the basics of manufacturing technology

7/29/2019 The Basics of Manufacturing Technology

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The Basics of Manufacturing TechnologyEE 498Professor Kelin J. Kuhn

Two lectures of materialGeneral rules for manufacturing design:

Keep the functional and physical characteristics as simple as possible Design for the most economic production method Design for a minimum number of machining operations Specify finish and accuracy no greater than are actually necessary

I. Conventional machining

A. Milling

Milling is the most versatile of the conventional machine tools. In concept, milling isvery straightforward. A cutter is held in a chuck which rotates at a controlled speed.The cutter is suspended over a work surface whose location can be preciselycontrolled. The part to be machined is securely fastened to the work surface, and thework surface is moved underneath the cutter. Appropriate choices of cutter type,depth of cut and speed determine the final shape.

A typical mill is shown below.


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There are two ways to cut using a mill. The edge of the piece can be cut (peripheralmilling) or the surface of the piece can be cut (face milling).

The piece can be milled where the work is fed against the direction of the rotating

milling cutter (up milling). This technique is best for surfaces with an initial roughfinish. Alternatively, the work can be fed in the same direction as the cutter (downmilling). This technique is best suited for intricate parts.

Cutters:

Cutters are typically fabricated from high speed steel in a number of shapes andsizes. However, cutters can also be obtained in carbide or diamond for special millingoperations.

Materials:

Materials best suited for milling are the softer metals and plastics. Aluminum andbrass are two commonly milled metals; Teflon and Delrin are commonly milledplastics. However, the ability to mill a metal is typically limited only by the hardness ofthe cutter. Special cutters can be obtained for milling harder materials andrefractories. Alternatively very sharp cutters are available for plastics and even wood.


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Set-up and fixturing.

A typical milling job requires more time to set up the fixturing to hold the job then toactually complete the job! Thus, fixturing is a critical part of the milling process.Fixturing may involve fabrication of a number of other pieces before the actual part is

manufactured.CNC and NC

Milling can be performed under computer control. Such mills called ComputerNumerical Control or Numerical Control mills are becoming increasing common insmall machine shops. There are numerous variations on these mills, the mostinteresting (at least for EEs!) are CNC mills that machine simple circuit boards.

Advantages of milling:

Very good for one-off objects Virtually any material can be milled with a proper cutter Complex parts with high detail Tolerances of 0.001" to 0.003" are possible Weights from a few grams to up to 100 lbs

Disadvantages:

A more reduced set of features possible. Certain features are not possible More materials waste than casting type processes Quite slow

B. Turning

Turning is the second most versatile of the conventional machine tools. Turning isuseful for producing parts with rotational symmetry.

In concept, turning is also very straightforward. The part is held (typically horizontally)in a machine called a lathe. One side of the part is clamped to a chuck, the other sideis held by a rotating support called a tail stock. A typical lathe is shown below.


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A cutter, held by a tool post, engages the material. The cutter can be fed transverselyinto the material or longitudinally along the material. In metal turning, the cutter isusually a single point cutter ground from a square piece of tool steel. Appropriatechoices of cutter type, depth of cut and speed determine the final shape. Typicalcutters are shown below.


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There are a number of possible machining operations with a lathe.

One of the most powerful uses of a metal lathe is in cutting screw threads. Screwthreads can be cut internally or externally as shown below.

Certain types of lathes termed screw machines are specifically designed to simplymanufacture screws. Conventional automatic screw machines can produce screws atthe rate of one every 3-5 seconds. Swiss type automatic screw machines (typicallymore accurate) produce precision screws at the rate of one every 10 or so seconds.However, Swiss type machines can produce intricate parts in far larger quantities.

Materials:

Materials best suited for turning are the softer metals and plastics. Aluminum and

brass are two commonly turned metals; Teflon and Delrin are commonly turnedplastics. The ability to turn a metal is primarily limited by the hardness of the cutterand the cutting speed. For example, turning carbon steel requires speeds up to 600sfpm and carbide tipped cutters.

Types of lathes:

Engine lathe: This is the typical lathe that you will see in a machine shop. It is limitedto one single point cutting tools and generally is used for prototype work.

Toolroom lathe: This is a smaller more precise version of an engine lathe. Oftencalled a jewelers lathe.


