Polymer and Ceramic Processing. Outline . Polymer Processing Techniques Polymer Additives Ceramic Fabrication Methods Glass Forming Particulate Forming Cementation. Forming Processes for Polymers . Compression Molding Injection Molding Extrusion Blow Molding. Processing Details.
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[hose and tubing, belts, rope and cable covers, sheeting and films]
Foam molding offers the possibility of increasing the size of a part without increasing weight and reducing the weight of a part with controlled change of properties. Chemical foaming involves mixing a chemical blowing agent with pellets prior to the pellets being fed into the feed throat of the molding machine.
Most engineering thermoplastic parts are fabricated by injection molding.
Fillers - Added to improve tensile strength & abrasion resistance, toughness & decrease cost. Examples: carbon black, silica gel, wood flour, glass, limestone, talc.
Plasticizers - Added to reduce the glass transition temperature Tg below room temperature. Presence of plasticizer transforms brittle polymer to a ductile one. Commonly added to PVC.
Stabilizers – Antioxidants, UV protection
Lubricants - Added to allow easier processing polymer “slides” through dies easier (sodium stearate).
Colorants - Dyes and pigments
Flame Retardants - Substances containing chlorine, fluorine and boron.Polymer Additives
Thermoplastics and thermosets
polymer and additives placed in mold cavity
mold heated and pressure applied
fluid polymer assumes shape of mold
In the extrusion of plastics, raw thermoplastic material in the form of small beads (resin) is gravity fed from a top mounted hopper into the barrel of the extruder. Additives (colorants and UV inhibitors in either liquid or pellet form) are often used and can be mixed into the resin prior to arriving at the hopper.
The material enters through the feed throat (an opening near the rear of the barrel) and comes into contact with the screw. The rotating screw (normally turning at up to 120 rpm) forces the plastic beads forward into the barrel which is heated to the desired melt temperature of the molten plastic (which can range from 200°C/400°F to 275°C/530°F depending on the polymer).
In most processes, a heating profile is set for the barrel where three or more independent PID controlled heater zones gradually increase the temperature of the barrel from the rear (where the plastic enters) to the front. This allows the plastic beads to melt gradually as they are pushed through the barrel and lowers the risk of overheating which may cause degradation in the polymer.
Ceramic Fabrication Methods
A glass is an inorganic non metallic material that does not have a crystalline structure. Such materials are said to be amorphous and are virtually solid liquids cooled at such a rate that crystals have not been able to form.
Typical glasses range from the soda-lime silicate glass for soda bottles to the extremely high purity silica glass for optical fibers.
Glass is widely used for windows, bottles, glasses for drinking, transfer piping and receptacles for highly corrosive liquids, optical glasses, windows for nuclear applications.
Most products have been blown glass. In recent times, most flat glass has been produced using the float process.
Mass produced bottles and decorative products are made using industrial scale blown glassprocess.
• Viscosity, h ,describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction.
-- relates shear stress () and velocity gradient (dv/dy):
hhas units of (Pa-s)
Glass or noncrystalline materials do not solidify in the same sense as crystalline materials. Upon cooling, a glass becomes more and more viscous with decreasing temperature.
Important in glass forming operations are the viscosity-temperature characteristics of glass.
viscosity = 10 Pa-s; glass is fluid enough to be considered liquid.
viscosity = 103 Pa-s; glass is easily deformed.
viscosity = 4x106 Pa-s; max temp. glass can be handled without altering dimensions.
viscosity = 1012 Pa-s; good atomic diffusion; stress relief.
viscosity = 3 x 1013 Pa-s; below strain point, fracture will occur before the onset of plastic deformation .
• Specific volume (1/r) vs Temperature (T):
• Crystalline materials:
-- crystallize at melting temp, Tm
-- have abrupt change in specific
volume at Tm
-- do not crystallize
-- change in slope in spec. vol. curve at glass transition temperature, Tg -- transparent - no grain boundaries to scatter light
Working range: glass-forming carried out
• Viscosity decreases with T
Sheet forming – continuous casting
sheets are formed by floating the molten glass on a pool of molten tin
at room temp.
-- Result: surface crack growth is suppressed.
-- removes internal stresses caused by uneven cooling.
-- puts surface of glass part into compression
-- suppresses growth of cracks from surface scratches.
Fully tempered glass is roughly 4 times stronger than annealed glass of the same thickness and configuration; residual surface compression must be over 10,000 psi for 6mm thickness, according to ASTM C 1048.
