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Mini-Seminar Dr. James Throne, Instructor 8:00-8:50 - Technology of Sheet Heating 9:00-9:50 - Constitutive Equations Applied to Sheet Stretching 10:00-10:50.

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Presentation on theme: "Mini-Seminar Dr. James Throne, Instructor 8:00-8:50 - Technology of Sheet Heating 9:00-9:50 - Constitutive Equations Applied to Sheet Stretching 10:00-10:50."— Presentation transcript:

1 Mini-Seminar Dr. James Throne, Instructor 8:00-8:50 - Technology of Sheet Heating 9:00-9:50 - Constitutive Equations Applied to Sheet Stretching 10:00-10:50 - Trimming as Mechanical Fracture

2 Mini-Seminar Advanced Topics in Thermoforming Part 3: 10:00-10:50 Trimming as Mechanical Fracture

3 Lets begin!

4 Mini-Seminar Advanced Topics in Thermoforming All materials contained herein are the intellectual property of Sherwood Technologies, Inc., copyright 1999-2006 No material may be copied or referred to in any manner without express written consent of the copyright holder All materials, written or oral, are the opinions of Sherwood Technologies, Inc., and James L. Throne, PhD Neither Sherwood Technologies, Inc. nor James L. Throne, PhD are compensated in any way by companies cited in materials presented herein Neither Sherwood Technologies, Inc., nor James L. Throne, PhD are to be held responsible for any misuse of these materials that result in injury or damage to persons or property

5 Mini-Seminar Advanced Topics in Thermoforming This mini-seminar requires you to have a working engineering knowledge of heat transfer and stress-strain mechanics Dont attend if you cant handle theory and equations Each mini-seminar will last 50 minutes, followed by a 10-minute bio break Please turn off cell phones PowerPoint presentations are available at the end of this seminar for downloading to your memory stick

6 Mini-Seminar Advanced Topics in Thermoforming For those concerned about hearing the plenary speaker at 1100 hours, please be assured that this mini- seminar will end promptly at 1050 hours… And, if for some strange reason, it doesnt, please feel free to leave… THERE WILL BE NO FINAL! You can download all PPTs at the end of this section

7 Part 3: Trimming as Mechanical Fracture Outline Fundamentals Fracture mechanics The mechanics of trimming Trimming accuracy Thin-gauge trimming Heavy-gauge trimming

8 Part 3: Trimming as Mechanical Fracture What is Trimming? Trimming is semi-controlled mechanical breaking The objective is to separate the formed part from the web, skeleton, unformed plastic around it The methods of trimming are strongly dependent on sheet thickness Trimming can be manual or robotic, it can take place on the mold surface or in a remote fixture

9 Part 3: Trimming as Mechanical Fracture Traditional trimming methods Manual - knives, serrated for foam Routers - hand-held, table-mounted fixed- position, multi-axis Band saws Circular saws - stationary, hand-held small- diameter, toothless saws for foam Abrasive wheels

10 Part 3: Trimming as Mechanical Fracture Traditional trimming methods, cont. Sharp-edged compression blades or dies - steel-rule, ground forged, machined Linear shear guillotines - one-sided, two-sided Flames Lasers Water jets

11 Part 3: Trimming as Mechanical Fracture Traditional Trimming Methods

12 Part 3: Trimming as Mechanical Fracture Mechanics of Fracture Three general fracture modes Mode I - tensile mode, fracture surfaces spread apart by stress Mode II - shear mode, fracture surfaces slide perpendicular to advancing crack Mode III - tearing mode, fracture surfaces spread by stress parallel to crack

13 Part 3: Trimming as Mechanical Fracture Mechanics of Fracture

14 Part 3: Trimming as Mechanical Fracture Mechanics of Fracture Mode III is a shearing action. Guillotine cutting of heavy gauge sheet and punch-and-die cutting of thin gauge sheet are Mode III Mode I is a compressing action. In-place trimming of thin-gauge sheet is Mode I

15 Part 3: Trimming as Mechanical Fracture Mechanics of Fracture Force needed to initiate a crack is substantially less than theoretical cohesive strength of polymer Cracks initiate at flaws or defects Consider a polymer having a small crack in length under plane stress (Griffith crack theory) where E is Youngs modulus and...

