MAE 183A Exam 2
A
The pattern that determines the geometry of the casting in expendable mold processes is usually made (a) oversized, (b) the same size, or (c) undersized relative to the dimensions of the casting?
Chaplets are metal supports of various designs used to hold the core in place in the sand mold.
What is a chaplet?
A cupola is a vertical cylindrical furnace equipped with a tapping spout near its base. Cupolas are used for melting cast irons
What is a cupola?
A machine tool can be defined as a power-driven machine that positions and moves a tool relative to the work to accomplish machining or other metal shaping process.
What is a machine tool?
A B D F
3Multiple Choice Q 11.4:Which of the following are advantages of die casting over sand casting (four best answers): (a) better surface finish, (b) closer tolerances, (c) higher melting temperature metals can be cast, (d) higher production rates, (e) larger parts can be cast, and (f) mold can be reused?
C
A misrun is which one of the following defects in casting? (a) globules of metal becoming entrapped in the casting, (b) metal is not properly poured into the downsprue, (c) metal solidifies before filling the cavity, (d) microporosity, and (e) "pipe" formation
A B
A three-plate mold offers which of the following advantages when compared to a two-plate mold (two best answers): (a) automatic separation of parts from runners, (b) gating is usually at the base of the part to reduce weld lines, (c) sprue does not solidify, and (d) stronger molded parts?
B E
According to the Merchant equation, an increase in rake angle would have which of the following results, all other factors remaining the same (more than one)? (a) decrease in friction angle, (b) decrease in power requirements, (c) decrease in shear plane angle, (d) increase in cutting temperature, or (e) increase in shear plane angle.
The molder, material, injection machine, and the mold.
According to the injection molding videos, what are the four primary elements that influence the injection molding process?
Other properties include: viscoelasticity, a combination of viscous and elastic properties which cause the melt to exhibit memory - the tendency to return to its previous shape, as exhibited by die swell in extrusion
Besides viscosity, what other properties of a polymer melt are important in plastics processing? Briefly define each of the properties identified
In plastic extrusion, a polymer melt is compressed to flow through a die orifice and thus the continuous length of the plastic assumes a cross-sectional shape that is approximately the same as the profile of the orifice.
Briefly describe the plastic extrusion process
A
Casting is a process in which molten metal flows into a mold where it solidifies in the shape of the mold cavity; the term castingalso applies to the part that is made by this process: (a) true or (b) false?
Die swell is the tendency of the extrudate to expand in cross-section dimensions immediately on exiting the die orifice. It results from the viscoelastic properties of the polymer melt.
Define die swell in extrusion.
The defects include: short shots, in which the polymer melt solidifies before filling the cavity; flashing, in which the polymer melt is squeezed into the parting surfaces between the mold halves and around ejection pins; sink marks, in which the surface is drawn into the molding by contraction of internal material; and weld lines where the melt has flowed around a core or
Discuss some of the defects that can occur in plastic injection molding
D
During solidification of an alloy when a mixture of solid and liquid metals is present, the solid-liquid mixture is referred to as which one of the following: (a) eutectic composition, (b) ingot segregation, (c) liquidus, (d) mushy zone, or (e) solidus?
B
For which of the following reasons is a green mold so- named? (a) green is the color of the mold, (b) moisture is contained in the mold, (c) mold is cured, or (d) mold is dry.
B
Shell molding is best described by which one of the following: (a) casting operation in which the molten metal has been poured out after a thin shell has been solidified in the mold, (b) casting process in which the mold is a thin shell of sand binded by a thermosetting resin, (c) sand casting operation in which the pattern is a shell rather than a solid form, or (d) casting operation used to make artificial sea shells?
A D
The two most common polymer foams are which of the following: (a) polyacetal, (b) polyethylene, (c) polystyrene, (d) polyurethane, and (e) polyvinylchloride?
D
Hot working of metals refers to which one of the following temperature regions relative to the melting point of the given metal on an absolute temperature scale? (a) room temperature, (b) 0.2Tm, (c) 0.4Tm, or (d) 0.6Tm.
Increasing temperature decreases both K and n in the flow curve equation
How does increasing temperature affect the parameters in the flow curve equation?
A polymer melt exhibits pseudoplasticity, which means that its value decreases with increasing shear rate.
