optimization of micro cellular injection molding parameters

S. J. A. Rizvi1, N. Bhatnagar2,*

1 Center of Polymer Science and Engineering, Indian Institute of Technology, Delhi, New Delhi, India2 Department of Mechanical Engineering, Indian Institute of Technology, Delhi, New Delhi, India

Optimization of Microcellular InjectionMolding Parameters

Microcellular foamed polymeric products have found variousapplications depending upon their cell morphology. The mi-crocellular injection molding process parameters have effecton the cell size, cell density and cell distribution. In order todevelop desired cell morphology for a particular application,the set of injection molding parameters is required to be opti-mized for achieving a correct process window. In this paperan effort is made to study the effect of various injection mold-ing processing parameters on cell morphology. Series of ex-periments were conducted for each of the processing param-eter and the samples were observed under Scanning ElectronMicroscope (SEM). The trends of cell size and cell density withrespect to the processing parameters were qualitatively ana-lyzed. The results are useful for the processors who are inter-ested in building up an understanding of this novel processand also to enable optimization of the set of processing para-meters so that the desired microstructure of a foamed polymercan be obtained.

1 Introduction

In microcellular injection molding, supercritical nitrogen orcarbon dioxide is injected into the barrel of injection moldingmachine with the help of specially designed injector at the me-tering zone. Single phase polymer – gas solution is then in-jected into the mold with the help of a shut-off nozzle. Thisprocess produces molded parts with a cell size of the order ofmicrons (1 to 100 microns) whereas the conventional cellularproducts have a cell size 0.1 mm to 10 mm (Yuan et al.,2003). The system is brought to the supersaturated state eitherby reducing the pressure (pressure induced phase separation)or by increasing the temperature (temperature induced phaseseparation) resulting in the nucleation and cell growth insidethe polymer matrix (Goel and Beckman, 1994, Kumar andSuh, 1990).

Injection molding process is one of the most promisingmethods for the mass production of parts with complex design.Now, it is also used to produce parts with microcellular struc-ture. Advantages of microcellular process are reduction of in-

jection pressure, clamping force, cycle time along with 10 to30% saving in raw material. It also eliminates the need of apacking stage and improves the dimensional stability of themolded parts (Kramschuster et al., 2005; Jacobsen and Pierick,2000).

Injection molded microcellular thermoplastics may havelarge difference in their properties depending upon their micro-structure. The applications of microcellular thermoplasticfoams are governed by the structure of microcellular foam.The microstructure of cellular foam is basically characterizedby the cell size, cell density and cell distribution. In order toproduce a microcellular foamed part of a thermoplastic target-ing one particular set of properties, it is very essential to opti-mize the injection molding processing parameters to achievethe desired properties. From the available literature it is ob-served that very few studies are being carried out to understand

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Intern. Polymer Processing XXIV (2009) 5 / Carl Hanser Verlag, Munich 1

* Mail address: Naresh Bhatnagar, III-178, Dept. Mechanical Engi-neering, IIT-Delhi, New Delhi-110016, IndiaE-mail: [email protected]

Fig. 1. Experimental setup of microcellular injection molding

Process variable 1 2 3 4 5

1. Injection rate (cm3/s) 95 100 105 110 1152. Back pressure (bar) 120 130 140 150 1803. Melt temperature (8C) 209 222 233 239 –4. Barrel residence time (s) 120 180 240 300 3605. Suck back (cm3) 5 10 15 20 25

Table 1. Processing parameters

the effect of various processing parameters like injection rate,melt temperature, back pressure, suck back stroke etc. on thecell size and cell density. However, in the manufacturing worldthe correct process window for any material at hand is desir-able to remain competitive. A need was therefore felt to studyand understand the underlying effects of variety of processingparameters in the microcellular injection molding processsince these are different than the conventional injection mold-ing process. An initial effort is made in this study to correlatethe microstructure with injection molding parameters. Ulti-mately, a correct understanding of the relations between pro-cessing parameters and microstructure will help in optimizingthe processing parameters for targeting a particular set of prop-erties in a mass production molding environment.

2 Experimental

2.1 Experimental Setup and Material

Fig. 1 shows the layout of the experimental setup, which pri-marily consists of microcellular injection molding machine

and a high pressure compressor (Battenfeld, Austria) and afamily mold for tensile, impact, flexural and HDT test speci-men as per the ASTM standards (designed in-house). This in-jection molding machine has a clamping force of 40 t and max-imum possible injection rate and injection pressure are116 cm3/s and 1500 bar respectively. This injection moldingmachine is equipped with 35 mm diameter screw with L/D ra-tio of 22 : 1. The high pressure compressor supplies continuoussupply of nitrogen gas at a maximum pressure of 300 bar.

