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  • Electrohydraulics for Blow Moulding

    Blow moulding: the essence of intelligent hydraulic power.

    Energy saving and reduced noise, from the hydraulic power pack to the Parison, from cylinders to

    the CE/PED certification.

    Blow Moulding is an application field characterized by electrohydraulic systems

    which combine high performances and low energy consumption (Fig.1).

    Hydraulic power packs are the heart of the presses and are designed in modular

    layout to allow single station machines to be transformed into twin-stations version

    and vice versa (Fig.2).

    The power packs are fitted with digital proportional valves for controlling the main

    axis of the machine with enhanced cycles for repetitive, large series production of

    bottles, plastic cans and hollow containers of every kind.

    The motions of these machines are very fast and involve large instantaneous flows,

    over 500 l/min, complied with the use of oleo-pneumatic accumulators supported by

    pump, with fixed or variable displacement. They allow both to improve the dynamic

    performance, thanks to the storage of energy, as well as to considerably limit the

    installed power on power pack.

    In fact, the sizing of the motor-pump units is based on the average volumes of oil

    required during the full machine cycle and not on the instantaneous flow-rates

    (Fig.3).

    The presence of accumulators requires the relevant certification of power units in

    conformity with the PED 97/23/CE Directive for under-pressure equipment (explo-

    sion risk), that imposes the use of certified valves for maximum pressure relief in the

    control system. These valves are free-leakage, PED approved by an authorized body

    and they are sealed after factory set. They are fitted in “safety and shut-off block”,

    including accessories for isolating and discharging accumulators as well (Fig.4).

    The Machines Directive 2006/42/CE in conformity with the ISO 4413 standard

    “General rules relating to hydraulic systems”, calls for presses with accumulators to

    shut-off all machine movements during operations with open gate via isolating val-

    ves fitted on the main pressure line and by venting the pumps outlet. It is therefore

    possible to carry out maintenance or tuning of the moulds in safety, even when the

    hydraulic system is switched on and under pressure.

    The safety valves are spool type or cartridge type with active piloting and free-leaka-

    ge, equipped with proximity sensor to monitor spool or poppet position, CE approved

    by an authorised body (Fig.5).

    Power packs original and unique design allows manufacturers to obtain the final cer-

    tification of the machine through simplified procedures.

    In Blow Moulding presses a low level of noise is required, according to the limit of

    75db (A) permited in working ambient.

    The use of vane pumps or internal gear pumps, silent couplings, anti-vibration dum-

    pers for motor-pump units with 6-poles electric motors, allow to realize power packs

    with noise levels of less than 70 db (A), even for installed power of over 60 kW.

    The high hydraulic power is partly dissipated in heat, thus requiring the adoption of

    efficient and well-sized cooling systems.

    Water oil heat exchangers with tube bundle or the more efficient plate heat exchan-

    gers are the best solutions to grant optimal thermostatic control of the oil.

    To improve the reliability of the machine and to lengthen its working life it is neces-

    sary to keep the fluid clean, because of contamination from mechanical wearing of

    the moving parts.

    Fig. 4 - PED safety block

    Fig. 1 - Blow Moulding Press

    (Magic MP spa)

    Fig. 2 - Modular power pack

    Fig. 5 - CE safety valves

    Fig. 3 - Accumulator system

    AR08K/E

    Lorenzo Tosi technical dept.

    Scoda Srl www.scoda.it

  • For this purpose, the adopted filtration units must have high filtering rate, less than

    10 micron, to guarantee long working life of the system and reduced maintenance.

    Off-line system is the best solution to make fluid filtering and cooling more efficent

    and indipendent from machine cycle. It is installed out from the main power supply

    line (Fig.6).

    The hydraulic circuit is developped in a compact and reliable design by using hydrau- lic manifold blocks with duly machined cast iron or steel plates, on which the valves and components are assembled (Fig.7).

    The pressure’s control in the press is realized by pilot operated relief valves, particu- larly smooth in commutation to avoid hammer shock; thanks to a digital programma- ble electronic transducer the range of accumulation pressure can be optimized.

    All the digital proportional valves are connected to the machine’s electronic control

    unit via the available fieldbus networks (Canbus, Profibus, Powerlink).

    Concerning dynamic axis motions like carriage and mould, single or double stage

    high performances digital servoproportional valves are used, also with regenerative

    spools to reduce the maximum flow rate.

    When dimensions of the cylinders and high motion speed require high flow rates of

    more than 300l/min, the main control valve will be coupled with specific hydraulic

    operated cartridge valves, called “flow amplifiers”, operating on the basis of the

    master (proportional) and slave (cartridge valve) principle.

    For mould and carriage axis, standard cylinders (ISO 6022) with round screwed

    heads are used, for pressure up to 250 bar (Fig.8), making the special floating bush

    cylinders obsolete.

    For the upper and lower nozzles and for Bobbing movement (raising-lowering extru-

    der), standard (ISO 6020-2) cylinders with square heads assembled with tie rods are

    used for pressure up to 160 bar.

    The control of the plastic film thickness, also called Parison, is operated by one or

    more servoactuators and it is operated in closed loop position control. Parison con-

    trol is composed by tailored servocylinder with integral LVDT position transducer

    monitoring the rod position and by high dynamic proportional valve with digital elec-

    tronic (Fig.9).

    The development and optimization of the above reported solutions are the result of

    close cooperation and synergy among specialists of electrohydraulics, machine

    manufacturers and electronic system integrators.

    Fig. 6 - Cooling and filtration

    Fig. 8 - Standard ISO/DIN cylinders

    Fig. 7 - Proportional manifolds

    Fig. 9 - Parison servoactuators

    Functional diagram of Blow Moulding Press

    Parison Proportional valve for pressure control Active safety cartridge Low-noise vane pump Servo-proportional blowpins Flow amplifier valve Servo-proportional for moulds motion control AP/CH Machine control electric unit

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