Injection–Compression Molding of Glass-Fiber Filled Phenolic Molding Compounds

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  • Lach et al./Deformation Behavior of Nitrogen-Alloyed Austenitic Steels at High

    nounced with increasing hardness. The difference instrength between specimens 3 and 4 amounts to 200 MPa inFigure 4 and 500 MPa in Figure 5.

    A metallographic study on quasistatically compressed spe-cimens shows localized plastic deformation, the internal com-pression cone is distributed heterogeneously in the cross sec-tion.[8] Suppressed cross slipping of dislocations can be maderesponsible for this behavior. Quasistatic compression testsusing cold-drawn and strain-aged steel P900N (curve 2, Fig. 4and Fig. 5) result in a slightly decreased flow curve comparedto curve 1 in Fig. 4 and Fig. 5. Metallographic investigationshowed that plastic deformation is extremely localized in anarrow isothermal shear-band across the entire specimen.[8]

    The specimen rapidly looses its macroscopic cylindricalshape and the compression flow curve becomes irregular, be-cause in the equation to evaluate the test data, it is assumedthat specimens will stay cylindrical.

    Nitrogen-alloyed austenitic steels are very strain-rate sen-sitive materials. Under solution-annealed conditions the dy-namic compression strength is twice that of the quasistaticstrength. From 430 HV30 onwards, cold work-hardened spe-cimens tested dynamically show adiabatic shear-bands.Work-hardening at 400 C prior to testing increases the ducti-lity of the material. The plastic deformation is localized inheavily work-hardened samples.

    [1] G. Stein, J. Menzel, Stahl und Eisen 1992, 112, S47.[2] H. K. Feichtinger, HNS 93: Proc. 3rd Int.. Conf., Kiev, 1993

    (Ed.: Inst. Met. Phys.), Ukrainian Academy of Science.[3] M. O. Speidel, in: Ergebnisse der Werkstoff-Forschung,

    Band 4, (Eds: M. O. Speidel, P. J. Uggowitzer), ThubalKain, Zrich 1991.

    [4] N. Paulus, Dissertation, ETH Zrich 1994.[5] P. J. Uggowitzer, in: Ergebnisse der Werkstoff-Forschung,

    Band 4, (Eds: M. O. Speidel, P. J. Uggowitzer), ThubalKain, Zrich 1991.

    [6] P. J. Uggowitzer, Habilitationsschrift, ETH Zrich 1992.[7] E. Lach, M. Scharf, Y. Tschamber, Stickstofflegierte austeni-

    tische Sthle. Teil II, ISL-Report R 129/97, Saint-Louis 1997.[8] E. Lach, A. Bohmann, M. Scharf, Stickstofflegierte auste-

    nitische Sthle. Teil III, ISL-Report R 101/98, Saint-Louis1998.

    Received: July 10, 2000

    InjectionCompression Moldingof Glass-Fiber Filled PhenolicMolding Compounds**

    By Edmund Haberstroh,* Joachim Berthold, andTim Jntgen

    Short-glass-fiber reinforced phenolic molding compoundsrepresent one of the newest material classes among thermo-sets. In general, these materials have considerable potential,since they maintain high strength under exposure to hightemperatures and chemicals. Economically, these materialsare very interesting and suitable for substituting for metals.Glass-fiber reinforced phenolic molding compounds (PF-GF)have established themselves as a group within the class of en-gineering plastics. Typical areas of application are primarilyfound in the automotive industry in the engine compartment(thermostat housings, coolant pumps) and in the brake sys-tem (control and hydraulic pistons).[13]

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    752 ADVANCED ENGINEERING MATERIALS 2000, 2, No. 11

    Fig. 4. Quasistatic and dynamic compression tests, steel P900N cold drawn to429 HV30.

    Fig. 5. Quasistatic and dynamic compression tests, steel P900N, cold drawn to540 HV30.

    [*] Prof. E. Haberstroh, J. Berthold, T. Jntgen

    Institute of Plastics Processing (IKV)Aachen Universitiy of TechnologyPontstrasse 49, D-52056 Aachen (Germany)E-mail: zentrale@ikv.rwth-aachen.de

    [**] The investigations set out in this communication receivedfinancial support from the Ministry of Economics and Technol-ogy (BMWi) and from the AiF e.V., to whom we extend ourthanks. Furthermore, we thank the Battenfeld company forproviding an injection molding machine and the RogersCorporation for their intensive support and cooperation as wellas for providing the test materials.

