Slot Die Coating Design
Slot die coating hot melt adhesives requires specialist systems to suit the specific requirements. Universal Converting Equipment is able to offer a range of slot die coating systems developed by Universal Adhesive Systems, ITW Dynatec and other manufacturers of slot dies. . Similar to full width and stripe coating, machine direction edge quality is controlled by:. Proper cavity design for uniform fluid velocity from edge to edge. Proper die shim thickness for optimized die pressure. Shim design to minimize flow disturbances at the edges of the slot opening. The Extrusion Coating Die. Precisely distributes the extrudate across a changeable exit width. Enable easy coating width changes. Manipulates the edge bead effect for efficient use of the coating materials. Slot Die Coating Systems Slot Die Coating Systems Hot melt adhesive slot die coatings require special systems for the respective requirements. Universal Converting Equipment is able to offer a range of slot die coating systems developed by Universal Adhesive Systems, ITW Dynatec and other slot machine manufacturers. In the world of slot die coating, thin is in. Or, to be precise, thin AND accurate is in. According to Scott Zwierlein of Frontier, “roll-to-roll slot die coating has become the industry standard for extremely thin and extremely accurate coating.” Let’s take a look to see if it’s the right coating method for you.
Coating Matters Advances in Slot Die Technology for Hot Melt Adhesives, Part 2
- Published: May 08, 2018
In this second and last part of the series, I will cover equipment design and operation.
In Part 1 of this series, I offered an introduction to the advances in slot die coating technology for hot melt adhesives. When it comes to fluid coating onto substrates, the equipment required falls into three areas: extrusion coating, hot melt coating, and liquid coating. What makes these three coating methods the same and what makes them different? I also talked about adhesive rheology (viscosity and elasticity). In this second and last part of the series, I will cover equipment design and operation.
EQUIPMENT DESIGN
So how do you develop the best equipment for such a complex application? Computer modeling. To develop an understanding of this three-dimensional equation of stress relaxation and viscosity response, finite element analysis can provide a window into handling hot melt adhesives properly. Without computer modeling, the manifold design will not consider the complex nature of the adhesive rheology and internal flow characteristics developed in manifold geometries with a slot die. Unlike ambient liquids that have self-leveling capability, hot melt adhesives need to be shaped and molded into the final application of a precision coated substrate.
However, this shaping and molding needs to be gentle enough not to develop additional and unnecessary stresses. The biggest variable controlling how the hot melt adhesive is shaped and molded is the lip opening. To reduce stresses on the adhesive, the lip opening needs to be wide enough to not place undue forces on the polymer melt but narrow enough to distribute the flow evenly across the width of coating. The lip opening is the most powerful variable in coating because it is the only adjustment that is volumetric in nature. As the lip opening is changed the resulting volumetric flow has a cubic effect on forces acting on the fluid. All other equipment variables are linear.
That doesn’t mean that the other equipment variables should be ignored. External to the slot die, the gap between the slot die and the substrate needs to be maintained flat and even to maintain a flat and even coating appearance. Because heat causes the metal to move, die bending adjusters may be required to flatten out the front face of the slot die to match the flatness of the substrate or backing roll. As hot melt adhesives are coated and influence the other equipment, by heating the backing roll, the slot die flatness may need to be further adjusted to match the contours of the steady state position of the equipment. So, unlike ambient temperature liquid coating, hot melt coating may have more variation at start-up. The faster the equipment can get to steady state, the less waste will be developed.
OPERATION
So, once the equipment is at steady state, what do you do if coating defects persist? One common coating defect in hot melt coating is ribbing; where the adhesive is confused as to whether to stick to the substrate or the slot die. In this case, decreasing the gap between the slot die and the substrate may encourage the adhesive to follow the substrate path. If this isn’t enough, the attack angle of the slot die can be adjusted. The attack angle is the rotation of the slot die around the lower corner of the upper lip of the slot die. This rotation will allow the lip faces to have the desired effect of improved meniscus development without the interference of the slot die with other components. When coating rubber bands, there are going to be some limitations to the ability to coat the rubber bands fast and thin.
The current upper limit of speed that I am aware of is 3280 feet/min (1000 m/min). This speed limitation seems to be more associated with cooling capability, but the speed could be lower for a thinner coating because of the stresses involved in the take-away process. This is a multi-variable problem and as the thickness of the adhesive coating decreases, the top speed will decrease also. The coating thickness of the adhesive is strongly connected to the internal strength of the adhesive. An adhesive chemistry can vary widely, and the effect on the ability to coat thin varies with the molecular character of the polymer.
