Gear wheels, made from POM (left) alJd

PA66, impose complex mechalJical alJd

tribological requiremelJts

.

AlternlltingI~< .

Controls Parts PropertiesProcess Optimization. Aetivetemperature-eontrol systems employing alternate

heating and eooling phases ean optimize not only the rheologieal eharaeteristies of

the proeess but also the morphology and henee the properties of molded parts

overall - without lengthening the eyele time. This has been eonfirmed by reeent

injeetion molding studies on gear wheels.

DIETMAR DRUMMERKARLHEINZ GRUBER

STEVEMEISTER

Dynamic, cydic or variothermmold-temperature contral is atechnology for boosting the quali-

ty of injection molded parts that is in-creasingly attracting the attention of plas-tics processors [1,2]. Essentially, the moldwall is heated prior to injection - this hasa beneficial effect on the resultant mor-

phology in the part, particularly in theboundary layer.In the ideal case, the tem-perature is raised ne ar to or above thecrystallization temperature of the poly-mer. Targeted morphology formation ofthe polymer benefits both the mechani-cal [3] and the tribological properties [4].

Dynamic Process Control

Mold-filling at elevated wall temperaturesconfers a number of advantages on theprocess, the economics, as well as the sur-

Translatedfram Kunststoffe4/2011,pp.46-49Article as PDF-File at www.kunststoffe-

international.com;DocumentNumber:PE110736

Kunststoffeinternational 4/2011

face finish and strength of the moldedpart. It. exploits the fact that non-cooled or

heated molds are easier to fill with hotmelt than well-cooled molds,

. reduces the required injection press ureand damping force,. reduces shear stress on the melt,

. maintains the holding pressure forlonger, even in the areas furthest fromthe gate,

. reduces residual stresses in injection-compression molding of optical parts,

. promotes surface effects, such as self-deaning and anti-reflection, the repli-cation of micro- and nano-structures,and the praduction of very smoothsurfaces and high-gloss finish es with a"piano finish",

. ensures more homogeneous orienta-tion of glass fibers in technical parts,

. lengthens the welding time for meltfronts, reduces weId lines, and

. lowers the risk of shrinking-inducedwarpage and improves the dimension-al accuracy and stability of moldedparts.

In variotherm temperature control, themold is not intensively cooled to the req-uisite demolding temperature until after

the cavity has been filled. As a result, thequality of molded parts can be increased,without the cyde time being extended.The variotherm method is easiest to real-

ize with an alternating temperature-con-trol system, which passes hot and cold flu-ids alternately through the existing cool-ing channels.

Process control in injection molding notonly influences the aforementioned rheo-logical effects, it also affects the internalproperties of a plastic part and hence itsmechanical properties. Thus, in the case ofsemi-crystalline polymers, a higher moldtemperature leads to greater rigidity andstrength, due to the increasing degree ofcrystallization, associated with reducedelongation at break. Evidence of this hasbeen pravided by experiments on moldmaterials of different thermal conductiv-

ity [5]. In contrast, marked temperaturegradients perpendicular to the flow direc-tion lessen the degree of crystallizationwithin the material and therefore promoteinhomogeneities, anisotropy and warping.

Conflicts Resolved

Controlling the temperature in the moldalso allows the tribological properties of ~

m

---. INJECTIONMOLDING

--- -- --- -

--- - - -- -

apart to be influenced in a tar-

geted manner (Fig.1, left). Thisfact is of particular interest forresilient machine parts, suchas thermoplastic gear wheels.The best-possible materialproperties in this caseare ob-tained by ahigh mold temper-ature. The process economics,though, require the shortest-possible cyde times.

Systematic studies at the In-

stitute ofPolymer Technology

QJL-E~QJc..EQJI-

-- -

Cycletime

QJL-E'"L-QJc..EQJI-

Heat Cool

Time Time

Fig.1.Lowthermalconductive moldmaterials improvemany part properties (left), but prolong the cycle time

(center); active alternating temperature control is more economical (right); the example shows the temperature

gradient in the cavity during an injection molding cycle.-

--- -- --- - --- --(LKT) at the University of Erlangen-Nuremberg, Germany, have shown thatvariotherm temperature control resolvesthe physical contradiction of the need forhigh temperatures during injection forgood morphology formation in theboundary area of the part and the needfor low temperatures for short cyde times.

