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Safety Footwear - Protection through well engineered design and manufacture

Published: 01st Mar 2010 in OSA Magazine

To consistently and effectively provide the necessary level of protection, safety footwear must be carefully designed and manufactured. A thorough consideration at the development stage of both the design and materials to be used can save both time and money during the testing and certification stage. A well-engineered product will also help to ensure there are few or no problems identified during ongoing compliance testing, thereby avoiding costly product recalls and the risk of litigation.

There are a number of standards for safety footwear throughout the world, which may have a number of differences in their requirements. However, all of them contain requirements for toe protection, penetration resistant inserts and electrical resistance properties.

The toecap impact and compression resistance

The toecap plays a vital role in determining the level of protection afforded to the toes by the footwear, but its effectiveness may be compromised by shortcomings in footwear design. Safety footwear protects the wearer’s toes through the principle of ‘defended space’. This means that the toecap provides a protective cell into which the wearer’s toes are placed.

Clearly, for the defended space principle to work, not only must the toecap have the necessary strength, but the outsole complex immediately below the toecap flange must also provide the required support. This will ensure that the crushing or impact force is efficiently transferred to the ground without the toecap becoming embedded in the outsole. The support to a toecap will be more effective if the thickness of the outsole is kept to an absolute minimum and it is produced from a hard compound. Quite clearly, this is in conflict with what is often required for a comfortable and fashionable product, so a balance must be reached.

The outsole design and bottom construction is also very important to the performance of the product in other respects: toespring, tread pattern, sole thickness and insole profile are all factors which can affect performance.

One point often overlooked is the location of the toecap onto the lasted upper. The toecap should be a snug fit and be located into position prior to the toe lasting operation, for example, with the use of a rubber hammer. This will prevent movement of the toecap during subsequent manufacturing operations, thus avoiding misalignment of the toecap in the finished footwear.

Most safety footwear is now supplied fitted with a footbed. This helps to improve underfoot cushioning and general comfort, plus thermal insulation. However, if the footbed covers the full length of the foot, it will extend into the defended space under the safety toecap. This has the effect of reducing the internal clearance in the protective cell, and will have a detrimental effect on the level of protection provided. It is therefore important to ensure that if footbeds are to be used, the making last has been designed with sufficient depth to accommodate them.

Penetration-resistant inserts

Penetration-resistant inserts in safety footwear come in two variations: metallic and non-metallic. Where steel penetration resistant inserts are used, care should be taken with their shape. Unless the insert has been designed specifically for the last being used, the insert may not extend to cover the full width of the insole. However, it is possible that the flange of the safety toecap will slowly dig into the outsole complex and pass over the outer edge of the penetration resistant insert, especially if the toe section of the boot is gradually compressed. The insert then springs inside the toecap and it distorts upwards and reduces the clearance under the toecap. To improve performance in the impact and compression test (and hence offer better protection in wear), the shape of the penetration-resistant insert must be compatible with the last bottom shape and toe cap.

 Most major safety footwear standards specify a maximum distance between the edge of the insert and that of the last’s featheredge and it is important that the relationship between the two is checked.

It is also important that the insert is securely located in place to prevent movement during manufacturing operations, such as injection moulding and vulcanising of the outsole. Metallic inserts are often supplied with holes in the toe and heel areas to allow them to be tacked to the insole. Where these holes are present in inserts for footwear intended for Europe, there should be no more than three each with a diameter no greater than 3mm. Alternatively, inserts can be secured in place with hot-melt adhesive as no holes are needed.

Recent developments in high-performance technical textiles have opened up the possibility of alternative materials and constructions that can provide the required level of protection. Such materials and constructions are most commonly used in sewn-in sock constructions (Strobel stitched), where they are adjacent to the foot or directly underneath the footbed. The sewn-in sock construction has distinct advantages when it comes to manufacturing cost, comfort and flexibility. In this construction, the insert is sewn directly to the edge of the lasted margin of the upper. This reduces the amount of leather used in the lasted margin (thus reducing cost) and also improves the flexibility of the footwear. When combined with a dual-density polyurethane outsole, the construction can provide a lightweight, flexible product with a good degree of comfort.

Slip resistance

Not all safety footwear standards contain requirements for slip resistance. Slip resistance is a complex property with many factors involved. However, well-designed footwear can help to reduce the risk of slipping accidents and injury.

On clean and dry surfaces, a tread pattern is not always necessary but, on contaminated surfaces – for example, where lubricants are present – an effective tread pattern is required to sweep aside the lubricant in much the same way as car tyre tread acts. The elements of good tread pattern design include soft flexible constructions that maximise contact with the floor, smooth flat wearing surfaces and leading edges in all directions.

Electrical properties

Most standards for safety footwear contain optional requirements for the electrical properties of the footwear. The most common category of footwear offering protection against electrical hazards is called ‘antistatic’ in the EN ISO, AS/NZS & SANS standards and called ‘static dissipative footwear’ in the ASTM & CSA standards. These test methods and their requirements are not identical, however, in all of these standards it is the purpose of the footwear to reduce excess static electricity by conducting the charge (from the body) to ground, whilst simultaneously maintaining a sufficiently high level of resistance to protect the wearer when exposed to hazards from incidental contact with live electric circuits.

In all of these standards, it is very important that the correct bottom stock components and materials are used to achieve the right electrical resistance properties and that the footwear is manufactured consistently to ensure good contact between components. Manufacturers should also be aware that electrical resistance can change with wear and to offer the longest period of protection, they should avoid engineering a product that gives initial results too close to either end of the range.

 

Author Details

SATRA offers the opportunity to work closely at the pre-production stage to try and avoid design problems which may adversely affect results. This is usually carried out by inspecting a sample and specification sheet for an initial assessment. This gives SATRA enough information to offer constructive advice and to highlight any potential problems that may occur. Many of SATRA’s customers have found this service invaluable as it can save development time and money on unnecessary tooling costs. As a Notified Body, SATRA also works with manufacturers and suppliers who wish to CE mark product intended for sales in Europe. Products are tested strictly in accordance with the European Personnel Protective Equipment (PPE) Directive and certified on this basis.

Readers interested in finding out more about how SATRA can work with them on safety footwear development and testing are invited to contact safetyfootwear@satra.co.uk or visit the website

www.satra.co.uk/safetyfootwearosa

Published: 01st Mar 2010 in OSA Magazine

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