Quality control is always a concern when working with an overseas vendor, and I can only imagine your frustration with the samples you have received. My advice having sourced from China and many other countries is that you need a person on the ground to make sure your expectations and specs are being met, so see if you can find an agent (not associated with the factory) to work on your behalf to save the time, cost and hassle of having to fly overseas. That said, unless you are at a scale where it makes sense, I have found over the years that it ends up usually being cheaper in the long run and much more efficient to produce domestically if you can. While overseas pricing can seem a lot cheaper on the front-end, if you run into issue (like you have), the savings end up evaporating. Hope this helps, and happy to jump on a call to talk in more detail.