Net Shape Forming

1
Net Shape Forming

• Net shape processing refers to any manufacturing
  process which creates an object in its finished
  form without the need for finish machining or
  other actions.
• Casting
• Injection molding

2
CASTING

• Casting
• Solid material heated to become molten, then
  poured into a cavity or mold that contains the
  metal in the desired shape while it cools and
  solidifies.
• Single step to form material.
• Virtually any configuration very versatile for
  designers.

3
Fundamentals

• Six Basic Requirements for Casting Process
• Mold Cavity (contains material in desired shape)
• Melting Process (melt needed quantity of material
  to proper casting temperature)
• Pouring Technique (introduce metal into mold, and
  provide escape for all air/gases)
• Solidification Process (allow for shrinkage
  without cracking or leaving voids)
• Mold Removal (break off or pull apart)
• Cleaning, Finishing, Inspection Operations

4
Definitions

• Pattern
• Shape of final casting, used to create mold.
• Sand molding sand is packed around pattern.
• Flask (i.e., snap flask)
• Rigid frame that holds molding aggregate (sand)
• Cope
• Top half of 2-part sand mold setup (flask, mold,
  pattern, and/or core)
• Drag
• Bottom half of 2-part sand mold setup

5
Molds
6
Definitions

• Core
• Sand or metal shape inserted into mold
• Pattern which creates internal features
• Riser
• Extra void to allow extra poured material
• Provides compensation for shrinkage
• Designed to be last material to solidify, so any
  voids should be in riser
• Gating system
• Network of channels to deliver material to
  cavities

7
Definitions

• Runners
• Horizontal channels in gating system
• Gates
• Controlled entrances to cavities in gating system
• Sprue
• Channels material from pouring cup to gating
  system
• Draft
• Taper on pattern/casting/cavities
• Permits casting to be withdrawn from mold

8
Runner system
9
Draft
10
temperature
11
Melting

• In order to melt the metal there are three basic
  heat sinks
• Heating from room temperature (T0) to melt
  temperature (Tm)
• Latent heat of fusion (Hf)
• Heating from melt to pour temperature (Tp)

12
Melting
13
Pouring

• Pouring a liquid into a sprue we can estimate the
  flow rate based on Bernoullis law

V1
Area
h1
This assumes no friction and is based on
equilibrium of energy
Vvelocity hHeight gAcceleration of gravity
V2
h2
14
Pouring

• If we further assume that at pouring the
  velocity0 , v10

V1
Then if we take the opening of the sprue to be
the reference point, then we can set h20
Area
h1
0
0
V2
h2
15
Pouring

• Then the equation simplifies to

V1
Area
h1
Or
V2
h2
16
Pouring

• Based on continuity

V1
Area
h1
Then the time to fill the mold (MFTmold fill
time)
V2
h2
17
Pouring
18
Solidification

• Freezing process of poured material
• Control of this determines
• Structural features properties
• Defects (porosity, voids)
• Cooling curve
• Cools to thermal arrest or freezing range
• Thermal arrest plateau due to latent heat of
  fusion
• Freezing range slope for metal with different
  melting, freezing points.
• Cooling slows during release of heat of fusion
• Complete solidification cools to ambient temp

19

• Chvorinovs Rule
• TSTtime of solidify
• VVolume mold
• ASurface area of mold
• nmold exponent (2)
• CmMold constant (min/cm2)

20
Solidification
21
Solidification

• Predicting Solidification Time
• Chvorinivs Rule
• Total Solidification Time Ts B(V/A)n
• B Mold Constant, determined by casting test
  specimens
• V Volume of casting
• A Surface area of casting
• n1.5 to 2.0 (assume a 2.0)

22
Shrinkage

• Stage 1 Shrinkage of liquid
• Depends on metals thermal properties
• Compensated for by riser(s)
• Stage 2 Solidification shrinkage
• Different for all metals (some expand)
• Design mold to start solidification away from
  riser, so molten material can feed into shrinkage
  void
• Stage 3 Solid metal contraction
• Compensation provided in in sand molds
• If rigid mold, may need to eject casting upon
  solidification