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Turret lathe: These are semiautomatic tools which perform essentially the samefunctions as an engine lathe. However, turret lathes are not limited to single cuttingtools -- thus several things may be happening to the part at the same time. In order tofacilitate tool changes, a six-sided turret is used in place of a tailstock.

CNC lathes: The same idea as a turret lathe, but on computer control. CNC or NClathes provide more rapid turn-around times for complex one-off parts.

Automatic screw machines: These are turret lathes, but arranged for continuous feedof bar stock. Some automatic screw machines have carried the idea a step furtherand have multiple spindles as well as multiple tools. Logically enough they are calledmultiple spindle bare automatic screw machines.

Swiss-type automatic screw machines: Swiss style machines work on a differentprinciple than the majority of lathes. In a Swiss-type machine, a revolving piece ofmaterial is fed through a bushing and then into the path of five radially mounted tools.The combination of the bushing and the radial tool mounting permits exceptionally

fine control of the cut. Swiss-type machines are almost exclusively used for theproduction of highly precise parts -- such as those in Swiss watches.

Precision:

Engine Lathes: 0.005" typical Turret Lathes: 0.003" typical Screw Machines: 0.003" typical Swiss-type machines: 0.0002" for special 0.0005" for typical

Advantages of turning:

Essentially the only way to make accurate radially symmetrical objects Virtually any material can be milled with a proper cutter Complex parts with high detail Tolerances of 0.001" to 0.003" are possible Weights from a few grams to up to 100 lbs

Disadvantages:

A more reduced set of features possible. Certain features are not possible More materials waste then casting type processes


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A side note on screws:

Although screw machines are typically used to fabricate precision screws and bolts --day-to-day screws and bolts are typically made using thread rolling machines. Inthread rolling, a blank is placed between two dies. One die moves, one die is

stationary. The blank is moved down the stationary die by the moving die and exitsthe machine as a completed screw.

Production versions of this use rotating planetary dies and can produce thousands ofscrews per hour.


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Rolled threads have certain advantages over cut threads. In particular, the rolledthread does not cut the material grain boundaries, but rather imparts a work-hardened surface similar to a forging. This tends to reduce stripping, as it is harder toshear across the grain of a rolled thread than a cut thread.


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II. Metal Casting

Metal casting is the process of creating objects by filling a cavity with liquid metal andletting the metal cool. Metal casting is approximately 6000 years old, as the first castobjects appear to be copper arrowheads dated from approximately 4000 BC.

A. Sand casting.

In spite of its innocuous name, sand casting is one of the most major industrial metalcasting processes. Sand casting accounts for over 90% of all metal poured forcasting.

The process of sand casting begins by fabricating a pattern for the final object. Thepattern is often two piece due to the construction of the mold. The pattern can bemade from virtually any substance including wood, foam, clay and plastic.

The mold which contains the sand is called a flask. It consists of two pieces, the top

or cope and the bottom or drag. The centerline divides the cope from the drag. Holescalled sprues feed molten metal into the flask and holes called risers allow airbubbles to escape.

To begin the casting process, the flask is broken into its two pieces. The pattern isinserted into the flask and the flask reassembled. Sand is packed tightly around thepattern. Then the flask is opened and the pattern removed. The sand imprint ischecked carefully, and appropriate risers and sprues added (if not contained on theoriginal pattern). Then the flask is closed and molten metal poured into the spruesuntil it emerges from the risers.

After the metal has cooled, the flask is broken open and the cast part removed. Thesand is cleaned and recycled back for the next casting operation. The sprues andrisers are removed and the part is cleaned.


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Tricks:

Either "green" sand (actually black) or dry sand is used for casting. In green sandcasting, the sand binder is kept moist with water. The part is cast as soon as possibleafter the pattern is removed. In dry sand casting, an organic binder is used -- and the

mold is baked after the pattern is removed. Green sand casting is more economical,dry sand casting has better dimensional tolerances.

To create a hole in the middle of a casting, a baked sand part called a core isinserted in the mold after the pattern has been removed. The core will be removeddestructively after the casting is complete -- leaving a hole in the middle of the part.

Polystyrene or Styrofoam can be used to create a one-time pattern for a specialtycasting. In this process, the pattern is inserted into the flask and left there. When themolten metal is poured over the pattern, it vaporizes and the vapor escapes from theriser holes.

