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MEASURING & CONTROL
Monitoring <strong>of</strong> <strong>aluminium</strong> extrusion dies
cleaning process by an optic sensor
A. Pascual Formoso 1 ; E. Piñeiro Ben 1 ; L. Herrero Castilla 1 ; C. Domínguez 2 , A. Llobera 2
1 2 Aimen Technology Center, Pontevedra, Centro Nacional de Microelectrónica (IMB-CNM, CSIC),
Barcelona, Universidad Autónoma de Barcelona, Bellaterra (Barcelona)
The aim <strong>of</strong> this work has been <strong>the</strong> assessment
<strong>of</strong> <strong>the</strong> use <strong>of</strong> a multiple internal
reflection sensor in order to monitor <strong>the</strong>
advance <strong>of</strong> <strong>the</strong> <strong>aluminium</strong> extrusion dies
cleaning process. The main goal <strong>of</strong> <strong>the</strong>
sensor is that it is able to measure different
initial concentrations <strong>of</strong> NaOH and it
shows <strong>the</strong> point in which <strong>the</strong> aluminate
starts to precipitate allowing to distinguish
states <strong>of</strong> <strong>the</strong> bath on concentration
ratios <strong>of</strong> [Al] / [NaOH] = 0.1.
The <strong>aluminium</strong> extrusion industry has a significant
economic importance all over Europe.
The process is carried out using extrusion dies,
steel disks with an opening cut through <strong>the</strong>m,
with <strong>the</strong> size and shape <strong>of</strong> <strong>the</strong> intended crosssection
<strong>of</strong> <strong>the</strong> final extruded product. An <strong>aluminium</strong>
billet pre-heated (500ºC) is forced
to flow through <strong>the</strong> die in order to get <strong>the</strong>
required pr<strong>of</strong>ile. At <strong>the</strong> end <strong>of</strong> <strong>the</strong> cycle, it is
necessary to remove all <strong>the</strong> remaining metal
inside <strong>the</strong> cavities <strong>of</strong> <strong>the</strong> die to reuse it in new
pieces production. In fact, <strong>the</strong> extrusion dies
are responsible for quality and process performance
and <strong>the</strong>ir cost is between 35-50% <strong>of</strong>
<strong>the</strong> cost <strong>of</strong> manufacture (Guía Tecnológica).
The ordinary cleaning method consists <strong>of</strong>
loading <strong>the</strong> dies into open tanks, containing a
solution <strong>of</strong> NaOH (caustic soda) at high concentration
and temperature near <strong>the</strong> boiling
point. The cleaning process may even exceed
10 hours, depending on die size and <strong>aluminium</strong>
quantity adhered to its surface. Generally,
<strong>the</strong> baths are changed after each use without
making a preliminary analysis to determine
if <strong>the</strong> bath still retains its cleaning properties.
Daily bath emptying (which can exceed 3,000
litre per day) causes a high cost due to <strong>the</strong>
new formulation reagents (water and sodium
hydr<strong>oxide</strong>) and energy. In addition, it generates
a large amount <strong>of</strong> hazardous waste (with
high basicity and high content <strong>of</strong> dissolved
<strong>aluminium</strong>) (Guía Tecnológica).
Several studies (Li et al, 2005a) have shown
that <strong>aluminium</strong> in a caustic solution at a temperature
approaching to boiling point leads
to sodium aluminate (NaAlO 2), resulting in
hydrogen gas (H 2 (g)). Sodium aluminate so-
lutions begin to run out, since a mesoscopic
point <strong>of</strong> view, due to presence <strong>of</strong> <strong>aluminium</strong>
hydr<strong>oxide</strong>s like gibbsite and bayerite (Harris
et al, 1999). Optical scattering techniques, like
Dynamic Light Scattering (DLS), have been
proven its feasibility to characterize <strong>the</strong> nucleation
and growth <strong>of</strong> <strong>the</strong>se species.