    1438-1656/00/1111-0752 $ 17.50+.50/0

  • Haberstroh et al./InjectionCompression Molding of Glass-Fiber Filled Phenolic Molding Compounds

    To process this material so that its very good mechanicalproperties can be taken advantage of fully, the injectioncom-pression molding process (which has been a prevalent alter-native to injection molding in the field of thermoset process-ing for some time) is often used. Owing to the low pressuresduring injection into the partly unclosed mold, this processallows fiber-sparing and gives less anisotropy in the moldedparts from undesirable glass-fiber orientations. For injectioncompression molding, no thorough process investigations areyet available to demonstrate quantitatively the advantagesover injection molding, for both process parameters and me-chanical properties of the product. This was the objective ofthe investigations reported in this communication.

    In recent years, the properties demanded of plastic mold-ings have increased constantly. Increasingly, precision partsare requested. Therefore, processes have been improved andnew processes have been developed to meet the list of re-quirements. Injectioncompression molding is used in severalvariations in thermoset processing.[4] In principle, the processis a combination of injection molding and compression mold-ing, combining the advantages of both. Injection molding of-fers a high degree of automation and manifold intermeshingoptions for example, whereas compression molding producesevenly distributed material compression and fewer orienta-tions.[5]

    The injectioncompression molding process can be di-vided into two basic steps: the injection phase and the com-pression phase (Fig. 1). Owing to the counter pressure on themold clamping unit of the machine, widening of the mold isprevented during injection. In the second process step, themold is closed entirely and with well-defined speed after theexpiration of the compression delay time, forming and com-pressing the molded part within the cavity.

    Illustration of the entire injectioncompression moldingcycle by means of the measured process parameters is shownin Fig. 2. It can be seen that a holding pressure phase followsthe compression movement. An open gate is used to allow acertain backflow of the injected material through the gatingsystem during the pressure build-up in the mold due to themold closing movement. It is advantageous if the meteringaccuracy of the machine are not very high. The plasticizing ofthermosets is characterized by injecting variable small melt

    volumes in the space in front of the screw, because there canbe neither non-return valves nor shut-off nozzles.

    The hydraulic pressure required is determined by thescrew stroke during the injection phase. The diagram showsthat the pressure is nearly constant during the entire injectionphase. The cavity pressure transducer does not produce a sig-nal during the injection phase because the melt is pushedagainst the walls of the partly unclosed mold without signifi-cant pressure build-up. Figure 2 shows an increase in the cav-ity pressure signal after the expiration of the compression de-lay time during the compression phase. Then the cavitypressure decreases again when the material is redistributedin the mold. The relatively low cavity pressure at the end ofthe compression phase can be explained by the back flow ofexcessive material through the gating system towards the ma-chine nozzle. The holding pressure phase follows; this is nec-essary to compress the sprue cone. During the holding phasethere is an increase in both the hydraulic and the cavity pres-sures. The pressure in the mold is then slightly higher thanduring the compression phase. We note that in our apparatusthe cavity pressure transducer is located relatively close tothe injection point and so flow processes near the sprue arestill registered.

    When comparing the parameters that influence the proper-ties of the molded-part between injection molding and injec-tioncompression molding, the significant additional factorsare compression gap and compression delay time. We alsoconsidered the volumetric flow rate, since injection into thepartly unclosed mold may result in less material stress in in-jectioncompression molding than in injection molding. Thereasons why we chose to study these machine parameters areexplained in detail in the following, based on their expectedeffects on the process.

    The compression gap is the opening movement of themold before the start of the injection phase. The effects of var-iation in the compression gap are shown qualitatively in Fig-ure 3. The compression ratio is the quotient of the volume ofthe partly unclosed mold and the volume of the molded part.If the compression gap (and thus the compression ratio) is in-creased, a noticeably different material distribution results. Amelt cake is formed around the injection point, which isthicker than the subsequent molded-part thickness, and its

    ADVANCED ENGINEERING MATERIALS 2000, 2, No. 11 753

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    Fig. 1. Process steps of the injectioncompression molding process. Fig. 2. Process parameters during injectioncompression molding with open gate.

  • Haberstroh et al./InjectionCompression Molding of Glass-Fiber Filled Phenolic Molding Compounds

    surface area is significantly smaller than the subsequentmolded-part surface. In contrast to injection molding (Fig. 3,left) the material flows under little pressure in the cavity. Thedegree of orientation that is imposed on the material duringthe injection phase therefore decreases with increasing com-pression gap. Under the influence of gravity, the material inthe mold slumps more or less rapidly downwards, depend-ing on the mold temperature, and hence melt viscosity, dur-ing the compression delay time.

