Avoiding metal hazards in finished footwear
Considering the issue of metal being left in finished footwear and how detection is a vital part of quality control.
by David Smith
An important consumer safety consideration is that footwear must not contain any undesirable elements which could present a hazard or risk to the wearer. This includes the possibility of metallic items left in the footwear from manufacture. The primary responsibility lies with the producer.
The ‘producer’ may be i) the manufacturer, ii) any company that represents itself as the manufacturer by putting its name on the product, iii) any person who repairs or reconditions the product, or iv) anyone in the supply chain whose activities affect the safety of the product. Retailers compel their suppliers (which may also be the producers) to ensure that potentially injurious metal objects are eliminated from their footwear – throughout the supply chain. Many retailers set out manufacturing codes of practice that specify procedures to guarantee a consistently safe product, and may levy severe penalties on suppliers and/or producers who supply goods containing metal pieces.
One of the main sources of potential risk in this regard is broken stitching needles. This is because it is impossible to entirely eliminate needles from the manufacturing process for the majority of footwear constructions. Where needles are in use, a needle control policy must be strictly enforced and monitored. Even with such controls in place, broken needles may still slip through the system and become stuck in finished footwear, where they present a latent hazard.
Tacks and staples (rather than cement or adhesive pads) are used by some manufacturers for lasting and bottoming operations. These too may find their way into finished footwear if strict controls are not enforced. Injuries may also result from other sharp metal objects, such as the sharp edges of poorly formed rivets or metal eyelets. It is clearly preferable to not use staples, tacks and needles at all, rather than trying to detect the presence of errant or misplaced items, but this is not always possible.
When metallic elements such as shanks will be present later in the manufacturing process, conveyor-belt systems with metal detection units are commonly employed to ensure that closed uppers do not proceed through the production process without first being checked.
Some machines detect all types of metals, including brass and copper. These systems are used where the product does not contain any metallic accessories, such as shanks, zips and rivets. When metallic accessories are present in footwear, it is necessary to identify needle or metal contamination among legitimate metallic items. This requires the use of a machine that can detect and differentiate ferrous metal fragments.
Metal detection must be used in conjunction with systems of control within the factory. As such, it represents the last line of defence – used at the end of the upper assembly operations – to confirm that no closed upper contains metal items or fragments. Metal detection is also used at the end of the production line for footwear supposed to be metal-free (such as children’s footwear) as a final check before the footwear is packed.
Metal detection is not an infallible solution and, due to a number of factors, machines can occasionally give false positive results or fail to detect some metallic items. The choice and operation of metal detectors requires some knowledge and forethought to obtain an appropriate level of detection for the type of footwear produced, and to ensure that once purchased, the machines are correctly calibrated and maintained.
As mentioned, in order to avoid metal contamination in finished footwear, factory controls need to be in place – carefully documented and rigorously enforced. Many potential sources can be eliminated through best practice and careful housekeeping. Factory controls usually focus primarily on stitching needle control, but often extend to cover any small metallic items present in the factory (such as hand tools). Some factories may specify that small hand tools such as scissors should be securely fastened to benches or workstations, unless these tools are issued at the start of each working day and recalled at the end.
A key first step in metal control is to make an inventory of all the metal items – both components and tools – commonly used or encountered along the production line, so that staff are aware of all the potential sources of contamination. Rivets, studs and poppers need to be tightly controlled so that stray items do not become incorporated in finished footwear. Simple and sensible precautions, such as fitting a lip to any bench where small metal items are handled, go a long way towards controlling their use.
There may be a 'metal-free' area around production areas. This strategy, as well as a tight needle control policy and excluding such obvious dangers as staples and pins from the production line, could even extend to the banning of tacks and metal fixings from notice boards in the vicinity.
As the control of stitching machine needles is the biggest issue for many factories, only the needles currently in use for the manufacturing process should be on the factory floor. In addition, they should only be in use for the job for which they were intended – not being used as picks or small knives. Spare needles must not be kept at stitching workstations, but instead should be held in a secure location to which only authorised personnel have access.
When needles are worn or damaged, they should be collected and returned, and a new needle issued by an authorised person. Needles should be disposed of according to local regulations – usually placed into a sealed container with an opening just big enough to permit entry of one needle at a time. A strict 'one in, one out' policy should be in place, with a new needle issued only upon return of a complete damaged one. The culture within the factory should be open, with staff actively encouraged to report needle breakages to avoid bad practice becoming commonplace.
If a needle is actually broken while in use, rather than becoming worn or damaged, work on that station should cease and efforts made to locate all the pieces of the broken needle. The fragments should be checked against an example of a whole needle to confirm that they have all been found. Only when all the pieces have been found and accounted for should a new needle be issued. If all the pieces cannot be accounted for, then the footwear being worked on must be bagged and removed from the workstation, along with shoes from any surrounding workstations. The workstation must be checked, preferably with a hand-held metal detector, and the remaining needle fragments removed.
There should be a clear metal detection procedure in place, and detection units should be placed at the end of the production line in such a way that it is impossible to circumvent them. This ensures that goods do not reach the packing area without passing through a detector. Detectors should ideally be located where they can be used to check all closed uppers, as well as between quality control and the packing section, to ensure that footwear intended to be metal-free is, indeed, so. Detectors should be sited away from sources of magnetic fields such as ceiling fans and the clutch mechanisms of certain stitching machines. The manufacturer or supplier of metal detectors should be able to advise on a suitable location. As previously mentioned, machines should be calibrated regularly and checked with test pieces.