Polystyrene (LG Polymers Pvt. Ltd. India, grade LGG108)with a melt index of 14.0 g/10 min (5 kg/200 8C) was used inthis experimental study. Industrial N2 with a purity of 99.5 %was directly used as a foaming agent.

2.2 Sample Preparation and Characterization

The barrel temperature of microcellular injection molding ma-chine was set at a range of 160 to 240 8C (from feed zone tonozzle) whereas the actual melt temperature (measured by atemperature sensor exposed to polymer melt in metering zone)inside the barrel was 215 8C ± 1 8C. The N2 gas was com-pressed and stored in high capacity accumulator at 200 barand during the metering operation this supercritical gas was in-jected with the help of an injector at a pressure of 30 bar abovethe pressure of the melt. The mold temperature was 25 8C andthe cooling cycle time was maintained between 100 to 120 s.A longer cooling time was kept deliberately so that the barrelresidence time of the order 120 to 180 s could be achievedsince longer barrel residence time allows a better mixing ofgas in polymer matrix. A fixed shot size of 55 cm3 was usedfor all the experimental study. The N2 gas was injected fromstart of metering to 40 cm3 of metering.

Fig. 2. SEM micrographs of fractured surfaceon the molded tensile test specimen for injec-tion rate (A) 100 cm3/s, (B) 105 cm3/s, (C)110 cm3/s and (D) 115 cm3/s; melt tempera-ture: 215 8C, back pressure: 120 bar, screwspeed: 50 mm/s, hold pressure: 80 bar, holdtime: 12 s, V-P switchover: volume specific(8 cm3), shot size: 55 cm3; Sc-N2 pressure:30 bar

A) B)

C) D)


S. J. A. Rizvi, N. Bhatnagar: Optimized Microcellular Injection Molding

2 Intern. Polymer Processing XXIV (2009) 5

Fig. 3. Effect of injection rate on mircocell diameter and density

The initial parameters for microcellular injection moldingprocess were based on recommended values obtained fromthe literature. Subsequently the process variables were gradu-ally varied to attain homogenous foaming conditions. Oncethe operating point was reached well within the process win-dow, experimentation was started. Table 1 shows the detailsof experiments and values of each process variable. Only oneprocess parameter was varied during each experiment at onetime to give genuine repeatability.

The molded specimens were produced by operating the in-jection molding machine in fully automatic mode for about 10to 15 shots. This allows the process to reach stable processingconditions and minimize the shot to shot variation of variousprocess parameters. The molded tensile test specimens wereimmersed in liquid N2 for 5 to 10 minutes before fracturingthem. Cryogenic fracture (at about –196 8C), which in this caseis much below the glass transition temperature, helps in avoid-ing distortion in cell structure. In order to eliminate the varia-tion of microstructure due to geometry of product, all the ten-

sile test specimens were fractured at identical geometriclocation i. e. 100 mm from the gate point. The fractured sur-faces were examined using table top scanning electron micro-scope (Hitachi TM 1000). The micrographs (Fig. 2) were ana-lyzed by “Image – J” (an image processing software) andaverage cell diameter and cell density were thereby measuredand calculated.

3 Result and Discussion

3.1 Effect of Injection Rate on Microstructure

As with any foam molding process, the microcellular processcannot be expected to produce class ‘A’ surface molded parts.Injection rate is one of the most critical parameter for surfaceappearance of molded parts. With a cold runner mold, the in-jection rate should be slow until the polymer enters the moldcavity. Once the flow front is established, the injection ratecan be increased to allow smaller cell diameter with uniformlydistributed microcells. As it can be seen in Fig. 3, average celldiameter decreases with increase in injection rate. Faster injec-tion rate is associated with rapid pressure quench or depressur-ization that causes supersaturation of polymer – gas solutionresulting in more number of nucleation sites. However, de-crease in cell density with increase in injection rate is an unu-sual phenomenon observed here and requires further detailedinvestigations.

