PT Online

When it comes to warm-up speed, formed part quality, and energy savings, halogen heating elements are tough to beat, according to two British custom thermoformers that are among the first to use these heaters. Sources at Thompson Plastics Group in Hull and Wokingham Plastics in Wokingham, Berkshire, confirm that halogen lamps provide heat-up times as short as 0.2 sec (versus 30-60 sec with quartz tubes), immediate response to setting changes, and ability to heat without scorching.

UNTRIED IN NORTH AMERICA

Halogen heaters have been used commercially for four years - but not yet on this continent. More than 20 thermoforming systems equipped with halogen heaters are operating in Europe and Japan. The machines were built by Geiss Maschinenfabrik of Sesslach, Germany. Geiss's "Flash" line of machines uses halogen tubes manufactured by Ushio of Japan. Each halogen element packs up to 50 w/sq in. of power and is rated at 2000 F maximum temperature.

Thompson Plastics and Wokingham Plastics each have one Geiss T7 Flash machine. Both companies typically use sheet from 0.117 up to 0.250 in. thick. Thompson, a large producer of parts for automotive and residential applications spent about 20% more on the halogen elements and an additional $166,000 for a transformer substation that was required for the high-energy halogen elements, says Andrew Eavis, Thompson's joint group chairman. The added capital cost is offset by energy savings, Eavis notes. Geiss says halogen heaters provide energy savings of up to 20%.

Quick heating and high-heat capability reduces cycle time and heats the sheet more thoroughly than do conventional elements, according. to the U.K. processors. Both companies claim to have experienced a 50% drop in heating time with the halogen elements, coupled with a drop in rejects due to surface defects like blistering or discoloration.

PENETRATING WAVES

At maximum temperature, halogen tubes emit most of their energy as infrared wavelengths of 1 micron. Plastics generally best absorb infrared at 3.1 to 10 microns. For that reason, the energy from the halogen tube tends to penetrate into the sheet and heat the core first, rather than being absorbed at the surface like the longer wavelengths are. Thus, halogen heats from the inside out, the opposite of conventional heaters.

More energy is needed to operate the halogen tubes. "But we can put the same energy or less into the sheet in a shorter period of time," says Eavis. The net result is said to be lower energy use.

Thompson has seen a 30% improvement in productivity in its forming of pigmented PS, ABS, and acrylic on the Hash machine in more than two years of use. Eavis believes it's the pigment in the sheet that is primarily excited by the incoming infrared.

Wokingham has processed pigmented and clear polycarbonate on its Flash machine since last January and has found no difference in cycle times, says managing director Terry Bradstreet. Wokingham makes electronics enclosures and children's play items.

A Siemens S7 computer controls the heaters in the Flash machines. Thompson has 400 elements in a 3 x 6 ft oven and Wokingham has 266 in a 3 x 4.5 ft oven.