Some form of coatings are used in almost every industry. They increase the durability and the functionality of the substrate being coated. When it comes to the types of coatings the number of options seem endless. There are also several coating methods, the techniques used to apply a coating to a substrate. Common methods of coating include dip, spin, spray, flow, and meniscus coating.
You may use more than one coating technique. However, picking a suitable method is important. That is because the coating method used can have a direct effect on the performance and success of both the coating and the object being coated.
So how do you know which coating technique to use for your application? Keep reading as we help answer that question and more. In this article, we will discuss different coating application methods, technical challenges, and tips for choosing the best method based on your application.
Common Methods of Coating
1. Dip Coating
What is it?
Dip coating is when the entire substrate is dipped (immersed) into a liquid coating. The thickness of the coating will depend on the coating’s viscosity and the specific substrate.
The dipping method is not always compatible with all substrates, such as those having deep grooves and blind holes.
This method makes it fairly easy to simultaneously apply a coating to both the substrate’s external and internal surfaces. The coated objects are removed from the liquid and dried by one of many curing methods.
Suitable applications
The dip coating technique is a relatively cheap and easy method. It can be used for both medical and industrial applications. This method is suitable for:
- High volume parts
- Small and simple-shaped objects
- Protective coatings
- Substrates with large surface areas
- Specialized, biomedical coatings
- Manual or automatic coating of small and large (bulk) batches
It is one of the most preferred methods for high-volume production. The number of dipping cycles, length of immersion and rate of withdrawal play a crucial role during such automated production.
Disadvantages
Disadvantages of the dip coating method may include streaks, uneven finishes or thickness, and others.
2. Spin Coating
What is it?
In this technique a Spin Coater is used to apply a thin, uniform coating on the surface of the substrate. The thickness of the coating is determined by the speed and time of spinning.
The coating is deposited at the center of the substrate, which is then rotated at a high speed. Centrifugal force causes the coating to spread and create a thin film on the surface of the substrate. The rate and time of spinning, the viscosity of the coating and the type of substrate play a crucial role in achieving the desired finish and durability.
Suitable applications
Spin coating methods are mainly used for applying medical coatings. The technique is sometimes used for industrial applications to a lesser extent. In general, the spin coating application method is suitable for:
- Small-sized substrates
- Achieving thin coatings
- Ensuring highly consistent and uniform thickness
Disadvantages
Under a few circumstances, the coating may develop swirl and comet-like patterns. It may also become too thin or thick depending upon the rate of spinning.
3. Spray Coating
What is it?
This technique commonly uses compressed air to atomize liquid coating and create a mist that rapidly flows towards the surface of the substrate.
It is the quickest and most flexible coating method. It can be used for complex shapes and intricate areas. A smooth, even coating film can be applied with controlled features such as the spray angle.
Depending on your equipment you can also spray both viscous and non-viscous liquid coatings into the spray gun for a consistent and uniform finish. Spray coating very thick formulations, such as insulative coatings, can also be accomplished with specialized equipment.
Spray Coating Technologies
The most common spray coating technology uses a spray gun (or machinery) and a compressed air source. This makes it suitable for both production line, manual, and even mobile coating applications.
The air pressure is typically between 0.2 to 0.5 MPa when flowing through the spray gun. It creates a negative pressure zone at the spray gun’s nozzle, which ejects the coating.
In addition, there are more advanced spray coating technologies used in biomedical coating applications. An example is ultrasonic spray nozzles, which use sound waves to create and deposit a fine mist.
Other spray coating technologies include (but are not limited to):
- Electrostatic spray coating
- Airless spraying
- Thermal spraying
- Plasma spray coating
Suitable applications
The spray coating method is one of the more common coating methods. In turn, it is commonly used across a wide range of industrial coating applications. These range from automotive paints to industrial applications and more.
This method of coating is also used for medical coatings, but to a lesser degree. Examples include advanced spray techniques for coating drugs and certain medical devices.
