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

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

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  1. 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

  2. TF Conference 2004 – ThroneFrictional Coefficients Between Plug and Sheet Objective To better understand the problems with theories and experiments in plug assist thermoforming

  3. TF Conference 2004 – ThroneFrictional 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

  4. TF Conference 2004 – ThroneFrictional 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

  5. TF Conference 2004 – ThroneFrictional Coefficients Between Plug and Sheet Think about this Question! In plug-assist thermoforming, what is sliding against what?

  6. TF Conference 2004 – ThroneFrictional Coefficients Between Plug and Sheet Outline • A discussion of the coefficient of friction issue • A sliding experiment • Some prototypical plug experiments • Conclusions

  7. TF Conference 2004 – ThroneFrictional Coefficients Between Plug and Sheet I. Some thoughts on the coefficient of friction

  8. TF Conference 2004 – ThroneFrictional 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

  9. TF Conference 2004 – ThroneFrictional Coefficients Between Plug and Sheet Historical views of coefficient of friction-I “The relation that the power required tomove 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.

  10. TF Conference 2004 – ThroneFrictional 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.

  11. TF Conference 2004 – ThroneFrictional 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.

  12. TF Conference 2004 – ThroneFrictional 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.

  13. TF Conference 2004 – ThroneFrictional Coefficients Between Plug and Sheet Coefficient of Friction Following discussion assumes that frictional effects are extant in plug-assisted thermoforming

  14. TF Conference 2004 – ThroneFrictional Coefficients Between Plug and Sheet Blunt-nose plug moving into sheet Contact may involve some sheet sliding

  15. TF Conference 2004 – ThroneFrictional 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

  16. TF Conference 2004 – ThroneFrictional 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?

  17. TF Conference 2004 – ThroneFrictional Coefficients Between Plug and Sheet Coefficient of Friction • Static CoF - Initiation of sliding between plug (and mold wall) and sheet

  18. TF Conference 2004 – ThroneFrictional 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

  19. TF Conference 2004 – ThroneFrictional 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!

  20. TF Conference 2004 – ThroneFrictional Coefficients Between Plug and Sheet Sliding Coefficient of Friction • Contact area increases with increasing load Plastic sheet Plug

  21. TF Conference 2004 – ThroneFrictional Coefficients Between Plug and Sheet Sliding Coefficient of Friction • Contact area increases with increasing load • Ergo, coefficient independent of load Plastic sheet Plug

  22. TF Conference 2004 – ThroneFrictional Coefficients Between Plug and Sheet • Are there other factors influencing the interaction between the plug and the sheet?

  23. TF Conference 2004 – ThroneFrictional Coefficients Between Plug and Sheet • Are there other factors influencing the interaction between the plug and the sheet? • Wear

  24. TF Conference 2004 – ThroneFrictional 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

  25. TF Conference 2004 – ThroneFrictional Coefficients Between Plug and Sheet Sliding Coefficient of Friction • Friction maximum, wear minimum in polymer transition region • Glass Transition Region

  26. TF Conference 2004 – ThroneFrictional Coefficients Between Plug and Sheet • Are there other factors influencing the interaction between the plug and the sheet? • Wear • Dry v. wet sliding

  27. TF Conference 2004 – ThroneFrictional Coefficients Between Plug and Sheet Dry v. “wet” sliding • Dry sliding assumes no lube between surfaces

  28. TF Conference 2004 – ThroneFrictional 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)

  29. TF Conference 2004 – ThroneFrictional 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

  30. TF Conference 2004 – ThroneFrictional 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

  31. TF Conference 2004 – ThroneFrictional 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

  32. TF Conference 2004 – ThroneFrictional 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

  33. TF Conference 2004 – ThroneFrictional Coefficients Between Plug and Sheet Two types of wet sliding • Boundary lubrication – low sliding velocity, low interfacial viscosity, high loading

  34. TF Conference 2004 – ThroneFrictional 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

  35. TF Conference 2004 – ThroneFrictional 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]

  36. TF Conference 2004 – ThroneFrictional 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

  37. TF Conference 2004 – ThroneFrictional Coefficients Between Plug and Sheet Two types of wet sliding Sheet does not move far under plug force

  38. TF Conference 2004 – ThroneFrictional Coefficients Between Plug and Sheet Measuring Frictional Coefficients

  39. TF Conference 2004 – ThroneFrictional Coefficients Between Plug and Sheet Traditional methods of measuring coefficient of friction • Weight sliding on inclined surface

  40. TF Conference 2004 – ThroneFrictional Coefficients Between Plug and Sheet Traditional methods of measuring coefficient of friction • Tabor “Abrasor” – stylus rubbing on rotating disk

  41. TF Conference 2004 – ThroneFrictional Coefficients Between Plug and Sheet Traditional… • Other methods • No methods entirely applicable to measuring plug-sheet friction…

  42. TF Conference 2004 – ThroneFrictional 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

  43. TF Conference 2004 – ThroneFrictional 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

  44. TF Conference 2004 – ThroneFrictional Coefficients Between Plug and Sheet Are We Answering the Question? In plug-assist thermoforming, what is sliding against what?

  45. TF Conference 2004 – ThroneFrictional Coefficients Between Plug and Sheet II. A sliding experiment

  46. TF Conference 2004 – ThroneFrictional Coefficients Between Plug and Sheet Traditional… • Other methods • No methods entirely applicable to measuring plug-sheet friction (except g but modified)

  47. TF Conference 2004 – ThroneFrictional Coefficients Between Plug and Sheet Plug-sheet friction – experiment • Consider figure below…

  48. TF Conference 2004 – ThroneFrictional 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…

  49. TF Conference 2004 – ThroneFrictional 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…

  50. TF Conference 2004 – ThroneFrictional 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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