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Low thickness formation
The low thickness formation, or low casting rate, founds its origins mainly in a bad packing of the particles in during the consolidation. The main actions to increase the casting rate are:
Use of a coarser clay
The use of a coarser clay instead of a finer one could help improving the permeability, therefore, the casting rate.
Imerys’ portfolio contains several clays that are showing a better permeability together with a good plasticity in order to maintain the green strength of the cast item.
Brazil: Paracast Plast
Use of a faster casting kaolins
The use of a faster casting kaolin will improve the casting rate.
Globally, Imerys kaolins are giving a better casting rate while keeping a high solid percentage, helping to manage with a good density and limited amount of electrolytes (defloculants).
Brazil: KaoCast RC2
India: KaoCast ND50
Other potential issues
The slip is fully deflocculated : in this case, the particles are so-well organized, that the permeability drops. If the materials in the casting slip are not fluid enough, we can recommend clays and kaolins that could help to increase density (Too low density or too high viscosity).
It could be of use to check the PSD of the non-plastic materials: for Fine Fireclay, check the amount of fine grains of quartz and chamotte, generally below 63 µm. Moreover, a finer PSD usually means higher shrinkage.
For VC, the PSD of quartz and feldspar have an impact on the permeability. But before changing for higher values, it makes sense to check also the water absorption, because a higher PSD is driving to higher water absorption.
A bad drainage of the mould after casting is leading to surface defects :
- drips on visible surfaces, that with impact the glazing, and remain visible after firing;
- slip hang-up that will potentially give cracks during the drying.
A better adjustment of the rheology, and especially a reduction of the thixotropy is mandatory to increase the drainage.
The demoulding of the piece is a critical stage, because the operator will have to handle a soft piece. If the slip properties are not properly adjusted, and the technological times not observed, some issues could appear in here, and sometimes only revealed after drying or firing :
Soft pieces, deformation
A too high thixotropy is usually the origin of too soft pieces, together with a too low density and over- deflocculation.
The cracks during the demoulding stage have 2 main origins : lack of plasticity of the cast piece and too high shrinkage in the mould.
Lack of plasticity: the plasticity is too low to tolerate the constraints given to the piece during the handling. We usually recommend to increase the plasticity of the clay, increase the clay percentage in the formulation, or use a more plastic kaolin :
Brazil: Vigor RC116
Too high shrinkage in the mould: the shrinkage of the cast piece is starting in the mould, but the lower part of the mould is any shifting. Thus, the piece is contracting itself on the mould, and some cracks can appear. The easiest way to solve this issue is to increase the slip density. Less water in the slip means less
shrinkage. (Too low density or too high viscosity)
Casting cords, stains, slip meet
These defects are directly linked to the interaction with both surface of the slip, air and plaster. During the mould filling, the surface of the slip has higher concentration of fine and/or colloidal particles, and also electrolytes.
When two fronts of casting slip are joining, this heterogeneous surface with a creamy aspect could form a slip meet, an improper joined plan, leading to a potential defect.
Air bubbles trapped close to the surface of the cast piece are coming from:
- Too fast stirring of the slip before casting
- Too high viscosity avoiding the migration of the bubbles before casting
- Too fast mould filling
- Wrong orientation of the casting bench