Holes, general

A whole host of conditions contribute to holes in paper webs and these are generally well known, but the actual causes are often fugitive. However, the ill effects of holes and the like are always costly so that no harm comes from re-stating some possible causes. Consider the following a laundry list with
no particular importance attached to the order of presentation.
1. Fiber lumps (slugs, bundles) breaking loose from approach system or headbox.
— source should be eliminated through frequent flushing or system boil-out.
— headbox should be free of dead or slow spots where stock can hang up and later break free.
2. Perforated rolls running at incorrect speed or stopped altogether result in slugs and eventually holes.
3. Perforated roll glands may be plugged with stock.
— need to have glands cleaned and gland water flow returned to normal.
4. Headbox should be periodically checked for fiber strings.
— ensure water flow to roll glands is normal.
5. Ensure that headbox, breast roll, couch, and dandy roll showers are operating properly, if present.
— clear any plugged nozzles.
— check all shower water for pH and temperature.
6. Check for worn or defective clothing.
— check for strands/threads sticking out from wet felts, etc.
— holes or defective patches on forming fabric need to be corrected.
— dirty or filled clothing should be cleaned or replaced.
7. Check and adjust all sources of vacuum (vacufoils, flatboxes, couch roll, suction press, Uhle boxes).
— if suction boxes are shut off, ensure that drains are open so they do not become flooded.
8. Holes near edges can arise when suction box end deckles are set in too far leaving edges too wet for press section.
9. Foam in headbox (or on forming fabric) can cause holes.
— check for excess size or defoamer usage.
10. Pitch spots on forming fabric or dandy can cause holes.
— check and adjust wire pit pH, if needed.
— check for pitch or slime build-up throughout wet end (foils, trays, troughs, etc.)
— check defoamer addition.
11. Broke scabs, dirt, and the like in or on the web will produce holes.
12. Holes may be generated on forming fabric if stock is too slow or head is too low.
— good reason for controlling total head rather than headbox level.
13. Picking can occur at couch if running too tight a draw for grade.
— draw should be sufficient to eliminate bellying out of sheet.
14. Pits, marks, bulges, etc. in press rolls result in web defects.
15. Dirty draw roll between couch and press can cause pickouts.
— if driven, check the speed.
16. Check and adjust press loading.
— excessive pressure can cause holes.
— be certain press rolls are clean.
— ensure that press fabrics properly conditioned.
17. Make certain doctor blades (press, dryers, etc.) fit properly.
— check doctors for bleed-through.
18. Check to ensure felt conditioning boxes are clean or they may contribute to problems.
19. Felts, especially wet felts, must be clean to avoid build-up of shankers (mats).
— check for dirty or plugged Uhle box.
20. Felt rolls, fly rolls and dryers should be clean to prevent pick up of debris which may cause holes.
21. If felts are too wet can result in pickouts;
— check felt dryer.
— increase wet pressing.
— slow machine down.
22. Size press solutions may become contaminated with paper debris or starch scabs that can pick out.
23. Improperly cooked starch can cause pickouts.
— often indicated by return screening problems.
24. Temperature or pH shocks can cause salting out of additives that will contribute to holes.
25. Repeating defect usually indicates holes in felts, fabrics, etc..
— repair or change felt if necessary.
26. Contraries adhering to dryer felt can rupture sheet.
— shankers are more likely if felts are running wet.
27. Dryer temperatures (lead-in after press and size press) are preferably graduated to prevent pickouts.
— too slow drying can also contribute to holes.
28. Excessively free or excessively slow stock can accentuate hole problem from above causes.
29. Check size press pond level and guard positions to avoid splashing and dirtying carrier ropes, etc.
— improperly cooked starch can cause holes that look like slime holes.
30. Dirty dryers prevent good web contact.
— wet spots may pick out later or cause problems at size press.
31. Smoothing press and/or top press rolls are dirty.
— clean up and check wire pit pH.
32. Check dryer felts.
— clean off any foreign material sticking to felt and rupturing sheet as it hugs dryers.
33. Ensure correct felt tensions for application.
34. Check that felt dryers and felt rolls are clean .
35. If identified as pitch or slime related, a proper deposit control program can eliminate.
— larger, tear drop shaped defects are generally due to slime/pitch control problems, starch scabs,
or inadequately dispersed additives such as gums, defoamers, retention aids, etc.
36. At wet presses, drier sheet picks less, and lower pressure (at same solids) reduces picking.
37. Increasing refining reduces picking as adhesion is virtually unchanged while cohesion is improved.
— with fillers, refining method appears immaterial but without fillers, separate refining of hardwood and softwood can show less tendency to pick.
38. Pickouts at dandy may result from moving wet line.
39. Check out machine room system (hoods, catwalks, ceiling, etc.) for places where water drops can originate.
— usually indicates a problem with machine room ventilation so increase volume and/or temperature of hot air supplied to ceiling to prevent condensation.
— use wooden strips on condensing areas.
— water drops often carry contaminants.
40. Check screening/cleaning system.
— check reject flows from screen system.
— check for holes in screen baskets.
41. Headbox operation:
— check pond level for fluctuation or wrong level.
— check recirculation (regulator pressure in tapered manifold).
— check for plugged tube(s) at manifold.
— headbox should be free of “dead” or “slow” spots where stock can hang up and later break free in clumps.
42. Check lump breaker rolls for proper operation.
43. Check all sources of vacuum for proper operation (vacufoils, flatboxes, couch roll, suction press rolls, Uhle boxes).
44. Sometimes, reducing wet end draws will eliminate strain holes.
45. Check calender operation.
— can get peel-back holes from calender.
((From the above, it is clear that holes may arise anywhere in the machine run. Some causes result in random holes — size and position — while others have a repeat pattern, usually smaller in size. Hope this list is useful.))