Improvements to the CO2 silicate processFoseco products: CARSIL sodium silicate blended with special SOLOSIL additions DEXIL breakdown agent Principle: The main drawbacks of the basic C
Trang 1Improvements to the CO2 silicate process
Foseco products: CARSIL sodium silicate blended with special
SOLOSIL additions DEXIL breakdown agent
Principle: The main drawbacks of the basic CO2process are:
poor breakdown of the bond after casting
poor core storage properties
rather low tensile strength
These properties can be greatly improved by special additives, while retaining the simplicity and user friendliness of the CO2process
The CARSIL and SOLOSIL range: These products are a range of sodium
silicate-based binders for the CO2 process They may be simple sodium silicates which can be used with DEXIL breakdown agent if required, or they may be “one-shot” products which incorporate a breakdown agent, or they may (like SOLOSIL) incorporate special additives to improve bond strength
as well as breakdown
Binders containing high levels of breakdown additives give improved post-casting breakdown but the maximum as-gassed strength is reduced and core storage properties are likely to be impaired The selection of an optimum binder for a given application is therefore almost always a compromise The requirement for high production rates and high as-gassed strength must be balanced against core storage properties and the need for good breakdown The range of binders includes some which are suitable only for the CO2 process, some which are suitable for self-setting applications and some which can be used for both processes
The commonly used breakdown agents are organic materials which burn out under the effect of the heat of the casting While solid breakdown agents such as dextrose monohydrate, wood flour, coal dust and graphite can be used, powder materials are not easy to add consistently to sand in a continuous mixer Liquid breakdown agents are easier to handle, they usually consist of soluble carbohydrates The best improve gassing speed without loss of strength Some are also resistant to moisture pick-up and their use has increased the storage life of high ratio silicate bonded cores Sucrose is the only common carbohydrate soluble in sodium silicate without a chemical reaction It is readily soluble up to 25% and many
sugar-or molasses-based binders are available Use of sucrose increases gassing speed but reduces maximum strength and storage properties Nevertheless silicates containing sugar are the most popular CO2binders because of the convenience of a binder in the form of a single liquid Molasses can be used
as a low cost alternative to sugar, but it is subject to fermentation on storage The Foseco CARSIL range of silicate binders is based on sugar Some are designed for use with CO2, others for self-setting (SS) with ester hardeners Some can be used for both processes
Trang 2Sodium silicate bonded sand 209
The CARSIL range of silicate binders
Product Ratio Additive CO 2 /SS Comments
CARSIL 100 2.5:1 Sugar CO2/SS Higher ratio for faster
gassing, take care not to overgas Can be used with ester hardeners CARSIL 513 2.4:1 Sugar CO2/SS Low viscosity binder for
easy mixing in continuous mixers
Moulds and cores
CARSIL 520 2.0:1 High sugar CO2 High breakdown, low
viscosity
CARSIL 540 2.2:1 Low sugar CO2 Suitable for moulds or
cores
CARSIL 567 2.2:1 High sugar CO2 High breakdown, good
for Al casting
Note: Some of the CARSIL binders were formerly known as GASBINDA binders in the UK.
The extent to which a core will break down after casting varies depending
on the type of metal cast Low temperature alloys such as aluminium do not inject enough heat into the sand to burn out the breakdown agent fully Indeed, the low temperature heating may even strengthen the core In such cases it is useful to add additional breakdown agents such as DEXIL DEXIL 34BNF is a powder additive developed for use with light alloys It also acts as a binder extender so reducing the silicate requirement The application rate is 0.5–1.5% It should be added to the sand and pre-dispersed before adding the silicate
DEXIL 60 is a pumpable organic liquid which is particularly suitable for use with continuous mixers
SOLOSIL
SOLOSIL was developed to improve on the performance of silicates containing sugar-based additives SOLOSIL is a complex one-shot sodium silicate binder for the CO2 gassed process It contains a high level of breakdown agent/co-binder and offers a combination of high strength and rapid gassing with good core storage properties and excellent post-casting breakdown
The binder is best used with good quality silica sand Addition levels of 3.0–4.5% are used depending on the application To take full advantage of
Trang 340 80 Gassing time (seconds)
120 5
10 15 20
3.5% Solosil
3.5%
Conventional silicate
the high reactivity, an automatic gassing system incorporating a vaporiser, pressure regulator, flow controller and gassing timer is advisable The high rate of strength development is shown in Fig 14.3 While the transverse and tensile strength developed by SOLOSIL binders are still somewhat lower than some organic resin binders, SOLOSIL generally proves more cost effective and overcomes problems of poor hot strength, veining and finning, gas pinholing and fume on casting which occur with some resin binders
Self-setting sodium silicate processes
The first self-setting process used powder hardeners The Nishiyama process used finely ground ferrosilicon powder which reacts with sodium silicate generating heat and forming a very strong bond The reaction also generates hydrogen which is dangerous Other powder hardeners (which do not evolve dangerous gases) include di-calcium silicate, certain cements (such as blast furnace cement and sulphate resisting cement) and anhydrite However, all powder hardeners are difficult to add uniformly to sand in continuous mixers, and their reactivity is difficult to control, since particle size and the age after grinding affect the reactivity of the powder When liquid hardeners based on organic esters were introduced, the use of powder hardeners was largely discontinued
Ester silicate process
Foseco products: CARSIL sodium silicate binders
CARSET ester hardener VELOSET special ester for very rapid setting
Figure 14.3 Strength development of SOLOSIL compared with conventional sugar/silicate system.
