Pressing Flashcards
(21 cards)
What are the press sections requirements for the felt?
- runnability
- paper quality
- felt performance
- felt life
What are the functions of a press felt?
- remove water from the sheet
- compress the sheet
- support the sheet in the nip
- even the pressure changes in the nip
- influence sheet properties
- transportation
What is the structure of a press felt?
- base weave = woven or non-woven centre, gives strength
- Batts = surface of the felt, needled to both sides of batt to form surface
When are felts replaced?
- end of running life (scheduled)
- plugging (compaction/contamination)
- wearing
- accidents/holes/vibrations
What is the most common raw material used in press felts and why?
Polyamide yarns
- good compression resistance / elasticity
- good abrasion resistance
- good tensile strength
- high water absorption
- good chemical resistance
What parameters are influenced by the Batt?
- contact area - finer batt = higher contact area
- smoothness & dewatering - higher contact area = higher smoothness and dewatering
- wear resistance - finer batt = lower wear resistance
- flow resistance - finer = higher flow resistance giveing hydraulic marks
- permeability - a higher fineness = less permeability
What are the main dimensions when talking about press felts?
- base weight
- air permeability
- batt layers
Compare compressible and incompressible base weaves
Compressible: critical for crushing but good for start up dewatering
Incompressible: better against vibrations but lower in start up dewatering, better against crushing
What are the different types of base weaves for press felts?
- single base felt = one woven base, either single, double or triple layer
- laminated felt = two or more individual base weaves joined together via needling
- nonwoven felt = base weaves made with another method other than weaving
- hybrid felt = combination of woven and non-woven base weaves
- seam felt = endless felt
What are the differences between single/double/triple layer base weaves and laminated base weaves?
S/D/T layer:
- light
- compressible
- low void volume
- plugs easily
- soft
- more nip dewatering
Laminated:
- heavier
- incompressible
- high void volume
- stays open
- more stiff
- more uhle box dewatering
What is the manufacturing process of press felts?
- Warping
- Weaving
- Base heat setting
- Pre needling and needling
- Finishing
What structure/density is best for press dewatering?
A more closed/dense structure is better for water transportation in the nip and gives higher hydraulic pressure
What is the difference between uhle box and press nip dewatering?
Uhle boxes remove water in the felt that has been pressed from the sheet.
Nip dewatering removes water pressed from the sheet and through the felt.
Compare nip dewatering with nip and uhle box dewatering
Nip only:
- felt water content is equal or lower after nip
- require dense and thin lightweight felts
- fast felt start-up
Nip & uhle dewatering:
- felt water content is higher after nip
- open and medium-heavy felts
- longer felt start-up time
Are press felts more effective when they are wet or dry?
Wet - they have more hydraulic pressure
What are typical edge issues for press felts?
- trim stealing
- edge drop off
- edge cracks, napping or flipping
What are the benefits of TMO felts?
TMO = TransMaster Open
- better felt surface = better surface contact, better smoothness, better dryness and runnability
- felt can be hydrophilic or hydrophobic depending on application
- longer felt life = impregnation prevents compaction
- shorter start-up time = due to impregantion
Why do press felts need to be cleaned?
- to reduce dirt accumulations
- to have less sheet edge failures
- to keep the felt open so it can absorb more water
- to have even profiles
What press felt cleaning/conditioning methods are there?
Mechanically:
- HP showers
- LP showers
- felt edge showers
- uhle boxes
Chemically:
- alkali or acid washing during shutdown
- continuous cleaning with detergents via showers
- intermittent washing using stronger chemicals during web break
Are HP showers recommended for new felts?
Not on the first day, but otherwise fine
What effect does nip dewatering have on press felts?
- Improved cleanliness
- higher water permeability
