Sfogliabile_AeL_n3_GIUGNO_2016

Foundry
Figure 4:
micro-graphic
image of the
section of a
structural cast in
an area in contact
with the core
Figura 4:
micrografia di una
sezione di un getto
strutturale in una
zona a contatto
con l’anima
Casts with better metallurgical properties
The last advantage is the better metallurgical structure of casts
in the areas that are in contact with the core. This aspect is determined
by the behaviour of the core when it comes in contact
with the liquid aluminium, since with an organic core the
capablity of absorbing heat is gradually reduced during cooling.
This phenomenon is shown by the graph in Figure 3, where
the behaviour of the two cores is compared. The curves (left
side) show how the organic core sand binder heats up instantly
reaching a peak of roughly 480°C, a temperature where the
capability of absorbing heat decreases. On the other hand the
inorganic core absorbs much more heat in a longer time interval.
The curve showing the speed at which the alloy solidifies
confirms this trend: in the interval during which the liquid
metalsolidifies,theinorganiccoresandbinderprovestobe
still relatively cool, unlike the shell moulding core. The last consideration
concerns the mould’s temperature: comparing the
two graphics it can be seen that in the case of inorganic cores
temperature is on average lower, a phenomenon that may still
be associated to the high thermal capacity. The consequence
isanincreaseofthespeedofcooling,thatcorrespondstoa
veryfinesolidifyingmicro-structure,anindexofveryhigh
mechanical performances. As an example, Figure 4 shows a
micro-graphic rendering of the section of an EN-AC-42100
structural cast obtained in an area in contact with the core. As
may be observed, the micro-structure is particularly fine (average
SDAS 23 µm); this implies high mechanical performanc-
es. – Rp0.2=248MPa; Rm=305MPa; A%=12% – that may be compared
to those in areas not in contact with the core.
Limits to the application
In consideration of the above statements, the use of inorganic
core sand binders implies considerable advantages.
There are, however, some limitations of this application: for
instance the cores may not be emptied like shell moulding
cores since the “lattice” of the binder forms on account of
aflowofhotairina“gassing”processthatresemblesthe
cold box method. The second disadvantage concerns the
sand’s regeneration: today there are mechanical cycles of
removalofthebinderfromthesandparticlesinsomepilot
plants, whose efficiency still needs to be improved. The last
aspect that should be taken into consideration is the drying
of the cores after moulding as they have been proved
to be sensitive to humidity, both in the sense of atmospherichumidityandofresidualhumidityduetothedryingprocess
itself. The hot air introduced during the “gassing” process
is meant to remove water completely form the mixture
that serves as a solvent for the binder. Sometimes part of
the water may however remain trapped in the core’s solid
parts if correct process parameters are not used. Finally, in
time residual humidity tends to migrate towards dry areas,
makingthewholecoremorefragileForthisreasonstocking
the cores in warehouses is also a critical issue , especially if
temperature and atmospheric humidity are high. ❚
I limiti d’applicazione
Allalucediquantodettoinprecedenza,l’utilizzodelleanimeinorganicheportadeivantagginontrascurabili.Esistonoperòalcunilimitiapplicativi:adesempio,le
animenonpossonoesseresvuotatecomequelleinshell
moulding in quanto la “reticolazione” del legante avviene
tramite passaggio di aria calda in
un processo di “gasaggio” simile al
cold box. Il secondo svantaggio è relativo
alla rigenerazione della sabbia:adoggiesistonodeiciclimeccanici
di rimozione del legante dalle
particelle di sabbia in alcuni impianti
pilota, la cui efficacia è ancora da
migliorare. L’ultimo aspetto da tenere
in considerazione è l’asciugatura
delle anime dopo formatura in
quanto risultano sensibili all’umidità,intesasiacomeumiditàatmosferica
che come residuo del processo
di asciugatura stesso. Infatti, l’aria
calda introdotta durante la fase di
“gasaggio”hailcompitodirimuovere
completamente dall’impasto l’acquachefunge
da solvente dellegante.
Può però succedere che parte
dell’acqua resti intrappolata nelle
zone massicce dell’anima se non si
utilizzano dei parametri di processocorretti.Infine,conilpassaredeltempol’umiditàresidua
tende a migrare verso le zone asciutte, infragilendo
l’intera anima.
Per questo motivo risulta essere critica anche la stoccaggio
a magazzino delle anime, soprattutto se le temperature
e l’umidità atmosferica sono elevate. ❚
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