Moulding suspensions

A suspension is a mixture which arises when solid particles are mixed optimally in a liquid. The suspended solid particles have a diameter of appr. 200-0.5 nm and the mixture is also called a colloidal dispersion . The liquid is the medium of dispersion. A clay suspension is suitable for the production of so-called hollow, non-rotation symmetrical articles, such as sanitary ware. Until the beginning of the 20th century these products were made by beating the clay into plaster of paris moulds, the so-called dies. Gradually people discovered not only the physically and chemical properties of suspensions but also how to change them and thus the technique of clay moulding developed and complicated shapes could be made. The science of colloid chemistry has been essential here. In the field of technical ceramics the moulding technique is also applied with other raw materials than clay. 

The solid particles in a suspension attract because of the Van der Waals forces and charged particles also attract due to Coulomb forces. The charge of the particles and consequently also the Coulomb forces can be affected by adding electrolytes. The relevant theory was already 

In the reaction of figure 9.16 a proton is bound to an OH group. This reaction is reversible, which means that the structure on the right can be converted back into the one on the left by adding e g. OH-ions. These remove the proton and use it to form a water molecule. The addition of OH- ions to the structure on the left causes a negative surface charge to be formed  

The surface charge of the suspended particles affects the viscosity of the suspension, but that was already explained in the chapter Colloid chemistry. 

With the viscosity of a liquid mass we mean the flow behaviour and this depends on a) the viscosity of the dispersion medium, b) the concentration of the solid, c) the shape and dimensions of the particles and d) the interaction between the solid particles themselves and the solid particles and the molecules of the dispersion medium. 

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J. C Van Suijdam The Rheology of Mould Suspensions. - M. Zlokamik Scale-Up of Surface. Aerotors for Whste Whter Treatment. 

Conventional filtration is used to separate large volumes of diluted cells suspensions (>1 L), extracellular products and mould suspensions because the mycelium has very low density and it is difficult to separate the fluid by centrifugation. ... 

Edirisinghe, M.J., Shaw, H.M. and Tomkins, K.L. (1992) Flow behavior of ceramic injection moulding suspensions. Ceramics Int., 18,193-200. 

Zhang, T. and Evans, J.R.G., Predicting the Viscosity of Ceramic Injection Mouldings Suspensions. To be published. 

In slip easting a thin slurry, or suspension, of clay in water is poured into a porous mould. Water is absorbed into the mould wall, causing a layer of clay to form and adhere to it. The excess slurry is tipped out of the mould and the slip-cast shell, now dry enough to have strength, is taken out and fired. The process allows intricate shapes (like plates, cups, vases) to be made quickly and accurately. 

A third type of suspension is that in which polymer particles are suspended in monomer which is then polymerised. This is, however, rather more a variant of the casting process in which monomer or low molecular weight polymer is cast into a mould and then polymerised in situ. 

It was claimed that the maximum continuous operating temperature in most chemical environments was 120°C and even 140-150°C in some instances. The major chemical applications were in the form of pipe and tank linings and injection moulded valve and pump parts. Coatings could be applied to metals by means of fluidised bed, water suspension and organic dispersion techniques. 

Since 1960, the inner core has been made from c/5-poly-butadiene by the compression moulding technique. This replaced the earlier material made from a suspension of barytes or bentonite clay in water and glycerine or the winding of rubber threads made from t /5-polyisoprene, either from latex or a dry rubber compound. A typical thread recipe is given Table 4. 

Preservatives may inelude disinfeetant and antiseptic chemicals together with eertain compounds used almost exclusively as preservatives. They are added to mar industrial, including pharmaceutical, products which may, by their nature, support the growth of bacteria and moulds causing spoilage of the product and possibly infection of the user. In the field of pharmaceutical preservation, addition of an inhibitory substance to a multidose injection (Chapter 21) or the prevention of growth in aqueous suspensions ofdmgs intended for oral administration (Chapter 18) are prime examples. 

The glass polyalkenoate cement can also be used to replace plaster as a mould in the slip process for pottery. It possesses the same property as plaster of Paris, of causing material to deposit on its surface from slip suspensions. So far this property has not been exploited in the manufacture of pottery. 

Plasticisers are compatible and miscible with their host polymer causing them to swell and allowing them to be used for coating, moulding, spraying (when used as a suspension in a liquid solvent) or calendering, extrusion and injection moulding (when used as a polymer melt). 

Melt the mixture III at 60°C, stir it into the suspension II and finally add I. When a homogeneous suspension has been obtained cast the sticks in preformed moulds. 

After a uniform suspension has been obtained, pour it into polyethylene moulds to produce suppositories, each of 2 g weight. 

Hunt et al. [ 17] have investigated the role of various coupling agents, such as zirconates, titanates, and zircoaluminates, in zirconia-polypropylene suspensions for use in the production of ceramics by injection moulding. All the coupling agents were observed to reduce the melt viscosity. 

Pouring clay suspension in a mould for producing a wash basin at Sphinx sanitary (by J.Aarsen). 

Controlling the stability of suspended particles, and the filtration characteristics of cast suspensions, are very important in the ceramics industry. Here the casting suspensions may comprise clays, silica, oxides, carbides or nitrides [357,793], In slip casting, a suspension is pored into a porous mould and allowed to stand. Water dif-... 

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