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Gauging system lights the way

BKL uses converting technology to make electroluminescent lamps; Betacontrol of America supplies the in-line gauging.

Electroluminescent (EL) lamps have become a popular light source for backlighting liquid crystal displays for watches, pocket pagers, and a host of other innovative products. These thin, flexible, lightweight-yet-rugged lamps deliver a cool, soft, nonglare light in even the smallest places. Making EL lamps was once a slow, labor-intensive process using expensive screen printing technology. BKL Inc., located in King of Prussia, PA, however, claims to have "reinvented" the way EL lamps are made, using technology drawn from the high-speed converting industry.

BKL uses a continuous process to apply luminescent coatings - to exacting tolerances - to the plastic alloy substrate used in the production of EL lamps. In this process, the thickness and uniformity of the coating are absolutely critical: Too much or too little can result in poor or no luminescence, or the quality of the light can vary from one area of the lamp surface to another. In addition, the high cost of EL coatings and the competitive nature of the business demands that a precise amount of coating be applied.

To guarantee precision and quality, BKL has installed an in-line gauging system from Betacontrol of America that ensures the performance and economic success of the product.

How It's Done

According to Jim Durante, coatings and lamination manager for BKL, the construction process of the lamp begins with two thermoplastics polyester film substrates. One is supplied with an indium tin oxide (ITO) coating already applied by the film vendor; the other is metallized by a sputtering process (supplier information is proprietary).

The ITO-coated polyester receives an additional thin coat (under 5 mils) of a phosphor suspended in a proprietary binder system. The metallized polyester substrate gets a barium titanate coating that is typically even thinner than the phosphor coating.

The process lines for the phosphor and barium titanate coatings were developed in-house by BKL. The substrates are fed from an unwind stand, and the coatings are metered onto the polyester as it moves past the coating head. Each substrate also receives a conductive bus, where a mechanical termination is subsequently attached. Then, the coated substrates pass into a drying oven for curing.

Finally, the two coated substrates are laminated together to produce the finished BKL roll-form EL material. Lamps are die-cut from the roll-form product to exacting tolerances using precision steel rule dies. Depending on the lamps' ultimate use, they can be encapsulated to protect the phosphor from moisture. Typical encapsulating materials include polyester and Aclar, a flexible thermoplastic copolymer made and sold by Allied Signal. After encapsulation, the lamp is trimmed and electrical terminations (supplied by AMP Inc.) are applied in the conductive bus. The lamp is then tested prior to shipment.

Coating is Critical

The most critical factors in the patented coating process utilized by BKL, reiterates Durante, are the thickness and consistency of the coatings. Typical coating thickness tolerances are approximately [+ or -]0.0001 in.

"Any inconsistency in thickness will result in brightness variation," explains Durante. "One side might be bright and the other dim unless we have precise control over thickness. Until a year ago we were measuring coating thickness using a snap gauge, but that is very inconsistent and very dependent on the skill of the person using it. It also could not give us a web-thickness profile.

"We wanted to control our process better," says Durante, "and we also wanted to measure closer to the coating head. With our previous gauge, we were forced to measure at a point 35 feet downstream from the coating head, because the coating had to be dry before the snap gauge could be used. So, by the time we discovered the problem, we had already generated at least 35 feet of scrap."

Given the high cost of the materials used in EL lamps, even a small amount of scrap could be costly, notes Durante. The new Betacontrol gauge, however, fits neatly in an area just 1.5 feet from the coating head.

The Betacontrol gauge uses an isotope of the radioactive element krypton to generate beta rays that travel through the material to be measured. The radiation level weakens as a function of the volume of coated film through which it passes. Given constant material density, these readings become a highly reliable measure of material thickness, says Durante. The polyester substrate material BKL purchases is itself produced to exacting tolerances. So, when its thickness is automatically subtracted from the beta gauge readings of total thickness, the result is a reliable measure of coating thickness.

No Contact Needed

Durante points out that the chief benefit of the beta gauge - and the one that allows it to be installed so close to the coating head - is that it is a completely contactless method of measuring. The coated substrate can be gauged while still wet, even before it passes through the curing oven.

BKL considered other gauges, Durante says, but none were completely contactless. And some even required two sensing probes: one to measure the substrate prior to coating, the other to measure the thickness of the coated substrate. The Betacontrol system requires just one, so there are fewer sensors to buy and maintain.

In addition, BKL wanted to ensure that thickness was consistent across the width of the substrate. To accomplish this, the other systems required several fixed sensors to take readings across the web. With the Betacontrol gauge, both the transmitter and receiver traverse the entire width of the coating surface, providing a "thickness profile" that documents consistent coating thickness and ensures even, consistent brightness in the finished EL lamp.

Fewer Rejects, Less Waste

Durante estimates that the Betacontrol system has helped BKL reduce its brightness variation by 20%. The new gauge has also reduced scrap significantly on run change-overs. But the most significant savings, notes Durante, are realized during a run. System software processes the data from the sensor, producing a profile of coating thickness. After manufacturing personnel establish upper and lower control limits, the system will automatically monitor the thickness and sound an alarm whenever the process begins to drift. BKL personnel can then adjust the coating head to eliminate the variation and prevent rejects before they happen.

The system is now being used to automate statistical process control and has eliminated some of the manual SPC readings that are taken off-line as a quality check. When the new system is fully implemented, Durante expects real-time data collection will eliminate manual measures almost entirely, cutting SPC labor costs as well.

"The Betacontrol system met all our criteria and offered the shortest payback period," adds Paul Lightkep, purchasing manager. "Price-wise, Betacontrol is very competitive, and the fact that we only had to have one scanner, which oscillates across the web, made it very attractive."

Supplier Information:

Betacontrol of America, Towaco, NJ; ph: 201/2634243; fax: 201/263-0477.

Allied-Signal Specially Films, Morristown, NJ; ph: 2201/455-3210; fax: 201/455-3507.

AMP Inc., Harrisburg, PA; ph/fax: 800/522-6752.

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