How does the viscosity of a polymer melt differ from most fluids that are Newtonian?
The reasons include: (1) its applicability to most materials; (2) its capability to produce a variety of geometries to a part; (3) it can achieve closer tolerances than most other processes; and (4) it can create good surface finishes
Identify some of the reasons why machining is commercially and technologically important
The four forces that act upon the chip are (1) friction force, (2) normal force to friction, (3) shear force, and (4) normal force to friction
Identify the four forces that act upon the chip in the orthogonal metal cutting model but cannot be measured directly in an operation.
The categories are: (1) extrusion, (2) molding, (3) forming of continuous sheets and films, (4) fibers, (5) foamed products, and (6) discrete formed sheets and films.
Identify the main categories of plastics shaping processes, as classified by the resulting product geometry
Chvorinov's Rule: TTS= Cm(V/A)2= CmL2. Since Cmcan be considered the same for both chicken andturkey, T6lb/T3lb= T6lb/(1 hr) = (6/3)2/3; T6lb= 1.59 hr = 1 hr 35 min.
If it took you 1 hour to cook a 3 lb chicken last time, how long would you cook a 6 lb turkey? Recall the discussion in the class
Because if the drawing stress exceeded the yield strength, the metal on the exit side of the draw die would stretch rather than force metal to be pulled through the die opening
In a wire drawing operation, why must the drawing stress never exceed the yield strength of the work metal?
B
In plaster mold casting, the mold is made of which one of the following materials: (a) Al2O3, (b) CaSO4-H2O, (c) SiC, or (d) SiO2
A E
In which of the following properties do plastic parts often compare favorably with metals (two best answers): (a) impact resistance, (b) resistance to ultraviolet radiation, (c) stiffness, (d) strength, (e) strength-to-weight ratio, and (f) temperature resistance?
C
Increasing strain rate tends to have which one of the following effects on flow stress during hot forming of metal? (a) decreases flow stress, (b) has no effect, or (c) increases flow stress.
Advantages of cold working are: (1) better accuracy, (2) better surface finish, (3) increased strength due to work hardening, (4) possible directional properties due to grain flow, and (5) no heating of work required
Indicate some of the advantages of cold working relative to warm and hot working
The flow curve is defined in Eq. (18.1) as Yf = Kεn
Indicate the mathematical equation for the flow curve.
E
Investment casting is also known by which one of the following names: (a) fast-payback molding, (b) full-mold process, (c) lost-foam process, (d) lost pattern process, or (e) lost-wax process?
Rolled products include flat sheet and plate stock, round bar and rod stock, rails, structural shapes such as I-beams and channels.
List some of the products produced on a rolling mill.
The three types are: (1) discontinuous, in which the chip is formed into separated segments; (2) continuous, in which the chip does not segment and is formed from a ductile metal; and (3) continuous with built-up edge, which is the same as (2) except that friction at the tool-chip interface causes adhesion of the work material to the tool rake face.
Name and briefly describe the three types of chips that occur in metal cutting.
The two categories are: (1) expendable mold processes, and (2) permanent mold processes.
Name the two basic categories of casting processes?
B
Production of tubing is possible in indirect extrusion but not in direct extrusion: (a) true or (b) false?
Viscosity of a polymer melt depends on (1) temperature and (2) shear rate. Also, (3) the molecular weight of the polymer affects viscosity.
Viscosity is an important property of a polymer melt in plastics shaping processes. Upon what parameters does viscosity depend?
Disadvantages include (1)limitations on mechanical strength properties;(2)porosity; (3)poor dimensional accuracy and surface finish for some casting processes; (4)safety hazards due to handling of hot metals; and (5)environmental problems.
What are some of the disadvantages of casting?
The factors include (1) pouring temperature above the melting point, (2) metal alloy composition, (3) viscosity of the liquid metal, and (4) heat transfer to the surroundings
What are some of the factors that affect the fluidity of a molten metal during pouring into a mold cavity?
General defects include: (1) misruns, (2) cold shuts, (3) cold shots, (4) shrinkage cavity, (5) microporosity, and (6) hot tearing. See Article 11.6.1.
What are some of the general defects encountered in casting processes?