Strict cleaning procedures should be in place. Cleaning staff should be in the habit of reporting any metal pieces found when cleaning, and should feel free to report such incidences without fear of reproach.
Finally, the choice of materials in footwear production will obviously have an impact on a metal detector’s ability to find errant metallic items. Trims and accessories are increasingly being made of non-ferrous materials, which makes ferrous needles more easily detected. Where steel shanks are used, or ferrous eyelets, metal detection ceases to be a valid quality control system.
Metal detection technology
Metal detectors have been in use in a variety of applications for over a hundred years. The basic design has not changed dramatically, and is often referred to as the 'balanced coil'. A coil is energised with alternating current, which creates an electromagnetic field. Particles of ferrous and non-ferrous metal passing a metal detector affect its electromagnetic inspection field, creating a disturbance that is detected by the machine. In industrial, conveyor-based systems of the type we refer to in this article, a transmitter coil 'illuminates' the samples as they pass through the machine, while receiver coils (often two of them) detect the presence of any illuminated particles. Different coil sizes are used for different applications, with the best machines using several coils for improved detection.
Many conveyor systems offer two modes of detection: 'AM' (for all metals) and 'FM' (for ferrous metals). Where some metalwork is present in finished footwear, the FM mode can be used to find broken needles or other errant ferrous elements. However, in footwear where no metal should be present, the less discriminating AM mode may be used.
Limitations of the technology
As with all measuring and monitoring equipment, calibration and maintenance are essential to ensure correct operation and reliable results. Test routines and best practice need to be agreed upon and documented, and machines need to be correctly set up for the products most often scanned. Two of the simplest controls on a metal detector are 'sensitivity' and 'discrimination'. Sensitivity is adjusted to allow the user to ignore local sources of interference that may affect normal operation, with the trade-off of some corresponding loss of range. In footwear applications, sensitivity must be adjusted so that external metallic items are not detected. Discrimination likewise allows users some control over what is detected. As discrimination is increased, smaller pieces of metal and background signals are ignored and, as it is increased further, only larger pieces are detected.
The usefulness of any industrial metal detection system is determined by the size of the aperture through which footwear samples pass – more specifically, the ratio of aperture size to product size. Orientation of metal objects within the footwear can affect their detectability, as can the shape of the objects. Every metallic object bounces back some of the signal transmitted to it by the illumination coil, behaving a little like an antenna. Some shapes are easier to detect than others – ring- or loop-shaped objects bounce back the most signal, while rod-shaped items, viewed end-on, offer the worst response.
It may sometimes be the case that background conductive properties affect successful metal detection within the footwear. For example, high levels of moisture or salt in the leather of certain footwear could lead to a false positive reading being given by the metal detector.
How shoemakers could learn from other industries
Metal detection is employed in many other industries, to prevent metallic items ending up in finished products. One of the areas in which metal detection is employed particularly fastidiously is the food industry, where any possibility that metallic debris from food preparation machinery and equipment enters the food itself must be eliminated.
Metal detection systems used at the end of food production lines are becoming increasingly advanced, driven by the demand for stringent quality control. In order to maintain reliable operation, machines need to be checked regularly with both ferrous and non-ferrous test packs, made up from fresh or frozen food products representative of the actual product that the system will be scanning. Suppliers of metal detectors may supply a range of controlled metallic pieces, each encapsulated in a small resin or plastic block. These can be slipped into the appropriate food product and then passed through the machine to confirm successful detection.
This checking process could be adopted by the footwear industry. Shoemakers could make their own calibration samples, featuring pieces of known metallic hazards likely to be encountered in the factory, such as eyelets, sewing needles and tacks. These samples could be produced from items of varying size, and encapsulated in different orientations.
If the incidence of false positives is a regular issue, and one that has discouraged manufacturers from more widespread reliance on metal detection, this perhaps indicates that devices used to check footwear are not calibrated or checked often enough with controls.
New technologies offer potential benefits
Multi-spectrum and multi-frequency machines represent two new technologies that offer greater sensitivity and reliability for industrial metal detectors. The most basic metal detectors use only a single operating frequency to generate their electro-magnetic inspection field.
Although economical, these single frequency detection systems are sometimes difficult to fine-tune when used on product lines that handle several different types of product, each with their own innate variations in material, construction and associated conductivity.
Multi-frequency machines offer the user a choice of frequency to be used in the inspection field. This frequency is either user-selected, or more commonly automatically chosen by the system, based on stored responses from previous products of that type. Even though these machines offer a choice of frequency, many still use only one frequency at a time. Multi-spectrum machines, on the other hand, use a range of frequencies simultaneously when inspecting products. Some multi-spectrum machines include software that enables the machine to ‘learn’ the varying responses of different metals to a range of frequencies, and of products that may have properties which confuse the detector. This advanced feature makes the system less prone to false positives when inspecting slightly conductive products, and more likely to identify the presence of all types of metal contaminants – both magnetic and non-magnetic.
How can we help?
SATRA has an ongoing investigation into metal detection systems used to indicate the presence of metallic items in finished footwear. We are interested in working with our members to investigate suitable and cost-effective technologies for identifying metal sources in footwear, as well as possible causes of false positive results. Please email email@example.com for further information.
This article was originally published on page 14 of the April 2020 issue of SATRA Bulletin.