Frictional Coefficients Between Plug and Sheet Problems with Theories, Experiments in Plug Assist

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Frictional Coefficients Between Plug and Sheet
Problems with Theories, Experiments in Plug
Assist

• James L. Throne
• Sherwood Technologies, Inc.
• Dunedin Florida 34698-3347

Paper presented at 2004 Thermoforming
Conference Indianapolis IN
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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Objective
• To better understand the problems with theories
  and experiments in plug assist thermoforming

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Caveat
• It is my intent to raise concerns about the ways
  in which we view the interaction between the plug
  and the sheet

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Caveat
• It is my intent to raise concerns about the ways
  in which we view the interaction between the plug
  and the sheet
• It is NOT my intent to provide answers to the
  questions about the interfacial conditions
  between the plug and the sheet

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Think about this Question!
• In plug-assist thermoforming,
• what is sliding against what?

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Outline
• A discussion of the coefficient of friction issue
• A sliding experiment
• Some prototypical plug experiments
• Conclusions

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• I. Some thoughts on the coefficient of friction

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Coefficient of Friction
• Frictional characteristics considered part of
  tribology
• Tribology is study of friction, lubrication and
  wear
• Traditional view is that all three are extant in
  thermoforming

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet
Historical views of coefficient of friction-I

• The relation that the power required to move a
  body bears to the weight or pressure on the body
  is known as the coefficient of friction.
• W.M. Davis, Friction and Lubrication, A Handbook
  For Engineers, Mechanics, Superintendents and
  Managers, The Lubrication Publishing Co.,
  Pittsburgh PA, 1903.

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet
Historical Views of coefficient of friction-II

• The coefficient of friction is the ratio between
  the resistance to motion and the perpendicular
  pressure.
• W.M. Davis, Friction and Lubrication, A Handbook
  For Engineers, Mechanics, Superintendents and
  Managers, The Lubrication Publishing Co.,
  Pittsburgh PA, 1903.

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet
Historical Views of coefficient of friction-III

• There is no other element in connection with
  lubrication that has received so much
  consideration as that of the coefficient of
  friction, and yet there is no other that is in so
  indeterminable a state
• Mr. Hall, Car Lubrication, ca. 1900 - cited in
  W.M. Davis,Friction and Lubrication, A Handbook
  For Engineers, Mechanics, Superintendents and
  Managers, The Lubrication Publishing Co.,
  Pittsburgh PA, 1903.

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet
Historical views of coefficient of friction-IV

• While the coefficient of friction must always be
  taken into consideration when designing and
  constructing machinery, it is not always
  practicable to calculate it with any degree of
  accuracy, and in fact it can only be determined
  absolutely by experiment.
• W.M. Davis,Friction and Lubrication, A Handbook
  For Engineers, Mechanics, Superintendents and
  Managers, The Lubrication Publishing Co.,
  Pittsburgh PA, 1903.

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Coefficient of Friction
• Following discussion assumes that frictional
  effects are extant in plug-assisted thermoforming

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Blunt-nose plug moving into sheet
• Contact may involve some sheet sliding

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Frictional Conditions
• Between Plug And Sheet
• Static CoF, no sliding (coefficient max)
• Sliding CoF, no static (coefficient zero)
• Some static, some sliding
• Slip-stick behavior
• or mold wall

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Frictional Conditions
• Between Plug And Sheet
• Static CoF, no sliding (coefficient max)
• Sliding CoF, no static (coefficient zero)
• Some static, some sliding
• Slip-stick behavior
• or mold wall
• Which of these are relevant when plastic
  stretches against plug surface?

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Coefficient of Friction
• Static CoF - Initiation of sliding between plug
  (and mold wall) and sheet

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Coefficient of Friction
• Static CoF - Initiation of sliding between plug
  (and mold wall) and sheet
• Sliding CoF- Continuation of sliding between plug
  (and mold wall) and sheet

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Coefficient of Friction
• Static CoF - Initiation of sliding between plug
  (and mold wall) and sheet
• Sliding CoF - Continuation of sliding between
  plug (and mold wall) and sheet
• Static friction value usually 100s to 1000s
  greater than sliding friction value, but not
  always!

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Sliding Coefficient of Friction
• Contact area increases with increasing load

Plastic sheet
Plug
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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Sliding Coefficient of Friction
• Contact area increases with increasing load
• Ergo, coefficient independent of load

Plastic sheet
Plug
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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Are there other factors influencing the
  interaction between the plug and the sheet?

