Plastics & Rubber World - September/October 2019 - 35

extrusion

manufacturers and toolmakers.
Steve Coxill, engineering services manager, says that some lines are more complex than others. "Some of the lines are
twin-strand extruders. While the other machines produce a single strand, these deliver two lengths of profile at the same time.
Where single lines produce around 225kg/
hour, these can achieve 450kg/hour."

Production line

The PVCu material used by Epwin is blended on site from five key constituents: PVC
resin, calcium carbonate, impact modifier,
heat stabiliser and titanium dioxide. The
mixed product, which has a consistency
similar to table salt, is moved around the
plant by a system of overhead pipes.
Each line is equipped with a special
weighing system at the inlet to the extruder. Coxill explains more: "As the material is made of a mix, you might have a
slight density change. To compensate, the
material is pre-weighed to check specific
density. If we want 450kg/hour but we're
getting 440, it speeds up the dosing screw
to get a little more material."
Profiles can be made in any colour, but
the majority of output is white. Colour is
changed directly on the line, with material changing from white to clear and then
clear to the new colour.
Coxhill goes on to explain the extrusion
process. "The raw material drops onto a
pair of contra-rotating screws. These compress and transport the material along a
heated barrel until it is gelled. At the end
of the screws the material enters the die,
where the flow is forced into the shape of
the product. When it leaves the die, the
material is around 200°C and has the consistency of chewing gum.
This pliable flow then enters a series
of blocks and plates called calibrators,
which shape and cool the product until it
achieves rigidity at about 30°C. The profile

a closed-loop water system cools freshly-extruded material
is pulled through the calibrators by what
is called the haul off. Effectively, this is two
opposing caterpillar tracks that grip the
top and bottom of the section.
"The screws push the material through
the die and the haul-off pulls the profile
through the calibrators," says Coxill. "They
have to work in perfect synchronisation,
or the product will be ruined."
Once the extruded PVC has been
cooled a PVC Nitrile gasket is added to the
profile. The process is similar to the larger extrusion line, but in a smaller scale. A
heating bar warms the application area
and the profile and hollow rubber extrusion are pressed together - no adhesive
required.
Printers mark the finished profile every
90cm. Should there be an issue with the
final product, this marking supports full
traceability, from material mix through to
stillage.
The final step in production is cutting
of the profile. This is completed using a
heated guillotine, rather than a straight
saw, as Coxill explains: "Heated to 195°C,
the blade is razor sharp. It cuts though the
product like it's not there, there's virtually
no noise and very little swarf."
Looking at the finished profiles as
they're loaded into transport stillages,
Coxill explains that while the various
chambers within the extruded part appear to maintain the form, they actually
provide paths for water drainage and improving the insulative properties.
"Five or six chambers give you the optimum thermal benefit," says Gerald Allen.
"You can add more chambers, but that
uses more material and doesn't return
any sizeable performance benefit."

machine hoppers receive material
from piping system across plant

Close up of cut profile reveals
functional interior design

SuPPort and metrology

laser welding helps support tool
maintenance, extending longevity

The second production area came online
a little more than a year before this visit.
From an empty warehouse, the structure
now contains a series of production lines
and related support equipment.
Features of note include the material

➡ Continued on page 36
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