Chrome Sand In Resin Bonded Systems And Inorganic Binders For Castables
What Is Chrome Sand?
Chrome sand is ground chromite, itself a naturally occurring mineral consisting of iron and chromium oxides. As an ore, chromite is the leading mineral for chromium production. As chrome sand, it is a black almost lustrous powder, it contains chromium primarily in the +3 oxidation state (and not the highly toxic +6 oxidation state). Chrome sand has a melting point of 2,040°C, making it highly suitable for metal casting, and is “almost chemically inert”(1). Chromite has found use as a refractory material for example in green sand casting, in addition to being a component in refractory cement, with its high chromia content ensuring a highly stable material that is largely resistant to wetting(2). Here, we will look at chrome sand’s application with resin bonded systems for molten metal casting applications.
Resin Bonded Systems
Resin bonded systems refers almost exclusively, in terms of chrome sand, when chrome sand is used as a component in the production of casting moulds for metals, and is always used with some kind of resin as a binder. The third major constituent is a hardener, to set the resin and solidify the bond. It is often stated that approximately 18% of all mined chromite is used for refractory purposes(3). In these scenarios, chrome sand is revered for its high temperature tolerance. Binders, in many cases, are organic-based and are typically resins. There are some instances where inorganic binders have been employed, usually to attain a highly specific property in the mould, the casting or the removal of the cast. As a general rule of thumb, the concentration of Cr2O7 in chromite/chrome sand for refractory and foundry applications should not be below 36%. Notwithstanding what is discussed below, some researchers have stated that long term performance at high temperatures of resins, both organic and inorganic, needs more research(4).
The function of the binder is to physically hold the aggregate together (in this case chrome sand, or a mixture of chrome sand and conventional silica sand), forming chemical bonds provides long term stability of the mold even at high temperature. Early casting molds employed core oil to bind together sand, which was eventually replaced with phenol/urethane-type binders in the 1960s and 1970s for many metal types, and furan-type binders mostly for ferrous metals only(5).
There are broadly two processes that utilise organic resins in the manufacture of casting molds; cold box and no-bake molding. Cold box molding is the process by which the slurry of chrome sand and binder are left to cure at ambient temperature, producing the mold.
No-bake molding, like cold box molding, uses no heat to cure. The difference is that in no-bake molding, the resin used is a quick setting one and a catalyst is often used. Resins chemically bind to the aggregate, providing strength. These methods contrast to green sand moulding in that no clay is used, and nor is any anthracite. They are also beneficial to use for large castings from an economic standpoint as no resources need to be assigned to heating processes. Typically, resin bonded moulds containing chrome sand are reserved for when the highest quality/precision casting is required, due to the overall higher cost than using conventional green sand casting - as chrome sand is considerably more expensive than silica/conventional sand.
Resin Bonded Castables (RBCs)
RBCs are characterised by the resin used alongside the sand, chrome sand and other additives. They are almost exclusively organic in nature - that is, they are comprised of carbon, hydrogen and oxygen, with no metals present. RBCs are used widely across the casting industry. Resins are often based on phenol or furan structures, or urethanes. Such resins in chrome sand casting have been viable for the production of cast products from molten magnesium alloys(6), titanium(7) in addition to the commonly used iron and steel. Organic resins are typically added to chrome sand moulds in ratios of no more than 10% by weight, with quantities larger than this contributing to making the mould difficult to form in the first instance due to viscosity, and harder to remove post-casting, and certainly not recyclable(8).
Some of the common types of organic RBCs are described below.
Ester hardened alkaline phenolic resins
In this RBC manifold, the binder is a low viscosity highly basic phenolic resin, combined with a liquid organic ester as a hardener. As little as 1.4% resin by mass can be used with chrome sand. This method’s casting ability is characterised by an ‘as cast’ finish on the metal - i.e. no wetting or formation of rough barbs of metal or dissolved sand is observed(9)
Typically used with pure silica sands, this process can be successfully utilised with chrome sands, and is particularly effective when a mixed silica/chrome sand is used. This resin is also alkaline.
Alkaline phenolic resins with carbon dioxide
An alkaline resin based on phenol is utilised with a coupling agent, which together with the chrome sand is placed into the box around the pattern. Carbon dioxide is blown through the material which causes hardening of the resin via a lowering of PH.
Up until the year 2000, the vast majority of resin-bound casting applications relied upon organic binders solely over inorganic ones. The reasons for this include better productivity, higher process reliability and overall better mechanical properties - plus an unmatched level of familiarity(10). Moreso prevalent in the casting of non-ferrous metals such as aluminium, inorganic binders have been used routinely since this time due to favourable environmental conditions in the foundry and a less complex process with aluminium.
One of the most popular inorganic binders in chrome sand castables is sodium silicate, also known as water-glass(11). Often cited advantages of inorganic binders are a lack of harmful emissions during casting (compared to an organic resin, such as phenolic, that decomposes on heating) and less maintenance. On the other hand, cores produced that contain inorganic binders are known for their affinity to water, and thus must be stored suitably and moisture must be excluded. Unlike with resins containing organic binders, chrome sand castables containing inorganic binders can be heat treated prior to use. Such heating is associated with greater compressive and mechanical strength in the castable, but no appreciable strength changes in the bonds formed between resin and sand in an unheated versus heated study(12). Heating can be performed via a conventional oven or using microwave heating apparatus(13) and has an overall hardening effect - hard and high strength molds are often associated with better surface finishes. A typical example of an inorganic binder resin would employ the resin at around 40 to 70% by weight, alongside other additives(14); with the overall amount of binder in the chrome sand casting module not exceeding 5% by weight.
Viscosity is one of the key parameters when designing a chrome sand resin system. If the mixture needs to be compacted, rammed or forced through an extruder, highly viscous mixtures would be sub-optimal. Furthermore, any mixture needs to be flowable into the casting chamber around the pattern. Resins oftentimes are the largest contributor to viscosity(15). As part of the overall balance of selecting resins for chrome sand castables, it is important to note that permeability of the resin in bulk will have an impact on the overall porosity of the casting - which should be monitored to ensure excess levels of porosity are not reached(16). Moreover, in addition to the resin, fillings and additives in the sand mold mixture play a role, especially with regards to viscosity and overall density(17). Thermal expansion is more a concern with inorganic binders due to their thermoplastic nature, where the sand core in the bound system can collapse under the intense heat and pressure exerted by the molten metal. This can be largely avoided via the use of commercially available coatings to disperse heat more efficiently(18).
In addition to normal box-type molding techniques, researchers have shown in a review that 3D printing can be used to create molds for chrome sand casting processes(19). Such innovation can produce highly specific molds for casting at low prices, in a process called ‘binder jetting’. Optimal ratios for chrome sand, binders and other parameters for casting applications can be determined computationally, enabling greater efficiencies in the sector as a whole(20). Furan-type binders are suspected carcinogens and their use is in consideration of being phased out worldwide.
As a professional in manufacturing and distribution, contact African Pegmatite as your primary chrome sand supplier.
- Chrome sand is used with resins (both organic/resin-based and inorganic) to produce high quality casting molds for the production of high-end casted materials
- The method is applicable to a variety of ferrous and non-ferrous metals
- Organic/resin-based binders are by far the most utilised binder class, with inorganic being less popular and reserved for non-ferrous applications
- Resin-based binders tend to be less expensive on the whole, but inorganic binders have fewer potentially harmful properties associated with them
- Binders work by physically and chemically holding together the chrome sand along with any other additives
- The selection of binder is only part of the story in creating an optimised chrome sand castable, other factors such as viscosity, additives and physical manipulation such as ramming need to be considered
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