tehnologia materialelor f41

7/29/2019 Tehnologia materialelor F41

1/22

Tehnologia materialelor F41

The Basics of Manufacturing Technology

EE 498

Professor Kelin J. Kuhn

Two lectures of material

General 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 is

very straightforward. A cutter is held in a chuck which rotates at a controlled speed. Thecutter is suspended over a work surface whose location can be precisely controlled. The

part to be machined is securely fastened to the work surface, and the work surface is

moved underneath the cutter. Appropriate choices of cutter type, depth of cut and speeddetermine the final shape.

A typical mill is shown below[1].
http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2_fn.htm#fn1http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2_fn.htm#fn1http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2_fn.htm#fn1


2/22

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)[2].

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 rough finish.

Alternatively, the work can be fed in the same direction as the cutter (down milling). Thistechnique is best suited for intricate parts[3].

Cutters:

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

operations.

Materials:
http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2_fn.htm#fn2http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2_fn.htm#fn2http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2_fn.htm#fn3http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2_fn.htm#fn3http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x23.gifhttp://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x22.gifhttp://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x21.gifhttp://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2_fn.htm#fn2http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2_fn.htm#fn3


3/22

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

are two commonly milled metals; Teflon and Delrin are commonly milled plastics.

However, the ability to mill a metal is typically limited only by the hardness of the cutter.Special cutters can be obtained for milling harder materials and refractories. Alternatively

very sharp cutters are available for plastics and even wood.

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 Computer Numerical

Control or Numerical Control mills are becoming increasing common in small machineshops. There are numerous variations on these mills, the most interesting (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 is useful

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 side is

held by a rotating support called a tail stock. A typical lathe is shown below [4].


4/22


5/22

rate of one every 3-5 seconds. Swiss type automatic screw machines (typically more

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 turned plastics. The

ability to turn a metal is primarily limited by the hardness of the cutter and the cuttingspeed. For example, turning carbon steel requires speeds up to 600 sfpm and carbide

tipped cutters.

Types of lathes:

Engine lathe: This is the typical lathe that you will see in a machine shop. It is limited to

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. Often called a

jewelers lathe.

Turret lathe: These are semiautomatic tools which perform essentially the same functions

as an engine lathe. However, turret lathes are not limited to single cutting tools -- thusseveral things may be happening to the part at the same time. In order to facilitate 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 NC lathes

provide more rapid turn-around times for complex one-off parts.

Automatic screw machines: These are turret lathes, but arranged for continuous feed of

bar stock. Some automatic screw machines have carried the idea a step further and have

multiple spindles as well as multiple tools. Logically enough they are called multiple

spindle bare automatic screw machines.

Swiss-type automatic screw machines: Swiss style machines work on a different principle

than the majority of lathes. In a Swiss-type machine, a revolving piece of material 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 the production 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


6/22

Advantages of turning:

Essentially the only way to make accurate radially symmetrical objects

Virtually any material can be milled with a proper cutter


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

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. In threadrolling, 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 exits the machine as a

completed screw[8].

Production versions of this use rotating planetary dies and can produce thousands ofscrews per hour[9].

Rolled threads have certain advantages over cut threads. In particular, the rolled thread

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 to shear across the grain

of a rolled thread than a cut thread[10].
http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2_fn.htm#fn8http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2_fn.htm#fn9http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2_fn.htm#fn10http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x29.gifhttp://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x28.gifhttp://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2_fn.htm#fn8http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2_fn.htm#fn9http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2_fn.htm#fn10


7/22

II. Metal Casting

Metal casting is the process of creating objects by filling a cavity with liquid metal and

letting the metal cool. Metal casting is approximately 6000 years old, as the first cast

objects 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 metal

casting processes. Sand casting accounts for over 90% of all metal poured for casting.

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 be madefrom 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. Holes called

sprues feed molten metal into the flask and holes called risers allow air bubbles toescape[11].

To begin the casting process, the flask is broken into its two pieces. The pattern is

inserted into the flask and the flask reassembled. Sand is packed tightly around the

pattern. Then the flask is opened and the pattern removed. The sand imprint is checkedcarefully, and appropriate risers and sprues added (if not contained on the original

pattern). Then the flask is closed and molten metal poured into the sprues until it emerges

from the risers.

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

removed and the part is cleaned.
http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2_fn.htm#fn11http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x211.gifhttp://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x210.gifhttp://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2_fn.htm#fn11


8/22

Tricks:

Either "green" sand (actually black) or dry sand is used for casting. In green sand casting,

the sand binder is kept moist with water. The part is cast as soon as possible after thepattern 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 hasbetter dimensional tolerances.

To create a hole in the middle of a casting, a baked sand part called a core is inserted inthe mold after the pattern has been removed. The core will be removed destructively 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 specialty casting.

In this process, the pattern is inserted into the flask and left there. When the molten metalis poured over the pattern, it vaporizes and the vapor escapes from the riser 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 many applications

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 (onefor the cope and one for the drag). A high polish and lack of pores is necessary in order tofacilitate removal of the pattern from the mold.

