TY - GEN

T1 - Potential for Cure Time Reduction in Rubber Injection Molding and its Analytical Prediction

AU - Friesenbichler, Walter

AU - Perko, Leonhard

PY - 2015/6/9

Y1 - 2015/6/9

N2 - In rubber injection moulding reduction of cure time is a long-term topic. Due to high part thicknesses in therange of several centimetres the cure time in most cases reaches amounts to several minutes. Therefore notonly the chosen curing temperature should be reached very fast but also a homogeneous crosslinking degreemust be achieved over the whole part volume. Reduction of cure time by means of shear heating isstate-of-the-art. Devices at the injection molding machine which generates coupled shear and elongationalflow are in practical use but up to now available software tools for injection molding simulation neglect theeffect of elongational heating.In [1] as engineering approach a new viscous model for the prediction of temperature changes in rubbercompounds flowing through conical dies and runner segments was deduced and experimentally validated. Theanalytical calculation program developed takes into account shear as well as elongational flow and allows aprecalculation of the temperature increase with accuracy of 4 %. As material data input shear and elongationalviscosity has to be measured. For SBR and NBR compounds it was shown that more than 80 % of thetemperature increase measured is caused by elongational flow.Systematic use of elongational flow generating dies in the injection mould (Figure 1) allows considerabledissipative heating [1]. For highly viscoelastic SBR and NBR compounds a temperature increase by 45 °C toscarcely 70 °C could be reached (Figure 2). Conditions precedent are conical dies with a half die angle ofhigher than 45° and high injection speed. This type of dissipative heating preferably appears around the centreline of the flow, and in combination with shear heating near to the wall a more homogeneous temperatureprofile at a higher temperature level is possible.Figure 1: Injection mould with elongational flow Figure 2: Measured and calculated bulk forgenerating dies tested temperature increase for NBRThe potential for reduction of cure time can easily be estimated in combination with a self-developedcalculator for cure time. For rubber parts with thicknesses higher than 4 mm the potential ranges from 5 % upto more than 30 % depending on mould temperature and wall thickness. By far the highest contribution toheating is caused by elongation as the polymer mass flows into a die. This was found to account for approx. 80percent of heat introduced into a NBR compound, compared to just 10 percent caused by shear at the diewalls.It is recommended to implement elongational flow generating dies in the machine nozzle area or in caseof cold runner systems directly at the gate. The level of reachable dissipative energy depends on the diegeometry of the maximum injection pressure available.This project was financially supported the Austrian Research Funding Agency FFG and the company partnersMaplan GmbH, Semperit technische Produkte GmbH and Erwin Mach Gummitechnik GmbH.

AB - In rubber injection moulding reduction of cure time is a long-term topic. Due to high part thicknesses in therange of several centimetres the cure time in most cases reaches amounts to several minutes. Therefore notonly the chosen curing temperature should be reached very fast but also a homogeneous crosslinking degreemust be achieved over the whole part volume. Reduction of cure time by means of shear heating isstate-of-the-art. Devices at the injection molding machine which generates coupled shear and elongationalflow are in practical use but up to now available software tools for injection molding simulation neglect theeffect of elongational heating.In [1] as engineering approach a new viscous model for the prediction of temperature changes in rubbercompounds flowing through conical dies and runner segments was deduced and experimentally validated. Theanalytical calculation program developed takes into account shear as well as elongational flow and allows aprecalculation of the temperature increase with accuracy of 4 %. As material data input shear and elongationalviscosity has to be measured. For SBR and NBR compounds it was shown that more than 80 % of thetemperature increase measured is caused by elongational flow.Systematic use of elongational flow generating dies in the injection mould (Figure 1) allows considerabledissipative heating [1]. For highly viscoelastic SBR and NBR compounds a temperature increase by 45 °C toscarcely 70 °C could be reached (Figure 2). Conditions precedent are conical dies with a half die angle ofhigher than 45° and high injection speed. This type of dissipative heating preferably appears around the centreline of the flow, and in combination with shear heating near to the wall a more homogeneous temperatureprofile at a higher temperature level is possible.Figure 1: Injection mould with elongational flow Figure 2: Measured and calculated bulk forgenerating dies tested temperature increase for NBRThe potential for reduction of cure time can easily be estimated in combination with a self-developedcalculator for cure time. For rubber parts with thicknesses higher than 4 mm the potential ranges from 5 % upto more than 30 % depending on mould temperature and wall thickness. By far the highest contribution toheating is caused by elongation as the polymer mass flows into a die. This was found to account for approx. 80percent of heat introduced into a NBR compound, compared to just 10 percent caused by shear at the diewalls.It is recommended to implement elongational flow generating dies in the machine nozzle area or in caseof cold runner systems directly at the gate. The level of reachable dissipative energy depends on the diegeometry of the maximum injection pressure available.This project was financially supported the Austrian Research Funding Agency FFG and the company partnersMaplan GmbH, Semperit technische Produkte GmbH and Erwin Mach Gummitechnik GmbH.

UR - http://www.pps-31.com/

M3 - Conference contribution

BT - Proceedings of the PPS-31

T2 - PPS-31 31st International Conference of the POLYMER PROCESSING SOCIETY

Y2 - 7 June 2015 through 11 June 2015

ER -