Thrust
in new directions
What
are the latest technological developments in extrusion press system?
N Radhakrishna Rao brings a few to light
In
last 20 years the design of hydraulic systems for powering the extrusion
presses has undergone drastic changes. These have resulted in cost effective
designs and state of art features in the drive and controls areas. Discussed
below are some these features in detail.
High
pressure variable delivery pumps
High
pressure variable delivery pumps have replaced the existing expensive
two-way of a two-way pump. They are more complicated than one way pumps
and semi closed pumps. The high pressure axial piston pumps have an
optimal design, show high efficiency with the latest fast acting proportional
and servo controlled and cost a fraction of the older two-way (two ports
delivery), complicated, bulky and difficult to maintain medium pressure
radial piston pumps.
Manifold
systems
The
older hydraulic systems used line mounted valves in two-way hydraulic
circuits. Such two-way circuits used both output ports. The line mounted
valves were located on valve stands or frames as close together as piping
considerations would allow. The piping was relatively complicated requiring
the use of many fittings, and therefore, many welded connections presented
opportunities for leaks.
Oilgear
Company’s distributed manifold systems is the most efficient versatile
manifold system available in the market today for extrusion presses.
It not only eliminates all of the inter-valve connections found in the
older valve system, but also reduces the high pressure piping between
the rear and front of press by nearly 50 per cent. Such manifold systems
are specially designed for extrusion presses and are equipped with the
standard slip in cartridges and operator heads. The distributed manifold
system consists of three principal manifolds – main ram, diverter, and
container or shear manifolds. A fourth manifold, that is the loader
or die changer, is a standard bar type manifold.
The
benefits of cartridge valve manifold blocks are reduced shocks in the
system, increased component life, reduced down time and versatility
and flexibility. The cartridge valves are cushioned plungers and orifice
operators, which are ideal for fixed displacement and accumulator systems.
They are fast acting, generate less heat, require less space and are
compact.
Programmable
logic controllers
Programmable
logic controllers (PLC) have gradually replaced the old contact relay
electrical systems for control press sequencing. PLC is actually a robust
industrial computer and has been in use for quite some time. It consists
of input out put interfaces, contacts for limit switches and pressure,
operator push buttons and switches, solenoid and pilot light outputs
to name a few facilities. The central processor of a PLC continually
scans the inputs and activates the outputs according to the sequence
stored in the programme. The hardware provides the following desired
benefits:
- Reliability
- Control
flexibility
- Performance
reporting through smart PLC (Extrusion Information system) recording
key operating parameters extrusion speed, pressure and die number
Closed
loop speed controller
The
Oilgear Company has specially developed an extrusion press controller
digital speed control to address the aspect of speed in extrusion presses.
Extrusion speed varies with pressure or load. The amount of variation
depends on the system efficiency or inefficiency of the press and main
pumps. This inefficiency is based on the internal leakage or Slip. Slip
varies with pressure and viscosity. As billet gets shorter and shorter
due to the extrusion process (billet extruded) the container friction
resistance decreases and pressure falls. Consequently slip decreases
and the ram speed increases in an uncontrolled manner. The introduction
of fast stroking servo controlled pumps, in combination with a stroke
measuring temposonic or encoder feedback measuring actual main ram speed
can provide a closed loop constant extrusion speed control. The benefits
of constant speed control are the following:
....CONTD
