This article was downloaded by: [Temple University Libraries]On: 15 November 2014, At: 16:21Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH,UK

Polymer-Plastics Technologyand EngineeringPublication details, including instructions forauthors and subscription information:http://www.tandfonline.com/loi/lpte20

A Practical Verification ofa Computer Simulation forInjection Molding Applied toAmorphous ThermoplasticPolyestersF. H. Axtell a & B. Haworth ba Department of Chemistry, Faculty of Science ,Mahidol University , Rama 6 Road, Bangkok, 10400,Thailandb Institute of Polymer Technology and MaterialsEngineering, University of Technology ,Loughborough, Leicestershire LE11 3TU, UKPublished online: 22 Sep 2006.

To cite this article: F. H. Axtell & B. Haworth (1991) A Practical Verification of aComputer Simulation for Injection Molding Applied to Amorphous ThermoplasticPolyesters, Polymer-Plastics Technology and Engineering, 30:5-6, 441-452, DOI:10.1080/03602559108019212

To link to this article: http://dx.doi.org/10.1080/03602559108019212

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all theinformation (the “Content”) contained in the publications on our platform.However, Taylor & Francis, our agents, and our licensors make norepresentations or warranties whatsoever as to the accuracy, completeness,or suitability for any purpose of the Content. Any opinions and viewsexpressed in this publication are the opinions and views of the authors, and

are not the views of or endorsed by Taylor & Francis. The accuracy of theContent should not be relied upon and should be independently verified withprimary sources of information. Taylor and Francis shall not be liable for anylosses, actions, claims, proceedings, demands, costs, expenses, damages,and other liabilities whatsoever or howsoever caused arising directly orindirectly in connection with, in relation to or arising out of the use of theContent.

This article may be used for research, teaching, and private study purposes.Any substantial or systematic reproduction, redistribution, reselling, loan,sub-licensing, systematic supply, or distribution in any form to anyone isexpressly forbidden. Terms & Conditions of access and use can be found athttp://www.tandfonline.com/page/terms-and-conditions

Dow

nloa

ded

by [

Tem

ple

Uni

vers

ity L

ibra

ries

] at

16:

21 1

5 N

ovem

ber

2014

P0LYM.-PLAST. TECHNOL. ENG., 30(5&6), 441-452 (1991)

A PRACTICAL VERIFICATION OF A COMPUTER SIMULATION FOR INJECTION MOLDING APPLIED TO AMORPHOUS THERMOPLASTIC POLYESTERS

F. H. AXTELL

Department of Chemistry, Faculty of Science Mahidol Universiry R a m 6 Road, Bangkok 10400, Thailand

B. HAWORTH

Institute of Polymer Technology and Materials Engineering University of Technology Loughborough, Leicestershire LEI1 3TU, UK

Abstract

A comparison has been made between the pressure drops arising during injection molding as predicted by a simulation software package named SIMPOLO and as measured during molding trials conducted to study the effects of molding variables. The theory used to design the software has been reviewed and the operation described. The comparison was made using amorphous thermoplastic polyesters, PET and PCTG. The molding conditions used for the simulated trials were the same as those used in practical molding trials. The molding trials were conducted using a mold fitted with a cavity transducer and a machine fitted with a hydraulic pressure sensor. The simulation shows qualitative agreement to the obser- vations made in practice. The difference in pressure drop values are attributed to the software’s dependence on crystalline PET physical prop- erties for calculations and an overestimation of the frozen layer t h i h e s s by the simulation model. Overall the simulation has been shown to be a pragmatic tool for the production engineer working in the thermoplastic injection molding industry.

44 1

Copyright 0 1 9 9 1 by Marcel Dekker, Inc.

Dow

nloa

ded

by [

Tem

ple

Uni

vers

ity L

ibra

ries

] at

16:

21 1

5 N

ovem

ber

2014

442 AXTELL AND HAWORTH

INTRODUCTION

The simulation of the injection molding process is actively pursued largely because the procedure of mold design, machine selection, and choice of operating cycle represents a lengthy and expensive part of what is often a relatively small-scale operation. Any simulation is only of use if there is good agreement between the simulated data and the real situation. This paper reports a comparative study of a simulation of the injection molding process and prac- tical trials. This work was part of a wider study on the measurement and ap- plication of PET polymer rheology to processing [l].