Trang 4Sodium silicate bonded sand 211
Principle: Sand is mixed with a suitable grade of sodium silicate, often
incorporating a breakdown agent, together with 10–12% (based on silicate)
of liquid organic ester hardener The acid ester reacts with and gels the sodium silicate, hardening the sand The speed of hardening is controlled by the type of ester used
Sand: Dry silica sand of AFS 45–60 is usually used As with all silicate
processes, the quality and purity of the sand is not critical; alkaline sand such as olivine can be used Fines should be at a low level Sand temperature should be above 15°C; low temperature slows the hardening
Additions: Sodium silicates with ratios between 2.2 and 2.8 are suitable, the
higher the ratio, the faster the set Silicates containing breakdown agents are usually used, additions between 2.5 and 3.5% are used depending on the sand grade The ester hardener is commonly:
glycerol diacetate fast cure
ethylene glycol diacetate medium cure
glycerol triacetate slow cure
Proprietary hardeners may be blends of the above with other esters The addition level is 10–12% of the silicate
Pattern equipment: Wood, resin or metal patterns can be used Core boxes and
patterns should be coated with polyurethane or alkyd paint followed by application of wax polish STRIPCOTE parting agent may also be used
Mixing: Continuous mixers are usually used; if batch mixers are used, the
ester hardener should be mixed with the sand before adding the silicate
Speed of strip: 20–120 minutes is common with normal ester hardeners.
Attempts to achieve faster setting may result in lower strength moulds because the work time becomes short With certain esters there is a tendency for core and mould distortion due to sagging if stripping occurs too early Faster setting can be achieved by using the special VELOSET hardener
Strength: The final strength achieved is:
Tensile 700 kPa (100 psi)
Compression 2000–5000 kPa (300–700 psi)
Coatings: Spirit-based coatings should be used.
Casting characteristics: No metallurgical problems arise with ferrous or
non-ferrous castings Breakdown is poor unless a silicate incorporating a breakdown agent is used
Trang 5Reclamation: As with all silicate processes, burnout of the bond does not
occur during casting and attrition does not remove all the silicate residue so that build-up occurs in the reclaimed sand, reducing refractoriness and leading to loss of control of work time and hardening speed The VELOSET system has been specially developed to permit reclamation (see below)
Environment: Silicate and ester have little smell and evolve little fume on
casting Silicates are caustic so skin and eye protection is needed while handling mixed sand
CARSET 500 Hardeners: These are blends of organic esters formulated to give
a wide range of setting speeds when used with sodium silicates, particularly the GARSIL series of silicates which incorporate a breakdown agent For the best results, the silicate addition should be kept as low as possible in relation
to the sand quality and the CARSET hardener maintained at 10% by weight
of the silicate level The speed of set is dependent on the sand temperature, silicate ratio and grade of CARSET hardener used
The CARSET 500 series of hardeners
CARSET 500
series
Gel times (minutes) at 20°C using various CARSIL binders CARSIL 540
2.2 ratio
CARSIL 513 2.4 ratio
CARSIL 100 2.5 ratio
Note: The gel time is the time taken for gelling to occur when silicate liquid is mixed with
an appropriate amount of setting agent The setting times may not be repeated exactly when sand is present, due to the possibility of impurities, but the figure provides a useful guide.
