A knit line is any line, visible or not, where two resin flows meet. Depending on the design of the mold and the material being injected, a knit line may present no problem at all, may be a cosmetic issue, or may cause a serious structural problem.
Good article. In many electromechanical assemblies, Agency requirements can matter also. A polymer may be the perfect match for the strength, finish, etc. needed for the product. However, that particular grade of polymer may not have the flame retardant properties needed to pass 94V-0 at the specified thickness in order to achieve agency certification for the entire product. It is important to also consider agency requirements up front during product development to make sure the entire product can be certified.
A spacecraft on its way to Mars is carrying core structures made with carbon-fiber composites. Launched November 18, NASA's Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft is the first designed for exploring and understanding the red planet's upper atmosphere.