16 Part 3: Trimming as Mechanical Fracture Mechanics of Fracture G* is the fracture energy, given as G* = 2(P+ ) where P is the plastic work done during yielding, is the surface energy of the polymer, K c is the fracture toughness or stress intensity factor

17 Part 3: Trimming as Mechanical Fracture Mechanics of Fracture Polymers with low P/ ratios are brittle (PMMA~5) Polymers with very high P/ ratios are ductile (vulcanized rubber ~ 500) It is nearly always the case that the energy to initiate a fracture is far greater than that needed to sustain crack propagation

18 Part 3: Trimming as Mechanical Fracture Mechanics of Fracture K c is the fracture toughness or stress intensity factor. It is written as K c = f( ) or C depends on crack geometry and surface being fractured PolymerK c (1000 lb/in 3/2 ) PS19.8 HIPS104 PE31.2 PMMA19.8

19 Part 3: Trimming as Mechanical Fracture Mechanics of Fracture An aside Nanoparticles have initial sizes substantially below the Griffith crack criterion As a result, theoretically, fracture is unlikely to initiate on a nanoparticle Meaning that, theoretically, nano-filled polymers should have impact strengths equal to those of the polymers themselves The functional word here is theoretically

20 Part 3: Trimming as Mechanical Fracture Mechanics of Trimming Five general mechanisms 1. Mode I In-plane unaxial compression (die-cutting)

21 Part 3: Trimming as Mechanical Fracture Mechanics of Trimming Five general mechanisms 2. Mode III antiplane pure shear (nibbling, shear cutting, punch and die)

22 Part 3: Trimming as Mechanical Fracture Mechanics of Trimming Five general mechanisms 3. Abrasion or abrasive cutting (grinding, filing,buffing, water jet cutting)

23 Part 3: Trimming as Mechanical Fracture Mechanics of Trimming Five general mechanisms 4. Brittle tensile fracture (routing, drilling, sawing)

24 Part 3: Trimming as Mechanical Fracture Mechanics of Trimming Five general mechanisms 5. Thermal (hot knife, hot wire, laser cutting)

25 Part 3: Trimming as Mechanical Fracture Trim Tolerance Limitation Local polymer shrinkage Polymer morphology Time-dependent part shrinkage after trimming Ductility of polymer Fixture/part rigidity Part temperature variation at trim line Registry accuracy

26 Part 3: Trimming as Mechanical Fracture Trim Tolerance Limitation, cont. Trim die temperature variation Part thickness, thickness variation Allowable wall thickness variation at trim line Die gap setting temperature v. trim die temperature Clamp frame stiffness Die flexing during trimming

27 Part 3: Trimming as Mechanical Fracture Factors in Selecting a Trimming Technique Sheet gauge Part size Number of parts Overall draw ratio Nonplanar nature of trim line Cut surface roughness tolerance Dimensional tolerance

28 Part 3: Trimming as Mechanical Fracture Factors in Selecting a Trimming Technique, continued Required speed of trimming Extent of fixturing Number of secondary operations [drilling, machining] Skill of operator/pressman Availability of desired trim equipment Availability of resharpening methods

29 Part 3: Trimming as Mechanical Fracture The mechanics of trimming depends on the sheet gauge Thin-gauge trimming Heavy-gauge trimming

30 Part 3: Trimming as Mechanical Fracture Thin Gauge Steel rule die cutting 1.In place, on the mold, in-situ 2.In machine, separate station 3.In line, separate machine

31 1. In-place trimming with steel-rule die Part 3: Trimming as Mechanical Fracture

32 Thermoforming – 1. Form and in- place trimming [GN] Part 3: Trimming as Mechanical Fracture

33 2. In-machine, usually with stacker

34 3. In-line trimming with canopy punch and die Part 3: Trimming as Mechanical Fracture