23
Molten Metal Problems

• Dross (Slag)
• Metal oxides from reaction with atmosphere
• Contributed to by erosion of furnace lining,
  crucible, ladles, mold, etc.
• Controlling dross
• Flux Covers surface of metal to protect it from
  air
• Skimming Remove dross from surface
• Dissolved Gas (Bubbles, or Gas Porosity)
• Controlling
• Melting pouring done in vacuum
• Pour slowly to avoid turbulence
• Degassing agent reacts with gas, floats to
  surface

24
Expendable Mold Casting Processes

• Single-Use Molds!!!!
• Multiple or Single-Use Patterns

STOP
25
Definitions

• Split Pattern
• Pattern divided into 2 segments along parting
  line
• Each segment creates half of mold
• Match Plate Pattern (used in our lab)
• Split pattern
• 2 segments are fastened separately to opposite
  sides of a match plate.
• Cope-and-Drag Pattern
• Match plate pattern
• 2 segments are fastened to separate match plates

26
Definitions

• Jolting
• Lift-Drop action in sand-mold process
• Compacts sand in flask with pattern
• Green Sand
• Molding sand consisting of sand and binders
• Shakeout
• Operations to separate components of casting
  process
• Molds/sand from flasks
• Castings from molding sand
• Cores from casting

27
Expendable Mold Processes

• Sand Casting (used in our lab)
• Expendable Mold
• Most common, highly versatile
• Sand with binder is packed around pattern inside
  flask, to create mold halves
• Sand is broken away after cooling of casting
• Requirements of sand
• Refractoriness (able to withstand high temps)
• Cohesiveness (able to retain shape after packing)
• Permeability (permits gases to escape through it)
• Collapsibility (permits metal to shrink, able to
  disintegrate to release casting)

28
Expendable Mold Processes

• Shell Molding
• Sand molding process
• Fine sand is bound with thermoset plastic
• Sand poured around hot pattern to create mold
• Good surface finish, dimensional accuracy
• V-process
• Vacuum molding process for sand casting
• Vacuum holds sand in place instead of binder
• Vacuum applied after sand is poured around
  pattern, and held until metal poured solidified

29
Vacuum
30
Expendable Mold Processes

• Processes with alternate molding materials
• Plaster molds
• Ceramic molds
• Expendable graphite molds
• Rubber molds

31
Expendable Mold Processes

• Investment Casting
• Wax pattern is coated with coated with molding
  material, or held in poured molding material
• Wax is melted or dissolved from set molding
  material, which is then ready for molten metal

32
Multiple-Use Mold Casting Processes

• Generally uses metal molds
• Common cast alloys based on
• aluminum, magnesium, zinc, lead, copper
• Molten metal forced into mold
• Limitations
• Lower temperatures lower melting point alloys
• Part size often limited
• Dies molds can be costly (similar to plastic
  molds)

33
Sources of Force for Molding

• Mechanical pressure (i.e. ram)
• Vacuum
• Gas pressure
• Centrifugal force

34
Definitions

• Cupola (Similar to blast furnace-not electric)
• Melting vessel made of refractory-lined steel
• Used mainly for cast iron
• Permanent mold casting
• Process using a reusable, segmented mold
• Mold is held together for pouring, opened for
  removal of casting
• Slush casting
• Produces hollow casting
• Metal is poured into mold, allowed to remain
  until a shell of the desired thickness is
  obtained
• Still-molten metal in center is poured out

35
Mold Life Factors

• Mold Material (must resist thermal fatigue)
• Alloy Being Cast ( its melting temperature)
• Pouring Temperature
• Mold Temperature
• must be balance between low temperature
  differences in pouring and avoiding mold erosion
• Mold Configuration
• must avoid large temperature differences in mold
  after pouring

36
Extrusion-The center of all polymer processing
37
Extrusion

• The basic and most common component of all
  polymer processing equipment
• Injection molding
• Blown film
• Profile extrusion

