How is your choice of doctor blade impacting the surface quality of your Yankee?

The condition of the Yankee surface is critical to both operating efficiency and tissue quality. Tests confirm that interactions with doctor blade materials can have a major impact on cast iron or metallized Yankee surface condition and wear rate – with potentially costly repercussions.

Tissue quality is dependent on many factors, not least the surface condition of the Yankee cylinder, which is subject to constant wear and tear by the action of the doctor blade.

Given that there is always some degree of wear and tear as the Yankee surface is doctored, a long-term ‘steady state’ condition simply isn’t possible – the surface will continually change as a result of friction, coating behavior changes, tissue structure changes and blade deterioration.

In most mills, operating parameters are normally adjusted as needed to take these factors into account to a point where, when productivity is sufficiently impacted, a decision must be taken to recondition the Yankee surface by grinding or polishing. As all tissue makers know, this is a very costly procedure.

To avoid this unwelcome expense, tissue makers would thus do best to strive for best practice operating parameters that minimize Yankee wear and stabilize the tissue-making process.

Wear mechanisms affecting the Yankee surface

There are three distinctly different causes of Yankee surface deterioration that are often grouped together under the general term ‘wear’. Friction, abrasion, and to a lesser extent, corrosion, are the mechanisms that attack the Yankee’s metallic surface. Although some mills have experienced occasional, serious corrosive problems, these are isolated instances and not something most tissue makers deal with on a daily basis. Friction and abrasion, conversely, are constant sources of concern.

Friction (adhesive wear)

Kinetic friction is now understood to be caused not simply by surface roughness, but also by chemical bonding between the surfaces. The more chemically similar the two surfaces, the greater the mutual solubility of the materials and the greater the degree of bonding.

 
 


Figure 1: Friction


As the surfaces momentarily bond and then instantaneously break free, material is transferred and energy released. Friction is therefore not a fundamental force, and so cannot be calculated from first principles, but instead must be determined empirically with painstaking attention to all details of the system in question.

One very telling indicator of friction is heat generation. In the Table below, the operating temperature of the creping blade was measured in systems with different material combinations:

  Cast iron Yankee surface Metallized Yankee surface
Steel creping blade 152º C 144º C
Ceramic creping blade 131º C 108º C

Table 1: Blade tip temperature (opposite Yankee surface)


It is clear that the ceramic blade tip, being nonmetallic, exhibits less heat build-up from friction, and implicitly it can be said that there is less material interaction.

It is also interesting to note that in these very simple mill measurements, the influence of the blade materials was dramatic, despite a ‘normal’ coating being present on the surface. As inefficient as the coating is in lubricating the blade/Yankee interface, it is still very important.

In the complete absence of this lubrication, far more rapid material loss occurs. In the Table below, laboratory tests of a ceramic blade against a small cast iron cylinder dramatically illustrates the reduction in wear a lubricated surface experiences compared with a dry surface:

Blade type Surface condition of cast iron Amount of loss (gms) Surface Ra (µ) before Surface Ra (µ) after
Ceramic Dry 0.25 0.36 0.33
Ceramic Release spray 0.05 0.30 0.26

Table 2: Short duration laboratory test of frictional results


 
 

These data confirm the importance of protecting the Yankee surface from friction and the resulting wear and tear. Even the hardness and wear-resistant properties of new-generation metallized surfaces are not immune to the devastating effects of adhesive metal transfer – indeed, BTG has seen several cases where relatively new metallized surfaces have been damaged by insufficient protection or improper choice of blade material.

In the Table below, a series of different types of ceramic-tipped blade and a conventional steel blade were run on a nonlubricated cast iron cylinder to generate a ‘worst case’ scenario for insufficient Yankee surface protection.

This kind of scenario could reflect periods such as machine wash-ups, coating pump failures, or other serious disturbances. From the results of this test, it’s clear that the steel blade can have devastating effects on the Yankee surface, and that the ceramic blades afford much greater tolerance in extreme situations:

Blade tip Material Cylinder Temp. Increase (ºC) Cylinder material loss (gms.) Surface Ra (µ) before Surface Ra (µ) after
Ceramic A 13 0.21 0.29 0.28
Ceramic B 29 0.32 0.29 0.27
Ceramic C 10 0.26 0.31 0.41
Steel 98 2.98 0.29 2.19

Table 3: Short duration lab test of frictional results – comparison of different blade types


Figure 2, below, shows photomicrographs of the sliding surface of two blade tips used in the above test. Examining these blade tips shows that while the ceramic appears quite inert to the conditions (Figure 2a), the steel blade exhibited plastic deformation as well as obvious material loss (Figure 2b).

Click here to see an enlarged picture

Figure 2a: Ceramic blade tip

             Click here to see an enlarged picture

Figure 2b: Steel blade tip

 
 

From a best practice standpoint, tissue makers would be advised to:

Abrasive wear
The second major type of wear faced by mills is abrasion, which can be thought of as the ‘cutting’ of one surface by the other.


