CO2-GAIM. Fluid-assisted

injection technology is an

established method for

producing complex hollow

parts that cannot be imple-

mented with conventional

cores. The choice of fluid

has a significant impact on

part quality and cycle time,

and the quality of the inner

surface. What is not so well

known is that carbon diox-

ide, acting as injection

medium, offers huge potential to reduce part costs and energy input and to im-

prove part quality – without additional effort.

MARCEL OP DE LAAK ET AL.

Fluid-assisted injection technology(FIT) is an umbrella term for injec-tion molding methods in which an

injection fluid displaces a liquid polymerfrom the core of a molded part. Hobson[1] in a patent dating back to 1939 de-scribed the ways in which this can hap-pen. This basic idea spawned severalmethods in the 1980s that can be classi-fied by the manner in which the melt isdisplaced. The most important are:� the short shot process,� the full shot or spill-over process,� the pushback process, and� the floating core process.

The various types of melt displacementaside, experiments were also conductedon different injection fluids in the late1990s that led to water-assisted (WAIM)and the gas-water-assisted injection tech-nique (TiK-WIT process) [2, 3].Water it-self makes an extremely attractive injec-tion fluid for high-volume production ofparts, primarily because of its great cool-ing action and its low cost. However, wa-ter has drawbacks for the process, becausethe parts need to be drained or dried andbecause leaks can give rise to serious dam-age and costs.

This article will now present anotherfluid, which is also well known but hasnot yet established itself for fluid-assist-ed injection, namely carbon dioxide(CO2). It is the physical properties of CO2,which are rather atypical of gases and willbe discussed later, that make this gas so

interesting for the gas-assisted injectiontechnique (GAIM).

Benefits of CO2 Gas-assistedInjection

A comparison of which fluid is suitablefor which parts reveals that water and CO2are equally favorable in terms of energyinput for fluid-assisted injection and at-tainable cooling or cycle times (Table 1). Inaddition, the GAIM process proceedsmore or less as readily with CO2 as it doeswith nitrogen (N2).

A look at the physical properties of CO2clearly shows its benefits [4]. Its densitystarts to increase markedly only from150 bar on, approaching that of water asthe pressure continues to rise (Fig. 1). An-other interesting aspect is that CO2 canbe liquefied at room temperature simply

Untapped Potential in Gas-Assisted Injection Molding

Various handles made bythe gas-assisted injectiontechnique (photo: TiK)

2

I N J EC T I ON MOLD ING

© Carl Hanser Verlag, Munich Kunststoffe international 3/2013

Translated from Kunststoffe 2/2013Article as PDF-File at www.kunststoffe-international.com; Document Number: PE111284

Internet-PDF-Datei. Diese PDF Datei enthält das Recht zur unbeschränkten Intranet- und Internetnutzung, sowie zur Verbreitung über elektronische Verteiler. Eine Verbreitung in gedruckter Form ist mit dieser PDF-Datei nicht gestattet.

3

I N J EC T I ON MOLD ING

by raising the pressure to 60 bar. Nitro-gen, however, cannot be liquefied at roomtemperature, and is a liquid only at cryo-genic temperatures (-196°C at atmos-pheric pressure).

CO2 is usually delivered as a liquid incylinders, cylinder bundles or tanks. Inthis physical state, the heat capacity, cp, ofCO2 (3.0 kJ kg-1 K-1) is three quarters thatof water (4.178 kJ kg-1 K-1) and almostthree times that of nitrogen (1.04 kJ kg-1

K-1). Another physical advantage of CO2is the high heat of expansion which thegas extracts from its environment as thepressure falls. The totality of these phys-ical properties explains the huge coolingpotential of CO2, which can shorten thecooling time by half, compared with ni-trogen.

A practical advantage of CO2-GAIM isthat the gas has a very good cleaning ef-fect. Annular gap injectors virtually stopclogging up and can be thoroughlycleaned by simply purging with CO2 be-tween two production cycles. The processis more stable as a result because there isno danger that the injector will gradual-ly clog up.

