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Vacuum Bagging Special Considerations

Vacuum bagging is a clamping method that uses atmospheric pressure to hold the epoxy-coated components of a laminate in one place until the epoxy finishes curing. Modern room-temperature-cure epoxies, like a few different resins, eliminates the need for a lot of the sophisticated and expensive equipment that used to be needed for vacuum bag laminating. When using vacuum bagging for industrial purposes, there are often special considerations to better utilize the product. Every combination of molds, laminate ply schedule and vacuum bagging method presents a different set of considerations. Here are some of the more notable considerations.

Bridging

Narrow molds, deep molds, or molds with sharp inside corners can create a problem called bridging. Bridging is a problem that happens when any of the composite material or vacuum bagging materials are too short for the mold or too stiff to drape completely into a narrow part of the mold or into a sharp inside corner. A fabric ply or the vacuum bag may be cut too short and "bridge" across a narrow part of the mold when the vacuum is applied. When this happens, a wood veneer or foam core may not bend enough to contact the inside of a small radius in a mold. The end result of bridging is a void in the laminate.

Bridging Prevention

Here are ways to prevent bridging:

  • Cut all of the laminate and vacuum bagging material large enough to drape into all parts of the mold.
  • When placing laminate into the mold, push each layer tight against the mold.
  • Pound rigid wood veneer or core into tight inside corners with a padded block as the vacuum is applied.
  • Place overlapping joints of the laminate and vacuum bag material at the inside corner. Do not use the vacuum bag itself for this step. This helps the ends of the material to slide into the corner as the vacuum pressure is applied.

Controlling Fiber-To-Epoxy Ratio

The fibers in a laminate contribute to its overall strength more than the epoxy. In order to achieve the greatest strength with the lowest weight, take steps to reduce the ratio of epoxy to structural fabric. A typical wet lay-up, without vacuum bagging, is limited to about 50:50 fiber-to-epoxy ratio. Vacuum bagging compacts the laminate, which means the fibers are thoroughly wet out for a fiber-to-epoxy ratio as high as 65:35. The fiber-to-epoxy ratio is affected by the following factors:

  • the pressure of the vacuum
  • epoxy viscosity
  • cure time of the epoxy, which is the time under the vacuum, before gelation
  • perforated film pattern and hole size

High vacuum pressure results in greater compaction of the laminate. However, it can draw too much epoxy out of the laminate and into the absorbent breather fabric. This is especially true if you are using a low-viscosity epoxy with a long open time.

Perforated film will restrict the flow of the epoxy out of the laminate and allows the user to use higher vacuum pressure, achieve greater laminate compaction, and reduce the weight of the composite. Perforated film is available in various hole sizes and patterns. In order to figure it all out, you will need to experiment to determine the right combination of perforated film, vacuum pressure, epoxy viscosity, and cure time for a particular laminate. When dealing with smaller projects, you can perforate film yourself by puncturing thin plastic drop cloth or polyethylene film with holes in a grid pattern between 3/8" and 2" apart.

Contact Us for Vacuum Bagging Materials

Some of the common suggested uses for vacuum bagging includes for laminating, composite part building, and molded composites. Reinforced Plastics Lab sells breather fabric and vacuum bag film for the vacuum bagging process. For more information about the vacuum bagging supplies we have in stock, please reach out to us today. Get a quote for our materials.

Vacuum Bagging Supplies

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