The growing interest <strong>of</strong> industry for control
and monitoring <strong>of</strong> production process
motivates research in <strong>the</strong> development <strong>of</strong> new
optical devices that allow <strong>the</strong>ir integration into
production lines (Harris et al, 1999). These
systems provide real-time information about
a process. One <strong>of</strong> <strong>the</strong>se labs on a chip device
are <strong>the</strong> multiple internal reflection (MIR) systems.
(Llobera et al, 2007).
The aim <strong>of</strong> this work has been <strong>the</strong> assessment
<strong>of</strong> <strong>the</strong> use <strong>of</strong> a low cost MIR sensor
fabricated in PDMS in order to monitor
<strong>the</strong> advance <strong>of</strong> <strong>the</strong> <strong>aluminium</strong> extrusion dies
cleaning process.
Method
Solution preparation: Due to <strong>the</strong> impossibility
<strong>of</strong> working with real extrusion dies at laboratory
scale, small pieces <strong>of</strong> <strong>aluminium</strong> have
been used for <strong>the</strong> simulations. Cleaning baths
are prepared using NaOH solution in a benchscale.
This bath is heated to <strong>the</strong> temperature
<strong>of</strong> work and <strong>the</strong> fragment <strong>of</strong> <strong>aluminium</strong> is put
into <strong>the</strong> bath. Once treatment time is completed,
<strong>the</strong> attacked <strong>aluminium</strong> piece is removed.
When <strong>the</strong> bath is cool
a dark gelatinous compounds
start to precipitate
(<strong>aluminium</strong>
hydr<strong>oxide</strong> and aluminates).
The <strong>aluminium</strong>
concentration in solution
and free sodium
hydr<strong>oxide</strong> concentration
are determined in
each test <strong>of</strong> experimental
set-up. The ratio <strong>of</strong>
free sodium hydr<strong>oxide</strong>
and <strong>aluminium</strong> in so-
lution gives an idea <strong>of</strong>
bath exhaustion.
Optical characterization: This work has been
carried out to characterize <strong>the</strong> optical properties
<strong>of</strong> samples such as refractive index, transmittance
spectral response and scattering.
The size distribution has been analyzed by
dynamic light scattering, from samples prepared
according to <strong>the</strong> procedure mentioned
previously (to 95ºC from a solution <strong>of</strong> initial
NaOH concentration <strong>of</strong> 19%). The size <strong>of</strong> particles
suspended in <strong>the</strong> bath has been determined
using a Spectrometer Autosizer 4800
<strong>of</strong> Malvern Instruments (PCS technique). The
refractive index has been measured at different
times (with a refractometer ABBEMAT-
HP) for so what‘s <strong>the</strong> bath depletion point.
Reference measurement system: Experiments
with laboratory samples have been
conducted to evaluate <strong>the</strong> behaviour <strong>of</strong> <strong>the</strong>
samples measured by <strong>the</strong> sensor and verify
<strong>the</strong> feasibility <strong>of</strong> <strong>the</strong> proposed detection
methods, as well as <strong>the</strong> needs and tolerances
<strong>of</strong> <strong>the</strong> coupling and sealing systems. For <strong>the</strong>se
tests, a setup similar to <strong>the</strong> proposed sensor
has been used, but discrete optical elements
were used instead <strong>of</strong> waveguides, as a reference
measurement system. This assembly allows
to develop transmission and dispersion
measurements at 90° simultaneously. The light
is injected through a multimode optical fibre
and collected with plastic optical fibres <strong>of</strong> 1
mm in diameter.
In order to improve <strong>the</strong> signal to noise ratio
and eliminate <strong>the</strong> contribution <strong>of</strong> light which is
Fig. 1: Attenuation and dispersion at 90° for solutions caustic soda with different
concentrations <strong>of</strong> <strong>aluminium</strong> Images: Aimen
28 ALUMINIUM · EAC CONGRESS 2011