ceramics glass plastic rubber

8/12/2019 Ceramics Glass Plastic Rubber

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Ceramics

Ceramics are inorganic nonmetallic materials.

Properties of Ceramics

Ceramic parts are hard, extremely strong in compression, highly chemical- and corrosion-resistant, nonflammable, and suitable for use at extremely high operating temperatures.Ceramic whitewares generally have good thermal shock resistance and low thermal

expansion. High modulus of elasticity and high radiation resistance are two additional

properties of importance in some applications. Most ceramics are dielectrics and, except forferrites, lack magnetic properties.

Excellent abrasion-resistant surfaces are possible. These surfaces also offer a pleasing

gloss or patina and can be vitreous and nonporous. In addition to their resistance to chemical

substances and corrosive materials, ceramics are relatively immune to fire, heat andweathering.

Generally, all ceramic materials are brittle. Tensile strengths are somewhat limited.There

also are some limitations in freedom of design because of processing complexities andinherent mechanical properties. Because of high firing temperatures, metal inserts cannot be

molded into ceramics.

The size of commercial ceramic components ranges from the very small electronic

components to large nose cones. Typical ceramic parts for mechanical applications arebearings, turbine blades, cams, cutting tools, extrusion dies, threadand wire guides, nozzles

for abrasive materials, wear plates, seals, valve seats, filters,pump parts, crucibles, and trays.

Typical parts for electrical and electronic applications include coil forms, tubes, insulators,lamp housings, printed-circuit boards, , resistor bases, vacuum-tube-element supports, and

terminals.

Design Recommendations for Ceramic Parts

Although technical ceramics can be fabricated into complex shapes, it is always desirable to

keep shapes as simple as possible for economic reasons. Tolerances also should be as liberalas the function of the component permits. It is important, from a structural standpoint, to

avoid problems that result from the low tensile strength and lack of ductility of ceramics.

Specific design recommendations for technical ceramics are as follows1. Edges and corners should have chamfers or generous radii to minimize chipping and

stress concentration and help forming.2. It is preferable to avoid large overhanging or unsupported sections.3. Pressed parts should be designed with as uniform a wall thickness as possible.

Differential shrinkage of sections of non-uniform thickness during drying and firing

causes stress, distortion, and cracking.

4. Other factors being equal, simple symmetrical shapes without deep recesses, holes, andprojections are preferable. Gently curved surfaces without abrupt break lines orangularity are normally preferred with most ceramic-forming processes.

5. When hollow pieces are cast a draft angle of at least 5 must be provided to facilitateremoval of the green body.


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6. Undercuts should be avoided in ceramic components if possible. Although someundercuts can be incorporated through the use of mold cores, machining is the normal

method for producing them. With dry pressing, machining is essential if undercuts arerequired. In all cases, costs are added.

7. Dry-pressed ceramics are subject to other design rules of powder-metal parts also butcannot match their close dimensional tolerances.8. Cavities, grooves, and blind holes in pressed parts should not be deeper than onehalf thepart thickness and preferably only one-third the thickness.

9. Extruded parts should be symmetrical, if possible, with uniform wall thickness.10.Holes in pressed parts should be large and as widely spaced as possible. Thin walls

between holes, depressions, or outside edges should be avoided. These walls should be at

least as thick as the basic walls of the part, especially if the part is small and thin-walled.

11.It must be remembered that distortions from shrinkage can cause fitting problems whenholes are used for fasteners and when holes in ceramic parts are to be aligned with holesin mating parts. Holes in ceramic parts may become slightly out of round after firing.

Multiple holes that are to be aligned with corresponding holes in other parts must be

further enlarged or elongated. The amount of the enlargement or elongation depends onthe allowable hole-to-hole tolerance of the two parts.

12. Molding of screw threads in ceramic parts is not feasible. Screw threads can bemachined in green ceramic workpieces, but they constitute a potential problem and it is

better to design parts without screw threads if possible.13.Ribs and fins should be well rounded, wide, and well spaced and have normal draft.14.Grinding after firing can produce ceramic parts of high accuracy, but stock removal rates

are slow, and the operation is expensive.15.Ceramic parts can be permanently joined to metal components by adhesive bonding,

soldering, brazing, and shrink fitting.


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Glass

Glass may be defined as an inorganic product of fusion which has been cooled to a rigid

condition without crystallization. Glass is an undercooled liquid of very high viscosity.Glasses are amorphous.

