Sticking vs. Picking in Tablet Coating: What’s the Difference?

Introduction

In the pharmaceutical coating process, surface defects such as sticking and picking can significantly affect tablet quality, coating uniformity, and production efficiency. These defects often occur during film coating when process parameters, coating materials, or equipment settings are not properly controlled. Understanding the differences between sticking and picking—and how modern tablet coating machines manage these issues—is essential for pharmaceutical manufacturers and procurement professionals seeking stable and high-quality production.

Overview of Tablet Coating Defects in Pharmaceutical Manufacturing

Tablet coating is a critical stage in solid dosage manufacturing. The tablet coating process not only improves the appearance of tablets but also enhances stability, taste masking, and controlled drug release. During coating, tablets rotate in a coating pan while a spray system applies a thin polymer film formed by specialized tablet coating materials.

However, improper operating conditions may lead to coating defects that compromise tablet quality. Two of the most common problems are sticking (also called twinning) and picking. Although both issues appear during the coating stage, they have different mechanisms and require different troubleshooting approaches.

For pharmaceutical manufacturers and procurement teams selecting a tablet film coating machine, understanding these defects is important for evaluating equipment performance, process control capabilities, and long-term production reliability.

Defining the Defects: Sticking vs. Picking in the Coating Pan

What is Sticking (Twinning)?

Sticking, often referred to as twinning, occurs when two or more tablets adhere together during the coating process. Instead of moving freely within the coating pan, tablets temporarily bond due to a wet coating surface. As a result, they rotate together until the bond breaks or the coating dries.

Sticking is particularly common during early stages of the pharmaceutical coating process, when the sprayed coating solution has not yet dried sufficiently. If multiple tablets adhere repeatedly, the result may be uneven coating distribution or deformation of tablet edges.

Key characteristics of sticking include:

• Temporary adhesion between tablets

• Occurs mainly during wet stages of coating

• May lead to uneven coating thickness

• Often caused by excessive spray rate or poor drying

If not controlled, sticking can slow down the entire tablet coating process, reduce batch uniformity, and increase rejection rates.

What is Picking?

Picking is a different type of defect that occurs when the coating layer partially removes or “picks” fragments from the tablet core surface. During the coating operation, portions of the tablet surface may detach and stick to the coating pan or other tablets.

This issue is often associated with weak tablet cores, excessive spray wetness, or improper tablet coating material formulation. Unlike sticking, which mainly involves tablet-to-tablet contact, picking involves damage to the tablet core itself.

Typical signs of picking include:

• Rough or damaged tablet surfaces

• Visible patches where the coating has pulled away the core

• Powder residues inside the coating pan

• Loss of tablet integrity

For pharmaceutical production lines using a tablet film coating machine, picking represents a more severe defect because it directly affects product quality and may cause regulatory compliance issues.

Root Causes of Sticking and Picking in Film Coating

Understanding the causes behind these defects helps manufacturers optimize the tablet coating process and prevent costly production losses. Most problems originate from three major factors: excessive wetness, poor atomization, and mechanical pan dynamics.

Over-Wetting

Over-wetting is one of the most common reasons for both sticking and picking in the pharmaceutical coating process.

During film coating, the spray solution must evaporate at a controlled rate. If too much liquid is applied too quickly, tablets remain wet for longer periods, increasing the likelihood of surface adhesion.

Over-wetting typically occurs due to:

• High spray rate

• Low drying airflow

• Excessive humidity in the coating chamber

• High solid concentration in coating suspension

When tablets remain excessively wet, they easily stick together inside the tablet coating machine. In severe cases, the softened surface may also cause picking if tablet cores are not sufficiently hardened.

To prevent over-wetting, manufacturers must balance spray rate, inlet air temperature, and airflow.

Poor Atomization

Atomization refers to how finely the coating solution is dispersed through the spray nozzle. In an efficient tablet film coating machine, the spray system produces small, uniform droplets that evenly cover tablet surfaces.

Poor atomization creates large droplets that deposit excessive liquid in localized areas. This uneven distribution can lead to both sticking and picking.

Common causes of poor atomization include:

• Incorrect spray nozzle pressure

• Blocked spray nozzles

• Improper spray gun alignment

• Incorrect viscosity of the coating solution

Large droplets increase localized wetness, making tablets stick to each other or damaging the tablet surface during drying.

High-quality tablet coating machines typically feature advanced spray systems designed to optimize atomization and ensure uniform coating distribution.

Mechanical and Pan Dynamics

The mechanical movement of tablets within the coating pan also plays an important role in preventing defects.