VELOSET hardeners: The VELOSET range is a series of advanced ester
hardeners for the self-setting silicate process They have been designed to give very rapid setting speed with a high strength, excellent through-cure and a high resistance to sagging Used in the VELOSET Sand Reclamation Process, they provide the only ester silicate process in which the sand can be reclaimed by a simple dry attrition process and reused at high levels equal
to those typical of resin bonded sands
Trang 6Sodium silicate bonded sand 213
Additions: There are three grades of VELOSET hardener VELOSET 1, 2 and
3 Binders of ratio 2.2–2.6 are used; lower ratios give inferior strength while
if higher ratios are used the bench life becomes too short The bench life obtained is independent of addition level The level is usually 10–12% based
on the binder If the sand is to be reclaimed, the addition level of 11% should not be exceeded
Bench life (minutes) at 20°C
When a choice is possible, always use the highest ratio CARSIL binder and the slowest grade of VELOSET hardener This provides optimum strength development
Mixer: Since VELOSET is rapid setting, it is preferable to use a continuous
mixer
VELOSET sand reclamation process: With the conventional ester silicate
process, dry attrition reclamation has occasionally been practised but the level of sand reuse is rarely more than 50%, which hardly justifies the capital investment involved With the VELOSET system, up to 90% reuse of sand is possible using mechanical attrition
The process stages are:
Crushing the sand to grain size
Drying
Attrition
Classification
Cooling
The reclaimed sand is blended with new sand in the proportion 75 to 25 During the first 10 cycles of reuse, the sand system stabilises and the bench life of the sand increases by a factor of up to 2 Also, mould strength should improve, and it is usually possible to reduce the binder addition level by up
to 20% yet still retaining the same strength as achieved using new sand Once the process has become established, it may become possible to reuse
up to 85–90% of the sand, Figs 14.4 and 14.5
Trang 7Figure 14.4 VELOSET reclamation, showing the variation in bench life after repeated use of relaimed sand, compared with conventional ester process.
Figure 14.5 VELOSET reclamation, ultimate strength characteristics of reclaimed sand, compared with conventional ester process.
Trang 8Sodium silicate bonded sand 215
Adhesives and sealants
It is often necessary to joint cores together to form assemblies, or to glue cores to moulds before closing the mould A range of CORFIX adhesives is available:
CORFIX
grade
4 Stove hardening 30 180–220 High viscosity gap
filling
8 Air hardening slow ambient For CO2and self-set
silicate
21 Air hardening fast ambient Any cold core
15–120 sec
140–180 Core assembly at high
rates, shell process
CORSEAL sealants
This is a group of core sealing or mudding compounds for filling out joint lines, cracks and minor blemishes in cores CORSEAL is available in two forms:
CORSEAL 2 is a powder which is mixed with water to form a thick paste (4 parts product to 3 parts water) The paste is applied by spatula
or trowel (or fingers) and allowed to dry for about an hour It may be lightly torched if required immediately
CORSEAL 3 and 4 are ready-mixed self-drying putties which are sufficiently permeable when full dry to prevent blowing but strong enough to prevent metal penetration into the joint Drying time depends on local conditions and the thickness of the layer applied but should be at least 30 minutes
TAK sealant
Small variations in the mating faces of moulds due to flexing of patterns or deformation of moulding boxes and moulding materials may result in gaps into which liquid metal will penetrate causing runout and flash This can be prevented by the application of TAK plastic mould sealant which forms a metal and gas-tight seal TAK does not melt at high temperatures and, if
Trang 9metal touches it, it burns to a compact, fibrous mass The TAK strip is laid around the upper surface of the drag mould, about 25 mm from the edge of the mould cavity and the mould is then closed and clamped TAK can also
be used to seal small core prints:
TAK 3 is supplied in cartridge form for extrusion from a hand gun; a variety of nozzle sizes is available
TAK 500 is ready-extruded material supplied in continuous lengths of
6 mm diameter
Trang 10Chapter 15
Magnesium casting
Casting alloys Magnesium alloy castings are used for aerospace, automotive and electronic applications Their main advantage is their light weight; typical magnesium alloys have a density of 1.8 g/ml compared with 2.7 g/ml for aluminium alloys Aluminium is the principal alloying constituent of magnesium-based casting alloys with zinc and manganese also present in small amounts Pressure diecasting is the most commonly used casting process and because
of the low casting temperature (650–700°C), hot chamber diecasting machines can be used Magnesium diecastings can be made with thinner walls than aluminium, allowing the overall weight of components to be substantially reduced and compensating for the higher alloy cost per kilogram Gravity diecasting and sand casting are also used, particularly for more highly stressed castings The use of high purity alloys with low levels
of Fe, Ni and Cu improves corrosion resistance allowing their use in automotive applications exposed to road salt The use of magnesium alloy diecastings in automotive components is growing rapidly as automobile companies seek ways of reducing weight Some vehicles already contain as much as 10–20 kg of Mg components The most popular parts made at present for production cars are: instrument panel substrates, cross car
Table 15.1 Commonly used magnesium alloys
AZ91 AZ81
The most common alloys for pressure and gravity die and sand casting
Housings, covers, brackets, chain saw parts, hand tools, computer parts etc
AM50 AM60
Both alloys combine strength, ductility castability and cold workability
Seat frames, instrument panels, brackets, wheels
AM20 Used for pressure diecastings
where high ductility and impact strength are required
Automotive safety parts