PC control breaks blow-molding mold

This case history expands upon the story behind one of our 2007 Innovator Award winners who reinvigorated a thirty-year-old rotary blow molding machine with new PCs to boost I/O speed and flexibility.

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By Jim Montague, Executive Editor

You can teach an old blow-molding machine new tricks, especially if you’ve got some new PC-based controls to make it happen. The Graham Wheel rotary blow-molding system has been making non-PET plastic bottles for more than 30 years, and reportedly manufactured nearly all of the world’s plastic motor oil bottles during that time.

In fact, the Wheel helped the oil industry convert from paperboard to plastic one-quart containers beginning in the 1980s, and it remains a dominant technology among high-output blow-molding systems. 

The Wheel comes in a variety of sizes from nine to 24 mold stations, and its manufacturer, Graham Engineering Corp. (GEC) of York, Pa., has made several size iterations over the years. The firm’s rotary blow molders are used at plants where opaque bottles are produced then immediately filled with automotive fluids, household detergents and cleansers and beverages, or are sold empty.

Wheel’s main component spins around a horizontally mounted axis. Up to six extruders feed plastic into a flow-head component to quickly form bottles in any type of thermoplastic, and create complex battle-wall structures with up to seven layers. The process begins with upward extrusion through a plastic parison tube at a precise location in relation to several rotating mold sets. These sets are located around the rotating axis, which closes just above the extrusion flowhead, where the continuous parison exits. This happens at the 9 o’clock position, where the parison is inflated by air pressure then is rotated around for a container discharge at the 6 o’clock position. The wheel’s operation is all-mechanical via opening and closing cams.

PC-Based Control Evolves
While some machine builders shy away from challenging their users and stick with PLCs as an excessively safe controls specification, Graham reports it long ago incorporated PC-based controls into its rotary blow molders. Wheel’s main control platform has been PC-based for more than five years (See Figure 1 below). However, the price of being an early adopter is that upgrades come at you faster, and Graham was no exception.

 

     FIGURE 1:  PC-BASED CONTROL MAKEOVER
  Wheel Rotary Blow Molding System
 

Graham Engineering’s Wheel rotary blow molding system experienced growing pains during the first five years it used PC-based control.

“As is the case with many products, there was demand for us to increase bottle production to meet various requirements due to meet expanding product offerings available to the consumer today,” says Dave Fiorani, Graham’s engineering manager. “As the speeds of the extrusion became faster, the need for more precise control of temperatures and parison programming were needed. In 2003, we decided that our existing PC hardware platform wasn’t going to keep up with our machine design migration. We needed to find a successor.”

When the durability and processing power limitation of its previous PC hardware vendor became apparent, Graham’s engineers evaluated industrial PCs from five major vendors using more than 20 critical performance criteria grouped into seven major areas, including:

  • Ability to buy commercial off-the-shelf (COTS) components from any PC store to replace failed components, adhering to ATX open standards
  • Software to map I/O capable of running on any PC
  • Integrated UPS function blocks to handle proper PC shutdown
  • Capable of direct communication to the HMI, with no OPC server required
  • Ability to provide an operator station with fully integrated pushbuttons
  • Highest speed processors currently available on the industrial PC market
  • Software updates available for download from the Internet, and upgrades available at no charge after initial license purchase

Flexibility Aids Efficiency
Following a lengthy evaluation, Graham chose Beckhoff Automation PCs with 2.4 GHz Pentium 4 processors running TwinCat software as the main control for its Wheel rotary blow molders. “We write all sequencing/temperature code and HMI code in house at Graham using TwinCat software and Indusoft’s HMI software for the interface,” adds Fiorani. “TwinCat allows us to do sequencing in real time. We write our own algorithms for PC-based control and to create motion and temperature profiles. This task is a lot faster now because we don’t have to rely on other suppliers for motion control cards. Ultimately, we’re a lot more seamless, and we can reduce the cost of our system because we don’t need discrete motion control on other products we’ve had to buy separately.”

Justin Kilgore, Graham’s senior electrical engineer, adds that “Most of our machines could be controlled using PLCs, but for a few specialized machine types, we need NC functionality. As a result, we use TwinCat NC across the board, so we’ll be able to use a standard software package.” GEC uses many of the languages provided in its IEC 61131-3-compliant programming environment, including ladder, function blocks, function block diagrams, structured text, and continuous function chart. (See Figure 2 below).

FIGURE 2: SPEAKING CLEARLY
Graham Engineering Software Solution

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