Akkerman, Remko

Lamers, E.A.D.

Wijskamp, Sebastiaan

English

University of Twente Research Information

IntroductionWarping of rubber pressed composite panelsS. Wijskamp, E.A.D. Lamers, R. AkkermanUniversity of TwenteTwente Institute of MechanicsP.O. Box 217, 7500 AE Enschede, The Netherlandsphone +31-(0)53-4892426, e-mail s.wijskamp@ctw.utwente.nlThe rubber pressing process is applied to producethermoplastic composite parts. The preconso-lidated laminate is reheated to its meltingtemperature in an infrared oven, then quicklytransported to the press where it rapidly cools underhigh pressure between the steel and rubber tool.A problem that occurs is that intended flat partsbecome curved after release. Normally, this so-called is assumed to be caused mainly bythe non-symmetrical cooling and resulting gradientin morphology and shrinkage during pressing.Here, it is believed that the non-symmetricalmechanical loading contributes considerably to theproduct distortions.warpingFigure 1 Warping of a woven fabric glass fibre reinforcedthermoplastic composite stiffenerObjectivesThe objective is to predict the warping of compositepanels in order to optimise the pressing processand the shape of the tool such that trial-and-errorcosts are reduced.MethodologyFor higher temperatures of the steel tool, thecooling was measured to be increasinglysymmetrical. The heat transfer of the panelspressed on the PTFE coated tool was as good asidentical on both sides. However, considerablewarping occurred.ResultsDiscussionThe non-symmetrical cooling is not the sole causefor the warping of rubber pressed thermoplasticcomposites. The different boundary conditions onthe steel and rubber tool side seem to induce alarge part of the distortions. Currently, FiniteElements analysis is applied to include the effectsof mechanical contact, visco-elastic materialbehaviour and assumed non-uniform loading dueto edge effects.Figure 3 Measured curvature of rubber pressed panels forvarious processing conditionsFigure 2 Measured temperature profile through the thickness ofa composite panel; the thermocouple on the steel side failedBefore, a one-dimensional coupled thermo-mechanical analysis including the shrinkagestresses due to non-symmetrical cooling wasproposed. Now, the temperature profile duringforming has been monitored by insertingthermocouples in rectangular flat panels. Thetemperature of the steel tool was varied. A testseries was performed with a thin PTFE foil on thesteel tool to analyse the influence on the coolingprofile. The resulting curvature of the panels afterrubber pressing has been measured with acoordinate measuring device.22.22.42.62.833.23.43.645 100 150 200 150 PTFEtool temperature (°C)curvature(1/m)1502002503003500.00E+00 2.40E-04 4.80E-04 7.20E-04 9.60E-04thickness (m)temperature(°C)0.5 s1.0 s1.5 s2.0 s0.25 s

Warping of rubber pressed composite panels

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