We have listed 20 common defects in ceramic former manufacturing. Defects increase rework, energy consumption and production cost. Finding and solving the root cause of the defect and reducing the defects improves the factory production efficiency and reduces the overall production cost.
1. Crack:
Common ceramic former defect where the body splits into two or starts to separate into two but joined in the end. Most common reasons are faster drying, handling, improper mould demoulding, mould preparation, mould life or process errors.
2. Pinholes:
A tiny hole in the surface of the piece which is less than 1.0 mm is called a pinhole. This is the major defect in the ceramic former industry. There are infinite number of reasons for the pinhole defects, some are: Higher RPM agitators in slip storage, pump type, pipe flow design, not cleaning the pipe, using wet mould and fast slip filling,
3. Water Mark:
These defects look like water flow marks on the pieces and can even be felt by touching it. This happens during the wet finisher or the inspector allows water to run down too much on the piece during the dry condition of the body, which leaves markings on the body. Using a high dense sponge may reduce the water holding capacity of the sponge and training of the spongers & inspectors to use the sponge after squeezing out the water for finishing.
4. Over Firing :
Over vitrification of the ceramic is marked as an over firing defect. These defective pieces can be warped/bent or have too many pinholes or glossy finish. This happens due to firing issues, kiln problems or body formulation issues.
5. Under Firing:
After firing the piece does not vitrify fully and body color will be darker. This defect causes the piece to have low strength, larger dimension and high water absorption. This defect happens majorly due to firing issues and sometimes may be body composition issues.
6. Flow Mark:
Flow marks are an uneven body surface which can be felt by touching or by visual. This defect does not happen due to mould or finishing issues. It happens because of slip rheological properties. Using a high fluid clay is the best solution to reduce this defect or use a good deflocculant.
7. Spray Defects:
Three kinds of spray defect one is that the over spray is done on the pieces, and another is under spray and this one is uneven spray. This can happen due to low gun pressure, erosion of needle/nozzle of the spray gun, distance between the gun and the pieces during spraying or unskilled labours. This defect can be minimised by regularly cleaning the spray gun, maintaining the spray pressure above 6 bar and proper training of the labourers.
8. Bend / Bad Shape:
The body or the finger bend defect is referenced to the misalignment / bad shape of the part of the formers. These defects can be marked as body bend or finger bend. The cause of the defect can be due to model design issues, wrong demoulding process in casting, warpage during the casting stage, over sponging in particular areas, over firing and bad body formulation.
9. Black Spot :
A tiny visible black speck or spot on the surface of the body is marked as black spot. Common reasons for this defect are falling in the kiln, iron contamination in the body or rust or dirt falling in the factory on the pieces before firing which are not removed before firing. By using magnets in the slip making & storage tank, ensure mould cleaning before slip filling the mould, vacuuming the kiln cars and cleaning the burners will reduce the black spot defect in the ceramic formers.
10. Bump:
A small raised portion in the body of the formers is marked as bump defect. This is not a common defect in the former, this may be due to contamination in the slip or firing issues. Proper slip circulation and sieving can reduce this defect.
11. Line:
A vertical line that is visible in the pieces after firing is line marking or casting lines. This happens when slip contains high PSD non-plastic material or slip not aged properly.
12. Chip Off:
If a small part of the piece in the former is broken that is called chip off. Normally this defect happens at the bottom of the formers. Sharp edges at the base increase the probability of the chip off. Proper chamfering of the former base reduces the chip and also proper cleaning of the kiln car/transportation trolley removes any sharp particles in the plate to avoid chipping and proper loading helps to reduce the chip off.
13. Colour Splash:
Color stains that are visible in the former body are marked as colour splash defects. This defect happens due to color from the base color glaze splashes on the body in the process or during handling transfer of one piece to another by human hand and other stains may be coming from the marking stamp ink.
14. Dunt:
Dunts are hairline cracks in the former body which mostly happens at the firing to cooling stage. This defect happens due to thermal shock in the body, this can happen during rapid cooling. To reduce this defect, slow cooling of the pieces should be done from 650 degrees to 130 degrees celsius. And also needs to check the residue in the slip due to Quartz powder.
15. Finishing Defects:
Improper finishing or excessive finishing of the formers are called finishing defects. Proper training helps to reduce this defect.
16. Handling Defects:
knocking/cracking of the pieces during handling are marked as Handling defects. Careful handling of pieces can reduce this defect.
17. Clay Bits:
Clay bits are broken clay particles found on the upper or lower surface of the glaze after firing.. This happens especially during the dipping process, where clay particles fall on the glaze and stick to the body while dipping. This defect can be reduced by cleaning the clay particle in the formers by blowing air before sending it to the dipping process. These clay particles are generally generated during the slot cutting process. So cleaning the base and inside the former is important.
18. Batt Wash Defects:
Kiln car wash (Alumina wash) sticking to the glazed area of the formers are marked as batt wash defects. This defect happens during firing when flaky battwash particles blow off and stick to the molten glaze. This defect can be reduced by cleaning the kiln car plate properly before loading.
19. Mould Marking:
Improper or vague mould markings are marked as mould marking defects. This usually happens due to higher mould age or mould knocking or handling during the wet cast stage.
20. Seam Line:
Seam line defects that are visible in grey color and tangible in the mould joint areas. This happens due to mould absorption difference between the sharp edges and the body area, another possibility is to have a gap between the mould due to ageing of the mould or design of the mould. These defects are especially high in the three mould joint location. Mostly by correcting the design will solve this defect. Another possible way is to do the fettling better to reduce the seam line defect.
These above 20 defects are the most common issues in the ceramic former production process.Did we miss any defects? Let us know how this article helped you to understand the defects and causes.
Post Comment