I have seen plenty of YouTube videos in which they use carbon fiber fabric to build various things. One such channel is Mike's Patey's channel where he makes amazing aircrafts from carbon fiber.

In these videos I usually see (but not always) that after they apply the resin to the fiber fabric, they put the structure in a kind of plastic bag and they vacuum pump it.

Why is that process? Is it hardening and giving some extra rigidity to the fiber in any way? Can this process be done by the use of pressure? (eg. a kind of mold that pressurizes the fiber from the outside so that the fiber also takes the correct shape).

  • $\begingroup$ Not quite sure of why my question got down-voted. If there is a reason, please let me know. It is always nice if we can correct our-selves. $\endgroup$ Commented May 12, 2020 at 11:01

2 Answers 2


Solar Mike's answer is accurate.

Carbon fiber has a resin to fiber ratio which provides the optimum strength. This is typically measured by weight. The amount of resin is applied to the fiber prior to enclosing it for vacuum application.

Once the vacuum begins, all of the air is removed from the fiber, forcing the resin into the voids, ensuring the strength aspect.

The pressure aspect will not remove the air from the enclosure containing the fiber. The voids within are going to be smaller but not non-existent. Another reason for vacuum bagging as a system is that it is not restricted to any specific container. Sufficient plastic and sealing will allow fiber constructions of immense size.

  • $\begingroup$ Have to vote for this, not because you are complimentary about my answer, but for the extra detail. $\endgroup$
    – Solar Mike
    Commented May 10, 2020 at 11:20
  • $\begingroup$ "Once the vacuum begins, all of the air is removed from the fiber, forcing the resin into the voids ..." I suspect that air is forced into the voids when after evacuation as air is let back in (or, as someone once explained to me, "as the vacuum is let out"). While evacuated there is no pressure to force anything anywhere other than air bubbles expanding and pushing resin out of the voids.. $\endgroup$
    – Transistor
    Commented May 10, 2020 at 11:23
  • $\begingroup$ @Transistor, while evacuated, there is one bar / one atmosphere / 14.7 pounds per square inch of pressure on the assembly. One would not use a solid vacuum enclosure, because the resin would cure before one might remove/release the vacuum. $\endgroup$
    – fred_dot_u
    Commented May 10, 2020 at 11:31
  • $\begingroup$ I'm not familiar with the process. Is it done in a chamber or in a bag? I've seen silicone moulding use vacuum: the liquid is poured around the part from which the mould is being made, it's popped into the vacuum chamber, pumped as low as the pressure will go and then released so that the liquid is forced into the voids. How is it different for carbon fibre? $\endgroup$
    – Transistor
    Commented May 10, 2020 at 11:47
  • $\begingroup$ Vacuum bagging of composites, not restricted to carbon fiber, is done by securing plastic around the entire assembly. Sometimes, it can be taped to a surface or fully enclosed in a flexible plastic bag. A solid enclosure such as that used for silicone molding and casting will not work. $\endgroup$
    – fred_dot_u
    Commented May 10, 2020 at 11:57

There are two reasons:

One is to remove trapped air bubbles


two to pack the fibres as densely as possible within the structure or weave of the fibre pattern wanted.


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