A new slot die for coating continuous-web substrates provides consistent, streak-free application of pressure sensitive adhesives (PSAs), achieving higher line speeds and greater accuracy than conventional roll coating, with less environmental impact, it was announced today by Extrusion Dies Industries, LLC (EDI) at ICE Asia (Booth 1-C05).

EDI developed the die specifically for PSAs, a hot melt adhesive subgroup. While roll coating speeds for hot melt application usually do not exceed 1200 feet/min. (365 meters/min.), the new die developed by EDI accommodates line speeds up to 2,000 feet/min. (610 meters/min.) and achieves coat weights as low as 0.5 mil (0.0127 mm). This is well below the conventional lower limit for hot melts of 1.0 mil (0.025 mm), according to Dennis S. Paradise, vice president of sales and marketing.

"As a completely enclosed, pre-metered system, slot die coating is more efficient and controllable than roll coating and other conventional open systems, in which only part of the adhesive delivered to the substrate is actually applied while the rest is re-circulated," said Paradise. "By avoiding this exposure of the adhesive to the environment, slot die coating prevents workplace emissions and contamination of the coating material."

Besides being available as a stand-alone system, the new EDI die can be supplied as part of a completely integrated, computer-controlled coating station incorporating melt delivery systems from ITW Dynatec and slot die equipment from EDI. "Resulting from a cooperative program between EDI and ITW Dynatec, the new coating station provides turnkey capabilities for tight-tolerance, high-speed application of virtually any type of hot melt adhesive, including PSAs," said Paradise. "Its performance is guaranteed by the partner companies."

How EDI's New Slot Die for PSA Coating Works

The method employed by the new die is contact coating, which involves applying the adhesive directly to a substrate, using the die lip to "wipe" the adhesive onto it. The keys to the advanced performance of the die are: 1) an innovative system for adjusting the cross-direction coat weight profile and 2) incorporation of a device that eliminates streaking and makes possible clear applications of PSAs.

Streaking is caused by gels or solid particles that lodge in the narrow lip opening and the interface between the die lip and substrate. In the new die, a motorized rotating rod in the lip area allows gels to pass through lip opening and coating interface.

At the same time, the new slot coating die includes a mechanism for adjusting the coat weight profile. Called a "restrictor bar," it is located between the manifold and the lip opening and spans the width of the flow channel. Coat weight adjustment is achieved by selectively turning bolts at regular intervals along this span to deform specific segments of the bar, inserting them into (or retracting them from) the melt stream and thus changing the height of the flow channel. This is a mechanism that has long since proven effective in dies for sheet extrusion.