35 Thin-gauge trimming Primary methods 1. In-plane uniaxial compression or die-cutting Cutting stress, = E, where E is elastic tensile modulus and is extent of strain Solid compressibility is reciprocal of bulk modulus, B which is related to the elastic tensile modulus: E = 3B(1-2 ) where is Poissons ratio, 0.3 < < 0.4

36 Part 3: Trimming as Mechanical Fracture Thin-gauge trimming Primary methods 2. Mode III antiplane pure shear or punch-and- die cutting Shearing stress, s = G s, where G is the modulus of rigidity or shear modulus and s is strain under shear force Shear modulus is related to tensile modulus: G = E/2(1+ ) where is Poissons ratio, 0.3 < < 0.4

37 Part 3: Trimming as Mechanical Fracture Thin-gauge trimming Primary methods – Compared Values for Poissons ratio PA =0.33PMMA =0.33 HDPE =0.35LDPE =0.38 Average =0.35 For average, E/G = 2.7, E/B = 0.9 Shear cutting force about 40% of that of compression cutting force (on average)

38 Part 3: Trimming as Mechanical Fracture Thin-gauge trimming Polymer response to compression cutting

39 Part 3: Trimming as Mechanical Fracture Thin-gauge trimming Polymer response to compression cutting of brittle polymers

40 Part 3: Trimming as Mechanical Fracture Thin-gauge trimming Compression cutting required force is function of Trim length Polymer type Polymer temperature Trim die temperature Sharpness of die General equation: F (force/unit length of trim) = a + b (polymer thickness)

41 Part 3: Trimming as Mechanical Fracture Thin-gauge trimming Various steel rule die designs Curve D is for dulled version of cutter B RPVC PS

42 Part 3: Trimming as Mechanical Fracture Thin-gauge trimming- 1 Steel rule die cutting force as function of cutter temperature, F = a + b(tk)

43 Part 3: Trimming as Mechanical Fracture Thin-gauge trimming- 2 Steel rule die cutting force as function of cutter temperature, F = a + b(tk)

44 Part 3: Trimming as Mechanical Fracture Thin-gauge trimming- 1 Steel rule die cutting force as function of sheet thickness, cutter temp = 20 o C, F = a + b(tk)

45 Part 3: Trimming as Mechanical Fracture Thin-gauge trimming- 2 Steel rule die cutting force as function of sheet thickness, cutter temp = 20 o C, F = a + b(tk) competitive packaging polymers

46 Part 3: Trimming as Mechanical Fracture

47 Thin-gauge trimming -Example Calculate the force required to die-cut 10-in x 10-in trays, 9-up from 40 mil sheet of a) HIPS, b) PET Trim length- 4 x 10 x 9 = 360 inches a) HIPS - 56.4 lb f /in x 360 = 20,304 lb f = 10 tons b) PET - 85.9 lb f /in x 360 = 30,924 lb f = 15.5 tons

48 Part 3: Trimming as Mechanical Fracture Heavy-gauge trimming Hand-trimming using saws, routers Automated, programmable multiaxial routers that include saws, drills

49 Within the past decade, CNC-driven multi-axis routers, borrowed from the woodworking industry, have become standard fare for high quality part production in heavy-gauge forming. 5-axis router - Quintax Part 3: Trimming as Mechanical Fracture Heavy-gauge trimming

50 Part 3: Trimming as Mechanical Fracture Three-Dimensional Trimming & Machining - The Five Axis CNC Router, K.J. Susnajara, Thermwood Corporation, 1999 Contents –Machine Basics –CNC Router Design –Holding the Part –Tooling –Programming –Accuracy –Economics Heavy-gauge trimming

51 Part 3: Trimming as Mechanical Fracture Accuracy Heavy-gauge trimming First Layer –Single v. multiple axis repeatability –Absolute positioning accuracy - Single v. multiple axis –Loaded v. unloaded repeatability, accuracy –Machine considerations Lead screw backlash Rotary resolution of servomotor Encoder resolution and stepping interval Rail linearity Machine alignment [square and perpendicular]