Any assessment of the energy neededto compress the fluid must distinguish be-tween whether compression takes placein the liquid or the gaseous state. If theCO2 is in liquid form, the correspondingpumps have a very high flow rate and theamount of energy needed for pressuriz-ing is more than two orders of magnitudelower than that for compressing gaseousnitrogen. True, nitrogen can also be pres-surized in the liquid state, but only froma cryogenic liquid-gas tank, which slow-ly warms up over a long period and thusrequires large amounts to be drawn offcontinuously.

A Process Flow Which Reducesthe Cycle Time

Before the injector forces the gas into theinjection mold and thus into the part, itis pressurized in the liquid state effective-ly from prevailing pressure (60 bar) in thecylinder bundle compresses the gas in theliquid state effectively and with a favor-able energy input from 300 to 400 bar. Asit is being injected, the CO2 expands intothe melt, displaces the liquid polymer andso forms the cavity in the part. During thesubsequent holding pressure time, the re-maining melt heats the CO2 so that, as aresult of this intense heat absorption, thegas passes into the supercritical state(Fig 2).

At the end of the pressure-holdingtime, the gas is discharged from the part.The drop in pressure causes a sharp dropin the temperature of the CO2, and a greatdeal of heat is extracted from the part.This enormous cooling effect can addi-tionally be utilized to specifically coolhotspots in the injection mold. To suc-cessfully employ CO2- GAIM, processorsmust observe a few important process de-tails. These concern � choosing the right injector, and � avoiding changes in the cross-section

of the supply line from the GAIM in-stallation to the injector.

The choice of injector is important for en-suring that gas-assisted injection pro-ceeds smoothly and that discharge isguaranteed [5]. True, this is also the casefor nitrogen but, because CO2 has a muchhigher density at a given pressure, it takeslonger to release it than to release N2 ifthe annular gap injectors are too small.The line from the GAIM installation tothe injector must not have any changes ofcross-section because these would lead toexpansion of the CO2 and the formationof dry ice, which would clog the line.

If users heed these tips, they can im-plement CO2-GAIM on most existingGAIM-injection molds that have beenpreviously operated with N2. Thus, as thefollowing case reports illustrate, the cycletime of many existing GAIM processescan be shortened by an average of morethan 25 % merely by replacing the gas andplant engineering (Fig. 3) – a joint devel-opment by Maximator GmbH, Nord-

Temperature

1,000

kg/m3

800

700

600

500

400

300

200

100

0

300

51

101

151201

251301

bar401

1

60100

140180

220°C

Pressure

Den

sity

900–1,000

800–900

700–800

600–700

500–600

400–500

300–400

200–300

100–200

0–100

Density of water

Fig. 1. The density of CO2 as a function of pressure and temperature. CO2 can be liquefied at roomtemperature by raising the pressure to 60 bar; as the pressure rises, its density approaches that ofwater (source: Maximator)

© Kunststoffe

Temperature

CO2Solid

(dry ice)

CO2Supercritical

CO2Gaseous

Gas-assisted injection

Gas release

Sublimation point-78.5 °C/1.013 bar

Triple point-56.6 °C/5.2 bar

Critical point31.0 °C/73.8 barWorking range of

cylinders/bundles

Working range oftank supply

CO2Liquid

104

103

102

101

100

10-1

bar

Pres

sure

-100 -80 -60 -40 -20 0 20 40 80 150 200 250 °C 300

Fig. 2. The phase diagram for carbon dioxide shows the thermodynamics of the CO2gas-assisted injection technique where the gas is supplied from cylinders or cylinder bundles (source: Linde/Maximator/TiK)

© Kunststoffe

Kunststoffe international 3/2013 www.kunststoffe-international.com

Internet-PDF-Datei. Diese PDF Datei enthält das Recht zur unbeschränkten Intranet- und Internetnutzung, sowie zur Verbreitung über elektronische Verteiler. Eine Verbreitung in gedruckter Form ist mit dieser PDF-Datei nicht gestattet.

4

I N J EC T I ON MOLD ING

Kunststoffe international 3/2013 www.kunststoffe-international.com

hausen, and Linde AG, Munich, both inGermany.