The tablet coating process relies on continuous tumbling motion to ensure that tablets are evenly exposed to the spray solution and drying airflow. If pan dynamics are poorly configured, tablets may not move freely, increasing the risk of defects.

Mechanical factors affecting coating performance include:

• Pan rotation speed

• Baffle design inside the pan

• Tablet load volume

• Tablet shape and size

If the pan rotates too slowly, tablets may cluster together and stick. If rotation is too fast, mechanical impact may cause coating damage or picking.

Modern tablet coating machines incorporate optimized pan geometry and adjustable rotation speeds to maintain stable tablet movement and prevent these issues.

Key Differences Between Sticking and Picking

Understanding these differences helps pharmaceutical engineers determine the appropriate corrective actions during the tablet coating process.

Troubleshooting: How to Prevent Sticking and Picking

Preventing coating defects requires a combination of proper equipment, optimized process parameters, and suitable coating formulations.

Several practical strategies can help minimize sticking and picking during tablet coating operations.

Optimize spray parameters
Adjusting spray rate and atomization pressure ensures that coating droplets are applied evenly without excessive wetness.

Improve drying efficiency
Maintaining proper inlet air temperature and airflow promotes rapid solvent evaporation, preventing prolonged wet surfaces.

Control coating formulation
The composition of tablet coating materials directly affects film formation. Polymer type, plasticizer content, and solvent system should be carefully selected to ensure strong adhesion and flexibility.

Monitor tablet core quality
Tablet hardness, porosity, and surface smoothness all influence coating performance. Strong tablet cores reduce the risk of picking.

Adjust pan rotation speed
Maintaining the correct pan speed ensures consistent tablet movement and prevents clustering inside the coating pan.

By implementing these adjustments, pharmaceutical manufacturers can significantly improve coating stability and product quality.

Overcome Coating Defects with Jianpai’s Advanced Tablet Coating Machines

While process optimization plays a critical role in preventing coating defects, equipment performance is equally important. A high-quality tablet coating machine provides precise control over airflow, spray parameters, and tablet movement.

JIANPAI specializes in advanced tablet film coating machines designed for pharmaceutical manufacturers seeking reliable and efficient coating solutions.

Key advantages of JIANPAI tablet coating equipment include:

Precision spray control
Advanced spray systems ensure uniform atomization of coating solutions, minimizing localized over-wetting and preventing sticking.

Optimized airflow design
Efficient hot-air distribution accelerates solvent evaporation and maintains stable drying conditions during the pharmaceutical coating process.

Intelligent control system
Automated parameter monitoring allows operators to adjust spray rate, pan speed, and drying temperature with high precision.

Enhanced pan structure
The specially designed coating pan improves tablet movement, ensuring even mixing and reducing the risk of mechanical defects.

Compatibility with various coating materials
JIANPAI machines support a wide range of tablet coating materials, including film coating polymers, enteric coatings, and sustained-release formulations.

For pharmaceutical procurement professionals, selecting a reliable tablet coating machine is a critical investment that directly affects product quality, regulatory compliance, and production efficiency.

Conclusion

Sticking and picking are two common defects that may occur during the tablet coating process, but they originate from different mechanisms and require distinct solutions. Sticking usually results from excessive wetness and inadequate drying, while picking is often associated with weak tablet cores or improper coating conditions. By optimizing spray parameters, improving drying efficiency, and selecting suitable tablet coating materials, manufacturers can significantly reduce these problems.

Equally important is the choice of equipment. Advanced tablet film coating machines from JIANPAI provide precise control over the entire pharmaceutical coating process, helping manufacturers maintain stable production, improve coating quality, and minimize defects. If your production line requires reliable and efficient tablet coating technology, partnering with JIANPAI can help ensure consistent results and long-term operational success.

FAQ

Can the coating formula cause picking?

Yes. The formulation of tablet coating materials can significantly influence picking. If the coating solution has poor adhesion properties or incorrect polymer ratios, it may fail to bond properly to the tablet surface. Excessive solvent content or improper plasticizer levels may also weaken the film structure, increasing the risk of picking during the tablet coating process.

What is “twinning” in tablet coating?

“Twinning” is another term for sticking. It refers to a situation where two tablets adhere together during coating due to wet surfaces. Twinning typically occurs when the spray rate is too high or when drying airflow is insufficient in the tablet coating machine.

How does pan speed affect tablet sticking?

Pan rotation speed controls how tablets tumble inside the coating pan. If the speed is too low, tablets may cluster together, increasing the chance of sticking. If it is too high, excessive mechanical impact may damage the coating layer. Proper pan speed ensures smooth tablet movement and helps maintain a stable pharmaceutical coating process.