Injection Pressure Influences Flow Behavior
New injection molding applications such as filters or light-conducting elements frequently require very thin walls. In order to fill such molded parts, very high injection pressures are used. Injection units for thin-wall injection molding accelerate the screw up to 800 mm/s in 30 ms for the injection process while providing an injection pressure of 2400 bar and more. This high pressure has a significant effect on the viscosity of the polymer melt. On the one hand, a higher pressure reduces the free volume in the polymer melt; at the same time, it intensifies the interactions between the molecule chains. Both effects cause the viscosity to increase. In actual practice, however, the influence of injection pressure on viscosity is usually not taken into consideration. This often leads to mistakes in simulation of injection molding processes as well as in the design of injection molding machines.
Research at the Institute for Polymer Injection Molding and Process Automation of the Johannes Kepler University (JKU), Linz/Austria, is focusing on this phenomenon. The researchers learned that the poor availability of pressure-dependent viscosity data is the reason why the effect of pressure is hardly ever considered. Even simulation packages that allow the pressure dependence of the viscosity to be taken into account when calculating the injection process only rarely have access to pressure-dependent viscosity data in their often very extensive material databases.
For this reason, the pressure coefficient β was determined as a function of the shear rate with the aid of a high-pressure capillary rheometer during their investigations. How strongly the pressure affects the viscosity is illustrated by the data for HDPE and PP: If the pressure is increased from 50 to 1000 bar, the viscosity of the HDPE increases by about 92 % at a shear rate of 50 s-1. Although the effect of pressure on the melt viscosity decreases with increasing shear rate, the viscosity still increases by 27% at a shear rate of 5000 s-1.
The results for PP are even greater. In this case, the viscosity increases by 122 % at a shear rate of 50 s-1 when the pressure is increased from 50 to 1000 bar. Even at a high shear rate of 5000 s-1, the viscosity increases by 86 %. These results show that, under the conditions encountered during the injection molding process, the viscosity is affected by the existing pressure conditions and that ignoring this effect is not justified at all.
Article: " Giving Pressure Greater Weight ", Kunststoffe International 2015/08
Would you like to subscribe to our Newsletters on plastics technology and profit from the latest information?