CASTING DEFECTS

Casting is tilt1 stage al which the fluid slip becomes a solid via the draining action qf the moulds. The different nature of these two states can cause several kinds of problem. In low pressure casting the defects created at this stage can almost always be repaired - structural collapse of very large cracks aside - while in high pressure casting, owing to ihe automated nature of the process, correcting a defect is more problematic.

CRACKS (N HOLLOW-CAST AND SOLID-CAST PARTIS

Present along both hollow and solid-cast parts: these are usually caused by problems related to the condition of the mould. On resin moulds they are localised as reduced-functionality portions that result in reduced thickness. Correction of such problems requires that the moulds he treated in an appropriate manner.

Another cause may he incorrect pressures and mould opening times: if pressure is insufficient there may be open fractures that are identified by jagged extremities without amy deformation in the surrounding parts, while excessive times cause closed cracks that are accompanied by deformation. These kinds of defect ure usually present in the weakest areas of the ceramic body (e.g. the interface between hollow-cast and solid-east parts).

Water stagnating in resin moulds can create a drop in slip density and compromise the continuity of the ceramic mass, causing cracks. Normally this defect is not continuous but, rather, appears at regular intervals when there is a build-up of liquid that has not he-en expelled during the various stages of the casting cycle.

PROBLEMS ON THE SURFACE OF THE PIECE

Surface quality w ill he seriously compromised - as will corporate image - where there are "snakeskin' surfaces or detachment of small portions of ceramic. The sanitarvware surface can be finished either green or after being dried, the result depending on the extent of the delect. If aesthetic qualities are compromised sanitaryware items are generally rejected.

Surface conditions or, more specifically, the hygroscopic content of a resin mould, are the self-evident causes of such delects. Prevention of such problems lies in proper mould preparation.

CASTING DEFORMATION

When the pieces are de-moulded they are still quite soft. They may be slightly deformed or tend to collapse; both obviously compromise quality. Cast piece characteristics may be heterogeneous across the formed thickness: external parts may be firmer than internal ones and moisture levels may also differ. Such dillerences create tension w ithin the pieces that are subsequently released in the (orni of cracks, usually in the hollow-east parts of the sanitaryware.

Mich defects cannot be repaired.

In traditional casting the delect is caused by a hardening time that is too brief for the specific characteristics of the slip being used in production and the specific mould conditions. Should the defect be evident it may have been caused by non-optimal preparation of the plaster mould (e.g. use of talc or failure to sprinkle with slip).

There may be deeper-rooted causes such as errors at the modelling stage, failure to achieve perfect evacuation of the slip or insufficient aging of the slip which mst s softening and a heterogeneity of its characteristics,

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Fig. 273 - Crack inside a washbasin chamber. The cause may be very small air bubbles inside I he body.

In high pressure casting erroneous timing of emptying or incorrect hardening times c an cause softening and deformation. llxcessive water content in the porosity of the mould may obstruct the hardening process, or could keep the walls of the piece too moist and so have a negative impact on its stability. The same effect occurs when de-moulding in multi-mould machines is too protracted as capillarity w ill transfer pie water contained in the mould to the piece. High pressure casting produces more intense particle packing and tends to generate ss deformation than traditional low pressure casting.

The easting cycle is highly susceptible to deformation;

• During slip drainage, supports for the walls of hollow -cast parts are removed. Removal of slip causes piece instability; when air (possibly compressed) does not take its place partial vacuums can form, detaching the just-lbrmed mass from the walls and thus deforming the sariitaryw are piece. Using air to exert pressure against the mass of liquid slip inside the chamber of the piece before beginning drainage, pushes the body against the walls of tht mould and so prevents inward collapse;

■ I lardening is achieved by applying compressed air on the inside of hollow-cast parts. This pressure must lie relieved before the mould is opened: il it is not the excessive pressure could deform the piece when the mould is opened.

When a sanitaryware piece is de-moulded it loses the physical support that was provided by the mould itself Gravity can take hold, causing collapse. In high pressure casting pressurised water and air are applied to detach the ceramic from the mould and can he a source of deformation. The higher the moisture content of sanitaryware the more likely it is to warp when the mould is opened. .Similarly the higher the water/air pressures used to detach the piece from the mould, the more likely it is to warp. 1 lowever delbrrnation may also lie created because of the opposite problem, that is, the application of pressures that are too low or too brief thus causing imperfect removal of the ceramic mass from the mould. Opening the mould then causes dragging of the sanitaryware arid deformation.

Because of the plastic nature of the de-moulded ceramic, handling must be carried out with great care; personnel must exercise maximum caution and provfch where necessary, support for the weakest parts.

Sometimes, despite taking precautions, the ceramic has an excessive tendenc towards deformation. These defects are obviated by making structural modification: designed to make affected parts less sensitive (i.e. stronger). Such changes may include the addition of support ribs, the use of hollow-cast parts to stabilise the baryeentre of the specific part or increased solid-cast thickness for greats strength,

Date: 2015-04-20; view: 3051