If fillers are added to produce a lower cost adhesive, the coating thickness achievable will rise. It is a balance between raw material costs and processability. For most adhesives, it is safe to assume that 1 mil (25.4 microns) or thicker can be coated. To coat thinner, tests should be run on the adhesive coating capability on a lab or pilot scale line to show proof of concept. I have seen some lower elasticity hot melts coat as thin as 0.5 mils (12.7 microns) successfully. Hot melt adhesives coat thicker also, but there are three main levels that coating can be grouped- > 3 mils (76.2 microns). In this region, a standard flexible lip slot die lip set should be utilized. 1-3 mils (25.4-76.2 microns). In this region, a rotary rod lip set should be utilized. < 1 mil (25.4 microns). In this region, a rotary rod lip set should be used, but is not guaranteed to have success. In the thicker regime, coating with a standard lip set would allow for a proximity coating technique. In proximity coating, the slot die is positioned the coating thickness away from the substrate with a precision backing roll supporting the substrate. The tall lip faces of 0.15-0.30 inch (3.81-7.62 mm) allow the formation of a meniscus and the sharp lip edges allow for release of the adhesive from the slot die. When the coating required drops below 3 mils (76.2 microns), a rotary rod lip design should be utilized to have streak free coating.
Without the rotary rod lip design, the lip opening and slot die to substrate gap starts to narrow to the point that adhesive gels or char can develop streaks that are persistent. The way a rotary rod lip set works, the slot die lip face is actually pushed into the backing roll to develop a deflection in the roll. This deflection creates longer path for the substrate to follow, resulting in a slip phenomenon where the adhesive gel or char that would normally get stuck in the narrow gaps clears the path. The backing roll needs to be made of rubber to allow for this deflection. As a general rule of thumb, the thinner you coat, the harder the durometer the rubber should be. It is common to see a 70 Shore A durometer roll for 3 mils (76.2 microns) of coating and a 90 Shore A durometer roll for 1 mil (25.4 microns) of coating. As presented in the explanation of the rotary rod lip design, the slot die itself is not the only equipment contributing to the ability to coat precisely. The backing roll, fluid delivery system and slot die positioner all provide some functionality to the success of adhesives coating.
In addition, the web handling, tension control and stability of the roll-to-roll coating line can reduce variability or contribute to coating defects. For all equipment involved it is critical to maintain temperature evenly. Variation in temperature translates into variation in rheological characteristics of the adhesive. As the rheology varies, flow varies and the equipment needs to be adjusted to compensate. When at all possible, get to steady state and stay there as long as possible.
CONCLUSION
When it comes to fluid coating, there are three options to consider. If the application lends itself to hot melt adhesive coating, keep in mind that you are working in the one area that is the cross section of extrusion and liquid coating. This complex area of operation has a lot of potential, but you need to do your homework to be successful. Understand the limits of the adhesive. Understand the limits of the equipment. Develop the best adhesive for the application and then model the equipment to optimize performance. Hot melt adhesive coating is growing in popularity, so the more you can develop a knowledge base the farther ahead your company will be.
Mark D. Miller, author of PFFC's Coating Matters column, is a fluid coating expert with experience and knowledge in the converting industry accumulated since 1996. Mark holds a Bachelor's degree in Chemical Engineering from the Univ. of Wisconsin-Madison and a Master's degree in Polymer Science & Engineering from Lehigh Univ. and a Juris Doctor from Hamline Univ. Mark is a technical consultant and CEO of Coating Tech Service LLC. He has worked in web coating technologies and chemical manufacturing operations and is a certified Six Sigma Black Belt trained in both DMAIC and DFSS disciplines. Coating Tech Service provides process troubleshooting and project management for precision coated products. Mark has extensive process knowledge in high precision coating applications including thin film photo voltaic, Li-Ion battery, and optical systems technology. Mark has been integral to new developments and technology that minimize product waste and improve process scalability.
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Types of roll coater methods
Roll coater methods can be classified based on the state of the coating fluid before coating and whether the coating amount is determined before or after coating. Although some systems employ several methods, this section describes examples of typical coating methods.
The classifications below are merely examples. There are various ways to classify the types, and some may differ from the descriptions below.
Open coating
This method usually has a coating head and a coating reservoir with pooled coating fluid that is exposed to the atmosphere. Typical examples are gravure coaters and reverse coaters.
Gravure coaters
- A. Backup roll
- B. Gravure roll
- C. Doctor blade
- D. Coating reservoir
A gravure roll with an unevenly engraved surface is immersed in the coating fluid in the reservoir. After the coating fluid on the surface of the gravure roll is wiped off with the doctor blade, the fluid remaining in the pits are transferred onto the web of the target (substrate). Changing the coating thickness requires changing the gravure roll to one engraved with a different pattern. Offset gravure coaters provide a smooth and even coated surface at high speeds.
