Carbon fiber precursor acrylic fiber bundle, method for thermally oxidizing part thereof, thermal oxidation oven, and process for producing carbon fiber bundle

A carbon fiber precursor acrylic fiber bundle comprising a plurality of high density parts as a portion thereof, wherein each high density part has a preceding density increase start point and a trailing density increase start point, wherein the high density parts satisfy requirements A and B:

• A:
• the high density parts have a maximum fiber density ρmax of 1.33 g/cm3 or higher, and
• B:
• an increase in fiber density between an intermediate density point and a maximum density region arrival point is 1.3×10−2 g/cm3 or less per 10 mm of a fiber bundle length;
• with the proviso that,
• the term “intermediate density point” is defined as the site which has a density ρm (=(ρ0+ρmax)/2) that is intermediate between the fiber density ρ0 of a non-high density part and the maximum fiber density ρmax,
• the term “maximum density region arrival point” is defined as the site Pr at which the increase in fiber density per 10 mm of the fiber bundle length becomes 1.0×10−3 g/cm3 or less, the increase in fiber density being represented by (ρr+1−ρr)/5 determined from a measurement in which density measurement points beginning with the density increase start point and located at intervals of 50 mm (P1, P2, . . . , Pr, Pr+1, . . . , Pn) are successively examined for fiber density (ρ1, ρ2, . . . , ρr, . . . , ρn), and
• the term “density increase start point” is defined as a site at which the fiber density is higher by 0.01 g/cm3 than the fiber density ρ0 of the non-high density part.