It is an awkward construction as a compound sentence. It should be changed into several sentences of far simpler construction, like this: "In addition, the stress difference is calculated before and after retracting the mandrel. The stress magnitude is calculated before retracting the mandrel and after retracting the mandrel. Retracting the mandrel indicates [or shows--BB] that the reduction of the stress magnitude is facilitated as the core number increases. The springback angles shown in Table 1 indicate increase as the core number increases. [The difficulty in the original sentence is cause by trying to articulate several distinctly different ideas in a single sentence. Technical writing, in particular, will always benefit from reductions to simple sentences. Learn to perform this simplification, and you will have much less difficulty.]

The sentence is a run-on and too long. Correction: "In addition, the stress difference before retracting mandrel and after is calculated and becomes larger with the increase of the core number. Thus, the reduction extent of springback angle in Table 1 increases with the number of cores."