Qt4.6: QTextDocument <HR> tag prints only very thin, almost invisible hair lines

When printing a QTextDocument with doc->print() I almost cannot see the horizontal rules inserted by <hr>开发者_运维知识库. When printing to PDF these are clearly visible. But when printed to a printer these lines are very very thin lines, almost invisible on the paper.

How do I fix this? I currently helped myself by inserting an <img> with a black pixel but this is very cumbersome as I have to exactly figure out the proper pixel width by trial and error.

<hr> tag supports the width attribute, which can be specified as an absolute or relative (%) value. E.g.:
<hr width=3>