Properties of GlassTransparency is the most important property of glass and accounts for most of its

applications. Glass is solid at room temperature and becomes plastic on heating to 1200C.Poor resistance of glass to thermal shock can be improved by tempering which also increases

mechanical strength.Glass products range in size from microspheres of fractional-millimeter diameter used as

fillers for plastics to large plate-glass windows. Typical pressed-glass components are

electrical insulators, baking dishes, food blenders, stoppers and stopcocks for laboratory

vessels, eyeglasses, and ornamental pieces. Typical blown-glass components are bottles andother containers, incandescent lamps, electron tubes, laboratory glassware, and television

picture tubes.Tubing and piping of glass, made by drawing, are used for laboratory, chemical industry, andhigh-temperature applications and thermometers. Flat glass for glazing, mirrors, tabletops,

and other purposes is made either by drawing or by rolling, which, in the case of plate glass,

is followed by grinding and polishing or by floating onto molten tin and drawinghorizontally. Glass powders are sintered to make filters and other porous objects. Glass fibers

are a major reinforcing medium for many products, for insulation and for fiber optics.

Design Recommendations for Glass Parts

1. Holes, cavities, and deep slots should be included in a part only if absolutely necessary.Holes are normally not punched through in the pressing operation but are machined froma thin web or hollow boss.

2. Walls should have uniform thickness,3. Part should be designed for compressive rather than tensile strength.


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4. Parts should be gently curved rather than sharp-angled shapes.5. Lettering or other irregular surface features may be incorporated in the direction of the mold

opening.6. Ribs and flanges can be incorporated in pressed-glass components, but not practicable in

blown parts.

7.

While bosses may be incorporated in some items like electrical insulators, they are normallynot practicable for general-purpose design and manufacture.8. Threads for bottle caps or similar connecting devices may be incorporated in blown-glass

parts.


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DESIGN OF PLASTIC PARTS:

Plastics are a group of synthetic chemical compounds which are at some stage plastic and couldbe shaped by heat, with or without pressure into useful products. They usually consist of one or

more chemical compounds of organic origin which have indefinite melting point and have

amorphous structure.Plastics are generally classified into thermoplastics or thermosetting resins. Thermosetting resinis hardened by heat, causing a chemical change which is irreversible. Thermoplastics are

softened by heating and harden on cooling. This process for thermoplastics can be repeated.

Due to their properties, plastics are considered important engineering materials. Plastics are:

Light in weight Resistant to most chemicals Resistant to corrosion Excellent electrical insulators Easy to shape into finished parts Sufficiently strong and hard for many applications

Some disadvantages are: they cannot withstand high temperatures, relatively low strength ascompared to metals.

Some design rules for plastic-moulded parts are:

1. Allow for shrinkage after moulding2. Do not give unnecessarily close tolerances. This increases cost3. Provide draft for easy removal of parts from the mould.4. Avoid undercuts which require cores or split moulds.5. Locate the mould parting in one plane as far as possible.6. Locate holes at right angles to parting surfaces. Avoid oblique holes.7. Avoid long cored holes.8. Avoid irregular shaped holes and projections. These are costlier than cylindrical ones.9. Locate all holes and projections in direction of mould closing.10.Locate lettering to be embossed or debossed on surfaces perpendicular to mould closing

direction.

11.Arrange ejector pin locations so that marks occur on concealed surfaces.12.All holding surfaces in components should be knurled.13.Some rules for plain and threaded inserts are:

a. Use brass inserts rather than steel as coefficient of thermal expansion of brass iscloser to that of steel.

b. Use inserts that are sturdy and not too small.c. Avoid long slender inserts as moulding pressure deflects them.d. Make plastic section around an insert heavy to hold the insert securely andprevent cracking.e. Make projecting portions of inserts cylindrical to minimize moulding cost.f. Design projecting threaded inserts with slight projecting unthreaded shoulder to

prevent plastic from filling the threads during moulding.g. Specify metal inserts for small threaded holes when thread will be subjected to

mechanical wear or stress.

14.Specify as shallow draw as possible. Deep draws require high moulding pressure.


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15.Design uniform cross section thickness and uniform distribution of mass.16.Design corners with good radii or fillets.17.Use ribs to add strength and rigidity to minimize distortion or warping.18.Avoid sharp discontinuities that cause stress concentration.19.Rib height should not be more than twice the thickness of rib section.20.

Break up large flat surfaces with beads, steps etc. to increase rigidity.

Design of some plastic components:

Springs

Newly developed, high strength plastics are preferred for plastic springs because magnitudeof stress can be kept lower as compared to yield stress. These are not suitable for very high

stresses as the plastic can show creep behavior. Fig. 1 shows conical springs designed with

plastics.

Fig. 1 Springs Using PlasticsBush BearingsPlastic is preferred for bearing material in chemically aggressive environments, without use

of any lubricant and needs no maintenance. Plastic bearings are cheap and easily replacewood, steel and zinc bearings. Fig. 2 illustrates two application of plastic a bearing material.