Materials:

Any metal that can be melted. Common metals include cast iron, steel, brass,bronze, aluminum alloys, and magnesium alloys.

Advantages:

Exceptionally economical Virtually no materials waste, as leftovers can be remelted and used again The castings can range from a few ounces to thousands of pounds The castings are isotropic Virtually unlimited freedom of shape

Disadvantages:

Dimensional tolerances of 1/16" are typical -- this is large for manyapplications

The castings have a work hardened (chilled) surface and cause significant toolwear in post casting machining

B. Plaster casting

Plaster casting begins with a highly polished pattern of wood, plastic or metal.Typically the cope and drag molds are made separately -- so the pattern is really a

half-pattern (one for the cope and one for the drag). A high polish and lack of pores isnecessary in order to facilitate removal of the pattern from the mold.

The fabrication of the mold begins with the cope or drag part of the mold flask. Abottom plate is placed in the mold flask. Then, the cope or drag half of the pattern issprayed with a material called "parting compound" to assist removal of the patternfrom the mold. The pattern is placed carefully onto the bottom plate. Next, the plasterslurry (70% gypsum and 30% strengthening materials) is poured into the flask overthe pattern. After the plaster has set, the mold is reversed, the bottom plate removed,and the pattern extracted. The molds are then baked.


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After the molds have set, molten metal is poured into them in the same way as withsand casting. Once the metal has hardened, the mold is broken open to retrieve theparts.

Tricks:

To create a hole in the middle of a casting, a plaster core can be used. The core willbe removed destructively after the casting is complete -- leaving a hole in the middle

of the part.

Plaster casting is somewhat more expensive than die casting (see next section) forlarge production runs. However, for runs of less than 1000, plaster casting is muchcheaper. Thus, it is ideally suited for development of test components before majorhigh volume manufacturing.

Materials:

Limited to non-ferrous metals. Ferrous metals react with sulfur in the gypsum. Typicalcast metals are aluminum alloys, yellow brass, zinc, magnesium alloys and copper.

Advantages:

Well suited for parts with thin walls, intricate detail and complex coring Walls may be cast as thin as 0.020" The castings can range from a few ounces to thousands of pounds Tolerances of 0.005" are possible Parts can be made with minimal post machining Excellent surface finish

Disadvantages:

Parts are typically small, less than 6" in any direction Non-ferrous metals only For production runs of over 2000 parts, metal molds are cheaper


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C. Investment or lost wax casting

Investment casting is also a very old casting technique. Jewelry assumed to be castby the investment process has been found dating back 3000 years. Investmentcasting is especially well suited for tiny intricate parts.

The basic idea is to create an expendable mold from wax or plastic. The expendablemold is then coated with a refractory material to form the casting mold.

2000 years ago, the expendable mold was carved carefully from wax. Sprues andriser would be added to the wax mold to create a total wax pattern. The wax patternwould then be covered with clay or plaster, allowed to set, and then baked. Uponbaking the wax would melt, leaving a one time pattern in the plaster mold.

Modern investment casting contains one additional step. Skilled model makers createmetal dies containing the primary patterns. Wax or plastic is then injected into thesedies to create the wax pattern. Typically, the wax pattern contains many patterns

gated together by sprues and risers.

The wax pattern is then covered with a refractory material. This could be done bydipping the pattern into a ceramic slurry -- or covering the pattern with somerefractory molding material. The mold is then baked and the wax or plastic allowed todrain or vaporize out. Molten metal is then poured into the mold.

Unlike the previous casting operations, getting the metal out of the mold is moredifficult in investment casting. Since the mold material is typically refractory -- it isoften difficult to remove. Chemicals, pressurized water and sand blasting are all usedto remove molds.

Materials:

Any metal that can be melted. However this process is best suited for high-temperature metals, precious metals, or metals difficult to fabricate by other methods.Thus, stainless steel, magnesium, and carbon or tool steels are commonlyinvestment cast.

Advantages:

Permits casting of materials difficult or impossible to fabricate with othermethods

Virtually no materials waste, as leftovers can be remelted and used again Allows exceptional detail and features difficult to machine Tolerances of 0.003" to 0.005" are possible A parting line is not necessary


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Disadvantages:

Typically restricted to less than 10 lb castings Typically restricted to less than 40" in total length Significantly more expensive than sand or plaster casting

D. Permanent mold casting (gravity die casting)

In gravity die casting, the metal is poured into the mold using only the force of gravity.The mold is typically a two piece mold clamped solidly together.

