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Processing
Copolymers (Thermoplastics)
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Thermoplastics
are understood to be plastics that become soft and formable
when heated to above their softening point and that become
solid again after cooling.
Examples of thermoplastics are polypropylene, polyethylene,
polyester, polyamide, duroplastic coating powder and cellulose
esters, etc.
Representative of a system
for processing the above thermoplastics is the system
shown below, in this particular instance designed and
delivered for polyethylene. Because of the low temperature
resistance of thermoplastics, the product has to be embrittled
with nitrogen (LN2). |
The
design bases on the following problem specifications:
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Feed material: polyethylene
Feed size: < 6 mm
Required end-product fineness:
99% < 150 µm
End-product throughput: approx.120 kg/h
1 = Feed bin
2 = Feed metering screw
3 = Metal separator
4 = Rotary valve
5 = Eddy-current screw cooler
6 = Contraplex pin mill CW
7 = Tumbler screening machine
8 = Automatic reverse jet filter
9 = Fan
10 = Automatic reverse jet filter
11 = LN2 distributor
and controller
12 = Control cabinet
A = Feed product
B = End product |
The plastic granules are charged from the feed bin together
with the coarse product from the sieving machine to the eddy-current
screw cooler for embrittlement. From there, they are conveyed
to the Contraplex CW pin mill where they are ground to approx.
99% < 500 µm (with a fraction of approx. 70% <
150 µm). The product is filtered in the silo-mounted
filter and conveyed to the downstream sieving machine. The 150 µm
end product is sieved there and in turn discharged as end product
via an automatic reverse jet filter. The supply of LN2
to the entire system is controlled by an automatic cryogenic
control unit. The system is insulated in order to minimise the
nitrogen consumption.
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