The purpose of thin stock screening is to remove debris from the dilute stock going to the headbox. Screening also deflocculates the stock and protects downstream equipment. Screening systems often use two or three "cascaded" stages in order to reduce fiber losses and improve overall debris removal efficiency. The design and operation of a screening system must take into account the requirements of the paper or board machine, desired sheet quality and properties, and characteristics of the particles to be removed.
•Identify the purposes of thin stock screening
•Identify the types of debris found in mechanical pulps, chemical pulps, and secondary fiber
•Identify the components and describe the operation of a vibratory flat screen
•Identify the components and describe the operation of pressure screens
•Explain why pressure screen holes and slots are tapered or relieved
•Identify and describe primary process flows
•Define "reject rate," "debris removal efficiency," and "long fiber removal efficiency"
•Describe the operations of cascaded screening systems
