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[April 2003]
Precision Movements for WC Manufacture
Making sure that a robot lines up to a WC with absolute precision and
ensuring that moulds are moved with a smooth motion profile were the key
criteria identified by Hoerbiger-Origa Ltd (Tewkesbury, Glos, England) when
it was called into Porvair Ceramics Ltd (King's Lynn, Norfolk, England).Porvair Ceramics is a leading innovator and manufacturer in the area of
pressure casting machinery and mould technology for the ceramic
sanitaryware, tableware and advanced ceramic sectors. The loads and
pressures involved in moulding larger items - such as WCs - require the
mould parts to be reinforced with a steel frame. These can weigh up to 1.2
tonnes each. Opening the moulds and transferring the unfired WCs is a task
that requires a feather-like touch despite the huge loads involved.Porvair's pressure casting machines have up to 16 moulds (eight on a WC
making machine). For a WC, each mould comprises four parts - top, bottom
and two sides. These are all suspended from an overhead beam which forms an
integral part of the machine's frame and they effectively move along the
beam as each is opened for demoulding. The support of each mould uses three
carriages, one of which also carries a pneumatic cylinder to raise and lower
the bottom mould.In use, the moulds are closed by a hydraulic ram and slurry is pumped into
the mould. Air pressure is then applied to form the hollow WC against the
mould's internal surface and also help drain excess liquid from the clay.Demoulding involves the sequential opening and closing of each mould in
turn. This process is fully automated, using a coupling device that is
traversed over the length of the overhead beam by a servo-driven linear
actuator. The actuator is synchronised with the operation of a demoulding
robot which travels along the axis parallel to that of the overhead beam so
that it is adjacent to each mould as it is opened.This actuator is over six metres long, so when building the machine Porvair
Ceramics found that the number of potential suppliers was limited. After
reviewing the alternatives, Porvair specified actuators manufactured by
Hoerbiger-Origa which has a track record in long stroke, high accuracy
units. Working with Hoerbiger-Origa's specialist motion engineers, it was
decided to use a servo-motor from Control Techniques - controlled by a
dedicated Rockwell PLC - which works alongside a machine-wide, PC-based
SCADA system.All of the mould suspension carriages on the Porvair machine are connected
to one another by a system of scissor linkages. The motion imparted by the
linear actuator to an individual mould part is proportionally transferred to
that mould's carriage, such that when the mould is fully open the robot is
centrally positioned between the two largest mould parts. Additionally, as
one mould is opened the preceding mould - which has already been demoulded -
is closed.For further information on this part of the machine's system, e-mail Ray
Barnes at: ray.barnes@hoerbiger-origa.com
ENDS