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Finding your perfect X-ray system

25 March 2018

There are several factors to take into account when selecting an X-ray inspection system, says Ciaran Murphy

The consequences of an unwanted foreign object in a food product can be far-reaching – the costs of a product recall, for example, and the possibility of a fine from the retailer.

Investment in an appropriate quality inspection system will, therefore, be money well spent but it still makes sense to take the time to ensure that the most suitable solution for a particular application is chosen – in terms of both performance and cost.  

X-ray technology is increasingly being adopted for product and pack inspection thanks to its versatility, when compared with more traditional quality checks such as metal detectors. X-ray inspection systems are able to look for a wide array of foreign bodies, including metal, dense plastics and rubbers, glass and stone. In addition, they can also carry out a variety of additional quality checks, such as missing, undersized and broken items, deformed product and packaging, under-filled compartments, the presence of cracks and fissures in products, and weight estimation.

Initial considerations
Two important initial considerations are the characteristics of the product and its packaging, and where the machine needs to be placed on the line.  Unpacked products such as raw materials for further processing, or meat and poultry, salad, vegetables and nuts may need to be screened at the very start.  Finished, packed product will be inspected much further down the line. Here, the placement of the X-ray inspection system may depend on whether the machine is required to assess primary packs or check for missing packs in a transit case.

In terms of the product and its packaging, the overall size will be important in determining the choice of machine. At Ishida, for example, the range of X-ray inspection systems includes a model with a chamber height of up to 390mm, able to handle large size items such as 25kg blocks of cheese or butter.  

In addition, while most X-ray systems operate from the top down, sending an X-ray beam through a product vertically, for taller pack formats, such as bottles, cartons and tubes, side-beam X-ray models are available which project the beam horizontally. As well as foreign bodies, this also gives the ability to check for the correct fill level and that the closure is in place.  

The characteristics of the product and the type of foreign bodies to be detected – or other quality checks that need to be performed such as missing items or deformed product – are critical in selecting an x-ray system with the appropriate level of sensitivity. X-ray inspection involves the projection of relatively low energy X-rays onto a sensor or detector. As the product or pack passes through the X-ray beam, only the residual energy reaches the sensor. Measurement of the difference in the absorption level of the X-ray energy between the product and a foreign body enables the foreign body to be detected.

The sensor within an X-ray machines is made up of small pixels similar to how a digital camera operates. The more and smaller the pixels, the better the resolution of the X-ray image that indicates the place of a foreign body. Some X-ray machines with smaller pixels will be better suited to identifying softer foreign bodies and at smaller sizes.  

Different X-ray machines will also have different levels of sensitivity. Entry levels systems, such as the Ishida IX-EN model, can detect foreign bodies with a reasonably high density.  Softer or less dense items like glass or rubbers will require machines with greater sensitivity.

Ishida X-ray inspection systems incorporate genetic algorithm software that is able to analyse image data over a number of generations to achieve an extremely high level of accuracy. Since many applications will require the detection of similar foreign bodies, the system can be optimised to look for these objects and help to eliminate recurring sources of contamination.

Generally speaking, the further down the line the inspection takes place, the more challenging the detection process becomes. The type of packaging may, therefore, also be a factor in the selection of the machine.  

However, some bulk and unpacked product also have inspection challenges. Meat and poultry, for example, a typical quality inspection requirement is the detection of unwanted bones in fillets. Because there is a relatively small difference in density between the bone and the meat, the bone can be particularly difficult to spot. 

This has led to the development of dual energy X-ray systems, which have two line sensors, one which takes images at high energy and one at low energy. This provides a better overall image of the product with a clearer contrast between the product and the bone or other foreign bodies.

In some instances, it may be necessary to install two separate machines at different points on the line to inspect bulk product prior to packaging as well as sealed packs; or to detect foreign bodies in sealed packs and then ensure that the correct number of packs is in each delivery case.   

The type of reject system is another important consideration. Typical choices available include arm, air, pusher and drop belts. The final selection should depend on the size and weight of the product and the speed of the line.  Many retailers also require confirmation that bad products have been effectively rejected and have not continued down the line.  

For bulk product, drop belts are a common means to remove product. An air system will mean that less product is lost but is more costly so may only be appropriate for higher value items.  

With any investment it is important to consider the total cost of ownership in terms of the initial purchase price of the machine and ongoing running and servicing costs, so never ignore this element when making your choice.

Ciaran Murphy is business manager EMEA – quality inspection control at Ishida.

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