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## MA2004 Manufacturing Process Engineering notes for first part of the course (wk 1 -7)

by: zzhong004 Notetaker

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# MA2004 Manufacturing Process Engineering notes for first part of the course (wk 1 -7) MA2004

Marketplace > Nanyang Technological University > Mechanical Engineering > MA2004 > MA2004 Manufacturing Process Engineering notes for first part of the course wk 1 7
zzhong004 Notetaker
NTU
GPA 3.52

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can be used as revision guide, and cheat sheet during exam if allowed.
COURSE
Manufacturing process engineering
PROF.
Prof. Chandrasekar
TYPE
Test Prep (MCAT, SAT...)
PAGES
4
WORDS
CONCEPTS
Engineering, manufacturing process.
KARMA
75 ?

## Popular in Mechanical Engineering

This 4 page Test Prep (MCAT, SAT...) was uploaded by zzhong004 Notetaker on Sunday March 6, 2016. The Test Prep (MCAT, SAT...) belongs to MA2004 at Nanyang Technological University taught by Prof. Chandrasekar in Summer 2015. Since its upload, it has received 367 views. For similar materials see Manufacturing process engineering in Mechanical Engineering at Nanyang Technological University.

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Date Created: 03/06/16
Dimensions   •   Dimensions:  linear  or  angular  sizes  of  a  component  specified  on  the  part  drawing.   •   Tolerances:  allowable  variations  from  the  specified  part  dimensions  that  are  permitted  in  manufacturing.     o   Bilateral  tolerance   o   Unilateral  tolerance   o   Limit  dimensions   •   Measurement:  provides  a  numerical  value  of  the  quantity  of  interest,  within  certain  limits  of   accuracy  and   precision.     •   Fixed  gages:  GO/NO-­‐GO  gages   o   Snap  gage  –  for  checking  outside  dimension   §   GO  limit:  check  the  maximum  allowable  size  of  the  exter nal  feature  (e.g.  maximum  outside   diameter),  i.e.  the  upper  limit  of  the  tolerance  or  the  maximum  material  condition.     §   NO-­‐GO  limit:  check  the  minimum  allowable  size  of  the  external  feature  (e.g.  minimum  outside   diameter),  i.e.  the  lower  limit  of  the  tolerance  or  the  minimum  material  condition.     o   Plug  gage  –  for  checking  internal  dimension   §   GO  limit:  check  the  minimum  allowab le  size  of  the  internal  feature  (e.g.  minimum  inside   diameter),  i.e.  the  lower  limit  of  the  tolerance  or  the  maximum  material  condition.     §   NO-­‐GO  limit:  check  the  maximum  allowable  size  of  the  internal  feature  (e.g.  maximum  inside   diameter),  i.e.  the  upper  limit  of  the  tolerance  or  the  minimum  material  condition.     •   Angular  measurements  (sine  bar):   •   Surface  Roughness  Equation   Lm n y R = y dx R = i = y a y +!+by + y +! k l a ∫ L m a ∑i=1 n n 0     R a  =  average  roughness        vertical  deviations  (absolute  value)  identified     by   y    =  vertical  deviation  from  nominal  surface  (absolute    script  i   value)   n  =number  of  deviations  included  in  Lm     Lm=specified  distance  over  which  the  surface   deviations  are  measured           •   Cutoff  length  is  a  sampling  distance  along  the  surface.   o   A  sampling  distance  shorter  than  the  waviness  eliminates  waviness  deviations  and  only  includes   roughness  deviations.  Typical  cutoff  length  is  0.8  mm  and   L  is  normally  set  at  5  times  the  cutoff  length.   m   Casting   •   Casting:  Process  in  which  molten  metal  flows  by  gravity  or  other  force  into  a  mold  where  it   solidifies  in  the   shape  of  the  mold  cavity.   •   Casting  is  a  net  shape  or  near  net  shape  process  capable  of  producing  complex  parts  of  various  sizes.   •   Mold:  Contains  cavity  whose  geometry  determines  part  shape:  Actual  size  and  shape  of  cavity  (pattern)  must  be   slightly  enlarged  to  allow  for  shrinkage  of  metal  during  solidification  and  cooling.   