Molds for casting iron or steel are made of graphite or other refractories. Molds forcasting aluminum, magnesium or copper alloy castings are made of iron or die-steel.

Molds incorporate the necessary sprues and risers, and also typically have pins forejecting the castings.

Materials:

Materials best suited for gravity die castings are materials with relatively low meltingpoints in order to be fluid in the mold. Typically iron, magnesium alloys, andaluminum alloys are cast using this process. Copper and zinc alloys tend to beinsufficiently fluid. Steel and steel alloys require special molds.

Advantages:

Reusable molds Good grain quality due to rapid heat transfer to the mold Virtually no materials waste, as leftovers can be remelted and used again Tolerances of 0.010" to 0.015" are possible Weights from a few ounces to up to 500 lbs

Disadvantages:

Expensive fabrication costs for the permanent mold Subject to warpage The need to machine the mold limits the type of features Reduced set of materials due to fluidity considerations


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E. Die casting

Die casting is one of the most common manufacturing processes. The basic idea isto force metal into a permanent mold using high pressure. The metal then cools(often assisted by water cooling of the die). The mold is then opened, and the casting

ejected.

Molds for die casting are quite elaborate. They are usually constructed of alloy steelin two pieces (called the cover and the ejector). The die must withstand hightemperature and pressure, so the die is typically made for chromium or tungstensteel alloys. In order to increase die life, and improve throughput, the die is usuallycooled with water, air or nitrogen.

There are two major types of die casting machines. Hot chamber die castingmachines are used for low melting point materials. A typical hot chamber machine isshown below. When the piston is raised, molten metal flows into the gooseneck. Themolten metal is forced out of the gooseneck and into the die by the plunger. These

machines are FAST typically operating at 150+ shots per minute.


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The cold chamber die casting machine is shown below. Die castings of aluminum,magnesium, brass and bronze are all made on cold chamber machines. In a coldchamber, the metal is fed first from the holding furnace into a chamber. The plungerthen takes that metal and forces it into the die. Cold chamber machines are typicallya bit slower than hot chamber machines.

Materials:

Materials best suited for die castings are zinc, aluminum, magnesium, copper, lead

and tin. High pressure die casting is generally limited to non-ferrous metals becauseof the difficulty in making refractory molds capable of withstanding the hightemperature and pressure.

Advantages:

Exceptionally fast Metallic or non-metallic inserts may be used Complex parts with high detail Reusable molds Good grain quality due to rapid heat transfer to the mold Virtually no materials waste, as leftovers can be remelted and used again Tolerances of 0.001" to 0.003" are possible Weights from a few grams to up to 100 lbs


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Disadvantages:

Expensive fabrication costs for the permanent mold Non-ferrous metals The need to machine the mold limits the type of features

Flash on parts Reduced set of materials due to fluidity considerations


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III. Forging

Forging is the process of shaping metals by deforming them with a hammer, a pressor rollers. Forged parts are characterized by a fibrous crystal structure. In suchstructures, the strength increases significantly along the lines of the grain boundary

flow. Forging is among the oldest of the metal working technologies.

Smith and hammer forging

Smith forging consists of making a part by banging on the heated metal with ahammer. This is the familiar forging process performed by blacksmiths on suchobjects as horseshoes. Hammer forging is simply a larger and more automatedversion of the same thing.

Drop forging

Drop forging is the most common of the forging processes. In drop forging, a heatedbar of material is forced into a die by a powered hammer. One half of the die isattached to the hammer and the other half to the anvil. In many cases, severaldifferent dies will be used for a single part, with the part transferring from die to die as

its shape becomes more well defined.

Impact forging

Impact forging is the same idea as drop forging, except two hammers are used, eachholding 1/2 of the die. Hammer forging is generally set up to only require one impact.Impact forgings tend to be of higher quality than drop forgings, apparently due to theeffects of the mutual impact on the grain structure.

Press forging

In press forging, like drop forging, heated metal is forced into a die. However, in

press forging, the die is not subjected to impact. Instead, the pressure is slowlyincreased over the course of several seconds. Maximum pressures may be as high


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as 10,000 tons. Press forging is a much quieter process than drop forging, andresults in parts which are at least equal in quality.

