Lost Cores for High-Pressure Die Casting

Abstract

Development of salt cores prepared by high-pressure squeezing and shooting with inorganic binders has shown a high potential of the given technology even for high-pressure casting of castings. Strength, surface quality of achieved castings, and solubility in water become a decisive criterion. The shape and quality of grain surface particularly of NaCl - cooking salts that can be well applied without anticaking additives has shown to be an important criterion. Thus the salt cores technology can cover increasingly growing demands for casting complexity especially for the automobile industry.

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of
FOUNDRY ENGINEERING
DOI: 10.2478/afe-2014-0045
Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences
ISSN (2299-2944)
Volume 14
Issue 2/2014
101 – 104
ARCHIVES of FOUNDRY ENGINEERING Volume 14, Issue 2 /2014,101 - 104 101
,
Lost Cores for High-Pressure Die Casting
P. Jelínek, E. Adámková*
Department of Metallurgy and Foundry Engineering, VŠB-Technical University of Ostrava,
17. listopadu 2172/15, 708 33 Ostrava – Poruba, Czech Republic
*Corresponding author. E-mail address: eliska.adamkova@vsb.cz
Received 04.03.2014; accepted in revised form 30.03.2014
Abstract
Development of salt cores prepared by high-pressure squeezing and shooting with inorganic binders has shown a high potential of the
given technology even for high-pressure casting of castings. Strength, surface quality of achieved castings, and solubility in water become
a decisive criterion. The shape and quality of grain surface particularly of NaCl – cooking salts that can be well applied without anticaking
additives has shown to be an important criterion. Thus the salt cores technology can cover increasingly growing demands for casting
complexity especially for the automobile industry.
Keywords: Innovative foundry technologies and materials, Product development, Salt cores, High-pressure die casting, Squeezing-
injection
1. Introduction
Classic metal cores used for high-pressure casting of castings
cannot entirely cover increasingly growing demands for shape
complexity of castings especially for automobile industry [1, 2].
Therefore the interest in development of soluble cores (Verlorene
Kerne, Lost Cores) that would also enable at the same time the
closed technological cycle is growing. This prerequisite is met by
cores from inorganic salts soluble in water. But the application of
them is limited above all by strength properties and both under
low temperatures (primary strength) and under increased
temperatures too. On the other hand it is also limited by casting
conditions – injecting of metal into the mould (the rate in ingates,
post-pressure, laminar filling, gating system). According to some
authors the salt cores are suitable for lower filling rates (under
35 m/s) with limited post-pressure, i.e. ideal e.g. for the
„rheocasting“ technology [3]. The contribution deal with
development of salt cores by way of high-pressure squeezing and
shooting with use of alkali silicate binders and their applications
on test high-pressure cast castings.
2. New processes of cores manufacture
• Sand cores – made by the classic PUR Cold – Box
technology (the polyurethanes binder) or Warm – Box (alkali
silicates). Blank cast holes are geometrically accurate but the
cores are wrongly collapsible (annealing is necessary).
Against penetration it is necessary to optimize the
granulometry of base sands and to use protective coatings.
Sometimes it is recommended to dip the cores in the binder
(resin, water glass) [3].
• Plastic cores (polyoxmethylen) – are made by working from
blocks. Removing is done by burning the core residues.
• Cores from low-melting metals, e.g. Zn-alloys (ZnAl4Cu1)
[3]. They must be melt out by additional annealing of
castings. But the surface is of a high quality corresponding to
that one obtained with permanent mould casting.
• Cores from inorganic salts
- Hollow cores combined with a metal tube [4]
- Full cores
For high-pressure casting the highest potential represent the
salt cores. Their use for gravity and low-pressure casting is known
for a longer time but their application for demanding conditions of
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102 ARCHIVES of FOUNDRY ENGINEERING Volume 14, Issue 2/2014, 101- 104
high-pressure casting necessitates next research. Three
technologies of sand cores preparation are developed in substance
(Fig. 1). Each of them has its own advantages and disadvantages
too.