Putting it Into Practice with Advantage

With the aid of the approach describedabove, i.e. simply by replacing the plantengineering and the injection gas, EngelFormenbau und Spritzguss GmbH, Sins-heim, Germany, shortened its cycle timefor manufacturing a refrigerator handlein the current series by 36 %. A thermalimaging camera is useful here because itenables the temperature distribution inthe part to be analyzed immediately afterthe handle has been automatically re-moved from the mold (Fig. 4). Direct com-parison shows that the handle producedwith carbon dioxide can be demolded ata much colder temperature than its coun-terpart produced with nitrogen. This notonly helps to shorten the cycle time, butalso improves the warpage behavior.

Similar success was achieved for a dip-stick guide tube produced by Gebr.Wielpütz GmbH & Co. KG, Hilden, Ger-many. The cycle time for this part wasshortened by 22 %. Made from a PA6.6GF30, the part has a very homogeneous(low) temperature of 75 to 80°C uponremoval, which is well below the normaldemolding temperature of 120 to 150°C(Fig 5). With this part, it is not the gaschannel which is crucial to shortening cy-cle times, but rather the machining stage,which the part needs following the injec-tion process.

Excessive shortening of the cycle timewould create hotspots at the thicker-walled areas of the holder and ribs. Thesehotspots are caused by small-sized slidersthat cannot be cooled with water. CO2serving as a fluid for gas-assisted injectioncould be used to effect spot cooling, with-out the need for further process equip-ment and requiring only a slight modifi-cation to the mold; this would shorten cy-

cle times even more, thereby increasingthe molded part quality even further.

Synergy Effects for ProductionCosts and Quality Optimization

Once a strategic decision has been takenin favor of CO2 as gas injection fluid, fur-ther technical ways of lowering produc-tion costs and improving part qualityemerge:� spot cooling of injection molds,� dynamic mold temperature control,

and � the cleaning of injection molds and

plastic surfaces (as pretreatment forpainting).

In spot cooling (Fig. 6), liquid CO2 is passedthrough a flexible capillary tube, 1.6 mmwide at most, to that spot on the moldingwhich needs cooling. There, the CO2 ex-pands into a chamber, which needs to beprovided in the cavity,and effectively coolsthe region around the expansion cham-ber [6, 7]. A combination of CO2-GAIMand spot cooling can be used to supply theCO2 gas to the hotspot and cool it in thesame mold while the gas is being dis-charged in the GAIM process.

A further application for CO2 lies inthe extremely fast dynamic cavity tem-perature control of injection molds andmold inserts (Fig. 7). This process, devel-oped jointly by Linde AG, gwk Wärme

Fig. 3. The newly developed CO2-GAIM module installation with integrated liquid booster is suitable forCO2 and N2 applications (photo: Maximator)

Fig. 4. The cycle time for producing this refrigerator handle was shortened by more than a third. Thisthermal image, captured immediately after the process, shows the higher temperature of a handleproduced by N2 (left) compared with that for CO2-GAIM (right) (photos: Linde)

Process Easy tohandle Fluid costsInvestment

in plantInvestment in

moldMaterial

costs

Process energyfor fluid-assisted

injectionInjector size Cycle time

CO2-GAIM ++ � 75 100 100 1 + 50

CO2-GAIM with flushing + – 75 105 100 3 � 40

GAIM ++ � 100 100 100 100 + 100

GAIM with flushing + – 100 105 100 300 � 80

TiK-WIT � + 150 120 130 1 – 50

FAIM cool � � 120 100 100 200 + 95

Table 1. Comparison of some fluid-assisted injection processes with different fluids. The percentages are expressed in relation to the nitrogen-GAIMprocess (= 100 %) (source: TiK)

Internet-PDF-Datei. Diese PDF Datei enthält das Recht zur unbeschränkten Intranet- und Internetnutzung, sowie zur Verbreitung über elektronische Verteiler. Eine Verbreitung in gedruckter Form ist mit dieser PDF-Datei nicht gestattet.

