Something fishy: HDPE brackets for Akva Group’s fish farm cages

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Austrian company Wittman-Battenfeld worked closely with Norwegian company Plasto to develop a new generation of large high-density polyethylene (HDPE) brackets for Akva Group’s fish farm cages.

Travellers who have visited the coastal areas of Scandinavia, Latin America or the Asian island regions will have noticed the enormous growth of “aquafarming” or “aquaculture” – fish farms, in short – in recent years. The most visible sign of fish farming is the clusters of rectangular or circular cages.

The Norwegian Akva Group is one of the world’s leading producers of circular fish cages, which it makes in diameters ranging from 13 to 64m. They consist of nets suspended from a circular float, which also functions as a service platform, and is made up of two concentric rings of watertight-welded HDPE pipes. These are held in position by injection-moulded HDPE brackets, which weigh up to 50kg and are manufactured by Plasto, a family-owned company based in Andalsnes, Norway. The large-volume injection moulding machine that produces these components was developed and manufactured by Wittmann Battenfeld, of Kottingbrunn, Austria, in close cooperation with Plasto.

 

Close-up of Plasto’s twin-tube platform in the company’s factory in Andalsnes, Norway

Wittmann Battenfeld was approached by Plasto with a detailed functional specification for the parts and the two companies worked closely together to develop production machine and ensure that it met all requirements. For Wittmann Battenfeld, it was a return to large component production, a first since the company became part of the Wittmann Group.

L to r: Plasto Managing Director Lars Stenerud, Wittmann Battenfeld Regional Sales Manager and Project Leader Edmund Kirsch; Christian Hiljemark, Managing Director of Battenfeld Sweden, along with a finished HDPE bracket.

Customised standards

The principal requirement was that the equipment should be based on a standard machine model. Wittman Battenfeld’s experience in previous projects led to the conclusion that the machine had to be not only optimally custom-built for this particular project, but also suitable for manufacturing other moulded brackets in smaller sizes and weights without the need for any further major changes. A shot weight of 50kg HDPE was the core specification but the machine had to have the potential to make even heavier parts.

“Following detailed analysis of sizes and weights in the spacer range, we knew that a 1,000t machine would suffice on the clamping side but that the injection side would require a special solution,” said Edmund Kirsch, Wittman Battenfeld’s Project Leader. “On the basis of these findings we chose a MacroPower 1000/19,000 as the basic model, which has a two-platen clamping unit. It was combined with the largest matching injection unit, which has a 165m screw.” Plasto suggested that the machine’s maximum injection capacity of 14,433cm³ should be increased to the required volume of about 85,000cm³ by means of a piston reservoir as an additional module. In order to implement the solution, Plasto recruited Plasdan, a Portuguese equipment manufacturer, as an additional partner. Plasdan contributed a “shot pot” unit: a 250mm plunger injection unit with a theoretical shot volume of 66,000cm³. Operating via two hydraulic cylinders, this was to be mounted above the machine’s plasticising unit on fixed mechanical anchorage points and connected with the melt stream coming from the machine’s injection unit by a manifold valve.

MacroPower 1000/19000 with Plasdan shot pot

Close-up of Plasdan shot pot

Intelligent design

The key to achieving large shot volumes with a standard machine is the intelligently designed manifold valve.

Close-up of the machine’s manifold valve

It directs the melt stream from the machine’s injection unit to fill the piston reservoir. Several consecutive plasticising strokes are directed into the “shot pot”. This sequential process was preferred over the more usual intrusion method because of the better melt quality. In the case of HDPE parts weighing up to 47kg, the cavity is filled solely via the piston aggregate. For larger shot weights the piston reservoir is filled first, then a maximum stroke is injected from the screw unit, which is then followed by the injection stroke of the piston aggregate. The screw unit can provide an additional volume of melt and inject it in addition to the plunger injection if required. This results in a maximum shot capacity of 94.8l or 68kg of HDPE. The machine can also be operated in a standard set-up, without the shot pot. Operational control is set by the machine’s Wittman-Battenfeld Unilog B6 control system.

Productivity through parallel operation

The essential reason for specifying the ability to adapt to different operations, including simultaneous plasticising and injection, was the need for the machine to make use of all options for shortening cycle time. Plasto’s contribution to the solutions included a gating system with nozzles placed in a flow-optimised position; a high-performance cooling system; and special holding pressure devices in the mould itself. These were added because the machine aggregate can only be used to apply the holding pressure for an extremely short time or maybe not at all, due to the high proportion of plasticising time. This function thus had to be assumed by separate holding pressure devices in the mould. The material pressed into the cavity by these not only helps to improve part surface quality but also ensures continuous contact between the plastic part and the cooled cavity wall. The use of all of these options has kept cycle times for the typical wall thicknesses of the product range to less than 20min.

The machine’s productivity is achieved both by optimised process technology and as a result of the system itself, such as smooth-running linear bearings in the clamping unit, which save drive energy; and short mould change times achieved by the completely free-standing moving platen.

The machine has now started production at the Plasto facility in Central Norway.



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