38
Extrusion

• Films and sheets.
• Covering on wire and cables.
• Profile Extrusion shapes e.g. rods, fibers,
  tubes, etc.
• Note Die Swell, Orifice design and Post
  forming
• Pipe Extrusion
• Sheet Extrusion
• Film Extrusion
• gt 0.25mm( 0.01 in) sheet
• lt0.25mm - film

39
Extrusion

• Has many functions
• Melt
• Mix
• Compound
• Pressurize

40
Extrusion

• Operation Principle five steps
• The extruder plasticated forced out through
  the die
• The die The hot molten soft plastics takes
  shape.
• Forming The hot material is further shaped.
• Post-forming The material is cut or further
  shaped.
• Secondary processing

41
Extrusion

• Is basically a feed screw with a heated barrel

42
Extrusion

• Why
• Plug heating is not effective because the thermal
  conductivity of most plastics is too
  low-proportional to electrical conductivity
• Heating from the outside-in is not efficient and
  results in high thermal gradients

43
Extrusion

• Thermal conductivity
• The amount of heat conducted through a sample
• Fourier Law of Conduction

44
Solution to heat flow

• Thin films
• Long slabs
• Particles
• Melt removal
• Pressure-induced melt removal
• DRAG-INDUCED MELT REMOVAL

45
Melt removal

• Pressure induced removal

F
Heated tool
46
Melt removal

• Drag induced melt removal

V
Heated tool
47
Extruder

• Hopper
• Screw The heart of the extruder
• Three-zone screw is the most used type
• (1) Feed zone
• greatest channel depth
• (2) Compression zone (transition zone)
• decreasing channel depth
• (3) Metering zone
• Assures proper delivery amount

48
Extrusion zone
49
The Screw is a melt drag design
50
Screw flow

• Need drag at barrel

51
Melt profile

• Classical melt cross section

52
Functions of a screw

• Convey
• Mix
• Plasticating (melting)
• Metering
• Venting

53
Transition (Compression) zone

• Promote both the compression and heating of the
  plastic granules.
• Uniformly tapered,
• Increasing root diameter
• Reduces the available volume between flights
• Compressing the granules.
• Air is purged back through the hopper.
• Heating,
• partly by conduction (15)
• mainly by friction from rotary shear (85)
• Mixed into a homogenous melt.
• one-fourth to one-third the entire screw length

54
Metering section

• Accurately controls amount of melt
• Assures smooth melt flow

55
Check valve

• Prevent back flow during injection
• Ball check valve
• Ring check valve
• Located at tip of screw
• Screen pack maybe at final section to trap
  contaminants

56
Two stage

• Release of entrapped volatiles moisture
• Better metering
• Better appearance, uniformity and properties

57
Twin Screws

• More is better (but at a cost)

58
Twin screws
From SPE
59
Twin Screw

• Use with reactive extrusion
• Often a modular design
• Relatively expensive
• More difficult to operate
• Good melting
• Good venting
• Can overload motor

60
Twin screws

• Modular design

61
Twin screws
Better conveying charteristics
62
Twin screws
Fully-intermeshing-co-rotating
63
Twin screws
No wiping
Wiping
No wiping
From SPE
64
Screw output

• Flow (Assuming no back flow!)

0
VbzMelt velocity in Z-Direction Fd and FpShape
factor WChannel width DScrew diameter NScrew
speed (Hz) hChannel depth LScrew
length PPressure ?Pitch ?Clearance flleakage
65
Screw sizes
15 in dia.!
66
Die swell

• Exit flow is larger than die opening
• D/Do
• Typically 1.12

Do
D
67
Die Swell
Die orifice Extruded profile
68
Die swell

• As a function of viscosity

?
Faster flow, more molecular alignment in flow
channel
D/Do
.
?
69
Die swell

• As a function of MW

Longer chains fold back on each other once
outside die
MW increase
D/Do
.
?
70
Die swell

• As a function of die temperature

Lower temperature reduce folding possibility
after existing
Decreasing temperature T
D/Do
.
?
71
Die swell

• As a function of die design (length/diameter)

