Vacuum Bagging Equipment
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. Molds and mold building are discussed in Section 3.
|Figure 2-1 Typical components of a vacuum bagging system.|
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'x8' panel, the same 20" Hg (10 psi) will yield over 46,000 pounds of clamping force spread evenly over the entire panel.
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.
|Figure 2-2 A typical vacuum pump capacity vs vacuum rating diagram. Note that the free air flow decreases as the vacuum pressure level increases.|
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 2CFM 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.
|Figure 2-3 A Gast Model 07061-40, 1/8 hp diaphragm pump. This pump displaces 1.2 CFM and will achieve a
maximum vacuum pressure of 24.0" Hg. It is a practical pump for small projects.|
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. If you find a used pump that you think will work for vacuum bagging, the vacuum and displacement ratings will give you an idea of the range of vacuum bagging you can do with it. If you are unsure about the pump, you can go through a dry run, following the procedures in this manual, to test the limitations of the pump. Keep in mind that the pump
should be able to hold a vacuum continuously until the adhesive reaches an effective cure, which may take as long as 8 to 12 hours depending on the hardener used and ambient temperature.
See Section 5.3 for cure time information. See Appendix C for a list of vacuum bagging equipment and material suppliers.
2.2 Vacuum bagging materials
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. See Appendix C for a list of vacuum bagging equipment and material suppliers.
2.2.5 Mastic sealant
Mastic is used to provide a continuous " target="_blank">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. WEST SYSTEM 883 Vacuum Bag Sealant is a ½" by 3/32" flexible adhesive strip that peels easily from the mold after use.
Generally, the better the airtight seal between the mold and bag material, the smaller the pump you'll need. Poor seals, or material which allows air leaks, will require a larger capacity pump to maintain satisfactory vacuum pressure.
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.
A control 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 b