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Fig. 2 Bush and Bearings Designed Using Plastics

GearsAccurate plastic gears are difficult to produce as they are to be moulded and requireconsideration for mould manufacturing inaccuracy and shrinkage. However, small and

medium sized gears are difficult to produce by machining in other materials and are preferred

to be made out of plastic.Fasteners in Plastic

Development with plastics has resulted into design of several new fasteners and fastening

methods. These plastic fasteners are specially made for non-plastic components. These

fasteners reduce assembly time. Five general kinds of plastic fasteners include:

Screw assemblies (plastic screws in toys) Snap and press fits (lunch boxes) Inserts (automobiles and vehicle bodies) Ties and twists (measuring instruments) Shrink wraps (packaging)


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Rubber

Natural rubber is a high molecular weight hydrocarbon polymer obtained from a milkyemulsion called latex by tabbing the bark of the tree Hevea brasiliensis(rubber tree). When

raw rubber is passed between hot rolls, physical and chemical changes take place and the

mass becomes plastic. Proper substances are added before vulcanization to control propertieslike strength, toughness and hardness and make it resistant to abrasion, chemicals, solvents,oxygen, light and oil. Accelerators like cotton are added to reduce vulcanization time.

Nitrogen compounds are added to increase age-resistance. Sulphur is added to increase

hardness. Vulcanization is simultaneous application of heat and pressure to shape and givedesired properties to rubber.Artificial rubber is synthesized rubber like material. The demand of automobile industry

required development of rubber like material which natural rubber did not meet. Buna S,

Buna N, Butyl rubber, neoprene and thiokol are most popular artificial rubber materials.

Rubber has several properties which make it useful as engineering material. It has large

deformability, low modulus of rigidity, absorbs shocks in cushioning and sealing, regainsshape after large deformation, can store more energy than other materials, damps vibration.

Rubber is bad conductor of electricity, impermeable to water and many gases too.

Rubber parts are formed to desired shape by mainly moulding and extruding. Injectionmoulding and dipping are most common production processes for rubber parts.

Design Recommendations of Rubber PartsRubber moulding is similar to plastic moulding and design principles are somewhat similar.

It is always better to make design simple, avoid projections, overhangs, undercuts etc. Some

design recommendations are:

1. Gating:Gating should be such that rubber in gate region permits easy removal of partand easy trimming.

2. Holes:Holes are easier to produce in moulding by using cores than by drilling.Holes should be shallow and wide as possible. Through holes are preferable to blindholes.

Blind deep holes having small diameter should be avoided. Blind holes should be tapered

or stepped.


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Sufficient wall thickness should be incorporated round holes to avoid tearing.

3. Wall thickness: Wall thickness of parts should be kept constant to have uniformproperties and avoid distortion.

4. Undercuts:Undercuts should be avoided. Undercuts can be produced by machining andnot by moulding. Internal undercuts should be avoided. Undercuts should not be verydeep and should be with radius.

5. Screw threads:Screw threads are moulded ony on hard rubber. Threaded metal insertsare used for soft rubber.

6. Inserts: Nonflush threaded inserts should be staggered for keeping uniform thickness.Inserts with flush heads are used. Inserts with sharp edges should be avoided.


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7. Draft: Draft in moulded parts varies with part design and nature of rubber. For softrubbers draft may not be needed. For soft thermoplastic rubber draft angle of 0.25 to 1while that of at least 1for hard rubber is provided.

8. Corners:Radii and Fillets are desired on corners for streamlines flow of rubber duringmoulding and minimize stress concentration.Low cost parts are often made by parting off from extruded sleeves. The square cornerformed by parting is often satisfactory for some applications.

Avoid sharp edges and minimum thickness of 0.8 mm is recommended.


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Minimum radius for forming of tubes should be atlease 1.5 times the outside diameter to

prevent cracking or warping.

Generous radii should be given in walls of bellows to minimize stress concentration andmake removal easier. Sharp corners should be avoided as these cause weak points.

9. Flash:Flash tends to form at mould parting line. Amount of flash permissible depends onthe part. For material saving and economy, mould is designed to produce thin flash or no

flash in case of precision moulds.

10.Parting line:Parting line should be in a single plane as far as possible.11.

Venting: Venting in the die is effective at final closing to avoid defects by releasingtrapped air during injection moulding.

12.Surface texture: Smooth, glossy surface on moulded rubber parts are costly. Mouldrelease agents are required to be added for smooth surface.

13.Shrinkage: Shrinkage is about 1.5% typically but it may vary between 0.4% to 4%depending upon the type or rubber.

14.Distortion:Exposed rubber surfaces are not truly flat due to shrinkage and other factors.15.Dimensional effect:Precise rubber parts are costly to produce due to cost of tooling and

strict process control. Dimensions of rubber may change after moulding due to relaxation

effect. Some moisture absorbing rubbers undergo dimensional change with time.

Rubber Parts1. Reinforced V belts, hose and tyres.2. Brake diaphragms3. Bellows and similar diaphragms4. Gaskets, washers5. O rings and metal insert seals6. Rubber bush bearings7. Flexible mechanical coupling8. Rubber pads for high frequency vibration damping.

ceramics glass plastic rubber

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