Our research is aimed at squeezing of crystalline salts and
shooting with use of a binder (alkali silicates).
Fig. 1. Review of technologies of salt cores preparation
3. Salt cores made by shooting and
squeezing
NaCl and KCl are included among the most frequently used
salts. Some deficiencies are solved by mixtures of salts with
minority share of carbonates, sulphates, and phosphates of
alkaline metals. Our first experiments resulted from NaCl and
KCl of chemical purity. In spite of the fact that physical-chemical
properties of both salts are not substantially different the KCl in
all cases shows considerably higher strengths both under low
temperatures and under high temperatures (up to 650 °C) too what
used to be explained in particular by different grain angularity and
shape [5]. Cardinal importance of the shape and quality of the
grain surface has been proved in case of NaCl (cooking salt) of
different origin and provenance [6].
Composition of cooking salt meets both hygienic (iodination,
fluorination) and technical requirements. In some salts the
additives of anticaking matters are found, and namely K
2
CO
3
,
CaCO
3
, MgCO
3
, SiO
2
, K
4
[Fe(CN)
6
]·3H
2
O. For determination an
importance of the grain shape 6 kinds of NaCl of fraction 0.06 –
1.0 mm were evaluated as follows:
1 – Rock salt with I, F
4 – Alpine salt with I, F
2 – Rock salt with I
5 – Sea salt
3 – Alpine salt with I
6 – NaCl, chemically pure, standard
and namely the grains of the shape of crushed material
(1, 2), of regular cube (3, 4) up to oval grains (5) and the
dipyramidal form (6). While the high-pressure squeezed cores
(104 MPa) were made from crystalline salts only, with the
shooting method the cores were made with the alkali silicate
binder and they were hot hardened (190 °C). Squeezed cores
achieve by 2 – 3 times higher bending strengths than shot cores
(Fig. 2). The core porosity correlates with it too and it ranges for
the squeezed cores under 6 % while for the shot ones it is
35 – 40 %. On the one hand the porosity helps in metal
penetration (deterioration of surface quality) but on the other it
accelerates dissolving. The lowest strength has shown the Alpine
salts recrystallized to grains of a cubic form. But the surface
analysis has proved the presence of anticaking matters (MgCO
3
,
CaCO
3
, Fig. 3) that prevent from recrystallization of etched grain
surfaces after compacting the cores. The highest strengths were
achieved (particularly in case of squeezed ones) in case of oval
grains of sea salt and the grains of the dipyramidal shape that are
most similar to the shape of SiO
2
grains.
Fig. 2. Comparison of strengths of salt cores squeezed and shot
from different salt kinds (mean value of 6 cores; fraction 0.063 –
1.0 mm; A = squeezed cores (104 MPa); B = shot ones (binder Na
– water glass 7.5 – 8.0 bars)
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ARCHIVES of FOUNDRY ENGINEERING Volume 14, Issue 2/2014, 101- 104 103
Fig. 3. Regular cubic grains of Alpine salts (samples No 3, 4) and
EDX analysis confirming the presence of anticaking additives on
the salt grain surface (MgCO
3
, CaCO
3
)
Further growth of cold and heat strengths is possible by
applying of composite salts resulting from the presumption that it
can be achieved by disturbing of long dislocation lines with finely
dispersed particles of high heat resistance present in the salt
matrix. The basis is formed by KCl and NaCl crystalline salts and
ZrSiO
4
, Al
2
O
3
, SiO
2
, Fe
2
O
3
of defined concentration and
granulometry were used as composites. Application of them has
shown as follows:
- the growth of primary strength of cores squeezed with force up
to 200 kN in correlation with the decrease of porosity
- additives increase the strength under high temperatures (650 °C)
of cores squeezed even with low pressures (52 MPa)
- maximum bending strength was achieved with Al
2
O
3
(> 9 MPa)
what is ca a treble of strength of the PUR Cold – Box cores
- residual strength after the exposure of 650 °C/ 1 h is by about
10 – 20 % higher that the primary strength
- quality of blank cast holes substantially increased (Ra < 3 – 5
µm)
- high kinetics of dissolution of cores in water is kept
Cores made by high-pressure squeezing are characteristic with
minimum porosity, high primary strength [7] but also with non-
uniformity of compaction in the squeezing height and especially
with high residual stress that resulted in formation of shrinkage
cracks. Those are already formed during disassembling of the core
box or during the proper pouring of castings.
