24 © Carl Hanser Verlag, Munich Kunststoffe international 11/2013

I N J EC T I ON MOLD ING

ROBERT ENDLWEBER

RUTH MARKUT-KOHL

JOSEF GIESSAUF

GEORG STEINBICHLER

In the MuCell process, the melt, loadedwith a physical blowing agent, foamsafter injection into the mold cavity.

This allows the raw material consump-

tion and part weight to be reduced whileimproving the part properties in manycases [1–8]. Compared to unfoamed in-jection molding, the use of this processminimizes warpage and the probability ofsink marks.

There are additional advantages inprocess control, part design and machinedimensions. In general, smaller injectionmolding machines with low clampingforce can be used, since the MuCellprocess does not require holding pressurefrom the screw. Foaming causes an “in-

ternal” holding pressure [1], which gen-erates a homogenous pressure profile inthe entire part. The magnitude of thispressure can be adjusted within limits andis usually lower than in standard injectionmolding. Due to the lower opening forcesin the mold, the clamping force is ulti-mately lower.

There are also two less well-knownbenefits of MuCell. The lower viscosityof the plastic melt offers either new de-sign options, since the flow paths can belonger [1, 5, 6], or the possibility of re-

Small Cells with a Big Effect

MuCell Foam Injection Molding is already used in many applications and is

currently being boosted by the increasing importance of lightweight construction.

The license-fee-free technology offers many advantages over standard injection

molding, and its full potential is by no means exhausted in practice.

The instrument panelsupport of the VWGolf 7 is producedfrom a glass fiber-

filled PP (FibremodGE 277 Al from Bore-

alis) by MuCell on anEngel duo 2700 injec-tion molding machine

(figure: Borealis)

Translated from Kunststoffe 11/2013, pp. 36–40Article as PDF-File at www.kunststoffe-international. com; Document Number: PE111532

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ducing the melt temperature and there-by shortening the cycle time [1, 3]. Ex-ploiting this potential requires a thor-ough understanding of the MuCellprocess.

Gas Supply – Supercritical butQuite Simple

Physical foaming by means of MuCelltechnology is based on developmentsmade at the Massachusetts Institute ofTechnology (MIT), Boston, USA, and ismarketed by Trexel. Engel Austria GmbH,Schwertberg, Austria, offers systems forthe MuCell process from a single suppli-er under the name “Engel foammelt.”Theadvantage for the user is that foaming isalso integrated into the injection mold-ing machine control and can be con-trolled and monitored via the machinedisplay.

The gas (N2 or CO2) is added in a su-percritical state, i.e. as a supercritical flu-id (SCF) (box p. 27), and, for this purpose,transferred to the gas supply unit at apressure level of about 440 bar. By meansof pressure controllers 1 and 2 and a cal-ibrated mass flow element, a constantmass throughput is generated at a definedpressure difference (Fig. 1). The quantity ofthe throughput is the result of the desiredgas content in the plastic melt and thegassing time during metering. Once themass throughput has been set, it remainsconstant throughout the cycle.

The gas feed to the melt is performedby means of an injector. When the melt isnot being injected with gas, the injectoris closed, but the throttle valve is openedso that the gas returns via a bypass. In thisphase, the pressure controller 3 regulatesthe operating pressure that is directlypresent at the closed injector.

In order to introduce gas into the melt,the injector is opened until the desired gascontent has been reached (“gas introduc-tion”). In this phase, the throttle valve tothe bypass is closed. When the injector isopened, the operating pressure falls spon-taneously. In the process, the operatingpressure adjusts to match the melt pres-sure directly below the injector. The lat-ter is measured and called “pressure at in-jector 1.”

Generating a One-phaseGas/Polymer Solution

The MuCell process requires a specialplastication unit. The screw, with a typi-cal length/diameter ratio of 24 to 25, isconstructed from:

� a plastication zone (feed, compression,metering zone),

� a rear non-return valve,which preventsthe gas-loaded melt from passing intothe plastication zone and foamingthere,

� a mixing zone for homogenization ofthe supercritical fluid in the melt,

� a front non-return valve with shorterstroke.

The injector is mounted on the meltcylinder directly in front of the rear non-return valve (in the flow direction). Af-ter the supercritical fluid has entered themixing zone of the screw, it is finely dis-persed by shearing and mixing process-es and by diffusion in the melt. A single-phase supercritical fluid/polymer solu-tion is established at the latest in front ofthe screw tip. To maintain this state, the

melt must be kept under pressure dur-ing the entire cycle. This is made possi-ble, even with the safety gate open, bymeans of a patented solution by Engel[9]. An interruption of the “active backpressure control” would result in a pres-sure drop and therefore foaming in themelt cylinder.

The process can be analyzed into a se-quence of defined single steps (Fig. 2). Af-ter injection, the screw is at the front po-sition with both non-return valves closed.Then the metering process starts. Themelt is transported forwards, and as a re-sult the two non-return valves (A) open.After a constant pressure is established be-low the injector, the gas injection canstart. During opening of the injector, thedelivery pressure adjusts to the pressurein the melt cylinder. The supercritical

Fig. 1. The MuCell gas injection unit is integrated into the injection molding machine control system.The system flow diagram of the gas supply unit is shown in the machine display. This integrationsimplifies the application and increases the process reliability (figures: Engel)

Fig. 2. The workflow for the MuCell process, A: Start of metering; B: Start of gas injection; C: End of metering; D: Start of injection

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fluid is then transported at constantthroughput into the melt (B).