The fabrication of the mold begins with the cope or drag part of the mold flask. A bottom

plate is placed in the mold flask. Then, the cope or drag half of the pattern is sprayed witha material called "parting compound" to assist removal of the pattern from the mold. The

pattern is placed carefully onto the bottom plate. Next, the plaster slurry (70% gypsum


9/22

and 30% strengthening materials) is poured into the flask over the pattern. After the

plaster has set, the mold is reversed, the bottom plate removed, and the pattern extracted.

The molds are then baked[12].

After the molds have set, molten metal is poured into them in the same way as with sand

casting. Once the metal has hardened, the mold is broken open to retrieve the parts.

Tricks:

To create a hole in the middle of a casting, a plaster core can be used. The core will beremoved 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) for large

production runs. However, for runs of less than 1000, plaster casting is much cheaper.Thus, it is ideally suited for development of test components before major high volume

manufacturing.

Materials:

Limited to non-ferrous metals. Ferrous metals react with sulfur in the gypsum. Typical

cast 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:
http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2_fn.htm#fn12http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x212.gifhttp://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2_fn.htm#fn12


10/22

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

C. Investment or lost wax casting

Investment casting is also a very old casting technique. Jewelry assumed to be cast by the

investment process has been found dating back 3000 years. Investment casting is

especially well suited for tiny intricate parts.

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

2000 years ago, the expendable mold was carved carefully from wax. Sprues and riser

would be added to the wax mold to create a total wax pattern. The wax pattern would

then be covered with clay or plaster, allowed to set, and then baked. Upon baking the wax

would melt, leaving a one time pattern in the plaster mold.

Modern investment casting contains one additional step. Skilled model makers create

metal dies containing the primary patterns. Wax or plastic is then injected into these diesto create the wax pattern. Typically, the wax pattern contains many patterns gated

together by sprues and risers[13].

The wax pattern is then covered with a refractory material. This could be done by dipping

the pattern into a ceramic slurry -- or covering the pattern with some refractory molding

material. The mold is then baked and the wax or plastic allowed to drain or vaporize out.Molten metal is then poured into the mold.

Unlike the previous casting operations, getting the metal out of the mold is more difficult

in investment casting. Since the mold material is typically refractory -- it is often difficult

to remove. Chemicals, pressurized water and sand blasting are all used to remove molds.

Materials:
http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2_fn.html#fn0http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2_fn.html#fn0http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x213.gifhttp://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2_fn.html#fn0


11/22

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 commonly investment cast.

Advantages:

Permits casting of materials difficult or impossible to fabricate with other

methods

Virtually no materials waste, as leftovers can be remelted and used again

Allows exceptional detail and features difficult to machine


A parting line is not necessary

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[14].

Molds for casting iron or steel are made of graphite or other refractories. Molds for

casting 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 melting

points in order to be fluid in the mold. Typically iron, magnesium alloys, and aluminum
http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2_fn.htm#fn14http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2_fn.htm#fn14http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x214.gifhttp://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2_fn.htm#fn14


12/22

alloys are cast using this process. Copper and zinc alloys tend to be insufficiently fluid.

Steel and steel alloys require special molds.

Advantages:

Reusable molds Good grain quality due to rapid heat transfer to the mold



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

E. Die casting

Die casting is one of the most common manufacturing processes. The basic idea is to

force metal into a permanent mold using high pressure. The metal then cools (oftenassisted 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 steel in

two pieces (called the cover and the ejector). The die must withstand high temperature

and pressure, so the die is typically made for chromium or tungsten steel alloys. In order

to increase die life, and improve throughput, the die is usually cooled with water, air ornitrogen.

There are two major types of die casting machines. Hot chamber die casting machines are

used for low melting point materials. A typical hot chamber machine is shown below[15].When the piston is raised, molten metal flows into the gooseneck. The molten 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.
http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2_fn.htm#fn15http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2_fn.htm#fn15


13/22

The cold chamber die casting machine is shown below[16]. Die castings of aluminum,magnesium, brass and bronze are all made on cold chamber machines. In a cold chamber,the metal is fed first from the holding furnace into a chamber. The plunger then takes that

metal and forces it into the die. Cold chamber machines are typically a bit slower than hot

chamber machines.


14/22

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 because of the

difficulty in making refractory molds capable of withstanding the high temperature andpressure.

Advantages:

Exceptionally fast

Metallic or non-metallic inserts may be used


Reusable molds

Good grain quality due to rapid heat transfer to the mold



Weights from a few grams to up to 100 lbs

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
http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x216.gif


15/22

III. Forging

Forging is the process of shaping metals by deforming them with a hammer, a press or

rollers. Forged parts are characterized by a fibrous crystal structure. In such structures,the strength increases significantly along the lines of the grain boundary flow[17].

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 a hammer.

This is the familiar forging process performed by blacksmiths on such objects as

horseshoes. Hammer forging is simply a larger and more automated version of the samething.