3.2 Effect of Back Pressure on Microstructure

Fig. 4 shows the qualitative effect of back pressure on cellularstructure. The quantitative analysis of the effect of back pres-

Fig. 4. SEM micrographs of fractured surfaceon the molded tensile test specimen for backpressure (A) 120 bar, (B) 130 bar, (C)140 bar and (D) 150 bar; melt temperature:215 8C, injection rate: 115 cm3/s, screw speed:50 mm/s, hold pressure: 120 bar, hold time:5 s, V-P switchover: volume specific (18 cm3),shot size: 55 cm3, Sc-N2 pressure: 30 bar

A) B)

C) D)



Intern. Polymer Processing XXIV (2009) 5 3

Fig. 5. Effect of back pressure on microcell diameter and density.

sure was carried out using image processing software as statedearlier and is further illustrated in Fig. 5. The average microcellsize was found to be decreasing with increase in back pressureduring the melt plasticization. At the same time cell densitywas found to follow a reverse trend for cell diameter as shownin Fig. 5. Initially cell diameter and density show reverse trendwith increase in back pressure. The cell diameter decreaseswith back pressure in Initial segment of the graph but in laterstage it gets almost stabilized. Since the solubility of N2 gas inpolymer increases with rise in pressure, increased back pres-sure accounts for increase in gas solubility resulting in highernumber of nucleation sites with comparatively smaller diam-eter of cells. After a certain limit, gas solubility approachestowards saturation limit and therefore further increase in celldensity and simultaneous decrease in cell diameter is hinderedand further increase in back pressure does not result in signifi-cant change in microstructure. This brings out an optimumrange of back pressure as a vital governing process parameter.

3.3 Effect of Melt Temperature on Microstructure

The optimum value of melt temperature for microcellular in-jection molding is, in fact, a compromise between cell structureand surface appearance. In order to optimize the cell structure,minimum feasible melt temperature is recommended. As thesolubility of gas decreases with increase in melt temperature,it leads to non-uniform cell morphology, whereas, higher melttemperature is usually recommended for better surface finish.However if the melt temperature is too low, then the moltenpolymer have higher melt strength and that will offer stiff resis-tance to the growing cells, resulting in higher density offoamed product. Fig. 6 and 7 show the effect of increasing melttemperature on the cell microstructure. Initial increase in celldiameter associated with reduction in cell density is the ob-vious result of increase in temperature as explained above, fol-lowed by decrease in cell diameter and increase in cell densitywhich can be attributed to a decrease in melt viscosity causingincrease in free volume that accounts for higher number of nu-cleation sites. Fig. 6D shows that at 239 8C melt temperature;there is a disappearance of microcellular structure and thismay be considered as the upper limit of melt temperature whileestablishing the process window for microcellular injectionmolding.

3.4 Effect of Barrel Residence Time on Microstructure

The barrel residence time in conventional injection molding isusually considered as the time required for entry of pellets toexit of melt from barrel of injection molding machine, howeverin the present case, the barrel residence time is considered astime span available after the end of metering to the end of in-jection, as shown in Fig. 8. The difference between cooling

Fig. 6. SEM micrographs of fractured surfaceon the molded tensile test specimen for melttemperature (A) 209 8C, (B) 222 8C, (C)233 8C and (D) 239 8C, injection rate:115 cm3/s, back pressure: 120 bar, screwspeed: 80 mm/s, hold pressure: 80 bar, holdtime: 5 s, V-P switchover: volume specific(8 cm3), shot size: 55 cm3, Sc-N2 pressure:30 bar

A) B)

C) D)




Fig. 7. Effect of melt temperature on microcell diameter and density

and metering time primarily attributes to barrel residence time(BRT). In case of parallel operation injection molding ma-chine, where metering time is larger than the cooling time, thebarrel residence time is almost negligible.

The solubility of gas in polymer also depends upon the gasdiffusion rate (for polystyrene/nitrogen the diffusion coeffi-cient estimated at 200 8C/27.6 MPa is 1.5·10–5 cm2/s andmaximum solubility is 2.0 %, Okamoto, 2003, Fig. 9). Duringthe gas polymer interaction, the increase in the barrel residencetime will eventually increase the gas solubility into polymermelt leading to high density foam with smaller cell diameterbut the initial experiments show a reverse trend. However, after180 s the curve shows the expected trend. The initial deviationwas investigated and interpreted as the contribution made byslight leakage of gas from the system with time. Finally, re-maining quantity of gas gets mixed with the polymer, and iffurther time is available then gas diffuses inside the core ofthe polymer, hence the solubility increases resulting in increasein cell density followed by reduction of cell diameter. In caseof very long BRT as shown in Fig. 10D, the hot gas under highpressure escapes from the shut off nozzle, leading to large re-duction of pressure in polymer/Sc-N2 mixture. The resultingmorphology may contain macro voids or no cell formation atall.




