Sequential extrusion joins different resins seamlessly.

Tubing and other products can now be extruded from a sequence of compatible but different resins having different durometers or other properties. This technique reportedly eliminates the headache and expense of secondary joining operations and the potential sacrifice of structural integrity that can result. In addition, multi-polymer or multi-durometer products can now be extruded on a single line and without downtime to change over from one polymer or durometer to the next.

First reported briefly in PT, April '96, p. 14, the new "Alternate Polymer" process from Harrel Corp., Norwalk, Conn., can combine two or more polymer components end to end during continuous extrusion. The process has been field tested for over a year, mainly to make medical tubing. However, the technique is also applicable to blown or cast film, sheet, pipe, or profile, says Holton Harris, Harrel president. He says there are no output limitations - simply use extruders sized to your desired rate.

The Alternate Polymer process is an extension of Harrel's patented Tube-Trol control technology for providing tightly programmed control of product dimensions, such as is required for "bump" or taper tubing. In this case, the programmed control has been extended to changing the extrudate composition, as well.

SET UP LIKE A COEX LINE

Harrel designed the AP-600 turnkey extrusion system specifically for alternating polymers. Harrel offers a complete two-component extrusion system at a 20-25% premium above a conventional coextrusion line.

The line set-up mirrors that of a coex line but differs in that it can shift from the extrusion of one polymer to another. Die designs require no special changes. "We have used a standard coextrusion die with the normal ports on the sides or the rear of the die," says Harris. However, the ability to change from polymer to polymer calls for changes in the extrusion equipment and especially in line control.

Gear pumps on all extruders are an essential part of the process. "Extruder output is not proportional to screw speed, it is highly non-linear. Further, the drive motor, screw, and the polymer in the flights have high inertia and cannot be quickly started and stopped," Harris comments. By contrast, a gear pump has much less inertia, so its speed can be changed much more quickly, Harris explains. It also prevents drooling of the polymer. He adds that the gear pump has a very linear output, so output becomes easy to control.

The Alternate Polymer process uses Harrel's Geartruder system, in which the gear pump is slaved to the extruder. This combination maintains a minimum pressure at the input to the gear pump, which ensures that the pump teeth are always filled with polymer. This gives the operator a known output of material per revolution.

The Geartruder also controls differential pressures from pump inlet to outer. A servo drive on each gear pump is said to ensure accurate changes in polymer proportions as one resin is gradually ramped down from 100% to zero and the other ramps up from zero to 100%.

Harrel uses a screw design that tolerates the varying pressures generated when the gear pump is turned on or off. As the process alternates from one resin to another, the extruder screw stops when its respective pump halts and starts up just a moment before the pump starts turning again in order to provide a minimum head pressure. "The screw must allow for pressure spikes," Harris says. He notes that changes to the metering section give the screw this capability.

CONTROL IS THE KEY

The Harrel system uses two Digi-panel controllers to execute the transition from polymer to polymer consistently and repeatedly. A controller on each extruder monitors functions such as the extruder and gear-pump rpm's, pressures, and temperatures.

In addition, one of the controllers monitors the company's patented TubeTrol process to control the dimensions of the product. The latter controller also regulates the polymer mix by changing the relative speeds of the gear pumps on the extruders. It also keeps track of the polymer proportions and makes corrections to the TubeTrol commands as the polymer mix changes. This feature addresses the fact that different polymers do not react identically to the same machine-control settings. The controller can also be used to trigger the cutoff device at the end of the line. All of the data for both controllers can be entered through front-panel keyboards.

Harrel has not attempted to find out how fast the system can execute a resin changeover. However, Harris speculates that a 100% change of polymer on a tube line operating at 100 ft/min can be executed within 2 to 4 in. of product.