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 Tositechnical dept.

Scoda Srlwww.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 valvesand 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

ParisonProportional valve for pressure controlActive safety cartridgeLow-noise vane pumpServo-proportional blowpinsFlow amplifier valveServo-proportional for moulds motion control AP/CH Machine control electric unit