What is Tablet Pitting?

Tablet pitting refers to the small depressions that appear on the surface of a tablet after coating with a sugar or film layer. In pharmaceutical production, many tablet defects are only revealed after coating. A tablet may appear flawless after compression, but once it enters the coating process, surface defects can become evident. Therefore, pitting on coated tablets may originate from pre-existing surface defects in the tablet itself or from issues during the coating process.

Let’s examine tablet pitting from the perspective of coating, understand its causes, and explore targeted solutions.

Causes of Tablet Pitting

Tablet pitting occurs when the stress applied during the coating process exceeds the mechanical strength of the tablet surface. This stress results from a combination of factors:

1. Tablet Surface Weakness: Tablets may already have indentations or fragile surfaces. During coating drum rotation, surface wear can cause depressions. Tablets with low hardness, high friability, or high porosity are more prone to pitting during coating.
2. Over-wetting During Coating: The hot coating solution sprayed during the process can excessively soften the tablet surface, leading to small pits.
3. Coating Formulation: Some coating solvent systems are more aggressive and may partially weaken the tablet surface.

In short, tablet pitting results from both intrinsic tablet properties and external stresses applied during coating.

Practical Methods to Prevent Tablet Pitting

The approach can be summarized as: “Strengthen the tablet, optimize the coating.”

1. Improve Tablet Quality

This is the fundamental solution. Optimize the formulation of binders and fillers and refine the compression process to enhance tablet hardness.

Formulation Optimization:

• Increase/Change Binders: Use binders with stronger adhesion or better suitability (e.g., replace starch with HPMC, or adjust the type and amount of PVP).
• Adjust Fillers: Choose fillers that compress well and form a strong structure (e.g., microcrystalline cellulose, MCC, which provides high strength and low porosity).
• Use Flow Agents and Lubricants: Agents like colloidal silica (flow agent) and magnesium stearate (lubricant) can help. Carefully control magnesium stearate dosage and mixing time; overuse or overmixing forms a hydrophobic layer, weakening inter-particle binding, resulting in “hard outside, soft inside” tablets prone to surface detachment.

Compression Process Optimization:

• Ensure Adequate Granulation: Use wet, dry, or fluid bed granulation to tightly bind active ingredients and excipients, reduce powder porosity, and increase particle density and strength.
• Adjust Compression Parameters:
• Main Compression Force: Moderate increase can significantly improve hardness and reduce friability without causing capping or delayed disintegration.
• Pre-compression Force: Proper pre-compression helps expel air, ensure uniform filling, and reduce internal stress points.

2. Optimize Coating Process

Create a gentler coating environment: moderate drum speed, control spray rate and atomization, and adopt mild drying curves (e.g., preheating first, avoiding high-temperature rapid drying).

Pre-Coating Preparation:

• Sufficient Preheating: Before spraying, gradually raise the tablet bed temperature to near the coating process temperature (e.g., 30–35°C). This stabilizes the tablet’s physical state and removes some moisture, reducing subsequent thermal stress.

Mechanical Stress Control:

• Lower Drum Speed: Maintain adequate rolling (“cascading” motion) while using the lowest effective speed. Impact energy increases exponentially with speed, so lowering speed directly reduces mechanical stress.
• Optimize Equipment: Modern coating pans with baffles or embedded exhaust systems promote smoother tablet movement and reduce collisions.

Spray and Drying Refinement:

• “Low Spray, High Atomization” Strategy:
• Reduce spray rate by 10–20% to prevent over-wetting.
• Increase atomization air pressure to create finer droplets, ensuring faster and more uniform surface coverage and drying.
• Implement Gentle Drying Curves:
• Avoid rapid high-temperature drying; lower inlet temperature (e.g., from 60°C to 50°C) to prevent surface binder softening and film skin formation that traps solvent.
• Ensure adequate airflow to remove solvent vapor without over-evaporation.

Select Appropriate Coating System:

• High-Flexibility Polymers: Water-based dispersions (pseudo-latex) form more elastic films than alcohol-based or simple HPMC coatings. For sensitive tablets, PVA-based coatings offer good flexibility and tear resistance.
• Use Plasticizers Wisely: Add plasticizers such as glycerin, PEG, or triethyl citrate to improve film flexibility. Optimize dosage via film stretch tests; insufficient plasticizer leads to brittle films, excessive amounts reduce strength.
• Adjust Coating Solid Content and Solvent: Higher solid content within acceptable viscosity reduces total liquid and drying time, limiting exposure to wet/heat stress. Avoid solvents that can dissolve or soften the tablet core.

Implementation Path

1. Diagnostics First: Test tablet friability and hardness uniformity. If friability is high, focus on tablet strengthening.
2. Process Check: If tablets are adequate, systematically review coating parameters; start with reduced drum speed and lower inlet temperature.
3. Formulation Assistance: If process adjustment fails, optimize or change coating formulation for better flexibility.
4. Integrated Optimization: Ideally, integrate tablet formulation, compression, coating system, and coating process in early development to prevent pitting at the source.

Related Questions and Practical Answers

• Why does higher press speed increase tablet pitting?
  Higher press speed increases impact energy, damaging weak surface areas.
• Can tablets with acceptable hardness still pit?
  Yes. Surface strength and internal structure are more critical than overall hardness.
• Does scaling up production increase pitting risk?
  Yes. Larger equipment typically imposes higher mechanical stress if parameters aren’t adjusted.
• Is tablet pitting a GMP concern?
  Yes. Visible surface defects can render products non-compliant.
• Can equipment design reduce pitting?
  Yes. Modern coating machines provide smoother movement and precise control, reducing tablet stress.

Summary

A successful prevention strategy must start at the tablet formulation and compression stage, combined with a compatible, gentle coating process and optimized coating formulation. Collaborating with experienced pharmaceutical equipment manufacturers ensures stable coating conditions and reliable tablet quality.

For any questions, feel free to contact us. Our pharmaceutical equipment engineers have over ten years of experience. If you need a reliable tablet coating machine or a complete tablet production line, we can provide the right solution for you.