In a proving mold for polyoxymethyl-ene (POM) gearwheelsmtle picture),thecompact cavity inserts are thermally in-sulated from the master mold by insulat-ing platens. The mold is maintained at aconstant temperature for the purpose ofprocess stability, and only the tempera-ture of the cavity inserts (Fig. 2) is activelycontrolled. The cavity inserts were builtup layer by layer from a steel powder us-ing laser cusing, based on the design da-

taoThe combination of insulationfrom the master mold

cp

-- -Fig. 2. The mold for

gear wheels has

conformal coolingchannels

'I

....

and conformal cooling channels con-duces to particularly rapid temperaturechanges in the cavity. Sensors near thecavity wall detect the prevailing temper-ature conditions directly, enabling theprocess to be specifically controlled, par-ticularlythe synchronization of the injec-tion pracess with the temperature-con-trol phases.

Injection at 1500(,Demolding at sOo(

For tempering the mold a variothermtemperature contral system (type: STWS200; manufacturer: Single Tem-periertechnik GmbH, Fig.3) was used. Thesystem employs water as the circulatingfluid and has a heating and a cooling cir-cuit with a hydraulic circuit -switching de-vice. The system has two separate circuits

in which water is held at dif-

ferent feed temperaturesand can actively providealternating heating andcooling (Fig.4).As a result,

~ it can effect an 80°Cchange in temperature

in the mold inserts withinlOs. The machine tells thevalve controller whether coldor warm fluid is to be con-

veyed into the mold circuit.For the type of POM used

(type: Hostaform C9021,

I f.

I 1 1'.' .

.

"

,,

'..~r .,,"t"

Fig. 3. The STWS 200

alternating temperature

control system can

effect heating and

cooling changes of

more than 100 oe

manufacturer: Ticona GmbH), this high-ly dynamic temperature control allowsthemold temperature to be brought to thelevel of the crystallization temperature:with the wall at a temperature of 150°C,the POM melt is injected into the cavity.The high wall temperature avoids the for-mation of boundary areas of low crys-tallinity or which are quasi-amorphous.At the same time, a homogeneous struc-ture can form throughout the part. Afterfilling and during the holding pressurephase, the part is cooled to below 80°C,which is appropriate for demolding thisparticular material. As the heating andcooling phases take approximately thesame time, the heating phase begins at theend of the cooling phase of the previouscyde (Fig.1. right). The cyde time is thusmuch shorter than is the case for coolingofless heat-conductive mold materials.

Improvements in Morphology

The cavity temperature significantly af-fects morphology formation. For techni-cally sophisticated parts, a cavity temper-ature of 100°C is common. The only wayto avoid the formation of a boundary lay-er that has a different microstructure is

to raise the temperature to 140oe. The re-sultant longer cooling time doubles thecyde time for the gear wheel under review(Fig.5). With active alternating tempera-ture technology, however, the part can bedemolded after an overall cyde time ofless than 20 S.

Differential scanning calorimetrymeasurements (DSC) confirm the uni-formity of crystallization across the thick-ness of the gear tooth. Thus, gears man-ufactured with the aid of variotherm tem-

perature contral have greater crystallini-ty, especially in the tribologically stressedboundary layer, than gears injectionmolded at a mold temperature of 100oe.The much narrower melting peak alsoconfirms the more homogeneous size dis-tribution of the crystalline structures.