When the set metering stroke has beenreached, plastication is terminated. As nomore material is transported, the pressurein the plastication zone of the screw falls.The resulting pressure difference betweenthe mixing zone and metering zone clos-es the rear non-return valve (C). Depend-ing on the geometrical design, wear andthe process settings, leakage of the meltcan now occur at the non-return valve.Consequently, the pressure below the in-jector falls. To a certain extent, this pres-sure drop is normal and has no effect onthe process quality.

With the start of the injection process,the screw moves axially forwards. Due tothe injection pressure in front of thescrew, a pressure different builds up at thefront non-return valve, which, apart from

friction mechanism between its outer sur-face and the melt cylinder, also leads tothe non-return valve closing (D). In thecavity, a pressure drop initiates the foam-ing process. The typical integral structure

with a non-foam skin and foamed coredevelops [2].

Keeping the Entire Process in View

To monitor the MuCell process, the con-trol of the Engel injection molding ma-chines records the time variation of pa-rameters during the injection cycle. Theoperating pressure of the gas supply unitand pressure below the injector are par-ticularly significant here. These two pa-rameters provide information about thequality and reproducibility of the super-critical fluid mixing behavior, and aboutthe closing behavior of the front and rearnon-return valves (Fig. 3).

In the process data log, which recordsprocess characteristics for each cycle in-stead of the curve profiles, deviations ofthe operating pressure or metering time,for example, can provide information

about a change in the process. For exam-ple,viscosity changes of the material, fluc-tuations in material drying and leaks inthe gas injection system can be identified.To meet the quality requirements for

175

150

125

100

75

bar

Time

MeteringInjection

Pres

sure

Pressure in front of screw tipMelt pressure under the injectorSupercritical fluid operating pressure before the injector

1

423

Δp

Fig. 3. Interpretation of the pressure curves helps to define a process window for the optimum gas content. 1: Start of metering; 2: Reaching a constant melt pressure below the injector; 3: Drop of the delivery pressure by opening the injector; 4: Closing the injector, end of gas injec-tion. Δp marks a pressure drop after the rear not-return valve is closed, which has no effect on the process quality

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Nitrogen content

15

%

5

0

-5

-10

-150

Visc

osity

cha

nge

0.5 1.0 1.5 % mass 2.0

225 °C210 °C Fig. 4. The pressure

curves during injec-tion allow the viscos-ity change of thepolymer melt to bedetermined, in thiscase a polypropylenemelt as a function ofthe nitrogen contentat 225 and 210°C

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300

bar

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150

100

50

0Close

to gateRemote

from gate

Compact

Pres

sure

300

bar

200

150

100

50

0Close

to gate

1 % N2

Pres

sure

Remotefrom gate

Cavity pressure sensorclose to gate

Cavity pressure sensorremote from gate

Gate

Fig. 5. Comparison of cavity pressures measured close to and remote from the gate (after half the cooling time) for production of a non-foamed and a foamed laptop cover. The difference during compact injection molding is obvious

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modern production, an integrated pres-sure or system test offers the possibility ofascertaining and logging the perfect op-eration of the gas supply unit.

New Design Options and theEffect of the “Internal” HoldingPressure

The dissolved gas reduces the viscosity ofthe melt, as is shown by the example ofan unfilled polypropylene (Fig. 4). At225°C melt temperature and a gas con-tent of 1.6 %, the viscosity falls by 10 %– and consequently also the filling pres-sure. This effect can be utilized to fill partswith longer flow paths or lower wall thick-nesses. Alternatively, the melt tempera-ture can be lowered to reduce the cooling

time. Thus, the temperature of a meltcharged with 1.6 % gas can be reduced by15°C to reach the original viscosity of thenon-gassed melt.

The pressure gradients that inevitablyoccur in conventional injection moldingbetween the gate and end of the flow pathare often the cause of non-uniformshrinkage and therefore warpage in thepart. In the manufacture of laptop coversof unfilled polypropylene, in standard in-jection molding, on the one hand, differ-ent pressure levels were measured close toand remote from the gate. In the MuCellprocess, on the other hand, the holdingpressure does not first need to be trans-mitted via long flow paths, but is already“included” in the melt and acts uniform-ly over the entire part (Fig. 5). This circum-

stance can also be used, contrary to therule of thumb, to produce ribs of the samethickness as the part without sink marks.Elimination of the external holding pres-sure reduces the maximum cavity pres-sure, so that the clamping force in the in-

jection molding of the laptop coverscould be reduced by 35 %.