    The size of the compression gap has an upper limit due tothe length of the vertical flash faces. A gap that is too big re-sults in a smaller contact surface of the melt cake with the hotmold walls, which means that the material will be heatedmore slowly. This also affects the cycle time negatively. Thelower limit of the compression gap is given by the conver-gence with the injection molding process, since the hydraulicpressure advantage of injectioncompression molding de-creases with reduction in compression gap. The compressiongap is commonly approximately 3 mm.[6,7]

    The compression delay time is of great importance becauseof the complex flow-curing behavior of thermosets. Duringthe delay the melt cake in the cavity is heated by contact withthe hot mold walls and it becomes more fluid due to the de-crease in viscosity. To form the molded part with minimalpressure during the complete mold closing movement of thecompression phase, the material needs to flow easily. Thusmaterial stress during compression is low with a low viscosi-ty. If the delay time is too short, the material is not fluidenough. The upper limit of delay time depends on the occur-rence of cross-linking. If the delay time is too long, cross-linkswill have started to form in the melt cake, which are then bro-ken during compression. The resultant high stress and dam-age to the molecular structure can lead to local weak spotswithin the molded part. This circumstance is illustrated sche-matically in Figure 4.

    The volumetric flow rate has an influence on the orienta-tion of the molecules and fillers within the melt cake. Thehigher the flow rate, the higher the anisotropy of the material.Anisotropy causes an increase in strength in the direction ofthe preferred orientations, but also a decrease in mechanicalstrength in the transverse direction. Anisotropy is also impor-tant in the injection molding process. The objective is the

    production of molded parts with the most isotropic proper-ties possible.

    A greater influence of the volumetric flow rate on the pro-cess results from the heating effect of higher flow rates, whichmodifies the flow-curing behavior. If the material enters thecavity at a higher temperature, the viscosity is lower, and themelt cake may slump downwards more quickly. During thesubsequent compression phase, this leads to longer flow dis-tances and local pressures that vary with position in themold. In the extreme case of complete descent to the bottomof the cavity, the consequence will be increased flash forma-tion.

    The tests were carried out with two glass-fiber reinforcedphenolic molding compounds from the Rogers Corporation,Manchester (USA), which have different filler compositions(RX 613 and RX 625). We used an injection molding machine,made by Battenfeld GmbH, Meinerzhagen, type BA 1300/630 CDC, which includes conventional compression controland a core compression unit.

    The experimental mold was developed at IKV. We chose asimple two-dimensional geometry, which consisted of a rec-tangle with an adjoined semi-circle (Fig. 5), so that we couldreadily determine warpage and shrinkage of the parts as wellas mechanical properties. The molded part measured175 mm in length, 150 mm in width, and 4 mm in thickness(the thickness could be varied between 2 and 6 mm by usingsteel distance plates). On the stationary mold plate, smallcrosses with defined spacing were inscribed, to enable deter-mination of the the shrinkage of the molded parts by means

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    754 ADVANCED ENGINEERING MATERIALS 2000, 2, No. 11

    Fig. 3. Influence of the compression gap on the melt distribution.Fig. 4. Influence of the compression delay on the cross-linkage.

    Fig. 5. Sample geometry.

  • Haberstroh et al./InjectionCompression Molding of Glass-Fiber Filled Phenolic Molding Compounds

    of a video system and a CCD-camera. Inserts could be usedin the mold to generate shapes with a rib or a sudden changein wall thickness.

    For both materials, we carried out a three-stage test seriesbased on a statistical test design. We hoped to carry out math-ematical process modeling subsequently. We measured sam-ple weight, shrinkage at three different locations, warpage,and fracture stress in tension and bending. Then we com-pared the results for compression-injection molding with in-jection molding.

    Figure 6 illustrates how the weight of the molded part isinfluenced by the volumetric flow rate. Weight increases withflow rate, and is higher for the injectioncompression processat all flow rates, which can be attributed to the compressionof the molded parts. The compression in injectioncompres-sion molding is flat (two-dimensional) over the entire moldedpart geometry, whereas the compression in injection moldingoccurs mostly on one spot around the injection point duringthe holding pressure phase. The rise in sample weight withflow rate can be attributed to the consequent reduction of thebackflow.[8] For these results the molded parts were producedwith sprue position II (compare Fig. 5).

    However, the behavior illustrated in Figure 6 depends verymuch on the sprue position in injectioncompression mold-ing. If the injection is almost concentric (sprue position I), thesample weights for injection molding increase with flow rate,as before, but the samp...

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