The operations include (1) trimming, in which the sprues, runners, risers, and flash are removed, (2) core removal, (3) surface cleaning, (4) inspection, (5) repair if needed, (6) heat treatment, and (7) machining.
What are some of the operations required in sand casting after the casting is removed from the mold?
Some of the reasons are: (1) many of the processes are net shape processes; (2) in general, less energy employed than in metalworking processes; (3) lower temperatures are required to process plastics than metals or ceramics; (4) great flexibility in geometry; and (5) painting and other finishing processes generally not required.
What are some of the reasons why the plastic shaping processes are important?
The functions are to (1) filter dirt and lumps, (2) build pressure, (3) straighten the flow and remove memory of the polymer melt.
What are the functions of the screen pack and breaker plate at the die end of the extruder barrel?
The functions are: (1) filter dirt and lumps, (2) build pressure, (3) straighten the flow and remove memory.
What are the functions of the screen pack and breaker plate at the die end of the extruder barrel?
Common die cast metals include: zinc, tin, lead, aluminum, brass, and magnesium.
What are the most common metals processed using die casting?
Cutting conditions include speed, feed, depth of cut, and whether or not a cutting fluid is used.
What are the parameters of a machining operation that are included within the scope of cutting conditions?
The two categories are: (1) single -point tools, used in operations such as turning and boring; and (2) multiple-edge cutting tools, used in operations such as milling and drilling.
What are the two basic categories of cutting tools in machining? Give an example of a machining operation that uses each of the tooling types.
The clamping units are: (1) mechanical toggle clamp and (2) hydraulic. In addition, there are hydromechanical units which combine hydraulic and mechanical actuations
What are the two basic types of clamping units?
The two types of forging machines are hammers, which impact the workpart, and presses, which apply a gradual pressure to the work.
What are the two basic types of forging equipment? How are they different?
(1) solid profiles, such as rounds and L-shapes; (2) hollow profiles, such as tubes; (3) wire and cable coating; (4) sheet and film; and (5) filaments (continuous fibers).
What are the various forms of extruded shapes and corresponding dies?
In machining, material is removed from the workpart so that the remaining material is the desired part geometry.
What distinguishes machining from other manufacturing processes?
The size effect refers to the fact that the specific energy increases as the cross-section area of the chip (to x w) decreases
What does the term size effect mean in metal cutting?
Trimming is a shearing operation used to remove the flash on the workpiece after impression die forging.
What is a trimming operation in the context of impression die forging?
Draft is the difference between the starting thickness and the final thickness as the work piece passes between the two opposing rolls.
What is draft in a rolling operation?
Flash is a thin portion of metal at the exterior of a casting that results from molten metal being squeezed into the spaces between the die halves of the mold at the parting line, or into the clearances around the cores and ejector pins.
What is flashin die casting?
The Antioch process refers to the making of the mold. The mold is 50% sand and 50% plaster heated in an autoclave and then dried. This mold has greater permeability than a plaster mold.
What is the Antioch process?
The sand is composed of silica sand, clay, and water.
What is the composition of green sand in the green-sand molding process?
A positive mold has a convex shape; a negative mold has a concave cavity
What is the difference between a positive mold and a negative mold in thermoforming?
An open mold is open to the atmosphere at the top; it is an open container in the desired shape, which must be flat at the top. A closed mold has a cavity that is entirely enclosed by the mold, with a passageway (called the gating system) leading from the outside to the cavity. Molten metal is poured into this gating system to fill the mold.
What is the difference between an open mold and a closed mold?
Deep drawing is a sheet metal forming process used to fabricate cup-shaped parts; bar drawing is a bulk deformation process used to reduce the diameter of a cylindrical work part.
What is the difference between deep drawing and bar drawing?
In true centrifugal casting, a tubular mold is used and a tubular part is produced. In semicentrifugal casting, the shape is solid; an example is a railway wheel. The mold is rotated so that centrifugal force is used to distribute the molten metal to the exterior of the mold so that the density of the final metal is greater at the outer sections.
What is the difference between true centrifugal casting and semicentrifugal casting?
Vacuum permanent-mold casting is a form of low- pressure casting in which a vacuum is used to draw molten metal into the cavity. Vacuum molding is sand casting in which the sand mold is held together by vacuum pressure rather than a chemical binder.