Fig. 8. Barrel residence time in typical injec-tion molding cycle

Fig. 9. Solubility of gas in a molten polymer (Okamoto, 2003)

Fig. 10. SEM micrographs of fractured sur-face on the molded tensile test specimen forbarrel residence time (A) 120 s, (B) 180 s, (C)240 s and (D) 360 s, melt temperature:215 8C, injection rate: 115 cm3/s, back pres-sure: 120 bar, screw speed: 80 mm/s, holdpressure: 80 bar, hold time: 5 s, V-P switch-over: volume specific (8 cm3), shot size:55 cm3, Sc-N2 pressure: 30 bar

A) B)

C) D)

3.5 Effect of Suckback on Microstructure

The cell nucleation and growth basically depends upon genera-tion of thermodynamic instability either by means of tempera-ture increase or by pressure quench. Thermodynamic instabil-ity causes the state of super saturation resulting in the end ofsingle phase homogeneous polymer/nitrogen mixture state,leading to cell nucleation. After metering, the suckback createssudden reduction in pressure which also leads to cell nucleationinside the barrel itself. The effect of suckback volume can beseen in Fig. 12 and 13. The cell size increases with suckbackvolume (accompanied by decrease in cell density). Further,the cell size homogeneity decreases with suckback volume.This is due to merger of cells during shear flow inside the moldcavity and runner system. Nucleation of cell inside the barrelmay cause the problem of sprue sticking. In some cases due toinsufficient back pressure, gas pressure (inside the barrel) maycause unwanted suckback.

4 Conclusions

In the present experimental study following observations weremade, which can lead to optimization of microcellular injec-tion molding process,1. Increase in the injection rate is always associated with de-

crease in the cell diameter.2. Higher back pressure increases the solubility of gas into the

polymer matrix resulting in smaller cell size and high celldensity.

3. Effect of melt temperature on the cell morphology is the neteffect of two qualitatively opposite factors i. e. solubility ofgas and viscosity of polymer melt. It is observed that initialincrease in melt temperature causes increase in cell diam-eter, as the viscosity of polymer melt is the key factor, how-ever, further increase in the melt temperature is associatedwith a decrease in cell size because the solubility of gas isin the governing position at higher melt temperature.

4. It is also observed that barrel residence time of gas – poly-mer solution has significant effect on cell structure. Thebarrel residence time upto 180 seconds causes increase incell density and decrease in cell diameter, after 180 secondsa reverse trend is observed due to possible leakage of hotgas from the system.

5. Suck back (pressure quench) after plasticization causes nu-cleation of cell inside the barrel itself and it was observedthat cell diameter increases with the suck back volume andit should be maintained at lower values for more uniformcellular structures.




Fig. 11. Effect of barrel residence time on microcell diameter anddensity

Fig. 12. SEM micrographs of fractured sur-face on the molded tensile test specimen forsuck back (A) 5 cm3, (B) 10 cm3, (C) 15 cm3

and (D) 20 cm3, melt temperature: 215 8C, in-jection rate: 115 cm3/s, back pressure:120 bar, screw speed: 80 mm/s, hold pressure:80 bar, hold time: 5 s, V-P switchover: volumespecific (8 cm3), shot size: 55 cm3, Sc-N2 pres-sure: 30 bar

A) B)

C) D)

References

Goel, S. K., Beckman, E. J., “Generation of Microcellular PolymericFoams Using Supercritical Carbon Dioxide: Effect of Pressure andTemperature on Nucleation”, Polym. Eng. Sci., 34, 1137–1147(1994)DOI:10.1002/pen.760341407

Jacobsen, K., Pierick, D., “Using Thermally Insulated Polymer Filmfor Mold Temperature Control to Improve Surface Quality of Micro-cellular Injection Molded Parts”, SPE ANTEC Tech. Papers, 1929 –19?? (2000)

Kramschuster, A., et al., “Quantitative Study of Shrinkage and War-page Behavior for Microcellular and Conventional Injection Mold-ing”, Polym. Eng. Sci., 45, 1408 –140? (2005)DOI:10.1002/pen.20410

Kumar, V., Suh, N. P., “A Process for Making Microcellular Thermo-plastic Parts”, Polym. Eng. Sci., 30, 1332–1339 (1990)DOI:10.1002/pen.760302010

Okamoto, K. T., Microcellular Processing, 1st Edition, Hanser Publish-er, Munich (2003)

Yuan, M., et al., “Microcellular Nanocomposite Injection MoldingProcess”, SPE ANTEC Tech. Papers, 691–695 (2003)

Date received: January 12, 2009Date accepted: August 24, 2009

BibliographyDOI 10.3139/217.2263Intern. Polymer ProcessingXXIV (2009) 5; page &–&ª Carl Hanser Verlag GmbH & Co. KGISSN 0930-777X

You will find the article and additional material by enter-ing the document number IIPPPP22226633 on our website atwww.polymer-process.com




Fig. 13. Effect of suck back volume on microcell diameter and density

optimization of micro cellular injection molding parameters

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