One of solution ways consisted in annealing of cores, and
namely to 180; 350; and 550/650 °C/2 h and residual bending
strength, the abrasive wear and the kinetics of dissolution in water
were studied at the same time.
It has been shown (Fig. 4) that annealing of cores from
mixtures 1 and 2 above 350 °C has no considerable influence on
residual strength when squeezing with force 100 kN while
extreme squeezing (200 kN) further on increases the bending
strength when after annealing (650 °C) it exceeds 9 MPa
(mixture 3).
From temperature of 175 °C the mixtures 2 and 3 exceed the
required bending strength limit for salt cores of 6.5 MPa [8].
Fig. 4. Residual bending strength of squeezed cores (100; 200 kN)
after thermal treatment (mixture of alkaline salts with 1 ÷ 3
additives)
With thermal exposure of the cores from mixtures 2 and 3 the
recrystallization processes of the salt system were completed. In
correlation with the growth of residual strength the abrasive wear
of cores considerably decreases (Fig. 5).
Fig. 5. Abrasive wear of squeezed salt cores in dependence on
thermal treatment (rotary screen 5 min., 60 r.p.m.)
Real porosity of cores decreased below 6.2 % with mean
diameter of pores of 0.0618 µm what is a precondition to
preventing the metal penetration and obtaining the perfect
smoothness of the blank cast hole. On the other hand this fact can
hamper the whole kinetics of cores dissolution.
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104 ARCHIVES of FOUNDRY ENGINEERING Volume 14, Issue 2/2014, 101- 104
An important finding was a fact that annealing temperature
has no negative influence on the kinetics of cores dissolution in
water. On the contrary the longest dissolution times had the cores
solidifying in the open air (48 h/20 °C) and after annealing to
180 °C (Fig. 6.). All the cores were dissolved within 32 min.
in 20 °C still water.
Fig. 6. Kinetics of solving the salt cores after thermal exposure in
water
High quality salt cores without shrinkage cracks made by
high-pressure squeezing were manufactured under conditions as
follows:
- by decreasing the pressing force to 100 kN
- by a shaped die head
- by thermal treatment of cores (annealing 350 °C)
- by protective spraying of the core box (based on BN)
- by carbonaceous additive for decreasing of internal friction
of composite crystalline salts
Results of these treatments on salt cores were checked on test
castings.
4. Test castings
Salt cores were checked in operating conditions of the joint-
stock company of KOVOLIS HEDVIKOV, a.s. on a casting
machine CLH 400 with compacting pressure in a chamber. The
cast AlSi9Cu3(Fe) alloy was injected with rate of 16 and 35 m/s
in ingates and the position of the test core towards the runner can
be changed by 0° (against the runner), 90° and 180°.
With use of cores from composite squeezed salts (100 kN) the
high smoothness of blank cast holes (Ra < 3 – 4 µm), geometrical
accuracy, and scavenging with freely flowing water (38 – 40 °C)
within 7 – 8 min. were achieved (Fig. 7).
5. Conclusion
Salt water soluble squeezed cores from composite mixtures
represent an extensive potential also for the technology of high-
pressure cast castings.
In case of shot cores from cooking salts of a higher shape
complexity with the presence of alkali silicate binders the further
development is required especially for surface protection with the
aim of achieving the higher smoothness of blank cast holes.
a)
b)
Fig. 7. a) Section through a test casting, a sample for
evaluation with the SEM and EDX techniques, b) lower part of
the test casting after washing-out the core
Acknowledgement
The research was realized with financial support of the
Technological Agency of the Czech Republic in the Alfa TA 020
11314 programme.
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