MI Band Heaters Allow Tighter Temperature Control Than Ceramic Knuckle Band Heaters

Problem:

A plastics processor was using ceramic knuckle heaters on a Krauss-Maffei plastic extruder to make profile shapes with PVC and TFE compounds. Both resins require operating temperatures in excess of 700°F (370°C). During operation, temperature swings in excess of ±75°F (±24°C) were experienced. The PVC and TFE extrusions responded poorly to these wide temperature variations and product quality suffered.

Initial attention was focused on the temperature controller to see if it was causing the wide temperature fluctuations. Further investigation revealed that construction characteristics inherent to the ceramic knuckle heaters were the true source of the problem. The ceramic knuckle's poor thermal conductivity was unable to conduct heat away from the resistance element fast enough, causing it to operate in excess of 1400°F (760°C).

When the power was shut off upon reaching the 700°F (370°C) set point, the 1400°F (760°C) heat stored in the ceramic knuckles' mass continued to be conducted into the barrel. This over-heating problem was compounded by the additional heat generated by friction in the process itself and the ceramic fiber insulation that trapped the combined heat against the barrel, raising the process/extrusion temperature beyond acceptable limits.

Solution:

The initial customer was advised by his Watlow representative to replace all ceramic knuckle heaters on one of his extruders with a complete set of MI bands. Although narrower in width, the MI band heaters covered the same amount of barrel surface area. The same day the MI band heaters were installed, the results were in: the over-temperature problem was gone. The lower mass, faster response and superior thermal conductivity of the MI band heaters kept the extruder operating within a ±5°F (±3°C) of set point. It also came up to operating temperature faster with MI band heaters. As a result, all 35 extruders in the shop were converted from ceramic knuckle heaters to Watlow MI band heaters.