Ceramic Manufacturing Flashcards
(29 cards)
Processing of traditional vs new ceramics
Traditional ceramics powder is mixed with water. New ceramics powder use other substances as binders
Comminution
Reducing particle size in ceramics process by mechanical energy in various forms such as impact, compression and attrition. Best on brittle materials. Two categories: crushing and grinding
Crushing
Reduction of large lumps to smaller sizes for further reduction. Accomplished by compression against rigid surfaces or by impact against surfaces in a rigid constrained motion
Types of equipment to perform crushing
Jaw crushers- large jaw toggles back and forth to crush limps against a hard rigid surfaces
Gyratory crushers-use a gyrating cone to compress lumps against a rigid surface
Roll crushers lumps are squeezed between rotating rolls
Hammer mills- rotating hammers impact the material to break up lumps
Grinding
Reduce the small pieces by crushing into a fine powder. Accomplished by abrasion and impact of the crushed minerals by the free motion of unconnected hard media, such as balls, pebbles or rods
Types of grinding
Ball mill
Roller mill
Impact grinding
Ball pill
Hard sphères mixed with the stock are tumbled inside a rotating cylindrical container. rotation causes balks and stick to be carried up the container wall then pulled down by gravity, creating grinding action. Often water is added so that ceramic is in the form of a slurry
Roller mill
Stock is compressed against a flag horizontal grinding table by rollers riding over table surface. Pressure of rollers is regulated by mechanical springs
Impact grinding
Particles are thrown against a hard flat surface in a high velocity air stream or a high speed slurry. Impact fractures the pieces
Impact of water in paste for shaping of ceramics
More wayer means more plastic and easily formed. However when formed, shrinkage occurs and can lead to cracking of product.
How to accommodate to problem of shrinkage during firing and drying
Other ceramic raw materials that do not shrink are added to paste
What are the four categories of the ingredients in a ceramic paste
Clay- provides consistency and plasticity for shaping
Non plastic raw materials- to prevent shrinkage
Other ingredients such as fluxes- melt during firing to promote singering
Wetting agents- improve mixing of ingredients
What is the types of shaping processes of clay and required water content of paste
Slip casting- 25-40% water leading to a slurry
Semi dry pressing- 15-25% so clay is in plastic condition
Dry pressing- 5% water, clay is basically dry with no plasticity
Slip casting
Slip is poured into a porous plaster of Paris mold so that water is gradually absorbed into the pasted to form, a firm layer of clay at surface. There’s solid and drain casting
Drain casting
Mold is inverted to drain excess slip after semi solid layer has formed, leaving a hollow part.
Solid casting
Time required. Additional slip must be periodically added to account for shrinkage
Manual vs mechanized method of plastic forming
Manual usually has higher water percentage so mixture is less stiff to mold
Hand molding vs hand throwing
Hand throwing is on a potters wheel
Semi dry pressing
Used high pressure to force mixture to flow into a die cavity. Flash is often formed from excess clay being squeezed between the die sections
Dry pressing
Binders usually added to provide sufficient strength in the pressed part for subsequent handing. Lubricants also used to prevent sticking. Di pe must be made or hardened tool sleel or cemented tungsten carbide to reduce wear. Geometry must be simple and amount and distribution of powder in die cavity must be right. K flash or shrinkage. No drying time.
Volume change of adding water to dry clay
No initial volume change as simply replaces air in the pores between ceramic grains. When more water is added grain become separated and the volume grows. Becomes liquid suspension of clay particles in water.
Volume change during drying
As water is removed, the piece shrinks. Occurs in 2 stages:
1. Rate of drying is rapid and constant as water evaporated from surface into surrounding. Shrinkage occurs
2. Moisture content deducted and little to no shrinkage occurs, drying process slows
How is the drying of clay usual accomplished
In drying chambers which temp and humidity is controlled. Water shouldn’t leave too rapidly making it more prone to cracks due to large moisture gradients in piece. Heating usually combination of convection and radiation, from infrared sources.
What is meant when a ceramic piece is green
It when it has been shaped but before firing (singering)
