Christopherus Bader, Erwin König and Christoph Schmidt

Quality from the Depth of the Mold Process Control

6/2013Volume 103 www.kunststoffe-international.com

Magazine for Plastics

SPECIALPLASTICS IN ELECTRONICS

from page 15

ExtrusionTechnical Products

under Flexible ProductionConditions 41

Injection MoldingIncreased Performance

also Makes Energy-EfficientTools Appealing 34

© Carl Hanser Verlag, München. 2013. All rights including reprinting, photographic reproduction and

translation reserved by the publishers.

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PRIAMUS SYSTEM TECHNOLOGIES AGRheinweg 4CH-8201 Schaffhausen / Switzerland

Tel. +41 (0)52 632 2626Fax +41 (0)52 632 2627info@priamus.comwww.priamus.com

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Quality from the Depth of the Mold

Process Control. To reproduce an injection molded part in the same quality, it is

not enough to use the machine settings as basis, since even an optimized process

changes continually. However, different part properties can be modified and con-

trolled as desired directly from the cavity. This means that processors no longer

need to elaborately test these features after the injection molding process.

Twin-cavity hot-runner moldof a cylinder housing forclutch slave cylinders. A pressure sensor and atemperature sensor areincorporated into both cavi-ties (figures: Priamus)

I N J E C T I ON MOLD ING

CHRISTOPHERUS BADERERWIN KÖNIG

CHRISTOPH SCHMIDT

All self-respecting injection moldershave full confidence in their ma-chine, just continuing to produce

while ignoring the high reject rates thatmake downstream testing of the finalmoldings necessary in the first place. Thetypical injection molding shop, therefore,still has a department that laboriouslyseparates the acceptable parts from theunacceptable.

Cavity pressure sensors have thereforebeen used directly in the mold cavity formany years to give feedback about partquality indirectly, for example from themaximum pressure. That is a step in theright direction, but it is by no means ad-

equate to describe all the quality features.For some time, cavity temperature sen-sors have been gaining in importance.They automatically detect the melt frontand use it for balancing hot-runner moldsor for automatic opening and closing thenozzles [1].

It is therefore all the more surprisingthat, in 2013, there are still machine sup-pliers who insist they can manage allquality problems solely via the machinesettings. Even with two cavities, this rais-es the question of how the machine cancompensate flow differences betweenone cavity and the next. And who canexplain why the standards for determin-ing the capability indexes recommendtesting during dry running – i. e. with-out a mold?

A New Dimension

It is not enough just to use one sensorto determine the flow of the melt in the

mold – and therefore its viscosity. Themelt must flow for a defined path in or-der for the measurement instruments todetect its shear rate and shear stress. To-day, a process developed by Priamus Sys-tem Technologies AG, Schaffhausen,Switzerland, can be used to automati-cally determine the flow properties ofpolymer melts during production, mon-itor them and control them to a desiredtarget value [2]. The cavity pressure sen-sor and cavity temperature sensor arepositioned in the flow direction and theincreases in the signals are evaluated asrequired.

Priamus goes one step further, addi-tionally controlling the strength and di-mensional accuracy of the molded partsby adjusting the melt and mold temper-atures, in addition to the injection andholding pressure profiles in each produc-tion cycle, depending on the process [3].What appears very laborious and compli-cated in theory is in practice quite easy to

Translated from Kunststoffe 6/2013, pp. 46–50Article as PDF-File at www.kunststoffe-international.com; Document Number: PE111347

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implement with the Fillcontrol systemfrom Priamus. But what actually motivat-ed processors not only to monitor theprocess directly at the part, but also tospecifically control particular quality pa-rameters? There is no simple answer, andit will differ depending on the industryand application.

For manufacturers of medical prod-ucts, the complicated and somewhat du-bious process validation procedures,particularly in the USA, are an expen-sive and not always effective quality as-surance method. Characteristics andproperties that have little to do withproduct quality are often validated.Those concerned are aware of this; how-ever there are no alternatives and it isdifficult to break with the ingrained tra-ditions of Total Quality Managementand Six Sigma. A system that allowsprocessors to simplify the validationprocess appropriately at the touch of abutton, however, changes this situationfundamentally.

In other sectors, such as the automo-tive supply industry, there are genuinequality criteria that have to be satisfied.Manufacturers must eliminate, for exam-ple, the risk of an airbag failing to openat the critical moment. In this case, thepractical benefit of a control system liesquite simply in controlling and reproduc-ing the quality feature reliably to the de-sired point.

Another aspect that concerns manycompanies in an age of globalization isthe question of how product quality canbe ensured at different production sites.Since what use is a process that has beenoptimized in Stuttgart,Germany,but can-not be so efficiently reproduced in Roma-nia, Spain or Malaysia?

