SPRAY COATING

In typical coating applications, a spray bar is utilized over a moving product transport to create a line of spray that coats the entire product as it passes underneath the bar. Flat fan nozzles are commonly used in such setups.

Although the product transport moves continuously, the product may be dispensed intermittently. This means that spraying may only be necessary when a product passes under the spray bar. When the cycle is fast, precise control is required, and this often calls for air-actuated or electrically-actuated spray nozzles.

Dosing can be controlled by both the flow through the nozzle and the speed of the transport. With electrically-actuated nozzles, dosing can also be controlled via pulse width modulation. Such nozzles can cycle up to 150 times per second. Therefore, by using very fast on/off cycles, flow can be reduced without affecting the continuity of the coating.

Hydraulic flat fan nozzles may be suitable for simple continuous spraying systems. However, systems that require stop/start spraying or other more precisely controlled on/off cycles will require air- or electrically-actuated nozzles. Air atomization of the spray may also be desirable in many coating applications as it can create a fine spray with small droplet sizes at low defect rates, which leads to better adhesion of the coating.

In a stop-start bed movement system, the product is moved under a spray bar and then halted while the coating is applied. The transport then moves the product on. This may involve several spray coating points, such as multiple applications of spray.

Typically, full cone nozzles are used to deliver an even coating of liquid to the stationary target. In many cases, air actuation of the nozzle is necessary to ensure a responsive on/off cycle. Air atomization of the spray may also be desirable to reduce the droplet size for better adhesion.

Pellet or tablet products require a different type of coating system. Running tablets along a conveyor would only coat one side and the sheer quantity of products would make turning each one impractical. Therefore, these types of products are typically coated in drum coating machines. The uncoated products are tumbled in a cylindrical drum while a spray of coating liquid is applied. The random movement of the products ensures that an even coating is applied to each one. This type of coating is very common in the pharmaceutical industry for tablet coating.

Usually, high-precision air atomizing nozzles are used in these applications. As the spray is directed at a moving mass of multiple tablets, it is desirable to create a fine mist. Finely atomized sprays lead to greater randomization of the liquid, resulting in a more even coating.

The fluidized bed coating process is similar to the drum coating method, as it involves moving uncoated tablets upwards in a chamber with the aid of an air current. The solid product is thus "fluidized", behaving like a liquid with randomized movement of individual particles. A spray is injected into the air current, which hits the moving tablets, thereby applying the coating. As the tablets are briefly suspended in an air stream, when they come into contact with the coating spray, the contact is potentially from all angles, allowing for an even 3D coating to be applied. This process depends on the randomized movements of the target pellets/granules so that they act in a liquid-like manner when they meet the also randomized droplets. Thus, the process emulates the mixing of two liquids.

The nozzles required for this type of application need to have well-controlled patterns, droplet sizes, and flow rates. Small areas of overspraying with increased liquid density can result in product clumping together. Therefore, Synergy nozzles, with their free air channels for activation, atomization, and spray direction, are the logical choice.

Accurate dosing is crucial in coating applications. Applying too much coating wastes money and may compromise product quality, while too little coating can result in subpar quality and rejected batches. Dosing can be particularly challenging when the production line is fast-moving, as even small variations in spray consistency can have a significant impact on the amount of coating applied.

Ensuring precise coverage is also essential in coating applications. Over-spraying can lead to wastage of expensive coatings and potential contamination of areas where the coating is not needed, while under-spraying can leave incomplete coverage and affect product quality. Therefore, it is crucial to ensure that the spray is directed accurately and consistently along the spray line.

At first glance, ensuring the spray stays on track seems to involve simple geometry. The spray angle of the nozzle can be used to calculate where the spray will be at a given distance from the target, and basic trigonometry can be used to determine the correct height of the bar to ensure coverage. However, there are additional considerations beyond a simple trigonometric calculation.

One of the key challenges is that most sprays do not deliver their liquid uniformly along the spray line. For example, fan spray nozzles tend to have a tapering of liquid distribution towards each end of the spray. As a result, it may be necessary to apply additional coating to ensure consistent coverage.

The size of drops in a spray coating system can have significant effects on the final coating quality. Finely atomized sprays have a higher surface area to volume ratio, which means they have more overall contact with the target and will adhere more quickly. This is particularly important in drum and liquid bed coating systems, where contact is randomized.

On the other hand, larger drops tend to bounce or break on impact instead of sticking to a surface. When they do form a layer, it tends to be uneven and thicker, meaning any required evaporation may take longer and result in an uneven coating. The prolonged presence of wet product increases the likelihood of product sticking together in liquid bed/pan coating systems or partially wet product on transport systems.

NOZZLES FOR SPRAYING VISCOUS LIQUIDS 

Spraying viscous liquids presents several challenges, particularly in the food industry where many viscous liquids must be sprayed precisely.

CONSISTENCY MEASUREMENTS 

Consistency is a measure of a liquid's resistance to shear stress and an approximate measure of its "thickness." It is measured in Pascal seconds, with water at 20 degrees Celsius having a consistency of 1.002 mPa.s. This is commonly reclassified as the unit centipoises (cps), with 1 cps equal to 1 millipascal second, such that water has a consistency of around 1 cps at room temperature.

AIR ATOMIZING NOZZLES 

Many air atomizing nozzles mix the liquid and air inside the nozzle before discharge from the opening. This is sufficient for low viscosity liquids like water, but for liquids much above 100 cps, issues arise. The larger drop sizes and air pockets that form in the chamber severely disrupt the spray pattern, leading to blockages and degraded coating quality. To overcome this challenge, external mix air atomizers should be deployed. These mix the air with the liquid stream after exit from the nozzle, avoiding the problems encountered by internal mix atomizers.