Figure 3: Abrasive wear


The degree to which this occurs is dependent on the roughness of the two surfaces, the hardness of the materials, and their ‘fracture toughness’. ‘Hardness’ is a concept most of us can readily envisage; ‘fracture toughness’ is perhaps not so clear. In the field of material sciences, fracture toughness = KIC (MPa√m). Perhaps the easiest way to think of this term is to imagine it as akin to brittleness. For example, a fine china cup can be very hard, but if dropped on the floor it readily shatters; it is not very ‘tough’.

The Table below lists several materials, along with their hardness and toughness values.

  Toughness, KIC [ MPa√m ] Hardness, HRC (Rockwell) Hardness, HRC (Vickers)
Cast iron surface 6-20 20-35 239-344
Metallized surface 4-10 55-65 594-834
Steel blade 50 50-55 513-594
Tool steel (blade tip) 8 60-65 700-830
Ceramic blade tip 3-4 65-76 834-1300

Table 4: Material properties


If hardness alone were a predictor of abrasion, then a material such as ceramic, which is much harder than a cast iron surface, would potentially cause high abrasive wear. But in fact the low toughness values of the ceramics prevent this damage, even if the two surfaces clash.

Looking at the values in Table 4, it would also be reasonable to assume that the abrasive wear between a cast iron surface and a steel blade would be quite high, because of the steel blade’s relatively high hardness and high toughness.

However, it’s critical to note that the hardness of steel, as well as its toughness, are much lower at elevated temperatures. Under normal operating conditions, the sliding surface of the steel blade tip reaches very high temperatures – much higher than the Yankee surface – resulting in plastic flow of any of the asperities that would abrade the cast iron surface. This argument again points to the primary wear mechanism being friction.

In the creping process, the most obvious form of abrasive damage manifests itself as large surface scratches on the Yankee, but the cumulative nature of millions of microscratches also takes its toll. Contaminants in the fiber furnish are often blamed for these problems. Sand, rust and metal particles could all be contributors to abrasive wear, so keeping the furnish as clean as possible is basic good practice.

Blade chatter
While deterioration of a Yankee surface due to friction is generally a slow and gradual process, the effects of blade chatter can develop relatively quickly, seriously affecting machine productivity and frequently requiring a surface regrinding.

The total cost of a chattering episode can be extremely high. The mechanism of doctor blade chatter has been well presented in some recent papers (for example, Archer, 2008), and reveal that the blade itself is often accorded far more blame than it deserves. In fact, the ‘stick-slip’ phenomenon (see McMillan, 1997) and the resulting damaging vibrations are not related to blade tip material, and are reported in equal measure across all blade types being used in the industry.

The strong and growing trend towards the use of ceramic-tipped creping doctor blades is being driven by a desire to improve productivity and tissue quality. From a global perspective, the use of the ceramic blades has not affected the frequency of blade chatter in either a positive or negative direction. However, in individual cases the introduction of a new blade type with different characteristics and behavior can certainly change the dynamics of the creping system.

The occurrence of chatter is an event that results from a complex chain of dynamic process conditions. With the introduction of a new ceramic blade, the dynamics change and the chain of events can diverge in different directions, with different end results.

The following brief case studies help illustrate this point:

Case 1
Edge chatter is perhaps the most common form of the problem and, if allowed, will ‘grow’ over time. As most tissue makers know, the area of the Yankee just outside of the web edge is problematic. It is difficult to properly adjust the profile of the blade holder in this area, the temperature is higher, and there is often fiber and chemical debris deposited from the felt. The end result is a hard build-up of organic material as shown in Figure 4a below.


Figure 4a


Figure 4b
In such a situation, when a more wear-resistant ceramic blade is installed, the more durable doctoring edge is able to clean this deposit off and prevent it from building back up for a much longer period of time. The chatter problem is effectively resolved as long as this clean condition exists (Figure 4b).

Case 2
In a second example, a coating package was being used that was ‘sensitive’, so that with every blade change most of the coating material was removed. With traditional steel blades, the rapid wear of the blade tip allowed the coating to build back up relatively quickly. But when the tissue maker introduced a ceramic blade, coating levels could not recover in an acceptable amount of time.
The quick remedy was to significantly reduce the creping blade loading pressure. But while this did bring the coating back to a more typical level, chatter rapidly developed across the entire face of the cylinder (Figure 5). In this case, the effective blade stiffness was dramatically reduced with the lower blade load, allowing the blade tip to vibrate at a lower frequency and higher amplitude.


Figure 5

Of course, system changes that can bring about chatter are not limited to the creping doctor blade itself – the list of process variables that can affect blade tip vibration is long. But in the case of ceramic-tipped blades, tissue makers should understand that introducing new improved technologies will generally necessitate some work to determine new optimum settings and operational procedures to take account of new elements.

Perhaps the most straightforward best practice is simply to observe! In the specific case of chatter, watching the Yankee surface with a strobe light is extremely valuable. Chatter will always occur in the coating layer long before it does any damage to the actual cylinder surface. By watching and documenting the effect of adjustments, damage can easily be avoided as new settings are developed for a new stable system.