What is the difference between vacuum permanent-mold casting and vacuum molding?
Thickness. Sheet stock has a thickness greater than 0.020 in (0.5 mm), while film stock is less than 0.020 in (0.5 mm) thick.
What is the distinction between plastic sheet and film?
The function of gates in an injection mold is to constrict the flow of molten plastic into the cavity, which increases the shear rate and reduces the viscosity of the polymer melt.
What is the function of gates in injection molds?
Chvorinov's Rule is summarized: TTS= Cm(V/A)2, where TTS= total solidification time, Cm= mold constant, V= volume of casting, and A= surface area of casting.
What is the relationship known as Chvorinov's Rule in casting?
Specific energy is the amount of energy required to remove a unit volume of the work material.
What is the specific energy in metal machining?
Vacuum permanent-mold casting is a form of low-pressure casting in which a vacuum is used to draw molten metal into the cavity.
What is vacuum permanent-mold casting?
The usual properties are: (1) strength - ability to maintain shape in the face of the flowing metal, (2) permeability - ability of the mold to allow hot air and gases to escape from the cavity, (3) thermal stability - ability to resist cracking and buckling when in contact with the molten metal, (4) collapsibility - ability of the mold to give way during shrinkage of the casting, (5) reusability - can the sand be reused to make other molds?
What properties determine the quality of a sand mold for sand casting?
Sand casting is the most important casting process.
Which casting process is the most important commercially?
Hot-chamber machines are faster because cold-chamber die casting machines require molten metal to be ladled into the chamber from an external source.
Which die casting machines usually have a higher production rate, cold-chamber or hot-chamber, and why?
A D F G
Which of the following are advantages and characteristics of hot working relative to cold working (four correct answers): (a) fracture of workpart isless likely, (b) friction is reduced, (c) increased strength properties, (d) isotropic mechanical properties, (e) less overall energy is required, (f) lower deformation forces are required, (g) more significant shape changes are possible, and (h) strain-rate sensitivity is reduced?
A
Which of the following are basic bulk deformation processes in metal forming (three correct answers): (a) bar drawing, (b) blanking, (c) deep drawing, (d) extrusion, (e) forging, and (f) stamping?
A D E
Which of the following are basic bulk deformation processes in metal forming (three correct answers): (a) bar drawing, (b) blanking, (c) deep drawing, (d) extrusion, (e) forging, and (f) stamping?
A F
Which of the following are desirable properties of the work material in metal forming (two best answers): (a) high ductility, (b) high tensile strength, (c) high yield strength, (d) low ductility, (e) low tensile strength, and (f) low yield strength?
A C D F
Which of the following casting processes are expendable mold operations (more than one)? (a)investment casting, (b) low pressure casting, (c) sand casting, (d) shell molding, (e) slush casting, and (f) vacuum molding.
B
Which of the following cutting conditions has the strongest effect on cutting temperature? (a) feed or (b) speed.
E F
Which of the following manufacturing processes are classified as material removal processes (two correct answers): (a) casting, (b) drawing, (c) extrusion, (d) forging, (e) grinding, (f) machining, (g) molding, (h) pressworking, and (i) spinning?
E F
Which of the following manufacturing processes are classified as material removal processes (two correct answers): (a) casting, (b) drawing, (c) extrusion, (d) forging, (e) grinding, (f) machining, (g) molding, (h) pressworking, and (i) spinning?
A D F
Which of the following metals would typically be die casted (more than one)? (a) aluminum, (b) cast iron, (c) steel, (d) tin, (e) tungsten, and (f) zinc
B C
Which of the following qualifies as a precision casting process (more than one)? (a) ingot casting, (b) investment casting, (c) plaster mold casting, (d) sand casting, and (c) shell molding
C
Which of the following stress or strength parameters is used in the computation of the maximum force in a forging operation (one best answer): (a) average flow stress, (b) compression strength, (c) final flow stress, (d) tensile strength, or (e) yield strength?
C
Which of the three types of chip would be expected in a turning operation conducted at low cutting speeds on a brittle work material (one answer)? (a) continuous, (b) continuous with built-up edge, or (c) discontinuous.
C
Which one of the following casting metals is most important commercially: (a) aluminum and its alloys, (b) bronze, (c) cast iron, (d) cast steel, or (e) zinc alloys?