Roll forging

Roll forging is used on short lengths of stock that require the additional strength

added by the forging process. Two counter-rotating wheels contain the die. Theheated rod stock is inserted into the wheels. The diameter of the stock is decreasedand its length increased. Typically, the rollers include a number of dies of decreasingdiameter. The operator consecutively moves the part through the dies until it reachesthe final form.


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IV. Powder processes

Some fascinating parts can be made by beginning with a powder rather than a rod orbillet. This process is one of the very few ways to create parts with controlledporosity. Such parts are of value in filtration applications for air and water.

Alternatively, parts can be formed with high porosity -- and then the pores filled withother substances such as metals or lubricants. Finally, powder based parts can beformed from material combinations virtually impossible with any other process.

The process begins with metal powders. Since molten material is not involved, it isnot necessary to select the powder based on melting point considerations. Thus, verypeculiar "alloys" may be constructed. In addition to the usual iron-base and copper-base powders, parts can be manufactured from stainless steel, aluminum, tin, nickel,titanium, chromium, graphite, silicon, metal oxides and metal carbides. Of specialinterest is the ease of using refractories such as tungsten, tantalum andmolybdenum. Additionally, unusual combinations of metals and ceramics are

possible with this process.

The process begins by filling the die cavity with the powder of interest. Then,punches enter the cavity and compress the powder. The powder is usuallycompressed cold. Typical pressures are 50-100 tons. The part is then ejected fromthe die cavity. At this point in the process, the part is very fragile and is called a"green compact".

The next step is to "sinter" the part. This consists of heating the part up to about 80%of the melting temperature of the constituents for about an hour in order to fuse thematerials together. The sintering process increases both the strength and the densityof the final part. This sintering process is usually performed under an inert gas toavoid oxidizing the parts. It is also possible to recompress the parts after sintering.This process, called coining, increases the final density of the part by about 80%.

Because the parts are porous, additional constituents can be added after sintering.For example copper or brass can be infiltrated into iron-based parts. Alternatively,waxes or greases can be impregnated into the parts.


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V. Pressworking (i.e. stamping, cutting, bending anddrawing

Pressworking describes a wide variety of methods for working with cold ormoderately warm (i.e. below the melting temperature) materials.

The three most common tools used in pressworking are the punchpress or press, thebrake, and the shear.

A. Stamping with a punch press

Stampings are produced by a machine called a press, or a punch press. A ramholding a punch is forced through the material into a die block. The combination ofthe punch and die block is often referred to as the die set. Presses range frommanual presses that can be operated by one hand, to huge 2000 ton presses.

Metal stampings are among the most versatile of the metal working processes. It is

often possible to redesign parts originally made using forging or die casting intosimpler lighter (and cheaper parts that can be fabricated using stamping processes.)

A very wide variety of metals and plastics can be used for stamping. About the onlyrestriction is that the material not be too brittle. Cold rolled steel, stainless steel,copper alloys, magnesium alloys, and soft-tempered aluminum alloys. One of themajor advantages of stamping is that the material may be coated, painted, or acomposite.

Production speed on stamping is unbelievable. Small parts can be produced at 20+parts per stroke at 10,000-20,000 strokes a minute. Even larger parts can be made

far faster than in any other type of manufacturing.

Advantages:

Unbelievably fast Wide variety of materials Reusable dies Tolerances of 0.005" to are possible Weights from a few grams to up to 100 lbs

Disadvantages:

Die and stretch marks Up to 25% scrap loss Springback of metal parts to cold forming Thickness range of 0.020" to 0.75" for stock

B. Bending using a brake

A device called press brake (or a brake) is used for making bends in materials.Brakes, like punches, come in size from simple tabletop units to huge systemsweighing many tons. Brakes may simply bend metal against a set object (mostcommon for the smaller units), or may use a die to set a particular bend radius.Examples of dies for brakes are shown below.


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C. Cutting using a shear

A shear uses a long straight die to cut metal. Shears, like brakes and punches, comein size from simple tabletop units to huge systems weighing many tons. Shearsprovide an advantage over other cutting techniques in that they leave a very clean

edge. There is no flash or slagging.


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VI. Plastics

There are two major classes of plastics, thermoset and thermoplastic.

