When the plant capacity utilize the 100% , to expansion ability of plant, you have limited sources to do so , in this situation what should be done?

Look into incorporating efficiencies within your capacity.
There are always gap in efficiencies and you should be able to highlight them if you look closely.
Also, you can think about working more than one shift.

In such a situation we should use cost controling techniques, in such a way that the plant is used more efficiently, e.g. labor and material being used should be used with more efficient way. And as a result costs on material and labor will reduce which will lead to more profitability.