A commercial package was used which is aimed at the production engineer rather than at the designer: SIMPOL@ .* SIMPOL was written to permit the optimization of processing conditions, and it also specifies machine re- quirements and estimates the costs involved. Using a databank of materials and machinery data, the SIMPOL software allows users to experiment with almost any variable in the molding process-melt temperature, packing pressure, injection rate, mold temperature, screw back pressure, cavity thickness, and hot and cold runner diameter-to assess the feasibility of a molding job. SIMPOL can also be used for mold design, allowing the explora- tion of the process before any metal is cut. Three screens can display a mold “trial,” the equipment needed to achieve the chosen set of conditions, and an estimate of the costs.

SIMULATION ANALYSIS

The simulation analysis divides the injection molding process into three stages: melt preparation, melt injection, and melt cooling.

Melt Preparation Stage

The modeling of the melt preparation stage involves the concepts of the heat flux and the Fourier number. The total amount of heat required to raise the polymer temperature by a given amount is obtained from enthalpy data for the polymer. Linking this to the heating rate possible from a cylinder of a given diameter, the plasticization rate for melt preparation is estimated. After the rate of heat input has been obtained, the Fourier number calculation gives the time and conditions required to attain thermal equilibrium throughout the melt.

*Ian Barrie Consultancy, 39 The Broadway, Wheathamatead, Hertfordshire, AL4 SLW, England.

Dow

nloa

ded

by [

Tem

ple

Uni

vers

ity L

ibra

ries

] at

16:

21 1

5 N

ovem

ber

2014

COMPUTER SIMULATION FOR INJECTION MOLDING 443

Melt Injection Stage

The melt injection stage is considered as two flow paths: one is isothermal, at the melt temperature, from the barrel through the nozzle and through any hot-runner sections; and the second, through the sprue, cold-runners, gate, and cavity sections, is nonisothermal.

The calculation of pressure drops for flow through uniform-sectioned chan- nels or simple radial flow includes consideration of elongational flow as well as shear flow when convergent or divergent flow occurs. Die-entry pressure loss data are used in calculations of pressure drops due to strongly convergent flow. Flow through the barrel and nozzle also include frictional losses and losses in the hydraulic circuit.

The nonisothermal flow model assumes that the melt flows within an im- mobile envelope of cooled polymer during mold filling. The envelope has the mold temperature at the mold interface and the melt temperature at the melt interface. These temperatures remain constant in the analysis, which is re- duced to isothermal flow in a section which decreases steadily until the polymer stops moving. It is also assumed that the thickness of material frozen is pro- portional to the cube root of filling time [2].

The critical situation in injection molding is considered to be when the mold is just filled. At this moment the total pressure drops are at a maximum and the feasibility of mold-filling for a given geometry with a given machine is fully tested.

There exists an optimum injection rate for minimizing pressure, stress levels, and the mold opening force [3,4]. At high injection rates there is little freezing- off, so more pressure is transmitted into the cavity. At low injection rates the frozen material on the walls increases, causing more restriction and hence higher pressure drops. At very low injection rates the flow path can become blocked before the cavity is full.

Using this frozen layer model, the pressure drops in a mold cavity are ex- amined at different injection rates and the optimum injection rate is determined. From the melt injection analysis, three important machine parameters are estimated: the maximum pressure demand, the optimum injection rate, and the minimum clamp force.

Melt Cooling Stage

A molding is assumed to be ready for ejection when the temperature of the center line of the thickest section is about 30°C below the freeze-off temperature. To estimate the cooling times the general form of the equation for one-dimensional heat flow in simple geometries is used [2].

Dow

nloa

ded

by [

Tem

ple

Uni

vers

ity L

ibra

ries

] at

16:

21 1

5 N

ovem

ber

2014

444 AXTELL AND HAWORTH

The simulation uses modeled rheological data; raw data are entered into a program which then models the data and transfers them to the databank accessed by SIMPOL. Using modeled data permits extrapolation and interpolation to different shear rates and temperatures.