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Are there other factors influencing the
  interaction between the plug and the sheet?
• Wear

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Sliding Coefficient of Friction
• Wear v. sliding friction
• Friction maximum in polymer transition region
• Wear minimum in polymer transition region

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Sliding Coefficient of Friction

Friction maximum, wear minimum in polymer
transition region Glass Transition Region
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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Are there other factors influencing the
  interaction between the plug and the sheet?
• Wear
• Dry v. wet sliding

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Dry v. wet sliding
• Dry sliding assumes no lube between surfaces

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Dry v. wet sliding
• Dry sliding assumes no lube between surfaces
• Plastics exude small molecules (low MW polymers,
  additives, processing aids)

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Dry v. wet sliding
• Dry sliding assumes no lube between surfaces
• Plastics exude small molecules (low MW polymers,
  additives, processing aids)
• Small molecules reside between plug and sheet

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Dry v. wet sliding
• Dry sliding assumes no lube between surfaces
• Plastics exude small molecules (low MW polymers,
  additives, processing aids)
• Small molecules reside between plug and sheet
• Small molecules transfer from sheet to plug

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Dry v. wet sliding
• Dry sliding assumes no lube between surfaces
• Plastics exude small molecules (low MW polymers,
  additives, processing aids)
• Small molecules reside between plug and sheet
• Small molecules transfer from sheet to plug
• Interface may go from dry to wet (or wet to dry!)
  as plug advances into sheet

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Are there other factors influencing the
  interaction between the plug and the sheet?
• Wear
• Dry v. wet sliding
• More than one type of wet sliding

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Two types of wet sliding
• Boundary lubrication low sliding velocity, low
  interfacial viscosity, high loading

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Two types of wet sliding
• Boundary lubrication low sliding velocity, low
  interfacial viscosity, high loading
• Hydraulic or hydrodynamic lubrication high
  sliding velocity, high viscosity, low loading

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Two types of wet sliding
• Boundary lubrication low sliding velocity, low
  interfacial viscosity, high loading
• Hydraulic or hydrodynamic lubrication high
  sliding velocity, high viscosity, low loading
• Boundary lubrication occurs during start/top
  activities as might be the case with plugs in
  contact with sheet

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Two types of wet sliding
• Boundary lubrication low sliding velocity, low
  interfacial viscosity, high loading
• Hydraulic or hydrodynamic lubrication high
  sliding velocity, high viscosity, low loading
• Boundary lubrication occurs during start/top
  activities as might be the case with plugs in
  contact with sheet
• Boundary lube friction 100s greater than
  hydraulic lube friction

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Two types of wet sliding
• Sheet does not move far under plug force

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Measuring Frictional Coefficients

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Traditional methods of
• measuring coefficient of friction
• Weight sliding on inclined surface

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Traditional methods of
• measuring coefficient of friction
• Tabor Abrasor stylus rubbing on rotating disk

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Traditional
• Other methods
• No methods entirely applicable to measuring
  plug-sheet friction

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Observations
• Frictional resistance is a complex issue
• Static v. sliding
• Dry v. wet
• The extent of sliding
• Boundary v. hydrodynamic effects

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Observations
• Frictional resistance is a complex issue
• Static v. sliding
• Dry v. wet
• The extent of sliding
• Boundary v. hydrodynamic effects
• Standard frictional devices may not give reliable
  results

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Are We Answering the Question?
• In plug-assist thermoforming,
• what is sliding against what?

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• II. A sliding experiment

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Traditional
• Other methods
• No methods entirely applicable to measuring
  plug-sheet friction (except g but modified)

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Plug-sheet friction experiment
• Consider figure below

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Plug-sheet friction experiment
• Consider figure below
• A is plug material, B is plastic sheet, p is
  applied load
• Plug material moved against sheet

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Plug-sheet friction experiment
• Consider figure below
• First, plug, sheet surfaces examined optically
  (100X or SEM)
• Plug mounted in fixture, load similar to applied
  plug force applied

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Plug-sheet friction experiment
• Consider figure below
• Sheet placed on hot plate, heated to forming
  temperature
• Plug heated to 20C of the sheet temperature

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Plug-sheet friction experiment
• Consider figure below
• Plug pressed against sheet
• Moved at slow rate ( 1 mm/sec, say) against
  sheet, then stopped

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Plug-sheet friction experiment
• Consider figure below
• Plug then lifted and indexed to new place on
  sheet
• Plug held away from sheet for period of time (10
  sec, say)

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Plug-sheet friction experiment
• Consider figure below
• Plug then placed against sheet and moved other
  direction at ( 1 mm/sec, say)
• After 10 (say) to-and-fro motions, plug removed
  from fixture and examined

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Plug-sheet friction experiment
• Consider figure below
• Plug surface examined optically (100X or SEM)
• Scrapings from plug surface chemically analyzed
  (FTIR) to determine matl transfer

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Plug-sheet friction experiment
• Consider figure below
• New plug examined, then heated and brought in
  contact with fresh sheet surface
• Plug moved to-and-fro 100 times, then examined
  optically and chemically