Drop forging

Drop forging is the most common of the forging processes. In drop forging, a heated barof material is forced into a die by a powered hammer. One half of the die is attached tothe hammer and the other half to the anvil. In many cases, several different 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 the

effects 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 slowly increased over

the course of several seconds. Maximum pressures may be as high as 10,000 tons. Press


16/22

forging is a much quieter process than drop forging, and results 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 bythe forging process. Two counter-rotating wheels contain the die. The heated rod stock is

inserted into the wheels. The diameter of the stock is decreased and its length increased.

Typically, the rollers include a number of dies of decreasing diameter. The operatorconsecutively moves the part through the dies until it reaches the final form[18].

IV. Powder processes

Some fascinating parts can be made by beginning with a powder rather than a rod or

billet. This process is one of the very few ways to create parts with controlled porosity.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 with other substances such as

metals or lubricants. Finally, powder based parts can be formed from materialcombinations virtually impossible with any other process.

The process begins with metal powders. Since molten material is not involved, it is not

necessary to select the powder based on melting point considerations. Thus, very peculiar

"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 special interest is the

ease of using refractories such as tungsten, tantalum and molybdenum. 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, punchesenter the cavity and compress the powder. The powder is usually compressed cold.

Typical pressures are 50-100 tons. The part is then ejected from the die cavity. At thispoint 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 the

materials together. The sintering process increases both the strength and the density of the


17/22

final part. This sintering process is usually performed under an inert gas to avoid

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 orgreases can be impregnated into the parts.

V. Pressworking (i.e. stamping, cutting, bending and drawing

Pressworking describes a wide variety of methods for working with cold or moderately

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 ram holding a

punch is forced through the material into a die block. The combination of the punch anddie block is often referred to as the die set. Presses range from manual 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 into simpler 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 only

restriction is that the material not be too brittle. Cold rolled steel, stainless steel, copper

alloys, magnesium alloys, and soft-tempered aluminum alloys. One of the majoradvantages of stamping is that the material may be coated, painted, or a composite.

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.
http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x219.gif


18/22

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 systems weighing many

tons. Brakes may simply bend metal against a set object (most common for the smallerunits), or may use a die to set a particular bend radius. Examples of dies for brakes are

shown below[19].

C. Cutting using a shear

A shear uses a long straight die to cut metal. Shears, like brakes and punches, come in

size from simple tabletop units to huge systems weighing many tons. Shears provide an


19/22

advantage over other cutting techniques in that they leave a very clean edge. There is no

flash or slagging.

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, their deformability

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, thermoset plastics 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.

Typical applications for plastics are given in the attached table[20].


20/22

Thermoplastics are typically molded using a process called injection molding. A typical

injection molding machine is shown below[21].

Pellets of the plastic are fed into a hopper and carried into the barrel by a screw or

plunger. The plastic melts in the barrel. When the proper volume of plastic has melted to

fill the mold (including the sprues and risers), the screw or plunger is forced forward,injecting the plastic into the mold.
http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2_fn.htm#fn21http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2_fn.htm#fn21http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x222.gifhttp://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x221.gifhttp://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2_fn.htm#fn21


21/22

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 because plastic shrinkage isquite different for different plastics.

Other plastic forming processes:

Compression molding: A mold is filled with pieces of thermoset plastic as well as various

fillers such as wood fiber, cotton and pigments. Heat and pressure is applied to the moldcavity to force the material to melt and fill the mold.

Extrusion: Extrusion is typically reserved for thermoplastics. The material is carried by a

screw to a heating chamber, and then forced through a heated die (much like toothpaste

through a tube). The extruded material then rests on a conveyor and is cooled by air orwater. The extruded lengths may be cut to length (as in plastic channel) or coiled in a

tube (as with pipe).

Blow molding: In a processes similar to glass blowing, thermoplastics can be blown up

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 allowed to

droop into a mold. The droop process can be gravity assisted, mechanically assisted,

vacuum assisted or air assisted. A typical example is plastic luggage.

VII. Other manufacturing processes

Abrasive jet machining: Abrasive jet machining uses a high velocity stream of abrasiveparticles as a machining tool. Abrasive jet machining is used for materials that 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 extended toactual fabrication of components. Grinding removes very little material (typically less

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 to create

"stamped" parts from materials that cannot be stamped by more conventional techniques.

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 are

manufactured from hard refractory materials which are difficult to machine

conventionally. EDM also allows for detailed stress free machining of many materials.


22/22

EDM is also used for machining of very small burr free holes such as jets in fuel injection

nozzles.

Laser beam machining: High average power laser beams (such as carbon dioxide lasers)and high peak power laser beams (such as Nd:YAG lasers) are used in a number of

machining applications. The high average power lasers are useful for cutting materialswith 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 isfor making the perforations in computer generated forms. High average power lasers also

find use in laser trimming and marking operations both in electronics and for mechanical

parts.

Ultrasonic machining: This is an unusual machining technique, usually limited to hard

brittle materials such as glass and ceramic. A tool vibrating at 20,000 to 30,000 Hz is

immersed in an abrasive slurry. The particles in the slurry become agitated and begin to

remove material from the workpiece. A typical application is fabrication of a ceramic

nozzle.

tehnologia materialelor f41

Documents