D/Do
Chains recover random orientation with flow
L/D
72
Die swell

• Die design

More die swell
Less die swell
73
Melt fracture

• Shear stress at 105 N/m2
• Irregular flow
• Limit flow
• Limit production
• Limit profits

74
Melt fracture

• Two driving forces
• Slip and stick-Melt sticks to wall then breaks
  free causing pulsation in pressure
• Skin rupture-Die swell causes pressure build up
  in melt at exist, then with sudden cooling the
  surface breaks

75
Melt fracture
Skin rupture
Slip and stick
76
Troubleshooting

• Melt Fracture
• Streamlining the flow channel
• Reduce shear stress
• Increase die temperature
• Opening die at land region
• Reduce extrusion rate
• Change die wall material (Ceramic insert)
• Change material (add processing agents)

77
Melt fracture-reductions

• Decrease entrance angle
• Increase temperature
• Reduce viscosity
• Reduce shear stress
• Increase die diameter (reduce stress)
• Reduce molecular weight

78
Die Design

• Three major parts

Manifold
Inlet channel
Land
79
Die Design
80
Die Flow (Newtonian flow)

• For a circular die
• For a rectangular die

RPipe radius µViscosity LPipe
length ?PPressure drop
WPipe width HHeight of opening µViscosity LPip
e length ?PPressure drop
81
Cross over of screw and die
Large die opening
Flow of typical screw
Q
Small die opening
?P
82
Blown film
83
Calendering

• 95 of sheet film products are PVC.
• A series of heated, revolving rollers
  progressively squeezed thermoplastics stock to
  the desired thickness in the forms of sheet or
  film.
• Products handbags, shoes, and luggage.
• Advantages minimum of cleaning
• Disadvantages expensive process

84
Trouble shooting

• Vent flow-Material coming out of vent
• Root cause is imbalance between stages
• Starve feeding
• Reduce screw speed
• Cool first stage
• Increase temperature in second stage
• Open die gap
• Check screen pack or use low mesh

85
Troubleshooting

• Air entrapment
• Root cause-air does not get out of screw in time
• Use large sized pellets
• Use high compression screw
• Shorten feed
• Use vented extruder
• Vacuum on hopper

86
Troubleshooting

• Gels-cross linked particles-two sources
• P-Gels from polymerization-call supplier
• E-Gels from extrusion
• Particles sticking to screw
• Look for dead spot
• Clean screw and look for scratches

87
Troubleshooting

• Poor mixing
• Add mixing section to screw

88
Injection Molding

• TSM 240
• Prof. Grewell

89
Applications
90
The Molding Process

• Five steps
• Heating and melting of material (Extrusion screw)
• Mixing and homogenization of melt (Screw)
• Injection of melt into mold
• Cool/Solidify
• Eject part

91
Typical Applications

• Automotive, appliance, computer, communications,
  medical and industrial
• Part thickness from 0.020 to 0.25
• Amendable to amorphous and crystalline materials
• Structural foams, inserts, two-shot molding
• High production rate

92
Typical Applications

• Complex parts
• Varying wall thicknesses
• Final part can have functionality (movement)
• Wide range of final finish
• Often no 2nd operations are needed
• gt60 billion pounds/year of plastics are produced
  in the US
• 32-Injection molding
• 36-Extruders
• 1850-1st patent by Hyatt brothers

93
Molding Machine
Hopper
Mold/Tool
Screw/Extruder
Ram
MoldFlow
94
Basic mold layout
95
Typical Cycle
Mold Open Time
Fill Time
Cooling Time
Hold Time
Mold Flow
96
Typical Cycle
Cycle Time Fill Time Hold Time Cooling
Time Mold Open Time
22 Sec.
1
9
10
2
Mold Flow
97
Limitations

• Intensive competitionlow profits
• Three shift operation often required (No ideal
  time)-Material must be purged
• High mold costs (10 K to 1 Ms)
• High equipment costs
• Long lead times on equipment and molds
• Often rework is needed during start up
• Process control can be challenging

98
The three factors

• Materials parameters
• Part geometry parameters
• Manufacturing parameters
• The final product is determined by these three

