Getting a handle on food machinery

David Greenwood Applications Engineer at WDS provides some insight into how to make the right decision when it comes to small parts for food machinery.

David Greenwood Applications Engineer at WDS provides some insight into how to make the right decision when it comes to small parts for food machinery.

In the case of food preparation, process and packaging machinery, it can be the small things that when overlooked cause disproportionately large problems.

The shape and profile of a small component, the material selected for its application, even just the wrong grade of material can cause a potential health or safety risk when misapplied on food handling machinery.

It is easy to overlook small parts when designing or servicing food machinery, but they are often exposed items that are open to touch, contact with foodstuffs and subject to harsh cleaning regimes.

Typically items include; levelling feet, castors and clamping wheels used to support the machine; hinges, toggles, bolts and clamps that provide access; and essential user interface items such as handles, grips, hand wheels, knobs and levers.

Materials used in the construction of food machinery must fulfil certain specific requirements: product-contact materials must be inert both to the product and to detergents and disinfectants under the conditions of intended use.

They must also be corrosion resistant, nontoxic, mechanically stable, and their surface finish must not be adversely affected by the conditions of intended use.

Non product-contact materials should also be mechanically stable, smoothly finished and easily cleanable.

Stainless steel and some plastics are the most common materials recommended for food grade machinery, although titanium can also be specified where weight is a higher priority than cost.

All normal or reasonably anticipated operating conditions including cleaning must be considered, these can often include high heat, humidity, exposure to acids and strong alkaline material; for which variables such as time, temperature and concentration also have to be considered as these can dramatically affect results.

Generally stainless steels offer excellent corrosion resistance.

When in actual contact with product the selection of the most appropriate grade will depend on the corrosive properties of the process and of the cleaning, including any antimicrobial chemicals used.

However, the choice should also be influenced by the stresses to which the steel will be subjected, its hardness and cost.

Where good resistance to general atmospheric corrosion is required, but the conditions of intended use will involve only contact with solutions of a pH value between about 6.5 and 8, low levels of chlorides (say, up to 50mg/litre) and low temperatures (say, up to 25C), the most common choice would be AISI304, an austenitic stainless steel, or its low-carbon version AISI-304L, which is more easily welded.

If both the level of chlorides and the temperature exceed approximately double these values, the material will require greater resistance to the crevice and pitting corrosion, which can result from chlorides concentrating locally.

AISI-316 is recommended here, but usually only used for internal working components.

Depending on the application, some polymeric materials may have advantages over stainless steel such as lower cost and weight or better chemical resistance.

This is particularly true of handles and hinges that do not have high load bearing requirements.

When choosing plastics, however compatibility with foodstuffs and ingredients, ie chemical resistance to oil, fat and preservatives and resistance to cleaning and disinfectants requires more consideration.

Both high and low temperatures have to be considered as some plastics do have a tendency to melt at very high temperatures and become brittle at very low temperatures.

Further considerations include: stress-crack resistance, hydrophobicity (ie the reactivity of the surface), absorption and abrasion resistance among others.

Polymers most frequently used in hygienically designed equipment are: acetal (homo- and copolymer) (POM); fluoropolymers such as ethylene-tetrafluoroethylene copolymer (ETFE), perfluoroalkoxy resin (PFA), polytetrafluoroethylene (PTFE, modified PTFE), polyvinylidene fluoride (PVDF) and fluorinated ethylene propylene copolymers (FEP); polycarbonate (PC); polyetheretherketone (Peek) polyether sulphone (PESU); high-density polyethylene (HOPE); polyphenylene sulphone (PPSU); polypropylene (PP); polysulphone (PSU); and unplasticised polyvinyl chloride (PVC).

If considering the use of polytetrafluoroethylene (PTFE), it must be taken into account that PTFE can be porous and difficult to clean.

But certain grades of modified PTFE and fully fluorinated copolymers such as PF A have been proven to meet EHEDG requirements for cleanability.

Hygienic food processing equipment should be easy to clean, in order to minimise the possibility of residue build-up and possible contamination by micro-organisms that can multiply at a frightening rate.

The rules for product contact surfaces are very strict, requiring maximum surface roughness of Ra 0.8um and a corner radius of 8mm.

External items must also adhere to surface smoothness requirements so as not to be too difficult to clean, surfaces must be smooth and free of imperfections such as pits, folds and crevices.

All surfaces that come into contact with product must be both easily accessible for visual inspection and manual cleaning, or at least it must be demonstrated that routine cleaning completely removes all soil.

The WDS 4food series includes a range of approved food grade stainless steel and plastic items to provide designers and maintenance engineers with less of a headache when trying to source or specify food grade components.

The WDS 4food range is covered in detail by two free catalogue available from WDS; one covering machine accessories and one dedicated to stainless steel parts.

Both catalogues provide a complete visual reference and full specification details for thousands of items.

CAD drawings for all the items are also included on a free CD for designers and many more are uniquely available to download as 3D CAD files in any native format from the WDS website.

(Source data for this article are reproduced with kind permission of the EHEDG European Hygienic Engineering and Design Group from "Hygienic equipment design criteria" Guidelines - Second Edition, April 2004).

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