Determining the Optimum Gas Content

It has been confirmed in many examplesthat admixing a supercritical fluid obtainspositive effects. The question now arisesof whether these effects can be increasedever further by increasing the gas content.Figure 6 shows the maximum injection

Nitrogen content

775

750

725

700

675

650

bar

0

Inje

ctio

n pr

essu

re

0.5 1.0 1.5 2.0 % 2.5

Fig. 6. Injection pressure and foam structure as a function of gas content for unfilled PP. First, theinjection pressure falls with increasing gas content until a plateau is established

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Above a critical pressure and a critical tem-perature, gas is in the “supercritical state“.This is the case for nitrogen (N2) above34 bar and -147°C and carbon dioxide (CO2)above 71 bar and 31°C. In the supercriticalstate, the properties change. The fluidsshow a behavior that is characteristic ofboth gases and liquids. High density andhigh solvent power – typical of liquids –are coupled with low viscosity and high dif-fusion coefficients, commonly consideredfeatures of gases. This state of mattertherefore offers the ideal conditions forhomogenization and dissolution of thephysical blowing agent in the plastic melt.

Supercritical Fluids!

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pressure required in the production oflaptop covers with different gas contents.Additionally, the corresponding foamstructures across the cross-section of theparts are illustrated. First, the injectionpressure falls with increasing gas contentuntil a plateau is established at 1.6 %,which defines the maximum soluble gascontent.

The microsections make clear how thefoam structure is continually refined andtherefore improved up to a supercriticalfluid content of 1.6 %. The reason for thisis that, with the gas concentration, the nu-cleation rate also increases. That meansthat a large number of small cells devel-ops. However, a further increase in the gascontent leads to a sudden increase in de-fects caused by undissolved gas. SEManalyses make the difference in size be-tween a fine-celled foam structure andsuch defects visible (Fig. 7). The optimumgas content varies from application to ap-plication and can be experimentally de-termined (analogously to Fig. 6). Here,the best foam structure is validated bylight microscopy analyses.

Summary

In an age of lightweight construction,MuCell technology is by no means an un-common process any more, and is in-creasingly being used for innovative ap-plications. Foam injection molding of-fers advantages in process engineering,machine dimensioning and part design.Exploiting the full potential requires fun-damental knowledge about the process.For example, the gas content has a directinfluence on the foam structure, andtherefore on the part properties. The ide-al gas content for a particular applicationcan be defined by interpreting pressurecurves and micrographs of the foamstructure. �

REFERENCES

1 Altstädt, V.; Mantey, A.: Thermoplast-Schaum-spritzgiessen. Carl Hanser Verlag, München 2010

2 Kirschling, G.: Mikroschäume aus PolycarbonatHerstellung-Struktur-Eigenschaften. Dissertation,University of Kassel 2009

3 Egger, P.; Fischer, M.; Kirschling, H.; Bledzki, A.: AStatus Report (1): Versatility for Mass Production

in MuCell Injection Moulding. Kunststoffe interna-tional 95 (2005) 12, pp. 66–70, PE103448

4 Egger, P.; Fischer, M.; Kirschling, H.; Bledzki, A.: AStatus Report (2): Versatility for Mass Productionin MuCell Injection Moulding. Kunststoffe interna-tional 96 (2006) 1, pp. 72–76, PE103461

5 Steinbichler, G.; Egger, P.; Wörndle, R.; Spiegel, B.;Wurnitsch, C.: Economical Structural Foam Mould-ing for Customised Component Properties: Gas -Not just because of the Cell Structure. Kunststoffeinternational 91 (2001) 5, pp. 24–26

6 Steinbichler, G.; Kragl, J.; Pierick, D.; Jacobsen, K.:Spritzgiessen von Strukturschaum. Kunststoffe 89(1999) 9, pp. 50–54

7 Stange, J.: Einfluss rheologischer Eigenschaftenauf das Schäumverhalten von Polypropylenen un-terschiedlicher molekularer Struktur. Dissertation,University of Erlangen-Nürnberg 2006

8 Kühn-Gajdzik, J.: Amorphe und teilkristallineMikroschäume im Spritzgiessverfahren. Disserta-tion, University of Kassel 2011

9 Engel Austria GmbH, Schwertberg/AT (2005): AT409 359 B, DE 101 53 331 B4, US 6 811 730 B2.Spritzgiessverfahren, 23.06.2005

THE AUTHORS

DIPL.-ING. ROBERT ENDLWEBER, born in 1984, istechnology manager in process technology develop-ment at Engel Austria GmbH, Schwertberg, Austria,robert.endlweber@engel.at

DIPL.-ING. DR. TECHN. RUTH MARKUT-KOHL,born in 1975, is development engineer in the processtechnology development at Engel; ruth.markut-kohl@engel.at

DIPL.-ING. JOSEF GIESSAUF, born in 1968, headsthe process technology development department atEngel Austria GmbH, Schwertberg;josef.giessauf@engel.at

PROF. DR.-ING. GEORG STEINBICHLER, born in1955, is head of research and development at Engeland director of the Institute of Polymer InjectionMolding and Process Automation at Johannes KeplerUniversity, Linz, Austria; georg.steinbichler@jku.at

Fig. 7. If the gas content is increased after reaching the point with the optimum microfoam structure (left), defects occur – large circular pores are formed (right)

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