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Plug-sheet friction experiment
• Consider figure below
• Only after material transfer level reaches
  constant value
• is resistance to sliding measured, using strain
  gauge or load cell

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Experiment 1
• EC Syntactic Foam, sanded w/180 grit, blown with
  oil-free air
• Normal stress 3 lb/in2
• Hot plate temp setting 162/-2oF
• 0.120 inch black GP-PS sheet
• Block held on sheet 10 s, moved 10 cm in 10 sec,
  then removed and cooled 15 s
• Force measured once block moved
• Concluded after 10 contacts

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Experiment 2
• EC Syntactic Foam, sanded w/180 grit, blown with
  oil-free air
• Normal stress 3 lb/in2
• Hot plate temp setting 162/-2oF
• 0.120 inch black GP-PS sheet
• Block immediately moved 10 cm in 10 s, then
  removed and cooled 15 s
• Force measured once block moved
• Concluded after 40 contacts

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Parameters
• Fixed
• Sheet, heater temperature
• Sliding length, times
• Measured
• Force
• Block surface temperature

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet
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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet
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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet
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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Visual Observations
• After 10 contacts in Expt 1, plug surface is
  smoother and grayer

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Visual Observations
• After 10 contacts in Expt 1, plug surface is
  smoother and grayer
• After 40 contacts in Expt 2, plug surface is
  gray, with small lt1 micron specks in surface

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Mea Culpa
• Syntactic foam blocks were sent to local
  university for FTIR reflectance and ESCA
  measurements in August 2001
• Blocks mysteriously disappeared (!)
• Retesting to obtain new samples has not yet begun

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Observations
• Hot plate experiments yield time-dependent force

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Observations
• Hot plate experiments yield time-dependent force
• Force is dependent on plug surface temperature

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• III. Some prototypical plug experiments
• Focus on ball or spherical plugs

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Plug design characteristics - Plug types
• Tapered, bullet- or bull-nosed

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Plug design characteristics - Plug types
• Tapered, bullet- or bull-nosed
• Applications
• Very deep draws
• Where thinning of sidewall is critical
• Where wall thickness is critical
• Where polymer chills rapidly

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Blunt-nose plug moving into sheet
• Contact may involve some sheet sliding

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Load Creep
• Rough acrylic under load increasing time or
  increasing temperature

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Load Creep
• Rough acrylic under load
• Red low temp

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Load Creep
• Rough acrylic under load
• Red low temp
• Yellow medium temp

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Load Creep
• Rough acrylic under load
• Red low temp
• Yellow medium temp
• Blue high temp

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Load Creep
• Rough acrylic under load
• Red low temp
• Yellow medium temp
• Blue high temp
• Note increasing contact area with increasing
  temperature

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Sheet stretching characteristics
• Sheet stretched in uniaxial, biaxial, equibiaxial
  and plane strain modes
• Mathematical models include
• Ogden doubly-infinite power-law
• Mooney form of Rivlin strain energy function
• Plane strain linear law
• K-BKZ viscoelastic model

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Sheet stretching characteristics
• Sheet stretched in uniaxial, biaxial, equibiaxial
  and plane strain modes
• Mathematical models include
• Ogden doubly-infinite power-law
• Mooney form of Rivlin strain energy function
• Plane strain linear law
• K-BKZ viscoelastic model
• Plugs stretch sheet in plane strain

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Plane strain stretching, cont.
• Top and side view of plug-assisted stretching
• Circles appear as circles from top

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Plane strain stretching, cont.
• Top and side view of plug-assisted stretching
• Circles appear as circles from top, are actually
  distorted

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Plane strain stretching, cont.
• Mooney-Rivlin equation
• sl (l2-1/l)(2C12C2/l)
• For plane strain
• (ll2-1)1/2(1-1/ll2)(F/2pr)(2C1-2C2)/to
• Where
• 1/l l lh t/to

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Plane strain stretching
• Force F required to stretch sheet to a depth d
  with a flat plug
• F 2pdE(T)to/ln (a/b)
• Where E(T) is temperature-dependent modulus, to
  is initial sheet thickness, a is plug diameter, b
  is sheet diameter

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Plane strain stretching, cont.
• Comparison of theories and flat plug experimental
  data
• JLT, 1986

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• 2004 Experiments on Spherical Plugs
• Into Circular Sheets
• 0.015-inch natural rubber sheet, 6.75-inch
  diameter
• Two wooden sphere diameters
• Small ball diameter 0.75 inch
• Large ball diameter 3 inch
• Force and penetration measured with scales

