This can be caused by using too much resin in the fiber, especially between layers.

Wrinkling may be due to a non optimized temperature history.

Raising the temperature causes a dramatic  (exponential) viscosity decrease. Meanwhile the crosslink reactions take place and at the gel point any resin flow can be prevented.

If the resin viscosity is too low and the resin is not reacting enough it can flow and accumulate towards less confined sites of the panel.

Chemo-rheology is a key word to find several papers around the aspects described above.

This can happen because of non-uniform distribution of resin between the layers, non-uniform temperature and pressure profiles and possible errors in the mold, especially for curved parts.

If there is a displacement (movement) of the bag (bleeder, breather and plastic cover) relatively to the laminate, wrinkling takes place, particularly in curved components. Instead of using a flexible plastic cover, if you use a rigid plastic cover, with the same shape of the component  being fabricted, avoiding the relative movement, the problem can be solved.

From my experience there are various mechanism which cause wrinkles, the one which I have studied is wrinkling caused by buckling of plies in when plies are out under compression when consolidated over complex geometries. See for example https://www.researchgate.net/publication/266488867_Out-of-plane_ply_wrinkling_defects_during_consolidation_over_an_external_radius

Article Out-of-plane ply wrinkling defects during consolidation over...

How do you repair the wrinkles to retain the structural integrity of the part?

Article Mechanisms of Origin and Classification of Out-Of-Plane Fibe...

Besides the vacuum bag method, fiber waviness / wrinkling is inherent to various manufacturing processes of fiber-reinforced composite parts. We recently published a review article on “Mechanisms of origin and classification of out-of-plane fiber waviness in composite materials – A review”. Numerous mechanisms of wrinkling have been analyzed, leading to several recommendations to prevent wrinkle formation not only during composite processing, but also at an earlier design stage, where generally several influence factors are defined. Additionally various types of waviness have been investigated and a classification scheme was developed for categorization purposes.

Dear Moslem Najafi,

Wrinkling may be due to a non optimized temperature history, different dimensions (single or non curved geometries), male-molded parts, de-bulking can cause wrinkles on the part requiring a cure and labor intensive.. Together with convex geometries, pad-up and joggle areas are double curved features that are known by the industry as a risk factor for out-of-plane fibre wrinkling. Raising the temperature causes a dramatic (exponential) viscosity decrease. If the resin viscosity is too low and the resin is not reacting enough it can flow and accumulate towards less confined sites of the panel. Such wrinkles can cause 40% knock-down on mechanical performance. Different types of out-of-plane fibre wrinkling that might occur when multistack forming a double curved geometry such as a pad-up or a joggle. The shear forces generated as a result of mismatches in the coefficient of thermal expansion of composite and tool, as well as the process of ply slippage that occurs during consolidation into radii. Wrinkling may be divided into two main groups; out-of-plane wrinkling and in-plane wrinkling. Out-of-plane wrinkling can, in most cases, be detected visually. In-plane fibre wrinkling might also occur however these wrinkles are often not detectable since they are only barely visible in the stack surface plies.

Ashish