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Mixing speed for two-part composite resins

Aviation Mechanic Airframe (AMA) practice question 20 of 30 on Non-Metallic Structures. ACS element AM.II.B.R5 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONWhen a two-part epoxy resin is being prepared for a wet layup repair, why must it be stirred slowly rather than rapidly?

  1. AFast mixing puts air into the mixture
  2. BFast mixing raises the fiber-to-resin ratio
  3. CFast mixing removes the dye from resin
Show the answer and explanation

Correct answer: A. Fast mixing puts air into the mixture

  • A — correct Air enters the mixture if the resin is mixed too fast, which is why it is mixed slowly and fully for three minutes.
  • B — incorrect The fiber to resin ratio is set by how much resin is applied to the fabric, not by the speed at which the resin is stirred.
  • C — incorrect A dye is added to some systems to show how well the material is mixed, and stirring does not remove it.

BACKGROUND

Epoxy resins, like all multipart materials, must be thoroughly mixed. Some resin systems have a dye added to make it easier to see how well the material is mixed, but because many systems do not, the resin must be mixed slowly and fully for three minutes. Air enters the mixture if the resin is mixed too fast, and a system that is not fully mixed may not cure properly. The edges and bottom of the mixing cup must be scraped so that all resin is mixed correctly. Large quantities of quick curing resin should not be mixed, because these resins produce heat after mixing and smoke from an overheating batch can burn or poison the technician.

STUDY TIP

Remember the three-minute slow mix and the scraping of the cup walls, since incomplete mixing and entrapped air are the two defects that mixing technique is meant to prevent.

KEY POINTS

  • Resin is mixed slowly and fully for three minutes.
  • Mixing too fast introduces air into the resin.
  • Only the amount needed should be mixed, in batches, to avoid overheating.

SOURCES

  • ama-airframe/ch07.txt "Mixing Resins"
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-31B Aviation Maintenance Technician Handbook – Airframe, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.