Solutions to common doctoring problems

Effective, trouble-free doctoring is essential for modern, automatic and computer controlled paper machine operation. This ensures paper machine runnability and product quality. Doctoring is a relatively simple process but it is still important. It has three basic functions in the paper machine:

  1. Shed the sheet during sheet breaks or sheet threading operations
  2. Remove water and contaminants from roll surfaces
  3. Creping (tissue)

Poor doctoring in the paper machine leads to substandard performance and quality. Stickies, fiber flocks, and water rings on the roll surface limit the manufacturing performance by causing holes and web breaks.  The breaks lead to unbalanced processes and poor paper quality. Poor doctoring increases the risk of damage to machine components and clothing when the sheet passes the doctor and wraps a roll.

The doctoring result is directly affected by:

  • Doctor construction
  • Doctor adjustments
  • Condition of doctor components
  • Condition of roll surface  

The doctoring result is indirectly affected by:

  • Roll cover material
  • Roll/Cylinder temperature
  • Stock properties
  • Temperature variations
  • Felt conditioning

BladeCarryMaintenance and troubleshooting

To make sure doctors perform as designed, it is essential to add doctor check-ups to mill maintenance agendas. For daily doctor maintenance when the machine is running:

  • Check roll surface for cleanliness
  • Check doctor for vibration
  • Check behind blade for cleanliness, especially inside rolls
  • Check lube/wash shower operation
  • Check oscillation

For daily doctor maintenance during a break (if necessary):

  • Change blade – clean the holder before new blade is installed
  • Clean the doctor. NOTE: Wash out behind the blade also. Make sure drive side doctor is also washed.

Causes of the most common doctoring problems

Causes of the most common doctoring problems

Some solutions for the most common doctoring problems

The most common solutions include:

  • Change blade material
  • Check blade dimensions (thickness and length/width)
  • Adjust loading according to blade type and application
  • Adjust or add blade lubrication showers
  • Check and service the doctor (oscillation, loading hoses, doctor alignment)
  • Check bearings condition
  • Recondition roll
  • Train personnel (handling of blades, regular maintenance of doctor and significance of doctoring)

For more tips on improving your machine doctoring, check out Valmet's Doctoring 101 white paper under Info Center/Downloads or contact your Valmet representative.