LIGHTEN UP!

The war against overweight is being waged on many fronts — low-fat foods, sugar-free drinks, and gruelling P90X-style workout regimes. But what about an overweight blow molded bottle? 

As with excess baggage on the human body, it’s all about overfeeding. 

Like every other type of plastics processor, extrusion blow molders have to squeeze every bit of production out of their operations to stay competitive. The problem is, blow molding is a complex process in which a large number of control tasks have to be managed: feeding granulates from the material supply, distributing the melt, and pressing the tube into the mold at the right moment. Probably the most complex control is the partial wall thickness control of the tube, which serves to distribute the material so that every bulge of the frequently complex moldings has the specified wall thickness; the wall thickness has to adhere to specifications, and so must the layer thicknesses if you’re running a multilayer container. 

But there’s the potential for a lot of variability here and, unlike film or sheet extruders, extrusion blow molders generally don’t have any kind of system, either a control in the molding machine itself or an online gauging tool, to tell them if material thickness or layer ratios are changing over time. As a result, many tend to process their bottles and containers with more resin than they actually need to ensure they’ll still have usable products.

That might sound prudent, but it’s more like throwing money out the window. “The actual overage may be small, but even if a molder is using just one per cent more resin than necessary, and it runs 20 million lbs. per year, that one per cent translates into 20,000 lbs., and $20,000 worth of wasted material,” said Alan Landers, product manager, blending and upstream products, with The Conair Group. “When you ‘overweight,’ the more material you run, the more you waste, and the dollars add up quickly.” 

A MATTER OF CONTROL

There are some fairly traditional alternatives to overweighting bottles, fuel tanks, and industrial containers, although these can cause headaches of their own. “One method is to program the parison, where you have a program cylinder that opens and closes the die to get you to a certain wall thickness; the disadvantage is that the wall thickness maintains all the way around your part,” said Chuck Flammer, vice president of sales, North America, with Kautex Machines Inc. “A second method is to use ovalized tooling, and make physical cuts to get more material out in certain sections of the parison; the problems are that it’s time-consuming, can add to cycle time, and can result in very thick and unwanted localized areas up and down the part.” 

Kautex offers several solutions. “Our static flexible die ring, or SFDR, has a bolt circle that goes around and flexes a piece of metal in and out to add material to a certain section without having to re-machine the tool,” Flammer said. “Our partial wall distribution system, or PWDS, consists of hydraulic cylinders that flex the die ring to create localized thin and thick sections; due to the fact that each PWDS actuator has its own wall thickness profile, it’s possible to move each side independently, thereby adjusting symmetrical and asymmetrical deformations, shifts, and any combinations of these to manipulate the radial die gap. It’s particularly well-suited for the manufacturing of blow molded fuel tanks through continuous extrusion.”

A third option is the company’s WDLS wall thickness layer control system, which uses measurements of the top and bottom flash weight and the net article weight to ensure that the wall thickness profile points remain at the correct locations of the molding, even over longer production runs. “The WDLS system allows extrusion blow molders to make very precise applications of material at precise points, including the thickest area of the part,” Flammer said. “And since the thickest area determines the length of cycle time, being able to minimize the material in that area can reduce the cycle time.” 

Kautex’s SFDR, PWDS, and WDLS systems are all supplied by Troisdorf, Germany-based blow molding equipment maker Handel Elke Feuerherm.

Davis-Standard LLC’s 3DX deformable die ring technology is an alternative to existing PWDS technology, offering a flexible die ring for improved control of radial wall thickness. Advantages of the system include innovative cylinder mounting to reduce heat from the head to avoid damage to cylinder seals, the company said, and a hydraulic safety function that prevents flexible die damage on accidental cold starts. In addition, a proportional pushout mechanism is designed to improve efficiencies on accumulator head machines. The flexible ring, hydraulic cylinders, and mounting features of the 3DX allows the blow molders to service the system themselves. “This technology, combined with the quick color changeover and parison control of our accumulator head technology, contributes to production of a consistent and cost-effective blow molded part,” the company said.

Kautex and other extrusion blow molding machine makers incorporate Simatic automation technology from Siemens into their machines — and Siemens has done a lot of work to help guarantee wall thickness. The company’s Simatic S7 blow molding system has two variations — the BMH 3100 and the BMH 3300 — that the company describes as well-suited for controlling wall thickness in extrusion blow molding. “The wall thickness profile is defined by a support value table with a maximum 128 equidistant support points,” the company said. “If fewer support points are served, the wall thickness program determines the support values which are not served by interpolation. The entire wall thickness profile is calculated with the served and interpolated support values and displayed on the HMI system. Simatic 7 systems also offer channels for multiple head or PWDS operation of continuous or accumulator head machines, up to 256 nodes for polynomial or linear interpolation, ejection speed controller with 256-step profile, tube length control, fill level control, and quality control.”

GETTING GRAVIMETRIC

There are also some solutions available beyond the extrusion blow molding machines themselves. Take gravimetric extrusion-line control systems. “Because these systems read and react to changes as they occur, you don’t need to set thickness setpoints unnecessarily high in anticipation of process variability in extrusion blow molding — you can really narrow the processing window,” said Conair’s Alan Landers. “Few extruders would attempt to run at a mere 2.5 per cent over gauge, much less at just one per cent over. However, with the additional control provided by the gravimetric extrusion-line control system, it’s not only possible, but easily cost justified. If you’re able to reduce your amount-over-gauge to something less than half of what you’re accustomed to — perhaps to even as little as 20 per cent — then the system can pay for itself and feed your profitability within weeks or months.” 

As with losing love handles in the real world, shedding unnecessary weight from extrusion blow molding applications can pay dividends.

RESOURCE LIST

The Conair Group (Cranberry Township, Pa.);
www.conairgroup.com; 800-654-6661
  Dier International Plastics Inc. (Markham, Ont.); 905-474-9874
  Turner Group (B.C. and Alberta) (Seattle, Wash.); 206-769-3707

Davis-Standard LLC — Blowmolding Systems (Bridgewater, N.J.);
www.bc-egan.com; 908-722-6000

Kautex Machines Inc. (North Branch, N.J.):
www.kautex-group.com; 908-253-6012

Siemens Canada Ltd. (Markham, Ont.);
www.siemens.ca; 800-263-7444