The Harold Moore range of metal detectable products are manufactured from polypropylene with a carefully formulated additive and the finished products have been approved for direct food contact under EU90/128. Red, green and yellow colours have recently been introduced in addition to the standard blue. One of the most common technical enquiries that we receive regarding our HM Metal Detectable Range is, “how much of your material is detectable?” The answer to this question is not as straight forward as it might first appear. Every food manufacturing plant that has a metal detector has it calibrated in a different way and set at varying levels of intensity. The type of matter going through the detector (particularly whether wet or dry), its size, speed and orientation also have an effect on the amount of material that can be detected. However, it is standard practice to calibrate detectors using ferrous ball bearing of known diameter. To assist in calibrating metal detection machines, Nayla have available a set of detectable plastic spheres and cubes of varying sizes. The set includes the following sizes (see picture): 8 mm cube, 6 mm cube, 4 mm cube, 2 mm cube, 20 x 10 x 2 mm plaque, 3 mm sphere, 6 mm sphere, and 10 mm sphere. The supplier of our HM Metal Detectable Range carried out tests using these shapes to find approximate equivalent ferrous calibrations. Two states were simulated – wet and dry. The minimum machine settings were used in each case where the detectable shapes achieved detection. Ferrous ball bearings were then passed through until the smallest was found that achieved the same reading on the detector. The table outlines the results obtained. The colour of the material does not affect the results.

Typical dry material setting

Typical wet material setting

As an example, the results imply the following:  If a factory runs an in-line metal detector for a dry food product, such as biscuits, set at 300 kHz at 0° and sensitive enough that a 0.8mm diameter ferrous ball bearing will trigger an alarm, then a 4 mm cube (or 0.06 g) of HM’s metal detectable plastic may be needed to set off the alarm. 

>= 4 mm actual size 

Another factory running a similar detection system set for a wet product, such as a microwave meal, set at 25 kHz at 90° and sensitive enough that a 0.8 mm ball bearing will trigger an alarm, should detect a 2 mm cube (or 0.01 g) of material. 

= 2 mm actual size 

However, since every plant uses different machines at different settings, we recommend that quality control staff carry out their own calibration test to ascertain their own requirements. The metal detectable test shapes described above are available now from Nayla Pty Limited for this purpose.