Inconsistent Cooking Times?

Loss of friction between the Teflon belts and the drive rollers can be noticeable, but not always clearly seen. Start with verifying the cooking time if the problem is not clearly visible. Place a marker on the Teflon belt, or make pace between products. With a stop watch, clock the cooking time from the point where the marker passes over the leading edge of the first heating plate to the point where it passes over the final edge of the last heating plate. Does the clocked time agree with the Set Point on the HMI?

Higher friction than normal can be the cause, but friction can be difficult to detect. First, stop the belts, release the belt tensioning system and try to move belts around by pulling them by hand. Can Teflon belts and rollers be moved as should? 

Secondly, remove the gear box from its drive shaft. Can the drive roller and its shaft be turned or higher than expected friction is noticeable?   

With no products in the cooking system, lift the lower Teflon belt off the heating plates and look for build-up of carbon, filth and burned particles. Build-up of filth produce greater friction and increase the ”pressure” as Teflon belts are elevated by the filth underneath.

If the heating plates are made of stainless steel (be very careful with aluminum plates), clean, brush and scrape them toughly to remove filth. If the problem is severe, apply a stronger detergent like ECOLAB P3-Topax 30, a similar product or apply ”oven cleaner” which can be bought at supermarkets on the heating plates conductive surfaces. Use protective googles, gloves, etc. while applying stronger detergents with a paint brush. Shield off cables, sensors and other sensitive parts which may be damaged by strong detergents. Follow the manufacturers instruction and rinse thoroughly with clean water.       

Drive roller friction. With the cooking system operating under load, open the doors, or remove covers for access to view the rubber coated drive rollers. Can slippage between the Teflon belts and the rubber coated drive rollers be seen? Be careful and place a finger partly on the edge of a Teflon belt, partly on its rubber coated drive roller. Can slippage be felt? Be careful and touch the rubber coated drive roller to feel if it provides expected friction and traction, or it feels slippery, hard or oily?

Inconsistent cooking time can, on larger cooking systems, be caused by water between the heating plates and the Teflonbelt. This phenomenon may occur after sanitation and start up in the morning. The symptom is usually that the Teflonbelts start rotating, but after ca 2 minutes they have pushed away the air between the heating plates and the belt, leaving only water between the surfaces which cause a ”vacuum”. Lift the Teflonbelt off the heating plates, place a broom shaft between to allow the waer to evaporate. 

Is the speed problem consistent or it varies over time? Do the Teflon belts move partly as expected and partly slower? Is it likely the seal (joint, bond) gets stucked somewhere? Look at scrapers and open belt washers slightly to release pressure. Are the edges of the Teflon belts intact or have they been damaged? Can they get stucked somewhere?  

Height. If the distance between the heating plates is less than it supposed to be, the pressure and friction increase. Assuming the parameter for height value on the HMI is the right for the product, then verify that the actual height is the same as the value on the HMI. Use a gadget (heat resistant) with known thickness, for example a spanner which is 9 mm thick. Change the parameter for height value on the HMI to the thickness fo the gadget +0.5 mm. With open doors, walk around the cooking system and insert the gadget between the heating plates at each corner of each heating plate. Is the height correct at all check points? Can differences in parameter value and actually height be the cause?   

Different speeds. The upper- and lower Teflon belts may have different speeds. With heat resistant gloves, pinch both belts by your fingers at the side of the cooking system to feel if the belts are running at same speed or not? Alternatively, mark the two belts with a marker-pen or attach paper-clips on their edges to better see if they have different speeds. 50 mm difference from the start to the end of the cooking system can be much for some applications.