A client asked how the flexibility of plastics could be tested. One of our engineers responded, and I thought it might be interesting to reproduce it here.
Better quality plastics are usually more flexible.
There is NO standard value for each type of plastic. Chinese manufacturers usually base their promises on what their resin manufacturer claims. Needless to say, there are all types of resins in China, from the very worst to very good ones.
The four tests listed below are useful for comparing those resins and the flexibility they give to the plastic.
1. Impact test: the plastic is hammered with a device. This test will tell how tough the plastic is. Higher toughness means “more difficult to be broken”.
Usually from this test we can also tell if it broke as a cut off, or if there was elongation before it broke. For certain applications, this is an important property to test.
2. Fatigue test: the plastics is flexed back and forth until its breaks. The higher the number of cycles, the better the plastic’s property.
3. Destructive pull test: this test will tell if the plastic breaks with longer elongation (better quality plastic) or a short elongation (lower quality).
4. UV resistance impact test: the plastics is placed a UV chamber, and then an impact test is performed (see No.1 above). What is interesting is the difference between the impact test results with and without UV exposure.
There are ASTM standards for testing these properties and manufactures should list the values of these properties for their materials. The values can then be compared for different materials and manufactures as you have stated.
However, from a quality standpoint in China, one of the biggest problems I have seen is contamination. Contaminants can reduce the properties of a material dramatically. Specifically, making it much less flexible than the pure material. This often results in parts breaking very easily.
To determine if a material is meeting its properties is a difficult process that requires producing a specific geometric specimen and testing it under precise conditions. We have found it much easier to break a piece of plastic off an existing part and inspect it under a microscope for contaminants. It is very easy to see contaminants such as fibers, metal pieces, and other foreign materials. This evidence is also easy to show a manufacture that they have a contamination problem.
Thanks Kurt for adding to the article. I have seen factories using 100% recycled plastic resin when their customer insisted on using 100% virgin resin, so I guess that might be issue No. 1. But contamination is definitely an issue — thanks for raising it!
I am talking about small suppliers here; big ones are a different league.
In my experience it’s useless to even talk to small suppliers about percentages of recycled material.
The best you’ll be able to accomplish would be a “yes, yes” then they’ll forget it when you turn your back.
The only thing that saves the day is a small simple and unquestionable test performed by both the producer and the internal QC (as Kurt above said).
I usually have them cut a rectangular shape piece from a fixed place of the product, where it is regular and smooth.
That is hanged horizontally by the border of a table, hanging out more or less 50% of its length.
Then I hang a known weight to the opposite end and see if the piece breaks or bends.
This at least prevents percentages too high of recycled material.
Interesting. Thanks for sharing this. Good idea.
With a bit of experience, it’s easy to see if virgin material was used by abusing a piece of plastic. But I like the objective and (hopefully) non-debatable test that you mention above.
Clean, unpainted, non-contaminated, recycled or reground material can be used in amazingly high percentages without affecting strength. Honda? (Carlos Castillo) published data showing that 90% unpainted regrind can be used without falling below the material specifications in an Impact strength test. The biggest problem in his case was painted regrind. Which used in percentages higher than 10% caused strength reductions below the material specs. But to insure no contaminants, we control the percentage of regrind.
Very interesting, thanks Kurt!