations, or may include some type of manifold within the envelope to help channel air to a single port. A variety of pipe or tubing can
be used for plumbing as long as it is airtight and resists collapsing under vacuum.
Vacuum hose is designed specifically for vacuum bagging and autoclave laminating. It is
available along with fittings, pumps, and other vacuum bagging materials from manufacturers specializing in vacuum bagging equipment. Because of its higher cost, this type of
plumbing system is most appropriate for large scale or production laminating operations.
Other types of wire reinforced hose may work, but they should be rated for crush resistance
or tested under vacuum for the appropriate length of (cure) time. Semi-rigid plastic tubing,
with adequate wall thickness, can be used for a plumbing system, but it is often awkward to
handle. If the laminate is to be post-cured during vacuum bagging, the tubing must also be
heat resistant. Plastic tubing that may be able to withstand vacuum at room temperature
may soften and collapse if heated.
Rigid ¾" PVC or CPVC pipe, elbows, T’s, and valves work well. They are low cost and
available at most local hardware or plumbing supply stores. The pieces do not need to be
cemented together and can be rearranged to suit any configuration. This type of plumbing
system, because of its low cost and versatility, is ideal for small scale or occasional laminating operations.
A vacuum port connects the exhaust tubing to the vacuum bag. It can be designed specifically for the purpose or built from commonly available materials. One of the simplest ports
is a hollow suction cup that sits over a small slit in the vacuum bag. Cups designed for use
with car top carriers can be easily adapted by drilling through the center of the cup.
Acontrol valve should be incorporated into the vacuum line to allow you to control the volume of airflow at the envelope. The control valve affects the rate of air removal, but not the
vacuum pressure. A second valve, the vacuum throttle valve, can be placed between the
control valve and the envelope. This valve, incorporated with a “T” fitting, acts as an adjustable leak in the system to control the envelope pressure. For convenience, valves should
be placed close to the envelope.
A trap should be incorporated into the line as close as possible to the envelope. The trap
collects any excess adhesive that gets sucked into the line before it reaches the valves or
pump and prevents a build up of adhesive in the line. A trap can easily be built with a small
section of pipe, a “T”, and an end cap.
A vacuum gauge is necessary to monitor the vacuum level/clamping force during the cure
time of the laminate. Most gauges read in inches of mercury from zero (one atmosphere) to
30 (inches Hg below one atmosphere). The reading of negative pressure inside the bag
equals the net pressure of the atmosphere pressing on the outside of the bag. To approximate this reading in pounds per square inch (psi), simply divide the reading by two. A vacuum gauge, available at most automotive stores, is modified by threading a hollow suction
cup (similar to the port) to the base. A 1½" PVC pipe cap, with a hole drilled and tapped to
match the gauge, will also work. The end of the cap is sealed to the vacuum bag with mastic.
A manifold is used in some situations to assist in air removal from the envelope. It can be a
thicker section of breather material or other material that provides a channel for air movement under the vacuum bag to a port. A ¾" PVC pipe with holes drilled along its length was
used in the applications shown later in this manual. Any hard object (such as the manifold)
placed under the vacuum bag can leave an undesirable impression in the laminate.
2.2.7 Mold Release
Mold release is essential for preventing the epoxy from sticking to the mold when laminating a part. There are generally three types of mold release used depending on the mold material and desired characteristics of the finished part. The most common type is a carnauba
based paste wax. This is usually put on in up to 5 layers for new molds and at least one layer
before each new part is molded. It is also a good idea to use something like PVA (polyvinyl
alcohol) over the 5 coats of wax on a new mold to help prevent sticking. Fine detail and
gloss level are obtained with the use of paste wax, but it can be difficult to buff anything
with a textured surface.
The second type of release is the semi-permanent formulation. Many different manufacturers provide liquid release systems that apply much easier than paste wax and last for multiple parts on one application of the product. Generally a sealer and a release are used to
provide the best results for new molds. Fine detail and gloss level are obtained as well as
texture since buffing to remove excess is not usually necessary.
The final type of mold release is of the general contaminant variety. This can range from
things like grease and Vaseline to toilet bowl wax, hair spray, hair gel or even clear packaging tape. These are generally used on rough or porous surfaces where detail, gloss, and texture are not issues for the final part. While not the prettiest, these release agents quick, cheap and widely available.
