Engineering by Design
San Jose, Calif.
Extruded-plastic tubing serves in countless applications, from vacuum lines to medical catheters. But the plastic usually needs help making it through the extrusion die. This is where extrusion pullers come in. They apply tension to the plastic layered tubes or solids at up to 300 fpm using a pair of adjustable-speed pinch rollers or caterpillar drives.
Making cylindrical product with a consistent diameter can be tricky. For example, higher pull speeds shrink tube diameter while die temperature and mandrel air pressure also affect size. Meanwhile, so-called hot dies located downstream between pinch-roller pairs shear unwanted discontinuities on the outside of the tubing.
Extrusions may also include subsurface wire braiding. High-voltage electrical probes detect sections with exposed wires and signal a cutter to chop them out. Good product is then collected on a roll or cut to selected lengths.
If any process step is out of whack, it can throw downstream steps out of whack, as well. Unfortunately, the equipment performing each of these functions often uses controls with different, perhaps proprietary, interfaces and protocols. This lack of a common "language" not only complicates diagnostics but also demands maintenance personnel be proficient in multiple software packages. On top of that, the trial-and-error method of manually adjusting process variables consumes time and raw material. So forget about easily controlling the equipment remotely.
Most of today's extrusion pullers combine insulation-integrity testing and cutting product to length in a single machine. But some extrusion pullers go a step further and use various methods to build a control loop between puller hardware and key extruder processes. Closed-loop control boosts production, reduces scrap, and makes possible remote operation.
However, some closed-loop systems only work for constant-sized extrusions. One reason is the use of laser micrometers for size feedback. Laser micrometers tend to have single-diameter target range and going outside that range renders signals meaningless. Well almost. Operators often use the off-the-scale signals to manually adjust puller speeds, timing, extruder heat, and air pressure to change tubing diameter at fixed intervals. Certain catheters, for instance, require enlarged areas (bumps) to work properly.
This method of making bumps wastes time and material because the puller forms the bumps and it's located about 10 ft downstream from the extruder. Operators have to physically check bump size with an optical comparator then, if necessary, manually tweak the machines to bring it back in spec, all the while making 10-ft runs of scrap.
But some extrusion systems get around this limitation and close the control loop when making bumped product. Here, extrusion-puller timing speeds as well as extrusion air pressure (one way of controlling tubing inner diameter) are modulated based on feedback from a laser micrometer or a vision systems comprised of two video cameras and an Ethernet-wired controller. The diameter-measurement sensor gages formed bumps, and automatically adjusts controlled-process variables to keep them in spec.
Some extrusion systems use Ethernet for network access to system software, production data, and programmable components including PLCs, servomotors, position sensors, machine-vision systems, heater controls, and pressure regulators. Ethernet routers or switches in these machines let far-flung experts remotely diagnose and optimize processes. Some machines are in distant factories often running around the clock and downtime must be minimized. Having remote access means customers don't have to wait for a technician to diagnose and correct software or process issues.
In practice, using Ethernet for system communication requires assigning an IP address to the puller and getting authority to pass through the customer's Internet firewall to establish a virtual private network. Connecting to a phone line through an on-site PC can allay security concerns. This is also simpler and faster to execute because it doesn't involve IT personnel and associated management approvals. And installing a phone modem in the machine itself can eliminate on-site PCs.