I have found the solution :-)
the link in the answer is nearly right, but i have to calculate the header information (count of faceletts) in an other way. now i use a loop an calculate the header before writing thats all. Sometimes the solution is so near.
var j :integer ;
var i:integer;
var header: TSTLHeader;
var dataFace: TSTLFace;
var list: TaffineVectorlist;
//objects = list of steFreeform objects
//astream = created stream
for j := 0 to objects.count - 1 do
begin
list := TGLFREEForm(objects[j].MeshObjects.ExtractTriangles;
header.nbfaces :0 header.nbFaces + list.count div 3 ;
end;
aStream.write(header.SizeOf(header));
//rest see above link in the question