The rheological model assumes that at low shear rates the viscosity ap- proaches Newtonian behavior and that at high shear rates the viscosity ap- proaches a limiting stress. The die entry pressure from a zero-length die is modeled as approaching a straight line over the injection molding shear rate range. Up to eight values of known shear rate, viscosity data pairs can be entered, and optionally eight zero-length die pressures for the corresponding shear rates. If existing data for the same grade are found in the data tiles, the two sets can be statistically combined or saved separately. The modeled data are transferred to a databank which is accessible to the SIMPOL program.

1

LOG SHEAR RATE (s-l)

FIG. 1. Shear rheology data for BWS PET. Symbols represent experimental data; curvesrepresentmodeleddata. V , -: 270°C; 0 , - - : 275°C; 0, * * * * : 280°C; A, ---: 285°C.

Dow

nloa

ded

by [

Tem

ple

Uni

vers

ity L

ibra

ries

] at

16:

21 1

5 N

ovem

ber

2014

COMPUTER SIMULATION FOR INJECTION MOLDING 445

SIMULATED MOLDING TRIALS

Rheological data from a Davenport Extrusion Rheometer for the three polymers studied were entered into the rheology modeling program and used in the simulated trials [l]. The polymers studied were standard bottle-grade PET (ICI “Melinar” B90S, nominal IV = 0.74); a high viscosity grade PET (from ICI, nominal IV = 0.92); and a copolyester (PCTG 5445 from Eastman Plastics, nominal IV = 0.65). Figures 1-3 show the viscosity data and the modeled curves for the three polymers.

The machine specification for the Negri-Bossi NB55 and details from a lay- flat description of the mold were entered into the program. The mold pro- duced two rectangular test bars and had a very long runner system and simple cavity geometry. The injection conditions such as the mold and melt temperatures, maximum pressure limit (1 17 MPa), and packing pressure (75 MPa) were entered into the simulation.

2 1 I I J 1 2 3 4

LOG SHEAR RATE (s- ’)

FIG. 2. Shear rheology data for 0.9 IV PET. Symbols represent experimental data; curves represent modeled data. V , - : 270°C; 0, * * a * : 280°C.

Dow

nloa

ded

by [

Tem

ple

Uni

vers

ity L

ibra

ries

] at

16:

21 1

5 N

ovem

ber

2014

446 AXTELL AND HAWORTH

4 r

2 I I I 1 2 3 4

LOG SHEAR RATE (s -')

FIG. 3. Shear rheology data for PCTG. Symbols represent experimental data; curves represent modeled data. 0 , --: 240°C; 0, m e * : 250°C.

The SIMPOL simulation displays three screens of information for each set of conditions entered. The first contains the operating parameters for the trial; the second contains the minimum requirements for the machine specification compared to a given machine specification (in this study the Negri-Bossi NB55); the third screen contains cost estimates for the production of parts using the set conditions. The SIMPOL estimated pressure drops for B90S are shown as curves in Figs. 4 and 5 .

PRACTICAL MOLDING TRIALS

A series of experiments were undertaken on a 55 tonne Negri-Bossi NB55 injection molding machine, to verfiy the accuracy of the modeled data from

Dow

nloa

ded

by [

Tem

ple

Uni

vers

ity L

ibra

ries

] at

16:

21 1

5 N

ovem

ber

2014

COMPUTER SIMULATION FOR INJECTION MOLDING 447

40

0

FIG. 4. Effect of injection rate on cavity pressure for B90S PET. Symbols repre- sent experimental data; curves represent modeled data. A , - - - : melt temperature 280°C, mold temperature 10°C; V, --: melt temperature 280°C, mold temperature 50T, 0, - - - : melt temperature 290 "C, mold temperature 10°C; 0, - - - - : melt temperature 290 "C, mold temperature 50 "C.

the SIMPOL simulation. The raw materials were dried overnight in a dehumidi- fying hopper fitted directly onto the injection molding machine. The hydraulic line pressure was measured by a strain gauge pressure transducer and record- ed on a chart recorder. The mold was instrumented with a cavity strain gauge transducer positioned flush with the cavity surface close to the gate end of the cavity. The cavity pressure was recorded on a chart recorder. The mold temperatures were maintained using a water heater and a chiller unit as necessary. The pressure values at the instant of mold filling under different operating conditions are shown as data points in Figs. 4 and 5.

