Edge Bead Defects

Have you heard of the common defect in slot die coating called edge bead? The defect can occur in either ambient temperature coating or hot melt adhesive coating.

Let’s define what edge bead is. When you are coating, the hope is that you can create the same coating thickness from edge to edge within that coated product. In proximity coating with backing roll, the fluid exiting the die goes on to the substrate and typically the material is coated with what we call a dry edge.  That area may be as narrow as a couple of millimeters or as wide as a centimeter on either side, depending on your product.

The area without liquid coating on the edges may be trimmed off or it may be necessary. In many cases, the fluid is even all the way across but builds up right at the edges. This buildup might be a millimeter or two. Once cured, it may still be a buildup that causes a solidified edge on your product. This can cause softening of your rolled product with hard ends and soft in the middle. This can also make it difficult or impossible to convert into smaller rolls, sheets, or whatever you are trying to develop. Therefore, reducing edge beads is helpful.

Even if you will be slicing off the final edge, the extra buildup of fluid at very least is a waste of fluid. Most likely it is causing more trim and handling. You can understand that reducing that fluid buildup is an economical solution.

To reduce the edge bead and make it less of an issue, understanding of why it is occurring is the key.

In ambient temperature coating, the internal design of die may result in forcing more material to the edges than allowing in the middle. All this excess fluid gets to the edge, and it hits the shim barrier. When this excess fluid exits, it has nowhere else to go and it just beads up on the end.

One possible cause for this may be that the internal manifold design is not appropriate, and you need to change the shape to encourage more flow to the middle than you have to the end. Does this mean that you need a new die? Not necessarily. One way to correct the problem is to use an insert inside the existing manifold to change the shape of it and encourage flow in a different way. Long term, you may want a manifold that is designed for your fluid, but if you are coating a variety of fluids, inserts are one way to overcome the issue.

Another cause for edge bead may be that the shim thickness is not accurate. When the shim is thinner and you bring the upper die body closer to the lower die body, you encourage the fluid to be pushed out to the end. If you’re getting edge bead and you feel confident that the manifold design is the best it can be, then the shim thickness might be off. You can change the shim thickness to have more flow in the middle. Assuming the shim is the control valve of the system, having thicker shim helps the fluid to naturally flow throw the middle of the die. If you have a center entrance fed die, flow coming into the die from the back middle is going to want to go out the front middle unless you squeeze it down and force it to the ends. For ambient temperature coating, typically you would have manifold shape and/or shim thickness control over the fluid flow from center to end.

The other solution for edge bead control is chambering. To understand the chambering solution, let’s assume that you are using 5 mil shim and you need to stage up to a size between 5 mil and 6 mil to not get the edge bead. If you can’t get the shim thickness accurate for what your fluid flow need is, then you probably need to look at chamfering the shim. But chamfering the shim or cutting kind of an edge release or constriction at exit for the die is something that does take computer modeling in order to get accurately right off the bat and just cutting off an edge of the shim is not going to necessarily solve your problem. It is kind of a micro or nano level control beyond the shim thickness.

Hot melt adhesive coating and ambient temperature coating are connected but have differences. In hot melt adhesive coating, you care about the viscosity of the fluid, the pressure inside the die, the flow material just like you do in the ambient temperature coating. But in addition, you also care about the elasticity of the polymer. So even in ambient temperature coating, you may have some elastic component, but the more fluidized, the more liquid that material is, the less the elastic character plays into that. In hot melt coating, the elastic component might actually be the bigger part of your flow control.

If you look at hot melt adhesive, you are basically taking 100% solids and you are controlling the flow of that 100% solid polymer by heating it. You are using temperature in order to make something that is rubbery to flow by heating it up. But depending on what your temperature is and how much sheer energy you are putting into it by forcing it through the slot die, you are going to either have the hot melt adhesive polymer act like a solid, act like a liquid or act like something that is rubbery in between that solid and liquid. The reason is not just the viscosity of the material, but also the G prime and G double prime, the loss of storage modulus, are effective parameters. One of those tells you the pure viscous nature of your polymer and the other tells you the elastic nature of your polymer.

If you are in the elastic nature of your polymer and you look at the final lip-land right before exit of the die, the point where the polymer is rheologically as it is exiting the die, if it is more elastic than it is purely viscous, then you may have a rubbery response or retraction of that polymer after it exits the die. This may create curl in your substrate and other types of defects like wrinkling. If you are avoiding edge curl at the point that the fluid exits the die, you have to be even more concerned about what state of the polymer is it in the viscous area, is it the rubbery plateau or is it moving through where it is acting like a solid, and how is it going to respond when it releases all that pressure, sees the outside area and then forms its final polymer?

Similar to ambient temperature dies, in hot melt adhesive dies, you can also control the shear rate by closing the lip faces down just like changing the shim thickness and you can encourage flow to the ends, and if you have too much flow in the middle, you can close down. If we are getting edge beads you may open up in the middle in order to encourage flow to the middle. Almost all hot melt dies have an adjustable lip instead of just one solid shim with fixed thickness, and you could make it variable from center to end in order to encourage flow differently as it exits the die which can be a benefit.

An operator may want to close down on the end of the die in order to stop flow completely so that the adhesive doesn’t leak out from the side, but so much compression force may push that polymer into a regime where it is actually acting more rubbery or if you haven’t pushed it enough, you may not have fluid. You should be doing calculations or running simulations on your flow to figure out what is the optimum gap in the die so that you are not forcing your hot melt adhesive to act like a rubbery material that might retract or cause issues.

In both ambient and hot melt coating, you are trying to look at the material and make sure that it is flowing to a point where it doesn’t have too much flow to the ends. In hot melt, specifically, you want to make sure that you are in the right flow regime for your polymer.  In ambient temperature coating, depending on how complex your fluid is, in addition to just getting the right manifold shape, shim thickness, and potentially chamber, you may also want to consider looking at if your material is acting more like a rubbery material, taking the same type of approach as hot melt.

A simplified explanation would be if you are trying to push rubber bands out and they are under a high amount of stress, they are going to retract when they exit as they are released from the stress. And then, when they retract, they are going to cause the issues stated earlier. The polymeric material behaves in a way similar to rubber bands in this instance. You want to have the fluid coming out of the die as relaxed as possible while still controlling flow.

In summary, in ambient temperature or hot melt coating to avoid an edge bead, first make sure your manifold shape is accurate. Secondly, consider the size of the opening and utilize shims to fine tune. Ultimately, understand the rheology of your material, so that you can determine the shear that is being placed on it by your equipment. This will help you make your material flow through the exit the way you want it to.