Low casting rate
Low thickness formation
Low casting rate or low thickness is generally caused by bad packing of the particles during consolidation.
The principal ways of increasing casting rate are:
i) Use of a coarser clay
Using a coarser clay instead of a finer one could help improve permeability, hence casting rate.
Imerys’ portfolio contains several clays with improved permeability together with good plasticity in order to maintain green strength of the cast item.
- UK: Hycast Rapide, Hycast FFC
- Brazil: Paracast Plast
- US: Martin #5/L-1, Martin #5
ii) Use of a faster casting kaolin
The use of a faster casting kaolin will improve casting rate. Generally speaking, Imerys kaolins improve casting rate while maintaining a high solid percentage, helping to manage density and limit amount of electrolytes (deflocculants).
- UK: LPC, Prosper, SPK
- Thailand : MRD Cast, MRD LW Cast
- Brazil: KaoCast RC2
- India: KaoCast ND50
Other potential issues
The slip is fully deflocculated: in this case, the particles are so-well organized, that permeability drops. If the materials in the casting slip are not liquid enough, we can recommend clays and kaolins to help increase density.
It could be of use to check the particle size distribution (PSD) of the non-plastic materials: for fine fireclay, check the amount of fine grains of quartz and chamotte, which are generally below 63µm. Moreover, a finer PSD usually means higher shrinkage.
For VC, the PSD of quartz and feldspar have an impact on permeability. But before changing for higher values, it makes sense to check water absorption, since a higher PSD will increase water absorption.
Bad drainage of the mold after casting can lead to surface defects:
- Drips on visible surfaces, that with impact the glazing and remain visible after firing;
- Slip hang-up that will potentially form cracks during drying.
Rheology will need to be adjusted and thixotropy reduced mandatory to increase drainage.
Demolding of the piece is a critical because the operator will have to handle a soft piece. If slip properties are not properly adjusted, and technological times not observed, issues may arise here, that may only come to light after drying or firing.
Soft pieces, deformation
A too high thixotropy is usually the cause of too soft pieces, together with a too low density and over deflocculation.
A better adjustment of casting parameters is necessary. See section on Slip preparation / Too low density or too high viscosity.
Cracks during demolding have 2 principal causes:
- lack of plasticity of the cast piece
- too high shrinkage in the mold
Lack of plasticity: We usually recommend increasing the plasticity of the clay, increasing the clay percentage in the formulation, or using a more plastic kaolin, such as the following grades:
- HK: Hycast Plastic, Hycast FFC, Hyplas 64S
- Thailand : Thaicast S1, Modiscast S4
- Brazil: Vigor RC116
- India: Certicast
- US: OM#4
Too high shrinkage in the mold: the shrinkage of the cast piece starts in the mold and the lower part of the mold shifts. The piece contracts therefore and some cracks may appear. The easiest way to solve this issue is to increase slip density. Less water in the slip means less shrinkage.
See section on Slip preparation / Too low density or too high viscosity.
Casting cords, stains, slip meet
Casting cords, stains and slip meet are directly linked to the interaction of the surface of the slip, air and plaster. During mold filling, the surface of the slip has higher concentrations of fine and/or colloidal particles and electrolytes.
When two casting slip fronts join, this heterogeneous surface with a creamy aspect may form a slip meet, leading to a potential defect.
Adjusting the rheological set up of the casting slip is the best way to control and avoid these defects, which are usually caused by a slip which too low thixotropy.
Air bubbles trapped close to the surface of the cast piece arise from:
- stirring the slip too quickly before casting
- too high viscosity preventing the migration of the bubbles before casting
- filling the mold too quickly
- wrong orientation of the casting bench
Adjusting rheology may help solve these issues. See section on Slip preparation / Too low density or too high viscosity.
Still having issues with casting? Have a question you need to ask us?