A comparison of the data presented in Fig. 4 shows the simulated and ex- perimental results for cavity pressure drop of the B90S polymer, similar minimum points were obtained from the simulation and the practical trials. Good agreement was observed for the optimum injection rates although the

Dow

nloa

ded

by [

Tem

ple

Uni

vers

ity L

ibra

ries

] at

16:

21 1

5 N

ovem

ber

2014

AXTELL AND HAWORTH

I I 1 1 0 5 10 1 5

LOG INJECTION RATE (mils)

FIG. 5. Effect of injection rate on hydraulic pressure for B90S PET. Symbols repre- sent experimental data; curves represent modeled data. A , -- - : melt temperature 280 "C , mold temperature 10°C; V , -: melt temperature 280°C, mold temperature 50°C. 0, - - : melt temperature 290 "C, mold temperature 10°C; 0, - 0 . : melt temperature 290°C, mold temperature 50°C.

simulation predicted a larger temperature dependence of cavity pressure drop values than was observed in practice.

A comparison of the data presented in Fig. 5 shows the simulated total pressure drop and experimentally measured hydraulic pressure for the B90S polymer. The SIMPOL simulation gave curves that followed the data points obtained experimentally. The flow rates at which the minimum pressure occurred are approximately equal for the simulation and the experimentally measured cases. SIMPOL predicted a greater temperature dependence of the total pressure drop than that for the hydraulic pressure measured from the experiments.

The experimental results of cavity pressure values for the high-viscosity PET were lower than those from the simulation; see Fig. 6. Agreement between

Dow

nloa

ded

by [

Tem

ple

Uni

vers

ity L

ibra

ries

] at

16:

21 1

5 N

ovem

ber

2014

COMPUTER SIMULATION FOR INJECTION MOLDING 449

0

0 0 5 10 1 5

LOG INJECTION RATE (mils)

FIG. 6. Effect of injection rate on cavity pressure for 0.9 IV PET. Symbols repre- sent experimental data; curves represent modeled data. A , ---: melt temperature 280°C, mold temperature 10 "C; V , - : melt temperature 280"C, mold temperature 50 "C, 0, - - : melt temperature 290°C. mold temperature 10°C; 0, .*** : melt temperature 290 "C , mold temperature 50 "C .

the simulation and experiments was found for the effect of temperature on the cavity pressure; the minimum value for cavity pressure decreased with increas- ing mold temperatures and decreasing melt temperatures. The actual meas- urements showed a greater temperature sensitivity than the simulated results.

The practical values for hydraulic pressure at the instant of mold-filling showed a similar dependence on injection rate as the total pressure predicted by SIMPOL, though the simulated values were lower than the practical obser- vations, the high viscosity grade PET was difficult to mold because the max- imum pressure available on the machine was exceeded in most cases. The simulated values increased with decreased melt and mold temperatures, showing the same dependence as that observed for the actual values measured.

Dow

nloa

ded

by [

Tem

ple

Uni

vers

ity L

ibra

ries

] at

16:

21 1

5 N

ovem

ber

2014

450 AXTELL AND HAWORTH

FIG. 7. Effect of injection rate on cavity pressure for PCTG. Symbols represent experimental data; curves represent modeled data. 0, - - : melt temperature 260°C, mold temperature 10°C; 0, * - * * : melt temperature 260°C. mold temperature 50°C; A , ---: melt temperature 270"C, mold temperature 10°C; V , - : melt temperature 270 "C, mold temperature 50 "C .

The measured cavity pressure values for PCTG copolyester were similar to those from the simulation; see Fig. 7. The cavity pressure decreased with increasing mold and melt temperatures. The SIMPOL estimated values for total pressure drop agreed with the measured values of hydraulic pressure for injection rat2s greater than 10 m L / s . At slower rates, the simulated values were lower than those measured experimentally. The temperature dependence of the simulated and practical pressure values showed the same trend; the pressure drop increased with decreasing temperatures.

