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Plastics extrusion: Embossed microstructures offer decorative effect options

Film or sheet requires a lotus look, sharkskin effect or something more? Better temperature control of chill and embossing roll surfaces could soon help extrusion processors add even more value to their products. The recent biennial plastics technology colloquium conducted by Germany's Institute of Plastics Processing (IKV; Aachen) provided a number of presentations targeting extrusion difficulties and possible methods to circumvent them.

; Aachen) provided a number of presentations targeting extrusion difficulties and possible methods to circumvent them. For example, IKV engineer Stefan Eilbracht demonstrated lab studies showing that when embossed on the surface with microstructures, then lotus, optical (reflection- or hologram-structure) or controlled sharkskin effects can be achieved even at production speed, and economically.

Quality of these effects often depends on better temperature control in the embossing area, says Eilbracht. He and his team, under the supervision of IKV director Walter Michaeli, showed two concepts for vario-thermal temperature control that directly increase the heat in the embossing area of both the embossing and backing rolls.

"The [cast line's] embossing roll fulfills a double function - to emboss and then as a chill roll to cool the melt. For embossing, the temperature needs to be high, above the setting temperature of the melt," explains Eilbracht. "But then the film needs to be quickly cooled in order to achieve production speeds."

Targeted heating at the right spot
He and his team came up with two methods by which a very small section of the chill and embossing roll surface achieves the right heat at or just prior to the area below the flat die slot introducing the melt to the roll surface. One method uses an external linear inductive heater while the other includes an infrared-diode laser. Both methods offer targeted heating at exactly that part of the moving metal roll surface where it is needed, rather than over the entire roll surface as with conventional systems. The two methods were trialed on the institute's Collin (Ebersberg) lab cast line using an induction system from Eldec Schwenk Induction (Dornstetten) and an infrared-diode laser system from Laserline (Mülheim-Kärlich; all Germany).

When a high AC current passes through the inductor it builds an alternating electro-magnetic field that produces a turbulent flow through the electrical resistance of the steel surface to heat at that particular spot. "This increased temperature is not realized by a heat transfer procedure for the entire roll, but rather only the outer surface of the roll," he says. This enables the roll to dissipate the heat faster so the roll can provide its following chill effect. The heat thickness of this surface layer depends on the frequency of the electro-magnetic field and the electrical properties of the roll material. Inductive heating, so says Eilbracht, functions best for metal surfaces with inferior electrical conductibility such as steel, rather than aluminum or copper.

One problem with this method, however, is the need for a large installation space at the die exit, which in most cases is not available, and the gap between the inductor and the roll surface needs to be extremely close.

Eilbracht says an alternative would be to use a laser in place of the inductor heating device located just prior to the flat die head. The advantages of this system include a high energy density, easy installation due to its compact size, and the polished roll surface trends to reduce surface heat absorption while the laser beams concentrate on the transparent film (due to their multiple reflection between the embossing-chill roll and backing roll). Negative is that this system tends to involve substantially higher investment costs than the inductive system and can require elaborate shop floor safety measures.

The comparative study showed that laser-based temperature control over the roll surface could allow better targeted temperature placement since heat camera results show the laser has a higher degree of efficiency than the induction method. Eilbracht says at a heating output of 30W/mm, the laser is able to provide a temperature difference of up to 50K which allows embossing at a temperature above the setting point while the following cooling phase is well below this temperature.

Further tests are needed, notes Eilbracht, to determine which temperature differences provide the best and most accurate reproduction of the surface detail. Also to be investigated is the interaction between the melt and the alternating electro-magnetic field produced by the inductive heating element compared to the infrared radiation of the laser beam for best temperature control. Should these tests prove successful, then extrusion processors will have some new decorative options to consider.

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