5

I N J EC T I ON MOLD ING

© Carl Hanser Verlag, Munich Kunststoffe international 3/2013

Kältetechnik mbH, Kierspe, Germany,and Iserlohn Kunststoff-TechnologieGmbH, was presented at Fakuma 2012.In it, CO2 is used to effect both heatingand cooling in the same mold insert. Toheat the cavity, gaseous CO2 is heated bymeans of a compressor and a turbo-

heater and passed through the confor-mal heating-cooling channels of themold insert. The gas is transported in aclosed loop. The mold insert is cooledagain by feeding liquid CO2 into thesame heating-cooling channels and re-laxing it. The cooling effect is similar to

the aforementioned spot cooling. Withthis technology, temperature gradientsof up to 20 K s-1 can be achieved forheating and cooling.

Conclusion

Using carbon dioxide as a fluid for thegas-assisted injection technique andslightly modifying the plant engineeringis a simple way for injection moldingshops to shorten cycle times and lowerpart costs. Since the process can be han-dled just as simply as the conversion fromnitrogen to CO2,CO2-GAIM can improveboth current and future GAIM applica-tions. The cleaning effect of the CO2 onthe injectors stabilizes the productionprocesses in the long term. In addition,CO2 offers an effective and inexpensiveway to solve temperature control tasks ininjection molding.�

REFERENCES1 PS-US 2331688: Method and apparatus for

making hollow articles of plastic material (1939)Hobson, R.

2 DE 103 39 859 B3: Verfahren und Vorrichtung zurHerstellung eines Kunststoff-Bauteils, welcheseinen Innenhohlraum hat (2003) Op de Laak, M.

3 Op de Laak, M.; Rupprecht, V.: Die Qual der Wahl.Kunststoffe 96 (2006) 9, pp 115-120

4 A.N.Other: Eigenschaften der Kohlensäure.Fachverband Kohlensäure-Industrie e.V., 1997

5 Eyerer, P.; Elsner, P.; Knoblauch-Xander, M.; vonRiewl, A.: Gasinjektionstechnik. Hanser Verlag,Munich 2003

6 A.N.Other: Technisches Handbuch Toolvac, Fir-menschrift der AGA Gas GmbH, 1995

7 Berghoff, M.: Kühlen kritischer Bereiche imWerkzeug mittels CO2-Temperierung. Diplomar-beit, Märkische Fachhochschule Iserlohn 1999

THE AUTHORSDIPL.-ING. MARCEL OP DE LAAK, born in 1970, is

a managing director of TiK-Technologie in KunststoffGmbH, Teningen, Germany; marcel.opdelaak@tik-center.de

DIPL.-ING. AXEL ZSCHAU, born in 1965, is a man-aging director of TiK-Technologie in Kunststoff GmbH,Teningen; axel.zschau@tik-center.de

DIPL.-ING. ANDREAS PRALLER, born in 1966,works for the Linde Gas division in Unterschleißheim,Germany, where he is project leader with responsibil-ity for developing and launching gas-based technolo-gies for the plastics-processing industry;andreas.praller@linde-gas.com

DIPL.-ING. (FH) MADS RASMUSSEN, born in1977, works in the Sinsheim technical office of Maxi-mator GmbH, where he provides plastics processorswith support on technologies for gas and water injec-tion technology and physical foaming;mrasmussen@maximator.de

Molded part

Capillarytube

CO2 CO2

CO2

Expansionchamber

Fig. 6. Schematicstructure of an injec-tion mold with spotcooling [6, 7]. The CO2cools the regionaround the expansionchamber effectively(figure: Linde)

© Kunststoffe

Fig. 5. At the end of the cycle, the temperature of a dipstick guide tube made from PA6.6 GF30 byCO2-GAIM is much lower than the normal demolding temperature for such materials, as shownhere by the thermal image captured immediately in the 2-cavity mold after the process (photos: Linde)

Fig. 7. The installa-tion for dynamic moldtemperature controlcombines rapid heat-ing and coolingchanges at tempera-ture gradients of upto 20 K s-1 (photo: gwk)

Internet-PDF-Datei. Diese PDF Datei enthält das Recht zur unbeschränkten Intranet- und Internetnutzung, sowie zur Verbreitung über elektronische Verteiler. Eine Verbreitung in gedruckter Form ist mit dieser PDF-Datei nicht gestattet.