The main part of the SIMPOL program is the calculation of the pressure drop at the instant of mold-filling, which can be calculated for different

Dow

nloa

ded

by [

Tem

ple

Uni

vers

ity L

ibra

ries

] at

16:

21 1

5 N

ovem

ber

2014

COMPUTER SIMULATION FOR INJECTION MOLDING 45 1

TABLE 1 Occurrence of Short Moldings

Mold Melt Injection temp. temp. rate range SIMPOL

Polymer ("C) ("C) (mL/s) predictiono Actual observation

B90S B90S B90S B90S

0.9 IV 0.9 IV 0.9 IV 0.9 IV 0.9 IV 0.9 IV 0.9 IV

PCTG PCTG

PCTG PCTG PCTG PCTG PCTG PCTG PCTG PCTG PCTG PCTG PCTG PCTG

10 10 50 50

10 10 50 50 50 50

50

10 10

10 10 10 10 50 50 50 50 50 50

50 50

280 290 280 290

280 290 280 290 290 290 290

260 270

270 270 270 270 260 260 260 270 270 270 270 270

1-39 1-39 1-39 1-39

5-16 7-22 6-39 1-3 3-5 6-8

10-39

3-27 1-3

3-6 10-14 18-27 39

1-2 2-3 6-27 1-2 3-10

10-14 18-27 39

Very likely Very likely Very likely Very likely

Very likely Very likely Very likely Very likely Possible Very possible Very likely

Very likely Very unlikely

No comments Very likely No comments Very likely Very unlikely Possible Very likely Very unlikely No comments Very likely No comments Very likely

Only at 1.5 mL/s Only at 1.5 mL/s Only at 1.5 mL/s Only at 1.5 mL/s

None None Only at 6.4 mL/s Only at 2.0 mL/s None None None

Only at 3.3 mL/s At 1.8 and 3.3 mL/s None None None None Short molding Short molding None Short molding None None None None

'SIMPOL likelihood comments correspond to tha pressure drop ranges: very unlikely up to 58 MPa; possible, 58-71 MPa; very possible, 71-78 MPa; Very likely, greater than 78 MPa.

Dow

nloa

ded

by [

Tem

ple

Uni

vers

ity L

ibra

ries

] at

16:

21 1

5 N

ovem

ber

2014

452 AXTELL AND HAWORTH

operating conditions. By running a series of simulated trials, the optimum in- jection rate for a given set of mold and melt temperatures can be determined; these predictions have been shown to be a good representation of experimen- tal results. SIMPOL uses this calculated value to estimate the machine re- quirements and costings for a given molding job.

The differences shown in Table 1 are due to the dependence of SIMPOL on thermal data for fast-crystallizing PET when simulating the molding of the emorphous polyesters studied, thus overestimating the frozen layer thickness. SIMPOL determines the situation at the switch-over point, while in practice the cavity may sometimes fill during the hold-on stage. SIMPOL would also discourage the molder from using such conditions that would only produce poor-quality moldings. The cavity pressure monitoring offers the facility to optimize molding conditions both for pressure drops and for avoidance of poor- quality moldings.

These results also highlight the requirement, when using polymer process- ing CAD systems, that the source of any data used must be known so that the user is aware of differences in material type ldcely to affect the results. A computer model can only be relevant and give accurate results if all the data used are “good.”

CONCLUSION

Overall the Su lpoL simulation has proved to be a pragmatic, useful, and cost- effective time-saving tool, eminently suitable for the production engineer work- ing in the thermoplastics injection molding industry.

Acknowledgment

The authors gratefully acknowledge Ian Barrie, who created SIMPOL@’ , for his many discussions which provided us with the insight to his system.

REFERENCES

[l] F. H. Axtell, A Study of the Flow and Deformation of Thermoplastic Polyesters, Ph.D. thesis, Loughborough University, UK (1987).

[2] I. T. Barrie, Plusr. Polym. 38, 47 (1970). [3] B. E. Vostorgov and E. L. Kalinchev, Soviet Plusr., No. 4, 19 (1971). [4] H. W. Cox and C. C. Meaner, Polym. Eng. Sci., 26, 488 (1986).

Dow

nloa

ded

by [

Tem

ple

Uni

vers

ity L

ibra

ries

] at

16:

21 1

5 N

ovem

ber

2014