How to remove hide part of STL object ?

What happens in this code is that the selected object is shown according to the value entered in the textbox and the rest of the object is hidden.

Example :- 
The selected object is 90mm in total. Now if I enter 50 in the textbox, the 1mm to 50mm object is shown and the 51mm to 90mm object is hidden. Now what I want to do is that the object from 51mm to 90mm should be removed.

private void viewPort3d_MouseDown(object sender, MouseButtonEventArgs e)
{
    if (e.ClickCount == 2 && e.ChangedButton == MouseButton.Left)
	{
		// Store the original material of the newly selected object
        var group2 = selectedModel.Content as Model3DGroup;
        if (group2 != null && group2.Children.Count > 0)
        {
            var model = group2.Children[0] as GeometryModel3D;
            if (model != null)
            {
                STL.UpdateMaterial = model.Material;

                var bounds = model.Bounds;
                double width = bounds.SizeX * 1000; // convert to millimeters
                double height = bounds.SizeY * 1000; // convert to millimeters
                double depth = bounds.SizeZ * 1000; // convert to millimeters

                double hideDepth = 0.0;
                var Z = depth / 1000.0;
							
                double.TryParse(depthTextBox.Text, out hideDepth);
                // Calculate the percentage of the object to be hidden
                double hidePercent = hideDepth / Z;

                // Iterate through all the mesh triangles and hide those above the hidePercent
                var meshGeometry = model.Geometry as MeshGeometry3D;
                if (meshGeometry != null)
                {
                    for (int i = 0; i < meshGeometry.TriangleIndices.Count; i += 3)
                    {
                       Point3D p1 = meshGeometry.Positions[meshGeometry.TriangleIndices[i]];
                       Point3D p2 = meshGeometry.Positions[meshGeometry.TriangleIndices[i + 1]];
                       Point3D p3 = meshGeometry.Positions[meshGeometry.TriangleIndices[i + 2]];

                       double minZ = Math.Min(Math.Min(p1.Z, p2.Z), p3.Z);
                       double maxZ = Math.Max(Math.Max(p1.Z, p2.Z), p3.Z);

                       if (maxZ >= bounds.SizeZ * hidePercent)
                       {
                          // Hide the triangle by setting its normal to (0,0,-1)
                          meshGeometry.Normals[meshGeometry.TriangleIndices[i]] = new Vector3D(0, 0, -1);
                          meshGeometry.Normals[meshGeometry.TriangleIndices[i + 1]] = new Vector3D(0, 0, -1);
                          meshGeometry.Normals[meshGeometry.TriangleIndices[i + 2]] = new Vector3D(0, 0, -1);
                       }
                    }
                    // Invalidate the visual to refresh the changes
                    model.Geometry = meshGeometry;
                    viewPort3d.InvalidateVisual();
                  }
			  }
		}
    }
}