o   Expendable  mold  processes  –  use  an  expendable  mold  which  must  be  destroyed  to  remove  cas ting.   o   Permanent  mold  processes  –  use  a  permanent  mold  which  can  be  used  to  produce  many  castings .   •   A  riser  is  a  reservoir  in  the  mold  which  is  a  source  of  liquid  metal  to  compensate  for  shrinkage  of  the  part  during   solidification.     •   The  heat  required  is  the  sum  of:     o   Heat  to  raise  temperature  to  melting  point.     o   Heat  of  fusion  to  convert  from  solid  to  liquid.     o   Heat  to  raise  molten  metal  to  desired  temperature  for  pouring.     •   Pouring  rate:  volumetric  rate  at  which  the  molten  metal  is  poure d  into  the  mold  (cm /s).   o   Too  slow:  metal  will  freeze  before  filling  the  cavity.   o   Too  fast:  causes  turbulence,  the  metal  flow  is  irregular  rather  than  smooth  and  streamlined  as  in  laminar   flow.     •   Turbulence:  metal  oxides  may  be  entrapped  during  solidification  and  degrade  the  casting  quality;  also   aggravates  mold  erosion.       ▯ ▯ ▯▯ ▯▯ ▯▯ ▯▯ •   Bernoulli's’  theorem:  ℎ▯+ ▯▯ + ▯▯ + ???? ▯ ℎ + ▯ ▯▯ + ▯▯ + ???? ▯  è ▯ ???? = 2????ℎ ▯   •   Continuity  law:  ???? = ???? ▯ ▯ ???? ???? ▯ ▯ ▯ •   Mold  Cavity  Filling  Time▯▯ = ▯  (friction  losses  and  possible  constriction  of  flow  through  the  gating  system   are  ignored  →  Filling  time  is  minimum)   •   Fluidity  is  the  capability  of  molten  metal  to  fill  the  mold  cavity.   èHigh  viscosity  =  Low  fluidity   n •   Chvorinov's  Rule:   TTS= m #\$                         m                    n  =  exponent  with  typical  value  =  2;  C  =  mold  constant   %& o   Casting  with  a  higher  volume -­‐to-­‐surface  area  ratio  cools  and  solidifies  more  slowly  than  one  with  a   lower  ratioèT TS  for  the  riser  must  be  greaTSr  than  T  for  the  main  casting.   •   Directional  Solidific ation:  To  minimize  the  effects  of  shrinkage .   o   Chills  -­‐  internal  or  external  heat  sinks  that  cause  rapid  freezing  in  certain  regions  of  the  casting.     §   Chill  placed  in  regions  where  there  is  a  larger  volume  of  metal   •   Quality  defects:  Misrun;  Cold  shut;  Cold  shot;  Shrinkage  cavity;  Microporosity;  Hot  tears;     •   Sand  Casting  Defects:  Sand  Blow;  Pin  holes;  Penetration;  Mold  Shift;  Mold  crack;     •   Product  Design  Considerations:   Geometric  simplicity ;  Corners  on  the  casting;  Section  thickness;  Draft  ang le;   minimize  use  of  core;  Dimensional  tolerances  and  surface  finish;  Machining    wances. •   Casting  processes:   o   Expandable  mold  processes:  Sand  casting  and  investment  casting.   o   Permanent  mold  processes:  Die  casting  (hot  chamber/cold  chamber  die  casting).  (hig h  chamber  has  high   productivity)       Sheet  Metalworking   •   Deformation  Processes :  Starting  workpart  is  shaped  by  application  of  forces  that  exceed  the  yield  stress  of  the   material.   F L −L σ e e = o σ = Ee •   Tensile  stress:    A o                L o         e      Tensile  strain:     o   Elastic  region:  Material  returns  to  its  original  length  when  the  stress  is  removed.   o   Plastic  region:  Permanent  deformation,  material  does  not  return  to  its  original  length  when  the  stress  is   removed.   δ F γ = •   Shear  stress:    A                                  Shear  strain:     •   Shear  stress  at  fracture  =  shear  strength   S.  {Shear  strength  can  be  estimated  from  tensile  strength:   S  ≅  0.7(TS)}   •   Sheet  Metalworking:  Cutting  and  forming  operations  performed  on  r elatively  thin  sheets  of  metal   •   Rollover  -­‐  Depression  made  by  the  punch  prior   to  cutting.   •   Burnish  -­‐  Smooth  region  resulting  from  penetration  of  the  punch  prior  to  fracture.   •   Fracture  zone  -­‐  Relatively  rough  surface  caused  by  the  fracture  of  the  metal  as  the  punch  goes  down.   •   Burr  -­‐  Sharp  corner  edge  caused  by  the  elongation  of  the  metal  during  the  final  stage  of  separation.   •   Shearing  èto  separate  large  sheets.   •   Blanking  èto  cut  part  perimeters  out  of  sheet  metal.   •   Punching  èto  make  holes  in  sheet  metal.   •   Clearance:  Distance  between  punch  cutting  edge   and  die  cutting  edge  (4% -­‐8%)  {  c  =  at  }   o   If  too  small,  fracture  lines  pass  each  other,  causing  double  burnishing  and  larger  force.     o   If  too  large,  metal  is  pinched  between  cutting  edges  and  excessive  burr  results.     §   For  a  round  blank  of  diabeter  D :     ü   Blanking  punch  diameter  =   D  -­‐  2c     b ü   Blanking  die  diameter  b   D     §   For  a  round  hole  of  dihmeter  D :     ü   Hole  punch  diameter  =h   D   ü   Hole  die  diameter h=   D  +  2c   •   Angular  Clearance:  allows  slug  or  blank  to  drop  through  the  die,  0.25 °  to  1.5°  on  each  side.   •   Cutting  Forces  Calculation:   F  =  S  t  L  =  0.7(TS)  t  L,     §   S  =  shear  strength  of  metal,   t  =  sheet  thickness,  and   L  =  length  of  cut  edge   •   Fine  blanking  is  a  shearing  process  which  produces  very  highly  precise  workpieces  with  completely  smooth,   tear-­‐free  sheared  surfaces.     •   Bending  is  the  forming  of  solid  parts,  where  angled  or  ring -­‐shaped  workpieces  are  produced  from  sheet  or  strip   metal.   •   Required  Blank  Length:  L=L 1L +2 , ▯  Ab  =  bend  allowance.   •   Bend  Allowance  Formula:   A =bπ (R+K tba     ▯▯▯ o   K ba =  stretching  factor:  If  Rba  <  2t,  K  = ba.33;  If  R    ≥  2t,  K  =  0.50   •   Springback  in  bending  is  seen  as  a  decrease  in  bend  angle  and  an  increase  in  bend  radius.   o   Methods  of  reducing:   Overbending  /  Bottoming   2 K bf wt F = •   Bending  Force:   D ,  For  V-­‐bendinbf  K  =  1.33;  for  edgbf  bending,   K  =  0.33.   •   Drawing  is  the  forming  of  smooth  (sheet)  blanks  into  hollow  parts.   •   Clearance  in  Drawing:  c  =  1.1  t   •   Major  Stresses  in  Flange  and  Wall:     o   Flange:  Compressive  hoop  stress  (may  cause  wrinkling)   o   Wall:  Longitudinal  tensile  stress  (blank  being  pulled  into  the  cavity,  may  cause  tearing)   DR = D b D p •   Drawing  Ratio  DR:   ,  Upper  limit:  DR  ≤  2.0;  i.e.  if  DR  >  2.0,  the  operation  is  not  feasible.   D −D r = b p •   Reduction  r:  (for  cylindrical  shape)   D b ,  Value  of  r  should  be  ≤  0.50.   •   Thickness-­‐to-­‐Diameter  Ratio  t/D b  to  be  greater  than  1%,b  As  t/D  decreases,  tendency  for   wrinkling  increases.   & # F = πD tpTS) \$D b −0.7 ! %D p " •   Drawing  Force:   .     0.015Yπ 2 2 Fh= {D b D + 2p2t + 2R d )} •   Blankholder  Force  or  Holding  Force:   4   •   Common  defects  in  drawn  parts:  (a)   wrinkling  can  occur  either  in  the  flange  or  (b)  in  the  wall  (due  to   compression),  (c)  tearing  (due  to  high  tensile  stresses  that  cause  thinning  and  failure),  (d)   earing  (due  to   anistropy),  and  (e)  surface  scratches  (due  to  poor  lubrication,  punch/die  not  smooth).     Polymer   •   Thermoplastics     o   Chemical  structure  remains  unchanged  during  heating  and  shaping,  can  be  recycled.     o   More  important  commercially,  comprising  more  than  70%  of  total  plastics  tonna     •   Thermosets     o   Undergo  a  curing  process  during  heating  and  shaping,   causing  a  permanent  change  (cross-­‐linking)  in   molecular  structure.     o   Once  cured,  they  cannot  be  remelted  or  recycled.   •   Viscosity  of  Polymer  Melts:   Fluid  property  that  relates  shear  stress  to  shear  rate  during  flow.   o   Due  to  its  high  molecular  weight,  a  polymer  melt  is  a  thick  fluid  with  high  viscosity.   