Thermoplastics are plastics such as polyethylene or styrene which can be softened

with heat. Thermoplastics can be molded, extruded and cast. However, theirdeformability with heat does pose problems in some applications.

Thermoset plastics include epoxies and phenolics. Thermoset plastics are typicallyformed from multipart solutions (a resin and a hardener). Once set, thermosetplastics do not flow or melt when heated.

Common Thermoplastics: ABS, Acetal, Acrylic, Cellulosics (cellulose acetate, ethylcellulose ....), Fluoroplastics (PVDF, ETFE, FEP ...), nylon, polycarbonate, polyester,polyethelene, polymide, polypropylene, polystyrene, polyurethane, polyvinylchoride(PVC).

Common Thermosets: Epoxy, Melamine, phenolic, urethane.


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Typical applications for plastics are given in the attached table.


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Thermoplastics are typically molded using a process called injection molding. Atypical injection molding machine is shown below.

Pellets of the plastic are fed into a hopper and carried into the barrel by a screw orplunger. The plastic melts in the barrel. When the proper volume of plastic hasmelted to fill the mold (including the sprues and risers), the screw or plunger is forcedforward, injecting the plastic into the mold.

Like in die castings, mold pressures tend to be very high -- often in the 3000 ton

range. However, unlike die casting, the mold temperatures tend to be rather low.Interestingly enough, molds made for one plastic cannot be used for others becauseplastic shrinkage is quite different for different plastics.

Other plastic forming processes:

Compression molding: A mold is filled with pieces of thermoset plastic as well asvarious fillers such as wood fiber, cotton and pigments. Heat and pressure is appliedto the mold cavity to force the material to melt and fill the mold.

Extrusion: Extrusion is typically reserved for thermoplastics. The material is carriedby a screw to a heating chamber, and then forced through a heated die (much liketoothpaste through a tube). The extruded material then rests on a conveyor and iscooled by air or water. The extruded lengths may be cut to length (as in plasticchannel) or coiled in a tube (as with pipe).

Blow molding: In a processes similar to glass blowing, thermoplastics can be blownup and then sealed in a mold. Typical examples include liter soft drink bottles.

Thermoforming: In this process, a sheet of thermoplastic is heated and then allowedto droop into a mold. The droop process can be gravity assisted, mechanicallyassisted, vacuum assisted or air assisted. A typical example is plastic luggage.


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VII. Other manufacturing processes

Abrasive jet machining: Abrasive jet machining uses a high velocity stream ofabrasive particles as a machining tool. Abrasive jet machining is used for materialsthat are sensitive to heat damage, or for forming thin sections of fragile brittle

materials. Abrasive machining is also used to mark or etch parts.

Grinding: The familiar process of grinding using an abrasive wheel can be extendedto actual fabrication of components. Grinding removes very little material (typicallyless than 0.001" at a pass). Thus grinding is used for precision finishing of surfaces.

Chemical milling: Chemical milling is a large scale version of the same photoetchingprocess used in integrated circuit manufacture. Chemical milling is usually used tocreate "stamped" parts from materials that cannot be stamped by more conventionaltechniques.

Electrical discharge machining: Electrical discharge machining removes materials by

application of an electric spark which vaporizes the material. EDM finds its mostcommon application in the making of dies for die casting or forging. These dies aremanufactured from hard refractory materials which are difficult to machineconventionally. EDM also allows for detailed stress free machining of many materials.EDM is also used for machining of very small burr free holes such as jets in fuelinjection nozzles.

Laser beam machining: High average power laser beams (such as carbon dioxidelasers) and high peak power laser beams (such as Nd:YAG lasers) are used in anumber of machining applications. The high average power lasers are useful forcutting materials with minimum heating. Applications range from fabric to large

sawmill sawblades. High peak power lasers are useful for making holes in materials.A very common application is for making the perforations in computer generatedforms. High average power lasers also find use in laser trimming and markingoperations both in electronics and for mechanical parts.

Ultrasonic machining: This is an unusual machining technique, usually limited to hardbrittle materials such as glass and ceramic. A tool vibrating at 20,000 to 30,000 Hz isimmersed in an abrasive slurry. The particles in the slurry become agitated and beginto remove material from the workpiece. A typical application is fabrication of aceramic nozzle.

the basics of manufacturing technology

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