Reverse coaters
- A. Backup roll
- B. Applicator roll
- C. Metering roll
- D. Coating reservoir
Reverse coaters usually consist of an applicator (coating) roll, a backup roll, and a metering roll. The backup and applicator rolls rotate in the same direction to transfer the coating fluid onto the substrate web. The coating thickness is determined by the gap between the applicator roll and metering roll as well as their speed. Care must be taken to prevent unstable rotation due to improper roll assembly, and the accuracy of the gap between the rolls must be noted.
Closed coating
This method prevents coating fluid from being exposed to the atmosphere before coating. This makes it easier to maintain coat quality depending on the coating fluid. Slot die coaters and lip coaters adopt this method.
Slot die coaters
- A. Die
- B. Die lip
- C. Coating
- D. Backup roll
- E. Substrate (web)
- F. Coating fluid
- G. Die head
Die coaters adopt technology used in extrusion molding to execute the coating process. Coating fluid is passed through a die to create a uniform film, and the film is applied on the substrate to achieve coating. Because the coating fluid is pressure-supplied to the die from an enclosed tank, the fluid is not affected by air, resulting in stable coating.
A slot die coater is a typical die coater that uses a die designed to ensure a uniform amount of coating fluid in the width direction. A shearing force smooths the coat beween the the end of the die, the die lip, and the backup roll.
In recent years, coating efficiency has been improved with more sophisticated coaters such as multi-layer slot die coaters that coat multiple layers simultaneously.
Lip coaters
- A. Backup roll
- B. Substrate (web)
- C. Lip nozzle
- D. Coating
Slot Die Coating Designs
Lip coaters include a numerically-controlled coating head that allows easier quality control. Coating fluid is supplied to the lip nozzle from a lower cavity to the upper cavity at the end of the lip nozzle, where it is applied to a moving substrate. The end of the lip is rounded to generate a shearing force between the tip and the substrate during coating application.
Post-metering coating
'Pre-metering' coaters are coaters that determine the amount of coating at the time of application.
Conversely, coaters that apply additional coating and then remove excess to achieve the target amount are categorized as 'Post-metering' coaters. Knife coaters and blade roll coaters fall into this category. The amount to be removed and the coating thicknesses are mechanically set based on the shape of the roll and the set gap.
Knife coaters
- A. Substrate (web)
- B. Backup roll
- C. Knife roll
- D. Applicator roll
- E. Pan
Knife coaters consist of a knife roll, backup roll, and liquid dam. The coating fluid in the liquid dam is transferred onto the substrate web through the gap between the turning backup roll and the fixed knife roll. The coating thickness is set based on the gap between the backup roll and the knife roll. With knife coaters, ensuring the accuracy of the backup roll assembly, knife roll blade, and adjustment of the gap between the rolls is important. It is also important to prevent jumping at substrate seams, which affects coating stability and can cause fluid leakage.
Blade roll coaters (with comma-type circular blades)
- A. Applicator roll
- B. Coating
- C. Backup roll
- D. Substrate (web)
Blade roll coaters are characterized by an applicator roll with a blade, which is called a circular blade, or a comma blade, for its shape. Coating fluid is pooled between the applicator roll and the substrate, and the blade on the roll wipes off excessive fluid to ensure an optimum coating thickness. This method is generally suited for highly viscous coating fluids or when forming a thick coating. However, some models can support various coating thicknesses.
Examples of other methods
In addition to the above, various other methods are available according to the coating purpose or the properties of the substrate or coating fluid. Some use applicator rolls in different shapes, various reverse rolls and offsets, or a combination of other rollers and methods. The following introduces a few such examples.
Slot Die Coating Head Design
Blade coaters
A doctor blade and backup roll maintain uniform thickness to allow smooth coating at high speeds. With this method, streaks (linear defects) are a concern.
Chamber doctor coaters
Coating fluid is supplied from a chamber, which prevents the emission of solvent smells and changes in viscosity while also allowing for easy blade replacement. This method can be used for direct gravure coating or reverse gravure coating.
Bar coaters
After coating, a bar makes the coated surface uniform. This system is suitable for small-quantity and high-speed coating of undercoats and topcoats.
Air knife (air doctor) coaters
These coaters are suitable for water-based coating fluids. After coating, pressurized air is applied to make the coated surface uniform. This method is less affected by the substrate thickness or uneven surfaces.