Tech Tip: Managing the Die, Coating Gap and Coating Bead in PSA Hot Melt Slot Die Coating
Slot Die Flow Channel Design
When coating a PSA hot melt with a slot die, the hot melt is fed via a drum unloader, melter or extruder and enters the die through a feed adaptor, into the throat of the slot die. That throat feeds the manifold. The main goal of the manifold is to provide distribution from the center of the die to the ends. A coat hanger-shaped manifold design is critical to achieving even distribution throughout the die. The wider the slot coating, the more challenging it is to get the distribution to the ends evenly.
The manifold design and land length are based primarily on rheology (how the material flows), and secondarily on process conditions.
The preland distributes the coating material evenly prior to the lip land. The lip gap, established by a machine gap or a body shim (if lane coating), can be adjusted and thus profiling the die lip to get as uniform of a coat weight as possible. So, though the overall thickness/weight of the coating is mostly based on the combination of flow rate and line speed, the fine distribution of that fluid is determined by the land length and lip gap. In general, with a larger lip gap, the pressure decreases and the melt flows heavier to the center. The tighter the lip gap, the pressure increases and redirects the flow to the ends.
But how does it redirect? The profiling of the lip gap adjusts the pressure inside the manifold to add more or less resistance to flow. This resistance to flow will change the velocity of the fluid, which will cause it to distribute more or less to the different areas of the die. Because you¡¯re adjusting pressure, with small adjustments, it takes a little bit of time for the pressure and distribution to stabilize (within 30 seconds up to a few minutes, depending on the amount of adjustment and fluid properties). Once you make an adjustment, the change in pressure has to make its way through the entire closed system back to the pump to stabilize. You will see some change, but not the full change right away.
Though the change in pressure and flow occurs within minutes, it is important for the operator to know that the change will not immediately show with the gauging scanner. It may take 10-15 minutes to see the full change given that the scanner takes a composite of several scans (to soften the machine direction variation) and give a true cross direction profile.
There is one other delayed response that occurs when profiling the fluid, but this one relates to the coating gap. Once a lip profile/pressure adjustment change is made to coat thicker or thinner (heavier or lighter) in one area, there is a change in the amount of the hot melt coated in that area, which applies more or less heat to the backing roll in the same area. The backing roll will slightly expand (when more heat) or constrict (when less heat), which will affect the coat thickness/weight accordingly. This coating gap profiling can take as much as 20-30 minutes (depending on the size adjustment) to show up as a coating profile change via the gauging system (for the same reasons mentioned previously regarding scan time).
The delay in coating profile changes showing up in the gauging system (via lip gap adjustments and/or the backing roll/coating gap expand/restricting) are very important things to take note of (and communicate to die operators) because we often hear about operators quickly making additional adjustments, after not seeing a significant enough change right away when they make an initial adjustment. This creates a less efficient process and more scrapped product due to making corrections.
Coating Gap
The hot melt adhesive is pumped into the die and exits in contact with a moving web. The web is usually supported immediately behind the slot with a rubber or steel backing roll. For coating hot melt PSAs, you¡¯re pushing into a rubber backing roll to shear the melt onto the substrate. Pressure(force) is needed to achieve a good wetting out of hot melt PSAs due to fluid characteristics of the melt and surface energy of the substrate. By using hydraulic pressure, you¡¯re creating the coating gap via the coating bead. Therefore, your operating coating gap is the coating thickness.
This makes the relation of the coating gap (along with lip face straightness and backing roll TIR) even more critical. The lip gap is profiled to provide a uniform coating, but the coating gap either maintains that coating or it can lose its flat profile. If the coating gap is larger in one area, that becomes the least resistant path so you will get more flow to the area. This least resistant path flow is very prominent in wipe coating (compared to draw coating), because of the pressure build up, given the close proximity to the roll. Therefore, a temperature-controlled backing roll (we typically see customers heat it to around 80¡ã C, significantly lower than the melt temperature) is utilized to try to minimize the amount of deflection that occurs as the hot melt is coated onto the substrate and heat transfers to that area of the backing roll. Some deflection of the roll in the center area still occurs (even given that it is temperature controlled) because the hot melt is causing it to heat up and expand in the coating area. Moreover, depending on the die design, you may also see some slight deflection in the die given the wire pocket. To combat the deflection in the roll (or possibly the die), the die is typically mechanically or thermally bent in on the ends via the positioner to facilitate an even coating gap across the die.
The ideal setup is to adjust the footprint so you are unable to pull out a 2-mil shim and back the adjustment out until the shim can be pulled out. When doing this (checking multiple spots across the die), the backing roll should be rotated every few seconds to prevent the continuous heat concentration from the die on a specific area in the roll.
Offset
Once the hot melt flows into the die, through the manifold, preland, and land area, it exits the die and forms a bead between the lip face and the substrate. The manipulation of that bead can occur via offset of the die bodies. Pressure distribution of the coating bead can influence the position of the meniscus.
To create an offset, typically shims are used in conjunction with an offset block system to adjust the lip faces either to be parallel or with an ¡°underbite¡± (with the lower body ahead of the upper body) or ¡°overbite¡± (with the upper body ahead of the lower body).
Both underbite and overbite setups are utilized when coating hot melt PSAs. We recommend starting with the lip faces flush and then figuring out if an adjustment is needed based on overspill, backspill, and/or coating appearance.
