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The invention of the electronic parison programmer in 1958 helped to make extrusion blow molding a more efficient and productive plastics manufacturing process. Denes B. Hun kar, an expert in process controls for injection and blow molding, developed the first system, which elevated extrusion blow from purely “black art” to a more scientific pro cess.
Up until that time, blow molders had only mechanical programmers, which cycled be tween two fixed positions for the die—one for the body and one for the neck and heel (which accounted for 75% to 80% of the bottle weight). Adjustments of either position were performed manually with a wrench.
Hunkar, who founded Hunkar Instrument Development Laboratories (now Hunkar Laboratories) in Cincinnati in 1962, developed a control system that enabled processors to vary part wall thickness and make more efficient use of resin. Until then, extrusion blow molders had no reliable, cost-effective method of distributing weight throughout the part. Typically, extrusion blow machines dropped heavy parisons and operators used unscientific methods such as starting and stopping the extruder in order to attain some sort of desired material distribution.
Hunkar’s parison programmer enabled the machine to vary the die/mandrel gap during parison extrusion. The parison geometry was programmed by specifying mandrel movement through a “pegboard” system of inserting pins in holes on a grid. The parison shape and thickness became dependent on the response speed of the control system with respect to mandrel actuation and the melt’s elasticity, relaxation time, and flow behavior. Today, parison-control systems can fine-tune parison geometry by plotting out up to 200 programming points.
Hunkar’s control system helped eliminate container thinning and blow-out problems and greatly facilitated lightweighting. The control mechanism also ignited the market for extrusion blow molded HDPE parts, which until then had been difficult to produce.
Parison programming also played a major role in the growth of handled parts. By selectively tailoring wall thicknesses, more complicated designs incorporating handles became readily attainable. Today, most extrusion blow molding machines are equipped with parison programming systems.
Very few readers of this issue can remember, or even imagine, what it was like when an injection mol...