Simply put, product contamination during heat processing causes rejects and costs you money. In some applications, reject rates due to contamination can reach 15 percent or more and cost hundreds of thousands of dollars annually.

This article will discuss six common sources of oven contamination and methods for eliminating them.

Common Cause No. 1
Particulates Released from Unwanted Coating

When applying a product coating, excess can end up on baskets, racks or the conveyor belt that carries the coated parts into the oven. This happens from coating

overspray or when paint rollers are not operating properly. The unintentional coating hardens and begins flaking off, contaminating the next product to enter the oven.

Solution. For ovens designed with a conveyor belt to carry products into the oven, use a non-stick conveyor material such as Teflon-coated fiberglass combined with a tacky-tape belt cleaner. Errant coating will not strongly adhere to the Teflon belt, and the belt cleaner will remove the dried coating before it accumulates and becomes airborne.

If parts are carried on baskets or racks, keep those carriers as clean as possible with frequent cleaning or burnoff, careful application of your coating and proper masking.

Common Cause No. 2
Condensate from Fumes Released by a Coating

Many paints and other coatings release volatile organic compounds (VOCs) while being cured. These VOCs can condense on surfaces in the oven — the interior walls, for example — that are below the dewpoint. Eventually, they harden and build up on the oven interior. The VOCs even can grow into stalactites that hang down from horizontal surfaces.

Once this buildup begins, the VOCs will begin to flake off and contaminate subsequent products entering the oven. The only way to remove the buildup is to scrape it off. Sometimes, it can be difficult or even impossible to get the oven clean again as the VOC deposits can develop in inaccessible areas such as behind structural members and inside sheet metal seams.

Solutions. Depending on your application and oven design, possible solutions include:

• Use an oven design that preheats the incoming makeup air. Preheating will greatly reduce condensation of the fumes inside the oven caused by localized cold spots. It can be designed into a new oven or retrofitted onto an existing oven. Find an oven supplier that is familiar with this feature as certain guidelines must be observed.
• Employ high efficiency filters inside the oven. A properly designed filtration system will pass the heated air through the filters 10 times or more per minute.
• When purchasing a new oven, make sure the interior is constructed of stainless steel, which is easier to clean and will not corrode. Be sure the oven interior is fully accessible so that personnel may wipe it down. The air supply ductwork should have removable access panels to allow cleaning of the duct interior. All edges must be ground smooth to avoid snag points where a cleaning rag can catch.

Common Cause No. 3
Condensate from Smoke Generated by the Product

Materials such as rubber, basalt, sintered metal and oily metal parts generate oily smoke when heated. Once heated, the oil takes the form of microscopic droplets that become smoke when airborne and are flammable at high concentrations. In addition, when the smoke contacts cooler surfaces such as door openings and seams in the insulated walls, they can condense and seep into seams, wick into the insulation or collect on the oven floor. Over time, the insulation becomes saturated and actually can ignite, creating a fire inside your oven.

Solutions. Depending on your application and oven design, possible solutions include:

• Increase the oven exhaust rate to remove the smoke being generated. A higher exhaust rate will reduce the concentration of airborne smoke and minimize condensation in the oven. Contact your oven manufacturer for recommendations on how to do this without negatively affecting your oven’s performance.
• Purchase an oven that uses a continuously welded, sealed interior. This will prevent issues with oil seeping into and saturating insulation via joints and seams.
• Clean the interior of your oven frequently. This sounds obvious, but it cannot be overlooked.

Common Cause No. 4
Dust or Dirt from the Factory Environment

If your factory is dirty, dust and other airborne contaminants can enter through door openings, fresh air inlets or the combustion fan and settle on your product.

Solution. Filter all air entering the oven. For the highest level of filtration, Merv 14 or higher filters should be used.

Common Cause No. 5
Combustion Fumes

Some coatings are sensitive to the products of combustion generated by oven burners. This can be difficult to diagnose. One symptom of contamination by

products of combustion is they often result in yellowing or discoloration of the coating.

Solutions. Depending on your coating and oven design, potential solutions may include replacing the heating equipment. Other solutions include:

• Electric heating may be best for the most contamination-sensitive applications such as premium-quality metal decorating because electric heaters do not produce products of combustion.
• If an electrically heated oven is not possible, indirect gas heating can be used. In this design, the burner fires through a heat exchanger, and the products of combustion are kept separate from the recirculated air in the oven.

Common Cause No. 6
Material-Handling System

In continuous heating processes, the slide of the conveyor belt or chain on its support structure can cause wear due to friction. As this wear occurs, tiny particles of material are released and can become airborne, finally settling on the product.

Solution. Use a low friction conveyor design in which the conveyor is supported on UHMW or Teflon wear strips instead of steel.

Proper oven design and maintenance minimizes reject rates. Follow these simple steps to reduce or eliminate product contamination and maximize your profit.

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