Are Chinese molds / products identical to US molds / products ?
I was reluctant to write this article and gave it much thought before I did. I spent several weeks evaluating the pros and cons. The current political debates go back and forth on the subject of import and export trade imbalances. The amount of jobs that it affects and the costs / savings also associated. But there is more to this story besides just cost. There is also part quality and durability. If you buy the same identical product in one country, is it the same from any other country?
Well I had the unique opportunity to build several molds in a few different countries all at the same time. There are serious pricing pressures to keep costs low and also volume requirements that kept shipping costs low as well. This drove building molds in multiple locations. This seemed simple enough and we expected little to no difference in product performance.
We started the project off and had 3 molds built in the US and 3 more in China. They were given identical product files and schedules. The US tools as to be expected were more expensive. I do not remember the exact costs and for this post, and cost was not the goal of this post.
The molds were built and sampled as typically expected and parts were sent to our office for review and approval. We went through our typical inspection procedures and went through a complete review of part weight, dimensional first article of inspection, and a tensile test to evaluate strength. We immediately noticed that the Chinese parts were consistently lighter. All 3 Chinese mold samples were all consistently lighter. The light went off as to ask what was going on.
We did a part inspection on all major dimensions and found that the US tools were all within .001 or so of nominal. All the Chinese tools produced parts .004 to .005 under nominal which was at the bottom of tolerance. Another light went off.
We put the sample on a tensile tester to test for product strength. We found that all the US parts pulled over 20 lbs while the Chinese parts only pulled at 12-14 lbs. That was a 40% reduction in strength on “identical” products. We had to fail all of the Chinese products and start evaluating how can products only a few .001ths be so much different.
Sitting around the lunch table, we discussed our findings with the rest of our team. We rain a few numbers on paper and finally discovered what is going on. It was all about saving material! Everyone knows that China has lower labor rates which inherently means that the material costs both for molds and for molded parts are by percentage higher. We we ran a few numbers. We did an experiment using a 1 inch cube. 1 inch x 1 inch x 1 inch = 1 inch ^3. Simple enough. Then we used our .005 tolerance and ran .995 inch x .995 inch x .995 inch = .985 inch ^3. That is only a 1.5% decrease in material. That did not seem like much at all. Where did the 40% strength reduction come from?
Well as it turned out, our product was long and thin. It did not have the same volume / size ration. We we changed our thought process. we then went with the 10.000 x 1.000 x .100 shape and resulted in 1.000 inch ^3 again. We applied the same tolerances of .005 and got 9.995 inch x .995 inch x .095 inch = .945 inch ^3 or a 5.5% reduction in weight. Also considering that the thinner wall was slightly harder to pack out, this may have increased it another few % points. We used our new dimensions to calculate the new expected strength of the product and found that it was weaker but not by the 40% we measured. Now we were stumped again. We knew that the polypropylene we required was 220,000 modulus. In order for the part to be that week, the modulus would have to be below 170,000.
We sent back to China the inspection report to China with our findings and after all the data was reviewed, they admitted that they substituted the 220,000 higher cost PP with a lower cost 168,000 PP that they had in stock. So from then on, we continued to test for strength and for stiffness using this method. We also found other products that had material substitution on. Moral of the story is that we had to check every shipment, every mold, every country from then on. This tensile test method proved very effective on catching many substitutions. It would not tell us what the material really was, but it would tell is that it was not correct. It saves lots of time and did not require much more expensive chemical testing to prove what the material actually was.