@ GarlHanserVerlag,Munich Kunststoffe international 4/2011

Mold ValveunitTemperature-control system

Mold ValveunitTemperature-control system

Fig. 4. Alternating temperature contral systems cause hot and cold fluid flow to flow

alternately through the temperature contra I circuit

Summary and Outlook

Targeted temperature control allows theproperty potential of semi-crystallinematerials to be better exploited, optimizesand homogenizes the part's microstruc-ture and prevents the formation of weakboundary layers. Unlike alternative ap-proaches, variotherm temperature con-trol of injection molds or targeted vario-therm temperature control of thermallyinsulated mold inserts conduces to a sub-

stantial reduction in cyde time.Variotherm temperature control yields

parts that exhibit improved internal andexternal properties and increases fidelityof reproduction of the injection moldingprocess. It offers the potential to exerttighter control over crystallization duringprocessing and actively utilize the specific

Lehrstuhl für Kunststofftechnik

UQiversitätMange,,-Nörnberg

D-91058ErlangenI GerJnany

jaitJ19 9131 85,.29100

,~ www.!kt,iiUili~!!!&~tiall;~en,d~,

Single TempenertechnikGmbtl

0-73269 HOChdorf I GerlQanyTEL +491153 3009-0

.~ www..sfng:l~!!ft~mpiida

Kunststoffeinternational 4/2011

temperature-time relationships of nude-ation and crystal growth through the useof precise temperature control. Especiallyin the case of gear wheels, with their tighttolerances and high service loads, it is pos-sible to improve part properties, such asthe fidelity of reproduction with regard tomeshing quality, mechanical characteris-tics such as tooth root strength, and tribo-logical properties such as tooth wear.

The influence of variotherm tempera-ture control on other target parametersin optical and technical parts, such asresidual stresses and associated fluid re-

sistance, surface properties and opticalproperties should be studied. The bene-fits which variotherm temperature con-trol offers injection mol ding processes asregards parts performance is by no meansexhausted. .

INJECTION MOLDING .

ACKNOWLEDGMENTS

Thesestudiesat the LKTwere supportedbythe fol-

lowing industrialpartners:. EngelAustria GmbH,Schwertberg,Austria,. HofmannInnovationGroupGmbH,Lichtenfels,

Germany

. OechslerAG,Ansbach,Germany. DurothermPlasticsGmbH,Haiterbach,Germany

. SimarGmbH,VaihingenjEnz,Germanyand

. Ticona GmbH, Kelsterbach, Germany,

asweil asthe GermanResearchFoundationas part

of ResearchGroup702.

REFERENCES

1 Schinköthe,w.; Walther,T: ReducingCycle

Times.Kunststoffeplast europe90 (2000)5,

pp. 17-19

2 Giessauf,J.; Pillwein, G; Steinbichler,G.:

VariothermTemperatureControlls Fitfor Produc-

tion. Kunststoffeinternational98 (2008)6,

pp.57-62

3 Lurz,A.; Kühnert,1.;Schmachtenberg,E.:Influ-

encesonthe propertiesof small andthin-walled

injectionmoldedparts- Part2:Importanceofthethermal conductivityof the moldmaterial.WAK

Journal of PlasticsTechnology4 (2008)1

4 Dallner,C.M.;Feulner,RW; Kobes,M. 0.;

Schmachtenberg,E: Einflussder Morphologieauf

die tribologischenEigenschaftenmontage-

spritzgegossenerMikrogetriebe.TribologyConfer-

ence,Göttingen2007

5 Jungmeier,A., Ehrenstein,G.w.; Drummer,D.:

New Aspectsof Process-InducedPropertiesof

Micro InjectionMolded Parts.Plastics,Rubber

andComposites:MacromolecularEngineering

39 (2010)7

THE AUTHORS

PROF.DR.-ING.DIETMARDRUMMER,born in

1971,is headof the Institute of PolymerTechnology

(LKT)at the Universityof Erlangen-Nuremberg,Ger-

many;drummer@lkt.uni-erlangen.deDIPL.-ING.KARLHEINZGRUBER,born in 1964,is

ManagingDirectorof SingleTemperiertechnikGmbH,

Hochdorf;kh.gruber@single-temp.de

DIPL.-WIRTSCH.-ING.STEVEMEISTER,born in

1982,is a scientific assistantat the LKT;

meister@lkt.uni-erlangen.de

Fig. 5. Alternating

temperature control

improves the mor-

phology of the gear

wheel, without

lengthening the cycletime

(photos:LKT/Single)

11