How to make the model surface close

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i use dynamic models to cut model into two seperate models , one of them is the blue picture upper,but the surface is open like the red rectangle in the picture , the opened surface can’t do 3d printer work . is it possible to use any extension or coding to make the opened surface close?

is use dynamic modeler make the [ Cap surface ] Option choosen ,the result is the picture below
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the closed surface is wired,what the doctor need is make the surface picture below closed
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