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet
Schematic of Plug Experiment Left Half-Initial
Plug Position
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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet
Schematic of Plug Experiment Right Half-Plug
Position During Sheet Stretching
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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet
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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Large Sphere Surface Treatment
• As-is rough-grained exterior pine
• Sanded w/200 grit paper
• Wood-putty-filled, sanded w/320 grit paper
• Filled, sanded, and polished
• Filled, sanded, polished and talc-coated
• Filled, sanded, polished and coated with lube

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Measurements
• Height of plug into sheet
• Force
• Diameter of spherical cap
• Calculations
• Area of spherical cap
• Area of truncated cone
• Thickness of sheet

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet
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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet
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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet
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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet
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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet
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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Thickness of free portion of membrane
• Two extremes shown in next figure

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Thickness of free portion of membrane
• Two extremes shown in next figure
• If sheet slides on surface (friction coeff0),
  thickness is uniform everywhere

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet
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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Thickness of free portion of membrane
• Two extremes shown in next figure
• If sheet slides on surface (friction coeff0),
  thickness is uniform everywhere
• If sheet sticks to surface (friction coeff1),
  sheet on cap is original thickness

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet
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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Experiment
• Draw a circle on the free portion of the sheet

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Experiment
• Draw a circle on the free portion of the sheet
• Press the plug into the sheet to a given depth

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Experiment
• Draw a circle on the free portion of the sheet
• Press the plug into the sheet to a given depth
• Measure the major and minor axes of the ellipse
  (a and b)

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Experiment
• Draw a circle on the free portion of the sheet
• Press the plug into the sheet to a given depth
• Measure the major and minor axes of the ellipse
  (a and b)
• Use the relative areal draw ratio equation
• Rarel r2/ab
• to calculate reduced thickness, 1/ Rarel

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Comparison of Theory and Experiment
• Penetration No Frict Max Frict Exptl
• 2.0 in 0.832 0.813 0.805
• 2.4 in 0.778 0.738 0.744
• Average of 9-10 experiments

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet
Solid Circles Measured Red Tk
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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Sheet Thickness
• What about the relationship between stretching
  force and sheet thickness?

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Sheet Thickness
• What about the relationship between stretching
  force and sheet thickness?
• According to plane strain theory, the force
  increases in proportion to the thickness.
• Does this hold true for this experiment?

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet
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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Observations
• The nature of the dry plug surface does not
  substantially affect the amount of force needed
  to stretch the membrane

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Observations
• The nature of the dry plug surface does not
  substantially affect the amount of force needed
  to stretch the membrane
• It appears that from simple measurements, the
  sheet adheres to rather than slides on the dry
  plug

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Lubed Interface
• So far, all experiments have been with a presumed
  dry interface

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Lubed Interface
• So far, all experiments have been with a presumed
  dry interface
• Remember the discussion on wet v. dry sliding?

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Lubed Interface
• So far, all experiments have been with a presumed
  dry interface
• Remember the discussion on wet v. dry sliding?
  What happens if the interface is lubricated?

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Lubed Interface
• Large plug coated with
• Heavy grease oil-based
• Glycerin water-based

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet
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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Observations
• So, what happened when the interface is
  lubricated?

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Observations
• So, what happened when the interface is
  lubricated?
• If its oil-lubed, essentially nothing..

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Observations
• So, what happened when the interface is
  lubricated?
• If its oil-lubed, essentially nothing..
• But if its water-lubed, stretching force is
  reduced

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Observations
• So, what happened when the interface is
  lubricated?
• If its oil-lubed, essentially nothing..
• But if its water-lubed, stretching force is
  reduced
• This obviously needs more study!

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• IV. Conclusions

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Conclusions
• From simple plug experiments, it is apparent (at
  least to me) that the interaction between the
  plug and the sheet is not clearly defined

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Conclusions
• From simple plug experiments, it is apparent (at
  least to me) that the interaction between the
  plug and the sheet is not clearly defined
• It is not apparent (again, at least to me) that
  coefficient of friction is an appropriate measure
  of this interaction

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Conclusions
• Simple measurements show applied force increases
  with increasing sheet thickness

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Conclusions
• Simple measurements show applied force increases
  with increasing sheet thickness
• Something is going on when the interface is
  lubricated. But why is the force reduced only
  when the lube is water-based?

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Conclusions
• Furthermore, one might anticipate that with
  thick-gauge sheet, compression and shear might
  also be important - in addition to (or instead
  of) sliding.

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• Conclusions
• Furthermore, one might anticipate that with
  thick-gauge sheet, compression and shear might
  also be important - in addition to (or instead
  of) sliding.
• But the question remains

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TF Conference 2004 Throne Frictional
Coefficients Between Plug and Sheet

• In plug-assist thermoforming,
• what is sliding against what?

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Questions?
129

• THANK YOU FOR YOUR ATTENTION!