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A Communication from the PLACE Div. of TAPPI

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The Role Of X-ray Sensor Technology In The Application Of Extrusion Coating And Lamination

by Gareth Joseph and Chris McGowan, NDC Infrared Engineering

X-ray is now a more accessible technology because the x-ray tubes for the plastics and converting industry (soft x-rays) have become far more stable and robust. Added to this, the price of x-ray tubes has reached a level that makes them more viable to use in industrial processes. Today X-ray sensors are not particularly common in the plastics and converting industries. Compared with beta sensors, they are rare species. This balance is set to change as the benefits and advantages of this technology start to be realized fully and introduced to industry. Added to this, the external factors of government legislation and user preferences means the demand for low energy or non-isotope technologies will grow rapidly.

The x-ray is a fairly simple device that has use in many everyday situations especially in the medical and dental fields. Electrons are “burnt off” a cathode plate and accelerated towards a heavy-metal anode by means of a high voltage applied between the two. The x-ray tube is a vacuumed capsule. When the “highly energized” electrons strike the anode, they release energy that is part of the electromagnetic spectrum—x-rays.

X-ray sensors offer many advantages for various applications including extrusion coating and laminating. Nevertheless, each process and line must be viewed as a separate project, and the specifications and requirements ultimately will define the most appropriate measurement solutions. Ultimately, a number of alternative sensor technologies always exist. The potential is also possible to use several types on one line such as a combination of GBS, x-ray, and IR backscatter. Selecting the correct gauging solution has critical importance because the costs of error and poor measurements can be alarming especially on the new high speed coating and lamination lines of today.

Comparative Design And Performance Of Primer Dryers For High-Speed Extrusion Coaters

by William R. Henry, Advance Systems Inc.

An extrusion coating line uses a hot air dryer to dry a primer coat. The one-sided, aqueous coating has a low solids content that must dry in limited space. The most effective and compact drying devices use high-velocity air impingement technology.

Flotation and slot-over-roll dryers are both effective for drying a primer coat at speeds below 300 m/min. The h factor for flotation air bars is inherently greater than for the slot-over-roll configuration, but the differences are small at these speeds. In process upgrades aimed at achieving significant speed increases from existing production lines, the performance gap between the air bar flotation dryer and slot-over-roll dryer increases considerably. As such, a flotation dryer will use a lower air temperature, less physical space, and less energy (gas consumption) per square meter of product.

At line speeds approaching 600 m/min for flexible packaging and 900 m/min for coated papers, the slots-over-rolls dryer may be subject to the negative affects of a fast moving web over a shallow-wrap roll. This includes cross-machine web instability and scratching caused by roll and web speed differences. The non-contacting nature of flotation air bars results in drying and web handling characteristics that are independent of line speed. Both the slot-over-roll and flotation dryer designs can handle drying requirements for a full range of production line speeds, but the inherent characteristics of the flotation design seem to provide a more energy efficient and stable result.

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