If positive material identification is important to you – and it should be – stay tuned and we’ll show you the right way to check and verify plastic resins. 

Hello, this is Gordon Styles, founder and CEO of Star Rapid. I’ve been involved in rapid prototyping and new product development for more than 37 years, and I’m happy to welcome you to another episode of Serious Engineering for serious engineers. 

On the table in front of me are seven black plastic boxes. Even to the practiced eye, each of these samples looks pretty much the same. But they are in fact different materials, each with its own unique chemical and physical properties. So, today’s challenge is to find out which is which, and we’re going to do so by exploring how positive material identification works. 

Now of course every product developer needs to be sure they are using exactly the material they specified for plastic injection molding. This affects more than just a part’s performance, durability, and cost. There are also legal requirements, such as with RoHS regulations in Europe, to demonstrate that plastic products don’t contain hazardous substances like cadmium or mercury. 

Given the importance of material verification, we’re amazed that PMI is still sometimes overlooked by many suppliers, or it’s done in a slipshod fashion. This might be because doing it right takes more time and effort, requires a skilled and well-trained staff, and it means a pretty substantial investment in equipment. 

In fact, before the advent of scientific metrology, there were other ways experienced molders had for identifying resins, but they were a bit haphazard. 

For example, one of the most common was the melt flow test, where a sample of material was heated to the melting point and the rate of flow was measured. You could even burn some resin and smell the tell-tale fumes that were produced. Clearly, we wouldn’t advise this for safety reasons as well as for accuracy. 

We still perform drop and impact tests, scratch tests, and we can measure specific gravity and density. Again, all of these methods can give you useful information, but they are not always practical, they take a long time to set-up and perform, and they can be destructive.  

That’s why when you need reliable and repeatable results for testing plastic resins in a modern manufacturing environment, there is no substitute for performing routine chemical and physical analysis using scientific instruments. Fortunately, there are some inherent characteristics of all physical elements that can be clevery exploited by sophisticated test equipment to yield accurate results.  

Let’s take a look at three of the most common methods of positive material identification for plastic resins, their advantages and drawbacks. 

X-Ray Fluorescence 

In this process, high-energy x-rays or gamma rays are bombarded onto the target surface. These rays interact with the electrons in the outermost orbit of the molecules of the sample, causing them to fluoresce, or emit a photon of energy. This photon has an energy signature that’s unique for each element in nature. 

Among the benefits of XRF is that it’s portable and non-destructive, so it can be used on finished parts or in the field with minimal sample preparation. However, it’s not ideal for providing exact chemical compositions in resins so we normally use it to check for the presence of banned substances. Note that the presence of x-rays could make it hazardous for long-term exposure. 

Laser Spectrometry 

Here a monochromatic laser beam of known frequency and energy is emitted, such as with this Polymax testing gun. A large percentage of this light will get reflected, unchanged, back to the detector. However, some of the light interacts with the molecules of the sample. When it does so, the light changes its energy state and produces a unique waveform that is characteristic for that material.  

The analyzer then compares this waveform to its internal database of known materials, yielding results of approximately 70% accuracy. This is normally good enough for simple field testing when you don’t want to damage the part. The reason we can’t guarantee 100% reliability is because the detector can be blocked by dirt on the sample or the lens, or due to incidental light leaking in from the ambient environment. 

Note that the database must be periodically updated when new formulations of material come along, using calibrated samples provided by the material manufacturer and tested in independent laboratories.  

Also, as with the XRF method, it helps to do multiple tests in a row. This is done to establish both the accuracy of the reading and its precision, or repeatability.  

Fourier Transform Infrared Spectroscopy 

Let’s just call it FTIR for short. We use this device when we want a much higher degree of accuracy and precision. Our Bruker Alpha FTIR, for example can routinely produce results of 98% accuracy or better against a database of known chemical spectra.  

It works on the principle that most molecules absorb light in the infrared, and convert this into molecular vibration measurable by its wavelength. Therefore, the instrument puts out infrared radiation and analyzes the resulting wavelength pattern. The beauty of the Alpha is that it transmits in many frequencies and does so very quickly. This means it can check all of the spectral data at once, rather than one frequency at a time which was common in the past. 

We use the Bruker to analyze golden samples of plastic resin pellets provided by the resin formulator. The resulting analysis then becomes a reference standard that we add to the machine’s memory. The Bruker Alpha is not especially portable, so we keep it only for laboratory use. 

Now, back to our little black boxes. What does the Polymax say?  

Now you can see why you shouldn’t settle for less than positive material identification on all of your projects. If you’ve found this to be useful information, please share it with others. Also give us a like, ding the bell, and stay tuned for the next episode when we discuss testing for metals.  See you there.