Tempered glass is manufactured through a process of extreme heating and rapid cooling, making it harder than normal glass.
The typical process to produce tempered glass involves heating the glass to over 1,000 °F, then rapidly cooling to lock the glass surfaces in a state of compression and the core in a state of tension.
When glass cools down to ambient temperature, the center plane of the glass contracts more than the surfaces. The contraction of the center plane pulls the surfaces into compression and the glass becomes very strong.
Tempered glass cannot be cut or drilled after tempering, and any alterations, such as edge-grinding, sandblasting or acid-etching, can cause premature failure.
Fabrication occurs on electrically heated horizontal furnaces that heat the glass to a uniform temperature of roughly 1200°F.
Ceramic rolls convey the glass through these furnaces at speeds regulated to ensure temperature uniformity and minimal optical distortions.
When the glass exits from the furnace, it is rapidly cooled by a series or air nozzles. This rapid cooling puts roughly 20% of the glass surface into a state of compression, with the center core in tension.
The brittle nature of tempered glass causes it to shatter into small oval-shaped pebbles when broken. This eliminates the danger of sharp edges. Due to this property, along with its strength, tempered glass is often referred to as safety glass.
Tempered glass breaks in a unique way. If any part of the glass fails, the entire panel shatters at once. This distinguishes it from normal glass, which might experience a small crack or localized breakage from an isolated impact.
Tempered glass might also fail long after the event that caused the failure.
Stresses continue to play until the defect erupts, triggering breakage of the entire panel.
Hs glass has greater resistance to thermal loads than annealed glass and, when broken, the fragments are typically larger than those of fully tempered glass.
It does not require the strength of fully tempered glass, and is intended for applications that do not specifically require a safety glass product.
Heat Strengthened Glass
Annealing float glass is the process of controlled cooling to prevent residual stress in the glass. It is part of the float glass manufacturing process.
Annealed glass can be cut, machined, drilled, edged and polished.
To anneal glass, the glass is heated and kept for a defined period of time to relieve internal stresses.
Carefully cooled under controlled conditions to ensure that no stresses are reintroduced by chilling/cooling.
P - Pressed Ware
R - Rolled Sheet
S - Ground & Polished
T - Tubing & Rod
U - Panels
B - Blown Ware
C - Cast
D - Drawn Sheet
E - Extruded
F - Frit & Powdered Glass
G - Gob & Strip
K - Special Cane
M - Multiform
The space shuttle makes use of ~25,000 reusable, lightweight, highly porous ceramic tiles that protect the aluminum frame from the heat generated during re-entry into the Earth’s atmosphere.
Green ceramic - A ceramic that has been shaped into a specific form but has not yet been sintered.
Typical Porcelain Composition
(50%) 1. Clay
(25%) 2. Filler – e.g. quartz (finely ground)
(25%) 3. Fluxing agent (Feldspar) -- aluminosilicates plus K+, Na+, Ca+
-- upon firing - forms low-melting-temp. glass
A liquid clay body (a slip) is poured into a plaster mold and allowed to form a layer on the inside cavity of the mold.
In a solid cast mold, ceramic objects like handles and platters are surrounded by plaster on all sides with a reservoir for slip, and are removed when the solid piece is held within.
For a hollow cast mold, once the plaster has absorbed most of the liquid from the outside layer of clay the remaining slip is poured off for later use.
The cast piece is removed from the mold, trimmed and dried. This produces a green piece that is then fired, with or without decoration and glaze.
The technique is suited to the production of complex shapes, and is commonly used for toilets, basins, figurines and teapots. The technique can also be used for small scale production runs.
-- heat treatment between 900-1400°C
-- vitrification: liquid glass forms from clay and flux – flows between SiO2 particles. (Flux lowers melting temperature).
micrograph of porcelain
• Drying: as water is removed - interparticle spacings decrease – shrinkage.
Drying too fast causes sample to warp or crack due to non-uniform shrinkage
Microstructure of a barium magnesium tantalate (BMT) ceramic prepared using compaction and sintering. (Courtesy Heather Shivey.)
Aluminum oxide powder:
-- sintered at 1700°C for 6 minutes.
Tape casting - A process for making thin sheets of ceramics using a ceramic slurry consisting of binders, plasticizers, etc. The slurry is cast as tape with the help of a blade onto a plastic substrate. Used for integrated circuits and capacitors
Slip = suspended ceramic particles + organic liquid