October 2, 2015
Bob Maine, Director of Engineering, Cambridge International
A May 2014 article in Food Processing Magazine describes the devastating fire that destroyed Cargill’s $100 million Booneville Arkansas facility.i The fire’s source: Plastic belting in a spiral conveyor. The result? The plant’s 800 employees lost their jobs and the 150,000-sq.-ft. facility remains shuttered today, more than six years after the fire.
Eliminating the potential for such catastrophic loss is one of the significant advantages metal conveyor belts hold over plastic. Stainless steel’s durable and sustainable nature also increases hygiene and food safety while decreasing time, cost and energy necessary to meet today’s stringent sanitation protocols. The following examines metal’s mettle in greater detail.
Extinguishing Fire Hazards Posed by Plastic
"Spiral coolers that use a plastic conveyor belt as opposed to a metal one, create the greatest concern for a fast spreading fire because the belt itself presents a combustible fuel load." This cautionary note was sounded by Liberty Mutual Insurance in a risk management guide issued for food processors on preventing plant fires.ii
Any plastic belting – mostly injection molded and constructed of polyethylene or acetal – is petroleum-based and therefore flammable. Plastic is particularly susceptible to fires created from exposure to heat sources in or near the conveyor. Three plastics commonly used in conveyor belting are polypropylene, nylon and acetal. The latter -- up to 40% more costly than nylon or polypropylene -- is very troubling for plant engineers because it burns ‘clear.’ Acetal’s invisible, fast moving flames are also a carcinogen hazard.
Protecting Against Abrasion & Plastic Contamination
A second advantage of metal over plastic belts is stainless steel’s inherent durability, resistance to abrasion and ability to operate at higher and lower temperature applications. Plastic belt abrasion leads to both product contamination and mechanical failure. The internal breakdown of polymeric links found in plastic makes surfaces brittle and creates shards that break off, and into, food during processing. This is especially troublesome as plastic is not detectable.
The most commonly used plastic in belt conveyors is polypropylene, which can only be used in environments where temperatures do not exceed 220 degrees Fahrenheit. While nylon has a higher temperature range, it is also hygroscopic and absorbs moisture with temperatures over 100 degrees Fahrenheit, resulting in swelling which can add up to 9.5% of its weight in moisture.
Satisfying Stringent Sanitation & Food Safety Protocols
A third area where metal conveyor belts are superior to plastic belting concerns sanitation and food safety. In recent years, there have been many examples of food borne illness outbreaks due to maintenance and cleaning of processing equipment including conveyor belts. As a result, food processors are paying more attention to food safety and seeking solutions that make it easier to clean and sanitize conveyor belts and other equipment while minimizing downtime on the line.iii
Stainless steel conveyor belts provide a non-porous sanitary surface and can be heated well above temperatures that bacteria can survive, typically 121°C for sterilization. Since most plastic conveyor belts cannot withstand temperatures over 100°C, they are cleaned with sanitizing chemicals.
Stainless steel, unlike plastic, is resistant to corrosive effects of sanitizing chemicals. Chlorine, one of the most commonly cleaning agents, attacks acetal, causing it to delayer. Metal surfaces – especially stainless steel - have lower rugosity making it easier to remove dirt or product from the belt’s crevices and pinpoints.
Moreover – and perhaps most importantly -- metal belts can be cleaned in place, eliminating the time, labor and expense to remove, wash and reinstall each one daily. The key is to allow access for cleaning by designing joints that prevent bacteria growth and increase ability for cleaning agents to both reach and drain.
Many plastic designs require either removing the conveyor belt for full immersion in a chemical bath or installing special cleaning systems to access the hinge points on the ends and sprockets. These fixes reinforce plastic’s inherent lack of cleanability and drainability.
Separating Science from Fiction
Whether processing poultry, cooling baked goods, or preparing ready to serve meals, the health, safety and cost benefits of metal conveyor belts for today’s food processing plant decision maker are incomparable. Metal belts can have high levels of open area without sacrificing strength or life. We invite you to examine the science behind conveyor belt design and construction and discover why metal matters.
Bob Maine holds a B.S in Mechanical Engineering from Virginia Tech and obtained further training in metallurgy, intellectual property and tech management at the universities of Maryland and Wisconsin.
i Food Processing Magazine, “Calculating the Cost of Conveyor Systems,” Kevin T. Higgins, May 7, 2014.
ii Liberty Mutual Property, Risk Management Guide, Spiral Coolers, 2007.
iii American Society for Microbiology, “Transfer of Microorganisms, including Listeria monocytogenes, from various materials to beef,” Graziella Midelet and Brigitte Carpentier, May 2002.