5 Reasons Your Internal Sizing Is Failing (and how to fix it)

Internal sizing is supposed to give paper controlled resistance to water, ink, and converting liquids by placing hydrophobes inside the sheet structure. When it fails, the symptoms are predictable: high Cobb, poor HST, edge wicking, ink feathering/bleeding, coating holdout issues, dusting, and unstable runnability due to deposits.

Most failures are not “bad chemistry.” They are process-condition mismatches that prevent Internal sizing agents (AKD, ASA, rosin systems, and hybrid programs) from being retained, distributed, and developed correctly.

Below are the five most common root causes in papermaking, with practical checks and corrective actions.

1) Wet-end pH is outside the workable window for your sizing system

Sizing chemistry is strongly pH-dependent:

• Rosin–alum sizing traditionally develops best in the acidic region (often ~pH 4–5), because aluminum chemistry is needed to anchor rosin to fibers; efficiency drops as pH rises above about 5.5 unless the program is redesigned (e.g., PAC-based neutral rosin systems).

• ASA and AKD are used in neutral/alkaline systems, but both are sensitive to hydrolysis and to the ionic environment; ASA hydrolyzes rapidly if conditions aren’t controlled.

  
  

What this looks like in production

• You hit target dosage, but Cobb/HST won’t move reliably.

• Sizing swings with small pH drift (especially on grade change, broke rate change, or ash fluctuations).

  
  

What to check

• Whitewater pH where the size is actually injected (not just machine chest pH).

• Alum/PAC dosing strategy (if rosin or ASA needs aluminum support).

• Local pH shocks from acidic/alkaline additive feeds.

  
  

Fix

• Align pH setpoints to the chosen chemistry and stabilize pH control around the addition point.

• If you must run neutral/alkaline with rosin, confirm you’re using an appropriate neutral rosin/PAC approach rather than “acid rosin logic.”

  
  

2) Hydrolysis and “age” are killing the size before it reaches the sheet

A very common hidden failure is that the size is chemically deactivated in the approach system.

ASA: extremely time-sensitive

ASA emulsions can start forming larger aggregates and lose effectiveness as hydrolysis progresses; shelf-life and residence time before the headbox matter a lot.

AKD: can hydrolyze under harsh conditions

AKD can hydrolyze under alkaline wet-end conditions (and the rate can increase with higher pH and temperature), reducing effective sizing.

What this looks like

• Sizing is better right after fresh make-down or fresh delivery, then gradually weakens.

• More deposits, spots, or runnability issues when you “push dosage.”

  
  

What to check

• ASA: time from emulsification to fan pump/headbox injection; emulsion stability; droplet size/quality; temperature profile.

• AKD: emulsion stability, storage conditions, water temperature, and high-shear points that can break emulsions.

  
  

Fix

• Inject ASA as close to the headbox as your system allows, minimize hold time, and keep emulsification consistent.

• For AKD, control conditions that accelerate losses (excessive alkalinity/temperature spikes) and protect emulsion integrity.

  
  

3) Charge imbalance (anionic trash) is neutralizing retention and stealing your chemistry

Internal sizing is not just a “dosage” problem; it is a colloidal delivery problem. If the wet end is overloaded with dissolved/colloidal anionic material (DCS), the cationic demand rises and retention of size droplets collapses.

High water closure (more recycle) increases dissolved ions and can disrupt additive performance, including sizing and retention. ASA performance is also known to be sensitive to wet-end conditions such as pH and anionic demand management.

What this looks like

• Sizing failure coincides with higher broke, recycled fiber, higher COD/TOC, or a change in furnish.

• You see higher whitewater solids, poor fines retention, and inconsistent drainage.

  
  

What to check

• Cationic demand / streaming current trends.

• Anionic trash sources: recycled fiber, coated broke, dispersants, excess starch carryover, stickies control programs.

• Conductivity drift (often a proxy for closure and ionic strength changes).

  
  

Fix

• Rebalance the wet end: correct the charge first (trash catcher, microparticle system tuning, retention aid sequencing).

• Re-validate where the Internal sizing agents are added relative to charge-control chemicals (order-of-addition matters).

  
  

4) Wrong addition point, wrong shear environment, or poor mixing is breaking the delivery

Even good chemistry fails if it never gets evenly distributed and attached to fibers/fines.

Mechanisms

• High shear can destabilize emulsions or change droplet interactions.

• Poor mixing creates local overfeed zones (deposits) and underfeed zones (unsized streaks).

• Adding too early increases contact time for hydrolysis (especially ASA) and losses to whitewater; adding too late can reduce contact time for adsorption and retention.

  
  

What this looks like

• Cross-direction sizing variability, streaks, spots, or edge issues.

• Higher deposit rate when you increase size dosage.

  
  

What to check

• Injection point relative to fan pump, screens, cleaners, and pressure pulsation points.

• Static mixer performance and dilution water quality.

• Whitewater size carryover (a sign of poor retention).

  
  

Fix

• Choose an addition location that provides rapid mixing but avoids unnecessary shear and residence time.

• Standardize dilution water and temperature so emulsion behavior is predictable.

  
  

5) Furnish and filler interactions are working against you (ash, hardness, and contaminants)

Furnish composition can flip sizing behavior even when the wet-end “numbers” look normal.

High filler / CaCO₃ systems

Neutral/alkaline papermaking often uses calcium carbonate; your sizing program must be compatible with that environment (one reason mills moved from acid rosin to AKD/ASA).

Hardness and calcium reactions (especially ASA deposits)

ASA hydrolysis products and calcium salts are linked to deposit risk and sizing inefficiency if conditions are not controlled.

Recycled fiber / OCC / contaminants

Recycled furnishes change surface chemistry and can consume retention capacity; AKD can work in recycled OCC systems, but the process still needs proper control for retention and development.

What this looks like

• Cobb rises when ash increases, when PCC/GCC source changes, or when broke rate spikes.

• “More size” causes more deposits instead of better performance.

  
  

What to check

• Ash type and level; filler surface treatment.

• Water hardness and calcium load.

• Detrimental extractives/stickies and pitch control effectiveness.

• Dryer section development: some systems (especially AKD) need proper curing to reach full hydrophobicity.

  
  

Fix

• Treat sizing as a full furnish + water-chemistry program, not a single-additive lever.

• When ash/furnish shifts, revisit dosage, aluminum support strategy, retention scheme, and addition point.

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A fast troubleshooting table (use this on the machine floor)

Symptom	Likely cause	Quick check	First corrective move
Cobb/HST swings with small pH changes	pH/aluminum mismatch	pH at size injection	Stabilize pH; adjust alum/PAC strategy
Good sizing after fresh make-down, then decays	ASA/AKD degradation	Emulsion age/residence time	Shorten time to headbox; tighten emulsification
High whitewater solids + poor sizing	Charge/retention collapse	Cationic demand/SCM	Rebalance charge; tune retention aids
Streaks, spots, deposits when dosing up	Poor mixing/shear	Injection point + mixer	Improve mixing, move feed, reduce shear exposure
Worse sizing at higher ash or hardness	Filler/hardness interaction	Ash/hardness trend	Adjust program for ash/hardness; control calcium effects

Practical takeaway

When internal sizing fails, assume the issue is one (or more) of these:

1. wrong pH/aluminum framework,

2. hydrolysis/aging before the headbox,

3. charge imbalance and weak retention,

4. poor addition/mixing/shear management,

5. furnish/filler/hardness interactions plus incomplete development.

   
   

Treat Internal sizing agents as part of a wet-end system: chemistry + water closure + retention + mechanical delivery. That’s how you convert “we added more size” into stable, measurable, repeatable sizing performance.