Novel Method for Rapid Determination of Thermoformability

Dr. Amit DhariaTransmit Technology Group, LLC, TXDr. Don HyltonMcConnell Co. Inc., GA

Outline®Properties –Thermoforming Process

relationship ®Current test methods ®Description of new test apparatus

/operation/ data acquisition ®Application and data interpretation®Future plans®Conclusion

Thermoforming Process®Extruding sheet stock ®Heating sheet above Tg®Stretching heated sheet in rubbery state®Forming stretched sheet®Cooling®Trimming ®Finishing –painting, printing

Structure - Properties -Thermoformability® Rate of change of strength with the

change in strain rate at forming temperature

®% Crystallinity – Breadth of rubbery Plateau

® Molecular weight, Molecular weight distribution, molecular architecture (branching, crosslinking) – MFR, Melt Elasticity

Other parameters® Density - % filler, type of fillers, degassing® Geometry – Thickness, area, multi-layered

structures, adhesion between layers® Residual stresses between and within in

extruded layer sheet stock® Thermal diffusivity (Cp, K. Rho)® Extrusion quality ( gels, unmelts,

thickness variation, grain patterns)® Color (IR absorption)

Current tests® Low shear melt viscosity (MFR, RMS)® Hot tensile test (2 in gauge length, 0.2 to 20

ipm, or 0.002 to 0.33 1/s vs. 10/1s for TF process)

® Melt Tension (Draw Force –Melt strength, Break Velocity –Draw down)

® Sag Test (sag distance, sag time)® Hot Creep Test® DMA (Relaxation time)

Major disadvantages of current methods® Most tests are conducted in melt or near melt

phase – TF at Tg + 30-40 F® Specimen do not reflect actual test geometry

(shape, size, thickness clamping mode)® Do not account for orientation, thermal stresses,

thickness variations® Isothermal environment, does not account for

transient nature during heating/ cooling steps® Effects of secondary process parameters can not

be evaluated® Results cannot be directly used.

What we want to know?® Will this material thermoform? ® Will this new material process the same? ® Will this lot process the same as the last one?® Why this lot does not process the same?® How much time is needed to heat the sheet?® How fast material will heat?® What is the right forming temperature range?® Will melt adhesion between layers survive

heating and stretching step?® Will material discolor, fed or degrade during

heating?

What processors want to know?®What is the maximum draw down ratio?® How fast part can be made? ®What is the MD and TD shrinkage?®Will material tear?® How much regrind can I use?®Will grains retain shape and depth?® Does extruded sheet have gels or

unmelts?

What Industry Needs?® A standard test method which reflects all unit

steps – heating, 3D stretching, forming, and cooling

® A test equipment which can be precisely controlled, is rapid, easy to use, provides repeatable and quantitative information, using the lease amount of material.

® Easy to use “Thermoformability Index” standard for comparing, contrasting effects of selected process/ material variables

TECHNOFORM TM

Patent PendingTTG

TECHNOFORM

Typical Data input® Mode of operation – Plug, vacuum, vacuum +plug® The heating element distance from the sheet

surface® The heating element temperature® The sheet temperature® Heat Soak time at given temperature® Plug velocity (2 to 400 mm/second)® Plug Delay Time ® Plug Temperature® Cooling time

Typical user Input Screen

Sag Distance

Thinning

Strainhardening

Forming Depth mm

Thermoformability Index=slope

Typical Data Output® Heating rate (Delta C/ time)® Sag distance ® Forming force (Stress) vs. forming

distance (strain)® Forming Force vs. time® Yield force® Forming force vs. actual temperature® Shrinkage

Heating rates for various plastic materials(Heater at 600 C, 3” from upper, 2” from lower)

30

80

130

180

230

0 20 40 60 80

t (seconds)

T (c

)

PPHDPEHIPSPVCABSAcetalPMMANylon

Effect of Crystallinity

05

1015202530

50 70 90 110 130

Forming distance, mm

Forc

e (N

)HDPE PP HIPS PETG ABS PMMA PVC

Effect of Forming Temperature

0

2

4

6

8

10

12

14

125 145 165 185

Temperature (C)

Fo

rce

(N)

ABS

PP

HDPE

HIPS

PETG

PMMA

ACETAL

Force100 = f (T, V, material)®F(ABS) =9.2348 -0.0547 T (R2 =99%)®F(PMMA)=7.1587 -0.0341 T(R2=98%)®F(PETG)=10.096 -0.0601 T (R2=92%)®F(HIPS)=9.6782 - 0.0503T(R2=93%)®F(HDPE)=5.2771 -0.0266 T (R2=86%)

Comparison of Acetal, Nylon, TPO

0123456

0 50 100 150Time (seconds)

Form

ing

dist

ance

(mm

)

Delrin (206)

PA (241 C)

TPO (161 C)

Effect of forming Speed on HDPE @ 150 C

Effect of Forming Speed on HDPE

0

2

4

6

8

10

0 50 100 150Distance (mm)

From

ing

forc

e (N

)

20 mm/sec30 mm.sec50 mm/sec

Comparison of filled vs. HMS-TPO

0

10

20

30

40

50

0 20 40 60 80 100

Distance (mm)

Forc

e (N

)40HMSTPO 20HMSTPO

40 FTPO 20FTPO

Effect of adding HMSPP in PP

01234567

40 60 80 100 120 140Form ing Dist ance, mm

Form

ing

Forc

e, L

bf

10%H MSPP 20%HMS PP 30% HMSPP

01234567

40 60 80 100 120 140Form ing Dist ance, mm

Form

ing

Forc

e, L

bf

10%H MSPP 20%HMS PP 30% HMSPP

01234567

40 60 80 100 120 140

Forming Distance, mm

Form

ing

Forc

e, L

bf10%HMSPP 20%HMSPP 30%HMSPP

Formability of HMSPP/PP Blends

02468

1012

10 20 30

% HMSPP

thic

knes

s (H

igh/

Low)

Effect of % Regrind on formability TPO20% regrind / Five Successive Extrusions

0123456

40 60 80 100 120

Forming Distance, mm

Form

ing Fo

rce,

Lbf

1st2nd3rd4th5th

Comparison of Test Methods

Relaxation Time (s) Vs. Force @ 75 mm depth

R2 = 0.9968

02468

1012

0 2 4 6 8

Relaxation Time (sec)

Form

ign

Froc

e (7

5 m

m)

PP, 165 C

HDPE,140CHIPS,160 C

Future Plans® Develop a standard “Thermoformability

Index” similar to Melt Flow Index; which can be used by industry as specification to describe thermoformability of material.

® Refine force-vs. displacement data into stress vs. strain data for each plug geometry

® Develop an integrated predictive modeling system like “Moldflow”.

Conclusions® A novel and simple test equipment is

developed which closely reflects all four unit steps of the typical thermoforming process and generates quantitative and repeatable information.

® The test data can be used in raw form to compare or contrast various materials and process parameters input or can be further modeled as a design or predictive tool.