Efficiency Gains and Control Improvements Using Different Barrel Heating Technologies for the Injection Molding Process - Xaloy - #1

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Bruce F. Taylor, Timothy W. Womer, Robert Kadykowski, Xaloy® Corporation, New Castle, PA

This paper compares the energy efficiency and control response of band-heaters with a new technology that uses non-contact induction to heat the barrel directly through an interposed layer of thermal insulation. Quantitative results from both laboratory injection molding machine runs and bench-top tests are reviewed. The effect of barrel diameter, surface condition and band-heater type on effi-' ciency and control response are also considered, as are the impli­cations of thermocouple depth.

Electric machines have significantly improved the energy efficien­cy of the injection molding industry over the last 20 years. Now, with this technology broadly accepted, the industry must find new ways to save energy, to remain a good global citizen and keep pro­duction costs down in the face of rising energy costs.

Melt-stream heating, particularly of the barrel, presents a compel­ling opportunity. The inefficiency of conventional barrel-heating technology is well understood. Typically between 30 and 70 % of the power consumed by band-heaters is wasted by radiation and convection to the surrounding environment. Eliminating these los­ses will reduce specific energy usage (i.e. kW/Kg of product) and allow machines to be pre-heated faster using the same power deli­very infrastructure, thereby reducing downtime to increase pro­ductivity.

Inherent characteristics of band-heaters also hamper temperature control response, limiting improvements in part-to-part quality uniformity and efforts to minimize change-over times. A band-heater's temperature must first rise above that of the barrel befo­re the barrel can be heated, and conversely, a band-heater's tem­perature must fall below that of the barrel before it can be cooled. The thermal mass of band-heaters (mass x heat capacity), and the thermal contact resistance (°C/watt) between them and the barrel, therefore significantly increase the thermal inertia of the melt-stre­am.

The recent introduction of alternative lower-mass radiant heating elements highlights the industry's growing recognition of the opportunity for improvement.

Another new technology that offers significant advantages is non-contact induction. Barrel heating using helical induction coils has been considered for decades, but was poorly applied. Past efforts often used inefficient low-frequency power supplies and always put the coils in direct contact with the barrel, undermining the compelling advantages of induction. Heat generated in the barrel was still allowed to escape to ambient and the coils' thermal mass wasn't removed from the equation. Contact with the hot barrel also increased the coil's electrical resistance, further undermining efficiency gains.

The nXheat^TM barrel heating solution (patent-pending) uses an optimized high-frequency power supply and a thermal insulating layer interposed between the barrel and coils to address the abo­ve issues and exploit the full potential of induction. All the heat is generated directly within the barrel and remains in the process. The coil's thermal mass is also eliminated, and coil resistive losses are negligible so the exterior surface is cool to the touch. Barrel heating efficiency approaches 100 % and temperature control response is significantly improved.

Various tests were performed to quantify energy efficiency gains and process control improvements, using a combination of bench-top setups and production runs on a laboratory injection molding machine. A sampling of the results is presented herein. Additional lab tests are underway and envisioned, as are monitored installati­ons on a spectrum of production machines.

Heating system efficiency was assessed two ways. First, the hea­ting system power required to process the same material and throughput was compared for band-heaters and induction on the laboratory injection molding machine. An identical part was pro­duced at a constant rate from multiple materials using two diffe­rent screw types. Next, for both band-heaters and induction the instantaneous heat input rate to a bench-top barrel segment (measured using an array of thermocouples located within the wall of the barrel) was compared to the instantaneous power con­sumption. Both tests also provided insight to control response. During the molding machine tests the same auto-tune controllers were used to control both the band-heaters and induction, allo­wing the resulting control interval times and melt-stream tempe­rature variability to be compared. In the bench-top barrel segment tests the lag in heat input and removal, into and out of the test barrel, when the heating means were turned on and off, was com­pared for band-heaters and induction.

Test were done on a Toshiba injection molding machine with a bar­rel ID of 36 mm and OD of 90 mm, using three (3) 250 mm long temperature control zones. Each band-heating zone comprised four (4) MICA-style band-heaters consuming approximately 3,200 watts/zone. Each induction zone comprised a helical coil driven by a specialized inverter adjusted to deliver 2,000 watts. The helical coil was wound about a grooved plastic winding sleeve, which sur­rounded a 19 mm thick layer of mineral-wool pipe insulation that was interposed between it and the barrel (Figure 1).

Eleven (11) bare thermocouples spaced along the barrel length were installed at a depth of 20 mm in heat-conductive cement. Three of these thermocouples, centered within the three zones, were used for control. A conventional on/off control scheme was employed using three conventional auto-tune PID controllers.