Vacuum bagging (or vacuum bag laminating) is a clamping method that uses
atmospheric pressure to hold the adhesive-or resin-coated components of a lamination
in place until the adhesive cures. The effectiveness of vacuum bagging permits the
lamination of a wide range of materials
from traditional wood veneers to synthetic
fibers and core materials.
Benefits of vacuum bagging:
• No clamps required—even clamping
pressure is provided by vacuum pump
and vinyl film.
• Control of resin content.
• Far easier, cleaner, and more evenly
applied than clamped by hand.
• Custom shapes.
• Materials are inexpensive and reusable.
You may recall that 1 atmosphere = 29. 92
inches of mercury (inHg), which is equivalent to 14. 7 psi. In rough terms, 2 inHg = 1
psi. Therefore, if you were to vacuum bag a 1
square foot laminate (144 inches square) at
20 inHg ( 10 psi), your effective clamping
force would be 1,440 psi evenly distributed
over the entire surface of the assembly.
THE EQUIPMENT
At a minimum you will need a vacuum
source and some type of film for bagging the
parts and applying pressure. Most systems
also include a vacuum gauge and a bleeder
valve of some sort in order to fine-tune the
applied pressure. Image 1 shows the
components of a typical vacuum system as
illustrated by West Systems. As you can see
from this illustration, the system is fairly
low-tech. This setup shows several layers—
bag, breather, release film, etc. While this is
one approach, my process used a simple
vinyl bag or tube, which is suitable for bonding flat laminations.
My setup uses a Gast vacuum pump that
is capable of a maximum of 24 inHg at 1. 5
cubic feet per minute (cfm). I have the
pump connected to a PVC cylinder, which
acts as a buffer between the pump and the
parts. This part is not entirely necessary,
but the cylinder gives me a place to mount
my vacuum gauge and bleeder valve, plus
ILLUSTRATION BY WEST SYSTEMS
the volume in the cylinder acts as a buffer
and smoothes out the delivery, which
makes it easier to read the gauge and adjust
the vacuum (i.e., pressure on the parts).
Finally, a vacuum line is connected from
the PVC cylinder to the vinyl bag, or tube.
While it is not entirely clear in this photo,
the tube gets clamped at both ends once the
parts have been placed inside.
When the struts were removed
from the bag they were straight
and solid—and I mean solid like
a baseball bat.
The pump unit is the most expensive
part of the system, depending on your
scrounging abilities. The Gast type pumps
are fairly expensive if purchased new, but
used models can be found for a significant
savings. An alternative might be a pump unit
found in a refrigerator—many have used
these pumps with great success as they are
Vacuum Throttle Valve Trap Port Gauge
capable of pulling 20 inHg or more. Others
have had reasonable results using a Shop-Vac type vacuum for small projects. Another
alternative is the kits or packages that can be
purchased from specialty suppliers such as
ACP Composites.
Most importantly, whatever pump you
use should be designed for continuous duty,
like the Gast type, or equipped with a pressure switch that will cycle the pump on or
off as needed to maintain adequate vacuum
levels. This is important because vacuum
pressure will need to be applied until the
adhesive or resin has cured, which could be
several hours or days. Pumps not designed
for continuous duty could burn up prematurely if not cycled properly.
Pump selection should be based on the
size of your project. In my case, I was constructing assemblies made of flat veneers
that did not require a high volume (high
cfm) or high vacuum (high applied pressure)
pump. Small and flat panels consisting of
few layers of glass or flat veneers can be
accomplished using a fairly small pump— 1-2
cfm at 5-6 inHg ( 2. 5-3 psi). Large panels or
intricate molds (such as aircraft cowlings)
Vacuum Control Valve
Flexible Vacuum Hose
Vacuum Pump
Vacuum Bag
Breather Material
Perforated Film (opt.)
Release Fabric
Laminate Plies
Mastic Sealant
Mold