Disadvantages
There are some disadvantages when it comes to spray coating. With traditional gun and compressed air spraying the main downside is waste/material loss due to overspray. In these applications the liquid coating utilization is around 50%. This can cause some issues with waste (inefficiency), cleanup, etc. Waste is significantly reduced
Other problems associated with traditional spray coating finishes include:
- Orange peel
- Pinholes
- Uneven spraying
- Sagging
- Spluttering
4. Flow Coating
What is it?
In flow coating liquid coatings are dispensed on the top of the substrates. The coating then flows using (controlled) gravity to evenly cover the entire surface. The thickness of the coating depends upon the gravity flow (flow rate) and viscosity of the coating.
This coating process has minimal steps, whether you are applying liquid coating manually or automatically.
Benefits:
Some benefits of using the flow coating method are:
- High coating utilization – excess coating flows into the collection tray and can be recycled
- Excellent transfer efficiency
- Once set up it is a more economical coating application
- Minimal space requirements
- Reduced waste
- High speed process
- Can be used for many substrates
Suitable applications
The flow coating method is used for industrial applications. It is recommended for flat horizontal sheets, panels having a large surface area, as well as more complex shapes. It is generally an alternative to dip coating, such as for coating large objects that cannot be dipped.
Disadvantages
While this coating technique is considered easy it does have some considerations.
- Suitable only for liquid coatings
- Upfront costs for set-up can be high, even if ongoing costs are low
- Preparation is critically important with this coating technique.
- Surrounding conditions such as temperature and humidity must be controlled to ensure a consistent and uniform finish. This may make it not suitable for certain applications.
5. Meniscus Coating
The Meniscus-guided coating (MGC) method is primarily used for applying uniform, high-quality thin films of polymers to packaging materials and solar panels. It is also used for specialized medical device coatings to a lesser degree.
There are a couple of meniscus coating techniques for the best finish and desired thickness, as follows:
- Dip
- Blade
- Slot-die
- Solution shearing
In general, a meniscus is translated over the surface of the substrate as the solution is spread using a coating head or viscous forces. The thickness and other essential properties of the thin film deposited on the substrate are directly influenced by the meniscus shape, the time required for curing, and the flow of the coating or solution over the substrate.
Coating utilization is very high. In fact, it is generally thought to be the highest in the category. It can be upwards of 99% using R2R processes during high-volume production.
Hydromer® Proprietary Meniscus Coating Method
Hydromer®, Inc. has developed a proprietary meniscus coating technique that meets the unique requirements of certain medical devices. This application technique is a good match for applying coatings to devices that are long (100+ cm) and have a small diameter, such as guidewires and long, small diameter catheters. Cardiovascular and neurovascular devices are good candidates for this specialty coating technique.
Find out more about Hydromer’s contract coating services.
Selecting the Right Method of Coating
Here is a list of things to consider while selecting the right coating method for your substrate:
- Complexity of the parts to be coated
- Desired finish and consistency in the coating
- Viscosity of the coating itself
- Production requirements and available equipment
- Curing methods used to solidify the coating
- Functionality, such as corrosion resistance and UV protection
- Environmental impact
Apart from the above, you must consider things like surface preparation, stickiness or adhesion properties, hazardous chemicals, and their environmental impact.
Hydromer® Hydrophilic Coatings and Contract Coating Services
Hydromer® is a leading manufacturer of hydrophilic coatings for both medical and industrial applications. Our advanced coating technologies can be used to coat prototypes and finished, market-ready devices. We excel in manually or automatically coating medical devices with unique geometries using the methods discussed above: Dip, Spray, Spin, Flow, and our proprietary Meniscus coating technique.
So if you need a custom coating to meet your specific product requirements or help applying it our team can help. From Contract R&D to contract coating and technology transfer services you will have a partner from ideation to production.
Our ISO 13485:2016 and 9001:2015 certified production facility is adept at building precise equipment to coat your device effectively. Get ready to launch your medical device equipment to the market quickly.
Contact our team at Hydromer with questions or to start your coatings project.
