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Inside Look: Slot Die and Comma Head Coating [Videos]

Posted by: Mike Rothaupt on 4/6/20 3:39 PM

When it comes to the coating phase, will your project use a comma head or slot die coating machine? While comma head coating is the most common method for coating transdermals, both machines have their own set of advantages. In this blog, I’ve included videos to show you each machine and discuss how choosing one or the other may benefit your project.  

Comma Head Coating 

In the video, you’ll see the comma head that we have here at Tapemark. 

While the comma head is in operation, your coating substrate comes through the bottom. Once in motion, it passes through a coating trough, which is holding your blend. The blend is then pulled up to the middle where it contacts the comma head. During contact, the comma head meters the blend to the pre-determined thickness. 

Comma Coating Advantages 

The primary benefit of this method is that it's fairly intuitive in its operation. The coating substrate meets very little resistance as it moves through the machine. The coating thickness is controlled by simply moving the comma head up and down. All said, the comma coating machine makes for relatively simple setup.  

Comma Coating Disadvantages 

During comma coating, the blend is exposed to the environment while it's in the coating trough, allowing for the possibility of solvent loss. This can be mitigated by having your trough covered, but some risk will still remain. 

Because the blend is getting pulled through by the release liner and then wiped at the comma head, any variation in either the release line or thickness or quality of the release coating will change how it's sheared. And those variations will result in a variation in your coating thickness. Choosing quality release liners for comma coating will help to mitigate this risk. 

Slot Die Coating 

In this video, you’ll see the slot die coating machine we have here at Tapemark. Bear in mind, we have the bottom of the slot die mounted to it so that we can discuss the geometry and how everything works.

So while it’s in operation, the coating substrate comes around the casting roll, and the elements would be in much closer proximity. The blend is fed through the back of the slot die through a pump with very little pulsation — typically you’ll see either gear pumps or diaphragm pumps being used. As the pumps force the blend through the back of the die, it’s spread through the internal manifold and then cast directly onto the substrate. 

Slot Die Advantages

The big advantage of this type of system is that it's completely closed. You don't have to worry about any solvent loss or contamination. And also, because the blend directly metered directly onto it, it's relatively independent of any variation in your substrate. As a result, you are able to achieve very high precision for your coat weights using the slot die method. 

Yield Benefits

The yield benefits with slot die kick in after the setup. The machine provides a very consistent method of coating. If you're in a very large scale process or you're going more towards commercialization, choosing slot die may be more beneficial. 

Stop and Start Capabilities

Another possibility is if you're running a process, which, for whatever reason needs to start and stop often, the slot die coater can go on and off coats almost on demand. 

Because it's a sealed system, you're not dealing with any solvent losses that can dry across the coating head, which happens during comma head coating. 

Sometimes, clients need to make shorter rolls to go off for sampling or do intermittent coating. Either way, this method will likely bring some advantages.

Slot Die Disadvantages

The main disadvantage of the slot die is that the set up for it can be quite cumbersome. It tends to take a lot longer to dial in on the process, so on some of the projects where you're really trying to utilize the amount of money available, it might not be as ideal. But on large scale applications, slot die coating really starts to show its advantages.


While comma head coating is the most common method for coating transdermals, slot die coating offers a whole host of additional benefits. I hope this post was helpful to you and your decision to include either comma head or slot die coating in your next project. 


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Topics: Coating

Mike Rothaupt


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