o   Most  polymer  shaping  processes  involve  flow  through  small  channels  or  die  ope  s.   o   Flow  rates  are  often  large,  leading  to  high  shear  rates  and  shear  stresses,  so  significant  pressures  are   required  to  accomplish  the  processes.     •   Viscosity  and  Shear  Rate :  Viscosity  of  a  polymer  melt  decreases  with  shear  rate,  thus  the  fluid  becomes   thinner  at  higher  shear  rates.     •   Viscosity  and  Temperature:   Viscosity  decreases  with  temperature,  thus  the  fluid  becomes  thinner  at  high er   temperatures   •   Casting:  Pouring  liquid  resin  into  a  mold,  using  gravity  to  fil l  cavity,  where  polymer  hardens,  and  both   thermoplastics  and  thermosets  are  cast.     •   Extrusion:  Compression  process  in  which  material  is  forced  to  flow  through  a  die  orifice  to  pr ovide  long   continuous  product  whose  cross -­‐sectional  shape  is  determined  by  the  shape  of  the  orifice.  Widely  used  for   thermoplastics  and  elastomers   •   Injection  Molding:  Polymer  is  heated  to  a  highly  plastic  state  and  forced  to  flow  under   high  pressure  into  a   mold  cavity  where  it  solidifies  and  the   molding  is  then  removed  from  cavity.   o   Injection  molding  is  the  most  widely  used  molding  process  for   thermoplastics.   o   Injection  system:  Melts  and  delivers  polymer  melt ;  Operates  much  like  an  extruder     o   Clamping  system:  Opens  and  closes  mold  each  injection  cycle   •   Two-­‐Plate  Mold  Features:   o   Cavity  –  geometry  of  part  but  slightly  oversized  to  allow  for  shrinkage.   o   Distribution  channel  through  which  polymer  melt  flows  from  nozzle  into  mold  cavity.     o   Ejection  system  –    to  eject  molded  part  from  cavity  at  end  of  molding  cycle.     o   Cooling  system  -­‐  consists  of  external  pump  connected  to  passageways  in  the  mold,  through  which  water   is  circulated  to  remove  heat  from  the  hot  plastic.     o   Air  vents  –  to  permit  evacuation  of  air  from  cavity  as   polymer  melt  rushes  in.   •   Three-­‐Plate  Mold:  Uses  three  plates  to  separate  parts  from  sprue  and  runner  when  the  mold  opens.   o   As  mold  opens,  runner  and  parts  disconnect  and  drop  into  two  containers  under  mold.     o   Allows  automatic  operation  of  molding  machine.     •   Compensation  for  Shrinkage :  D  =  D c  + p D S p +  D p ,    S  =  shrinkage  value.   •   Defects  in  Injection  Molding :  Short  shot;  Flash;  Sink  marks  and  voids;  Weld  line.   •   Compression  Molding:  A  widely  used  molding  process  for   thermosetting  plastics.     •   Transfer  Molding:  Modified  from  compression  molding,  polymer  enters  the  mold  cavity  as  a  fluid.  Thermoset   charge  is  loaded  into  a  chamber  immediately  ahead  of  the  mold  cavity,  where  it  is  heated;  pressure  is  then   applied  to  force  soft  polymer  to  flow  into  heated  mold  where   it  cures.   o   Pot  transfer  molding  -­‐  charge  is  injected  from  a  "pot"  through  a  vertical  sprue  channel  into  cavity.   o   Plunger  transfer  molding   –  plunger  injects  charge  from  a  heated  well  through  channels  into  cavity.     •   Compression  vs.  Transfer  Molding :   o   In  both  processes,  scrap  is  produced  each  cycle  as  leftover  material,  called  the   cull.     o   The  thermoset  scrap  cannot  be  recovered.     o   Transfer  molding  is  capable  of  molding  more  intricate  part  shapes  than  compression  molding  as  the   polymer  enters  the  cavity  as  a  fluid,  b ut  not  as  intricate  as  injection  molding.     •   Blow  Molding:  in  which  air  pressure  is  used  to  inflate  soft  plastic  into  a  mold  cavity.     o   Extrusion  Blow  Molding   o   Injection  Blow  Molding   •   Thermoforming:  Flat  thermoplastic  sheet  or  film  is  heated  and  deformed  into  desired  shape  using  a  mold.   o   Only  thermoplastics  can  be  thermoformed

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