Plastic is one of the most common materials in use today, with the unfortunate result that unwanted plastic contaminants end up inside of packaged goods. Plastic contaminants range from gaskets and o-rings to scraper blades, or even pens and safety glasses. X-ray detection has proven to be one of the most reliable ways of detecting metal, glass, bones, stones and other foreign material present in food packaging – but how well can X-ray detect plastic? The answer isn’t so clear cut. No X-ray system can detect all plastics. However, Peco InspX’ X-ray systems are designed to detect higher density plastics. In order to understand why all plastic can’t be detected and how to choose the best X-ray system to find plastic contaminants in your package, we’ll need to understand how X-ray systems work.
How Does X-Ray Detection Work?
X-ray systems for product inspection operate similarly to other X-ray systems you may be familiar with, such as those in hospitals or at airport security. A beam of X-rays is produced using an X-ray generator and is projected towards the object being inspected. The density and composition of the object determines how much of the X-ray is absorbed. In our analogy, as the X-ray passes through the package being inspected, shadows are cast based on different densities. Whatever “shadows” (X-rays) that pass through are then captured by a detector array, which renders a 2D representation of the object. The software then analyzes this image and looks for deviations (or defects) – the denser the contaminant, the more contrast appears and the easier it is to detect.
Given that that contaminant density is one of the biggest factors in successful detection, it is useful to compare the densities of various common materials. Water has a density of 1000 kg/m3, gravel has a density of 1250 kg/m3, glass has a density of 2579 kg/m3, and stainless steel has a density of 7600 kg/m3. Differentiating between these materials is very straightforward.
In contrast, Plastics have a range of densities– with most of them close to the density of water. Polyethylene (PET) has a density of 750 kg/m3, low-density polyethylene (LDPE) has a density of 920 kg/m3, high-density Polyethylene (HDPE) has a density of 950 kg/m3, polycarbonate (PC) has a density of 1200 kg/m3, and acetal has a density of 1410 kg/m3. A few higher density plastics exist, such as PTFE (Teflon) at 2200 kg/m3 and Viton at 1800 kg/m3. As expected in food inspection with X-Ray systems, the closer the contaminant’s density is to water, the harder it will be to detect.
Increase Your Odds of Detection
If the plastic contaminants you are attempting to find are too close in density to your product and you are unable to reliably detect them using an X-ray inspection system, analyze your process to determine if there are any opportunities to substitute for high density plastics, such as PTFE or Viton. If possible, avoid plastics similar in density to water such as HDPE, UHMW, and LDPE. Additionally a new group of X-ray detectable plastics have entered the market such as Sustarin C® (which is a replacement for acetal) or Polystone® (which is a replacement for UHMW). Specialty manufacturers of common manufacturing contaminants such as “band-aids” and protective gloves offer X-ray detectable alternatives. Another approach to improve the detectability of plastic contaminants is to utilize an X-ray system with triple beam architecture. These systems split the X-ray source into three beams, which allows for three different perspectives of a contaminant, thereby eliminating any blind spots and increasing the chance of detection.
Whether you are using a single beam or triple beam X-ray inspection system, best practice techniques will ensure the best chance of detecting contaminants:
- Inspect across the minimum thickness of your product.The less material the X-ray passes through, the more contrast you’ll see from a contaminant.
- Present the product with as much consistency as possible. When inspecting a bulk product like nuts or dried fruit, consider integrating a vibratory table to more evenly distribute the contents, thereby increasing the contrast from a contaminant and increasing the odds of detection.
The best way to determine which plastics are detectable in your product is to send in samples to our state-of-the-art Testing Lab in Burlingame, California.