52 Heavy-gauge trimming First Layer –Machine Design Lead screw backlash Rotary resolution of servomotor Encoder resolution and stepping interval Rail linearity Machine alignment [square and perpendicular] Head alignment - effect of crashes Head worm spur gear tooth accuracy, backlash Part 3: Trimming as Mechanical Fracture Accuracy

53 Heavy-gauge trimming Second Layer –Servo System Tracking –Inertia during acceleration, deceleration –Vibration, push-off, flexing –Speed –Tool length accuracy –Tool-to-collet tightening –CAD/CAM interpretation of curves [splines] –Trimming of part v. computer trim path Part 3: Trimming as Mechanical Fracture Accuracy

54 Heavy-gauge trimming Third Layer –Overall part size variability Molding temperature Raw material formulation Cooling characteristics –Polymer flexing under trim load –Bridge flexing during carriage movement –Dynamic flexing and bending v. speed –Polymer reaction to push-off –Bending, flexing of tool under load Part 3: Trimming as Mechanical Fracture Accuracy

55 Heavy-gauge trimming Third Layer –Thermal exansion, contraction Different materials in router Polymers being trimmed Tool dimensional change during trimming –Polymer warping, distortion during trimming –Trim direction v. grain in polymer Part 3: Trimming as Mechanical Fracture Accuracy

56 Heavy-gauge trimming Conclusion - Repeating an accurate position in space is far easier than achieving that accurate position in space. Part 3: Trimming as Mechanical Fracture Accuracy

57 Heavy-gauge trimming The mechanics of chip-breaking [Saws, drills, routers, mills] Part 3: Trimming as Mechanical Fracture Chip Work Cutter

58 Heavy-gauge trimming Factors Affecting Cutting Characteristics of Plastics Part 3: Trimming as Mechanical Fracture

59 Heavy-gauge trimming Multiple Edged Tool Cutting Part 3: Trimming as Mechanical Fracture

60 Heavy-gauge trimming Multiple Edged Tool Cutting Tooth depth of cut, g is: g = V p sin /U V is the feed rate, p is the tooth spacing, U is the peripheral blade speed, U=pDN, D is the diameter of the cutting blade, N is the blade speed (RPM). Part 3: Trimming as Mechanical Fracture

61 Heavy-gauge trimming Multiple Edged Tool Cutting The angle is given as = cos -1 [(h-b/2)/R] where h is the cut-off height or distance between the saw centerline and the bottom of the plastic sheet, b is the sheet thickness, R is the saw radius, R = D/2 Part 3: Trimming as Mechanical Fracture

62 Heavy-gauge trimming Multiple Edged Tool Cutting Feed rate, V, proportional to blade speed, N, and diameter, D Feed rate, V, inversely proportional to tooth spacing, p [Arithmetic holds for multiple-edge routers as well] Part 3: Trimming as Mechanical Fracture

63 Heavy-gauge trimming Drilling Part 3: Trimming as Mechanical Fracture

64 Heavy-gauge trimming Drilling Depth of cut per drill tooth, d: d = (s/n) sin /2 = (V/nN) sin /2 n is number of teeth on drill (one or two), N is the drill speed, V is the axial feed rate, s is drill feed speed, s = V/N, and is the point tooth angle Part 3: Trimming as Mechanical Fracture

65 Heavy-gauge trimming Effect of Drill Geometry on Drilling Conditions Drill ParameterDrill Condition Point angleRotational drill speed Rake angleDrill feeding speed Relief angleWork temperature Helix angleCooling provisions Shape of flutesNature of the hole Part 3: Trimming as Mechanical Fracture

66 End of Part 3 Trimming as Mechanical Fracture

67 Part 3: Trimming as Mechanical Fracture End of Mini-Seminar

68 Mini-Seminar Advanced Topics in Thermoforming THANK YOU FOR YOUR ATTENTION!


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