In our experience, an overbite can be helpful when coating thin PSA coatings. Thinner, light weight coatings are more difficult because the coating gap and lip gap are so small, resulting in a small coating window. Overbite is used for shearing/appearance and creates a little more of a melt well upstream. This shifts the position of the meniscus on the lower lip and stabilizes the coating bead to allow for more material to be available to coat (melt well). This also helps avoid air entrainment or voids.
For thick or higher weight, but low viscosity, PSA hot melt coatings, an underbite can be utilized to allow a larger melt well, which enables thicker wet coatings to clear but keeps the lower lip close to the roll, so you don¡¯t have spilling upstream past the die lip.
Typically offset is setup in 1 mil increments. Starting with 0.001¡± and then increasing until the coating defect is resolved. If the issue is drastic, then an increase by 2-3 mil may address the issue quicker. Too much overbite can cause back spill due to pushing the meniscus beyond the lower lip face. If there¡¯s too much underbite, you could see some coating issues because the downstream coating bead might not have a clean release at the lip exit point (causing ribbing).
Angle of Attack
The ¡°attack angle¡± refers to the angle of the die lip faces (controlled by the die positioner) in relation to the backing roll and substrate. The attack angle can influence the pressure drop and flow velocity in the coating gap. Typically, a positive converging angle, with the downstream lip closer to the substrate is utilized to increase the pressure drop (due to a decreasing coating gap), flow velocity, and shearing of the hot melt PSA. This shearing also aids in the surface appearance of the coating.
The attack angle adjustment maintains a parallel, non-diverging flow path which uses the relationship of the lip face to the circumference of the roll. The proper set-up of the die to the roll at 0¡ã of offset is the die¡¯s lower lip land, extended as a plane, passing through the center of the backing roll. The axis of the rotation should be the apex of the lower lip land and its lip face.
If the position of the lower lip exit is parallel and in line with the center of the backing roll, then either more or less angle is needed (depending on the direction).
We recommend utilizing the following formula as a good starting point for attack angle:
Wet lip face length (in.) / (backing roll diameter [in.] X .0175) = starting attack angle (degrees)
*Formula for non-rotary rod style lip inserts and a rubber backing roll is multiplying the calculated ¡°attack angle¡± by 2. The doubling of the attack angle for a rubber roll compensates for the compression of the roll.
*For rotary rod style inserts, start with a 0-degree angle.
These are starting points and can change depending on appearance, how much is carrying over, etc.
Too little attack angle (diverging angle, with upstream lip closer to the substrate) can result in air entrainment and unstable coating results, like ribbing or chevrons, are likely to occur in the coating. The coating is likely to have a duller finish as well.
Too much attack angle (converging angle, with downstream lip closer to the substrate) can push the meniscus beyond the lower lip face. This under spill will likely cause air entrainment as air is entrapped during the destabilization of the meniscus, and causes voids in the coating, unstable coat weight uniformity, and excessive wear on the upper lip.
Most of the time, a positive angle of attack (typically within 2.0¡ã or less) is used to increase the pressure drop (due to a decreasing coating gap) and shear the hot melt PSA. This produces a glossy, smooth appearance. The angle of attack is an ¡°on the fly¡± running adjustment, to be adjusted slowly (measured to 0.1¡ã) until the coating appearance is acceptable.
Again, these are general recommendations which are dependent on rheologies, surface tension of the fluid, surface energy of the substrate, and other properties of the PSA coatings. The substrate also matters and can affect these settings. PET (polyester), paper, and foil can coat differently and have different substrate surface energies that affect the adhesion of coating and manifestation of defects.
Using the correct attack angle and lip face offset means converters see greater results in their coating process by reducing coating defects to improve finish and stabilizing the coat weight uniformity. This also helps protect the life of their equipment by operating at an optimal gap for minimal die wear.
Rotary Rod
The rotary rod is used with a lip design primarily to help pass through contaminants to prevent them from getting stuck between the die and the substrate, causing die lines and streaks. It is typically recommended when coating 50 microns or less due to the small coating gap that occurs with thin coatings. The rotating rod can also help have a slight effect on the surface appearance. The speed at which it rotates is subjective to the desired appearance.
Typically, rotary rod lip faces are run flush because the rotary rod already extends out past the lips.
The recommended starting speed is 10 rpm and can be adjusted accordingly. If gels aren¡¯t passing well (resulting in streaks), then the speed can be increased. If you want higher shear, then you can slow down the rod to get a better surface finish. The aim is to produce a smooth surface finish while avoiding streaking.
General Set Up
The following initial set up for the die follows these steps:
The aim is to use the largest possible "wet lip face to substrate gap" with the smallest attack angle that will apply a smooth and acceptable coating. This prevents leakage past the lower, dry lip and promotes consistency and repeatability with the least amount of adjustment.
Conclusion
In conclusion, the diverse needs of PSA tape manufacturers utilizing slot die coating highlight the importance of accessible, practical guidance to optimize this precision coating method. Whether addressing the challenges faced by newcomers lacking internal expertise or supporting seasoned manufacturers grappling with the loss of legacy knowledge, this technical blog aimed to bridge the gap. By exploring best practices and offering actionable advice on die setup and operation¡ªon offset, angle of attack, and lip gap adjustments¡ªthis discussion seeks to empower companies to enhance efficiency and performance. Ultimately, the goal is to equip manufacturers with the tools and understanding needed to fully leverage slot die coating for both existing and new equipment, ensuring consistent, high-quality outcomes in hot melt PSA production.