A C E
Which three of the following are sections of a conventional extruder barrel for thermoplastics? (a) compression section, (b) die section, (c) feed section, (d) heating section, (e) metering section, and (f) shaping section.
Reasons why friction is undesirable in metal forming include the following: (1) it inhibits metal flow during deformation, causing residual stresses and product defects; (2) it increases forces and power required; and (3) it increases wearing of the tools.
Why is friction generally undesirable in metal forming operations?
orthogonal cutting is useful in the analysis of metal machining because it simplifies the rather complex 3D machining situation to 2D.
Why is the orthogonal cutting model useful in the analysis of metal machining?
Turbulence causes the following problems: (1) it accelerates formation of oxides in the solidified metal, and (2) it causes mold erosion or gradual wearing away of the mold due to impact of molten metal.
Why should turbulent flow of molten metal into the mold be avoided?
A C
The principal components of an injection molding machine are which two of the following: (a) clamping unit, (b) hopper, (c) injection unit, (d) mold, and (e) part ejection unit?
roughing operation is used to remove large amounts of material rapidly and to produce a part geometry close to the desired shape. A finishing operation follows roughing and is used to achieve the final geometry and surface finish
Define the difference between roughing and finishing operations in machining.
Increasing strain rate tends to increase the resistance to deformation. The tendency is especially prominent in hot forming operations.
Describe the effect of strain rate in metal forming.
Extrusion is a compression process in which the work material is forced to flow through a die orifice, thereby forcing its cross-section to assume the profile of the orifice.
Extrusion is a fundamental shaping process. Describe it.
The functions of gates in an injection mold are: (1) to increase shear rate to increase viscosity and temperature of the polymer melt, (2) the thinner cross- section of the gate freezes more rapidly to seal off the cavity; and (3) parts can be more easily broken off the runner at the gate.
Gates in injection molds have several functions; name them.
D
Given that Wm = weight of the molten metal displaced by a core and Wc = weight of the core, the buoyancy force is which one of the following? (a) downward force = Wm + Wc, (b) downward force = Wm - Wc, (c) upward force = Wm + Wc, or (d) upward force = Wm - Wc.
D
The average flow stress is the flow stress multiplied by which of the following factors? (a) n, (b) (1+n), (c) 1/n, or (d) 1/(1+n), where n is the strain hardening exponent.
(1) feed section, in which the feed stock is fed from the hopper and heated; (2) compression section, in which the polymer changes to a viscous fluid; and (3) metering section, in which pressure is developed to pump the plastic through the die orifice.
The barrel and screw of an extruder are generally divided into three sections; identify the sections.
B
The chip thickness ratio is which one of the following? (a) tc/to, (b) to/tc, (c) f/d, or (d) to/w.
B
The flow curve expresses the behavior of a metal in which of the following regions of the stress-strain curve? (a) elastic region, or (b) plastic region.
(b) Drag flow is the forward motion of the melt caused by the Archimedian screw principle in the barrel. The resistance to forward flow is called back pressure flow.
The forward movement of polymer melt in an extruder barrel is resisted by drag flow, which is caused by the resistance to flow through the die orifice: (a) true or (b) false.
(a), (b), (c), and (d). Degree of polymerization (a) is correlated with molecular weight, and rate of flow (c) is related to shear rate. The other parameters, (b) and (d), are probably more recognizable to the reader as correct answers to this question.
The shear viscosity of a polymer melt is affected by which of the following (more than one)? (a) degree of polymerization, (b) polymer type, (c) rate of flow, (d) temperature
Name the three most common machining processes.
The three common machining processes are: (1) turning, (2) drilling, and (3) milling
A split pattern is a pattern that consists of two pieces; a match-plate pattern consists of the two split patterns attached to opposite sides of a plate.
There are various types of patterns used in sand casting. What is the difference between a split pattern and a match- plate pattern?
C & D
Turbulence during pouring of the molten metal is undesirable for which of the following reasons (two best answers): (a) it causes discoloration of the mold surfaces, (b) it dissolves the binder used to hold together the sand mold, (c) it increases erosion of the mold surfaces, (d) it increases the formation of metallic oxides that can become entrapped during solidification, (e) it increases the mold filling time, and (f) it increases total solidification time?