Methods of manufacture

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2 Methods of manufacture For Ceramics (see (b)) Crush raw materials Shape the crushed raw materials (various means) Dry & fire Apply finishing operations, as needed; to achieve required dimensional tolerances & surface finish For Glasses (see (a)) Mix, then melt raw materials in a furnace Shape in molds (using various techniques) -Technique selected depends on the shape & size of the part Strengthen if desired -By chemical & thermal means -By lamination with polymer sheets 2

3 Several techniques available to process ceramics into useful products -Choice of method depends on particular ceramic, and shape of product -Pottery, tiles, ovenware: low control of materials, processes -High-tech components & cutting tools: process control more rigorous 3

4 Ceramics: general procedure -Crush or grind raw materials into very fine particles -Generally grind in a ball mill (as for PM) either dry or wet -Wet crushing is more effective (no dust/loss) -Size (through a sieve), filter, & wash the particles -Mix particles with additives to impart desired characteristics -Binder: for holding ceramic particles together -Lubricant: To reduce internal friction between particles; to improve mold release -Wetting agent: To improve mixing -Plasticizer: To render the mix more plastic & formable -Agents: To control foaming & sintering -Deflocculent: To make ceramic-water suspension more uniform (reverses electrical charge on particles from attract to repel) -Shape, Dry, Fire -Optional: finishing (e.g. machine or grind to improve dimensions & surface finish) 4

5 Slip Casting The most common casting process (for ceramics) A slip is a suspension of colloidal (small particles that do not settle) ceramic particles in an immiscible liquid (insoluble in each other), which is generally water. The slip is poured into a porous mold, made of plaster of paris. The slip must be fluid enough with low viscosity to flow easily into mold. Pour carefully (Avoid air entrapment) Allow mold to absorb some water from the suspension. Invert mold (to pour out remaining suspension) The product is now a hollow object, lining the cavity. Trim top; Open mold; Remove part May machine the green part (with care, to avoid damage) 5

6 Doctor-blade Process Used to make thin sheets of ceramics (<1.5 mm or 0.06 ) The slip is cast over a moving plastic belt The thickness of the slip is controlled by a blade 6

7 Plastic Forming -May be carried out by various methods including extrusion and jiggering, shown here. Extrusion ((a)) Clay mixture (20-30% water) is forced through a die opening by a rotating screw Constant t cross section; limits on wall thickness for hollow o shapes May perform subsequent shaping operations Low tooling costs; high production rates Jiggering ((b)) Clay slug is extruded and formed into a bat over a plaster mold The part is jiggered on a rotating mold; The jiggering motion forms the clay bat by means of rollers or templates. The part is dried and fired Formed ware removed from mold Produces axisymmetric parts; limited dimensional accuracy; automated production improves productivity 7

8 Dry & Fire to impart proper strength and hardness Drying is critical because of the tendency of part to warp or crack -Cracking may be caused by variations in moisture content and thickness -Reduce warping & cracking by close control of atmospheric humidity and ambient temperature Loss of moisture causes shrinkage (up to 20%) In a humid environment, evaporation rate is low; reducing the moisture gradient Firing involves heating part to elevated temperature in controlled environment Some shrinkage occurs during firing Firing imparts strength and hardness -Develops strong bond between the complex oxide particles in the ceramic -Reduced porosity 8

9 Finishing Operations may be required because firing causes dimensional changes These finishing operations are all described in some detail in later chapters in text Process selection is an important consideration because of the brittle nature of most ceramics, and the additional costs The effect of the finishing operation on the properties of the product should also be considered 9

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11 Float Method -For producing sheet glass Heat in furnace Feed molten glass into a long bath in which the glass floats over a bed of molten tin. -Bath is in a controlled atmosphere, temperature 1150 o C Glass (at 650 o C) moves over rollers into a lehr, a chamber where it solidifies 11

12 Two more processes for producing glass sheets: Drawing, Rolling Drawing ((a)) Pass ( pull ) molten glass through a pair of rolls (arrangement similar to an old fashioned clothes wringer) The solidifying glass is squeezed between the rolls, emerging as a flat sheet and rolled away on smaller rolls Rolling ((b)) Molten glass is squeezed between powered rollers, forming the sheet The surfaces of the glass may be embossed with a pattern using textured roller surfaces; the glass surface becomes a replica of the roller surfaces Glass sheet produced by drawing or rolling has a rough surface appearance imparted from contact with the rolls 12

13 Glass Tubing manufacture Molten glass is wrapped around a rotating hollow mandrel and is drawn out by a set of rolls -The mandrel may be cylindrical or cone shaped Air is blown through the mandrel to prevent collapse The machines may be horizontal, vertical or inclined Used to make fluorescent tubes May also make solid rods (no mandrel) 13

14 The blowing process to produce a glass bottle (a common discrete glass product) Blown air expands a hollow gob of heated glass against the inner walls of the mold The mold usually is coated with parting agent (oil or emulsion) to prevent glass from sticking to the mold In blow and blow process, a second blowing operation is employed to produce the final shape Acceptable surface finish for most applications Difficult to control wall thickness Economical process for high rate production 14

15 Another method to produce discrete glass products: Pressing Similar to closed die forging A gob of molten glass is placed into a mold and pressed into a confined cavity with a plunger Mold may be one-piece (shown here) or split (next image) Controlled cavity produces product with higher dimensional accuracy than by blowing Can produce thin wall sections by press and blow in which the pressed part is then subjected to air pressure 15

16 Use split mold process to produce non-flat shapes and shapes from which the plunger cannot be extracted, such as bottles 16

17 Centrifugal Casting of Glass (Aka spinning ) Process is similar to that used for metals The centrifugal force pushes the molten glass against the wall where it solidifies 17

18 Glass can be strengthened by a number of processes Discrete glass products may be subjected to annealing and other finishing operations to impart desired properties and surface characteristics Thermal Tempering The surfaces of the hot glass are cooled rapidly by a blast of air The surfaces shrink; at first tensile stresses develop on the surfaces As the bulk cools, it contracts, forcing the already cooled surfaces to contract, developing residual compressive surface stresses; the interior develops residual tensile stresses Compressive surface stresses improve the strength of the glass (similar to metals) Tempered glass shatters into large number of smaller pieces when it is broken - This is because of the high amount of energy stored in residual stresses 18

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22 Case Study

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