Confidence Is Good, Control Is Better

Whereas process monitoring and hot-runner balancing systems have been insuccessful use for many years, the qual-ity controllers of the Fillcontrol systemcan be explained in detail with referenceto two examples. The Austrian automo-tive supplier Magna Auteca from Krot-tendorf/Weiz produces over 275 millionplastic parts per year, mainly adjustmentmotors for side mirrors, on over 30 in-jection molding machines. It has beenusing Priamus systems since 2005 forautomatically balancing hot-runner sys-tems, ensuring that all the parts are al-ways filled simultaneously. In the pro-duction of the base plate of these adjust-

ment motors in a typical eight-cavityhot-runner mold (Fig. 1), the diameter ofthe injection moldings is a critical qual-ity parameter. This dimension is cur-rently carefully controlled in a down-stream operation after the parts havebeen demolded, which is to be eliminat-ed for cost reasons.

What is required, therefore, is not aconstant machine setting but a constantprocess. A lengthy series of experimentswas therefore carried out to investigatethe quality controllers of the Fillcontrolsystem and characterize this parameter’sdependency on the different machine set-tings. After the cavities have been bal-anced in each individual cycle, the diam-eters of the eight base plates can be selec-tively changed solely with the aid of theshear rate controller (Fig. 2). This is doneby changing the injection profile of themachine proportionally until the set tar-get value of the shear rate in the cavity hasbeen reached. Automatic control of thisdiameter within the required tolerancesis thus possible and subsequent measure-ment is not required.

The System Finds the Right Way Itself

A similar series of tests was performed atEnsinger GmbH in Rottenburg amNeckar, Germany. The tests were per-formed on a cylindrical housing for

clutch slave cylinders (Fig. 3) producedfrom a HAT-PPA filled with 50 % glassfibers (grade: Grivory HTV-5H1 black9205; manufacturer: EMS-Chemie AG,Domat/Ems, Switzerland). To consistent-ly meet the high requirements on theburst strength, surface roughness, di-mensional constancy and absence offlash, the quality fluctuations of the plas-tic melt must be compensated. A cavitypressure sensor and cavity temperaturesensor were therefore incorporated intoboth cavities of the hot-runner mold inorder to determine the melt flow in theform of shear rate and shear stress, cyclefor cycle directly in the mold (Fig. 4). Theviscosity is determined automaticallyfrom the data at the point when the meltreaches the respective sensor position.The pressure sensor is additionally usedto control the compression, the temper-ature sensor to control the shrinkage andfor automatic switchover to holding pres-sure.

To determine the influence of the fourcontrollers on the different quality pa-rameters, parts were first injectionmolded and evaluated under the opti-mized process conditions. Then the con-trol parameters (injection profile, hold-ing pressure profile, melt temperatureand mold temperature in dependenceon the cavity pressure) were deliberate-ly altered to create unacceptable partsoutside the quality tolerances. The Fill-

Fig. 1. Eight-cavityhot-runner mold formanufacturing baseplates for adjustmentmotors of side mir-rors. For automatichot-runner balancinga temperature sensoris incorporated intoeach cavity, and apressure sensor isadditionally incorpo-rated into one cavityfor quality control

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control system eventually corrected theinjection molding machine settings,based solely on measurements in themold, until the originally optimized pa-rameters were reached again (Fig. 5). Theeffects of the controllers can be recog-nized by means of the flash height (Fig. 6),with the change explained by the differ-ent flow properties or viscosities of theplastic melt.

Summary

A constant machine setting is by nomeans sufficient for constant part quali-ty. The studies have demonstrated thatdifferent quality parameters of the mold-ed part can be automatically regulated toa desired target value, avoiding the needfor expensive downstream controls. Theprerequisite for this is a cavity pressure

Fig. 3. The production of this cylinder housing of a PPA-GF50 makes strictrequirements on the burst strength, surface roughness, dimensional con-stancy and absence of flash

Fig. 4. The positions of the cavity pressure and cavity temperature sensorin the cylinder housing. The prerequisite for automatic process control isthat the pressure sensor is always positioned before the temperature sen-sor in the melt flow

Part dimension and deviation from nominal value

42.10mm

42.0041.9541.9041.8541.8041.7541.7041.6541.60

0.24

0.14

0.04

-0.06

-0.16

-0.26

Balancing on

mmShear rate50 [1/s]

Shear stress139,125 [Pa]

Viscosity2,776 [Pa·s]

Compression263 [bar]

Shrinkage78 °C @ 5 bar1

Part

rad

ius

Dev

iatio

n

2 3 4 5Cavities

6 7 8

Actual valueDeviation

v

T

p

pT

42.10mm

42.0041.9541.9041.8541.8041.7541.7041.6541.60

0.24

0.14

0.04

-0.06

-0.16

-0.26

Balancing on

mmShear rate150 [1/s]