99
Material parameters

• Amorphous, semi-crystalline, blends and filled
  materials
• Amorphous 5 shrinkage
• Crystalline 10-15 shrinkage
• Pressure-Volume-Temperature (PVT) behavior
• Viscosity/Melt flow
• Mold release (internal/external)

100
Part geometry

• Wall thickness
• Number of gates
• Gate location
• Gate thickness and area
• Type of gate (manually or automatically trimmed)
• Constrains (ribs, bosses or inserts)

101
Manufacturing parameters

• Fill time
• Packing time
• Mold temperature
• Melt temperature (NOT Tg/Tm but the temperature
  of the melt)
• Pressure

102
Classifications of equipment

• Delivery
• Amounts/size of shot
• Fraction of ounces to 300 lbs
• Most commonly 1-16 oz
• PS is used as rating material (also purge
  material)
• Clamping force
• Force to hold mold closed
• 1 to 12,000 ton
• 2-3 tons/in2 of surface cavity (including runners)

103
The process

• Filling
• Mold closes, screw rapidly moves forward, frozen
  polymer skin forms at mold walls

Melt
Clamp force
Lab
Mold flow
104
Preparing shot
Lab
105
The process

• Packing Time (Holding)
• Cavity filled, packing begins, cooling occurring

Lab
106
The process

• Cooling
• Packing complete, gate freezes off, cooling
  continues
• Screw moves back and begins plasticating resin
  for next shot
• Mold Open
• Cooling completes, mold opens

Moldflow
Lab
107
Hot runners

• There is continuous melt all the way to the cavity

Heater band
108
Hot Runners
Electrical Connectors
109
Hot Runners-The heater
110
Hot runners-Advantages

• Material savings
• Shorter cycle time-Do not have to wait for runner
  to solidify
• Smaller machine-smaller shot size
• Automation-no need to remove runners
• Gates can be located anywhere
• Cascade control

111
Hot runners-Limitations

• Costs
• Complex molds
• More troubleshooting
• Thermal damage
• Complex controls

112
Hot runners-Cascade control
113
Runnerless gate-stop

• The nozzle is at the part
• Saving similar to hot runners
• Geometry is limited

114
Balance runners

• Similar flow paths

115
Gates

• The simplest is the sprue gate
• Circular (Sprue dia,df)
• dfgtSmax1.0 mm (larger than thickest Smax part to
  assure last to solidify)
• Draft angle 1-4 degrees
• Fan gate or edge gate
• Long parts
• No/little gate marks
• Ring gate
• Tunnel gate (all around center section)

116
Gates-issues

• Appearance (minimize)
• Stress (minimize)
• Pressure to pack (maximize)
• Filling (maximize)
• Orientation (uniaxial with load)
• Shear degradation
• Weld line location

117
Fountain flow

• Describes the phenomena of how plastic flows in
  a mold
• Material that first enters shows up at the
  surface near the gate
• Material that enters the cavity last, shows up
  in the center downstream
• Has direct influence on molecular and fiber
  orientation at the part surface

Melt
118
The mold

• Knock out pins
• Knock out plates
• Knock out rings
• Driven by knockout plate

119
Mold layout

• Clamp stroke
• Maximum distance moving platen will allow
• Maximum daylight
• Furthest distance mold can separate
• Clamp speed
• Maximum speed which platen can be closed
• Tie rods
• Support platen

120
Clamps

• Two type
• Hydraulic (1 or 2)
• High clamp forces
• Higher costs
• Higher horse power
• Easily adjustable
• Clamp speed controllable
• Slow speed

121
Hydraulic clamps
122
Clamps

• Two type
• Toggle
• Limited clamp force
• Lower costs
• Lower horsepower
• No feedback on load
• Not controllable speed
• Fast

123
Toggle clamps
124
Toggle clamps
125
Clamp force

• ForceInjection pressure x part area

126
Co-Injection molding

• Multiple colors
• Multiple materials

127
Co-injection molding

• Machine layout

Extrusion screw 2
Extrusions screw 1
128
Co-injection molding

• Two shot mold

129
Co-injection molding

• Single shot rely on fountain flow