Shear stress199,351 [Pa]

Viscosity1,330 [Pa·s]

Compression270 [bar]

Shrinkage77.8 °C @ 5 bar1

Part

rad

ius

Dev

iatio

n

2 3 4 5Cavities

6 7 8

Actual valueDeviation

v

T

p

pT

42.10mm

42.0041.9541.9041.8541.8041.7541.7041.6541.60

0.24

0.14

0.04

-0.06

-0.16

-0.26

Balancing on

mmShear rate251 [1/s]

Shear stress228,552 [Pa]

Viscosity912 [Pa·s]

Compression282 [bar]

Shrinkage78.7 °C @ 5 bar1

Part

rad

ius

Dev

iatio

n

2 3 4 5Cavities

6 7 8

Actual valueDeviation

v

T

p

pT

Fig. 2. The diametersof the eight baseplates dependent ondifferent shear rates,measured directly inthe cavity. The shearrates are deliberatelychanged via the in-jection profiles. Inthis case, the processwas already balancedbefore the shear ratewas controlled

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sensor and a cavity temperature sensor,with a host computer interface for com-munication with the injection moldingmachine.

The future will show to what extent val-idation processes, too, can be simplifiedand made more meaningful with thisprocess. It is already foreseeable that it will

be possible to transfer processes opti-mized in this way from one machine toanother and from one production site toanother.�

Fig. 6. The flash height of the injection molded parts depends directly on the flow behavior of the melt, and therefore on its viscosity. The Fillcontrol sys-tem determines and regulates, e.g., the viscosity of the melt in the form of shear rate and shear stress, resulting in the flash height of the parts. In the fig-ure at the right-hand side, significantly higher flash can be seen because of a lower melt viscosity

0.08

mm

0.06

0.05

0.04

0.03

0.02Production

settingsSettingschanged

Controlled byFillcontrol

Flash height

Cavity 1Cavity 2

14

µm

10

8

6

4

2Production

settingsSettingschanged

Controlled byFillcontrol

Surface roughness Rz 6

Cavity 1Cavity 2

109.0

108.5

108.0

107.5

107.0

106.5

g

Productionsettings

Settingschanged

Controlled byFillcontrol

Part weight

Cavity 1Cavity 2

230

bar

220

215

210

205

200

195

190

185Production

settingsSettingschanged

Controlled byFillcontrol

Burst pressure

Cavity 1Cavity 2

Fig. 5. Effect of the four controller parameters (injection profile, holding pressure profile, melt temperature and mold temperature in dependence on thecavity pressure) on the various quality parameters. They were first determined under optimized process conditions and then deliberately changed. Thesystem eventually automatically corrected the parameters back to the optimized values

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© Carl Hanser Verlag, Munich Kunststoffe international 6/2013

ACKNOWLEDGEMENTThe authors would like to thank – Günter Bauernhofer, process manager of the plas-

tics injection molding shop at Magna Auteca, andthe employees Josef Koller and Andreas Klein-burger for their support with the trials,

– Reimar Olderog, injection molding departmenthead, and Dr. Dirk Weydandt, production managerfor injection molding at Ensinger, for providing thesystem and the capacities for performing the trials.

REFERENCES1 Bader, C.; Zeller, S.: The Discovery of the Melt

Front. Kunststoffe international 100 (2010) 6, pp.27–31

2 Bader, C.; König, E.: Taking Control. Kunststoffe in-

ternational 102 (2012) 6, pp. 39–433 Bader, C.; Kristiansen, P.M.: Hitting the Nail on the

Head. Kunststoffe international 101 (2011) 6,pp. 6–11

THE AUTHORSDIPL.-ING. (FH) CHRISTOPHERUS BADER, born in

1962, has been managing director of Priamus SystemTechnologies AG, Schaffhausen, Switzerland, since2001.

DIPL.-ING. (FH) ERWIN KÖNIG, born in 1964, hasbeen managing director of Priamus System Technolo-gies GmbH, Salach, Germany, since 2001.

CHRISTOPH SCHMIDT, born in 1971, as plasticstechnologist is responsible for process optimization atEnsinger GmbH in Rottenburg-Ergenzingen, Germany.

SPECIAL REPRINT from Kunststoffe internat. 6/2013

© Carl Hanser Verlag,Munich. 2013. All rights includingreprinting, photo-graphic reproductionand translation reserved by the pu-blishers.

6/2013Volume 103 www.kunststoffe-international.com

Magazine for Plastics

SPECIALPLASTICS IN ELECTRONICS

from page 15

ExtrusionTechnical Products

under Flexible ProductionConditions 41

Injection MoldingIncreased Performance

also Makes Energy-EfficientTools Appealing 34

www.kunststoffe-international.com