The injection molding cycle is comprised of a sequence of individual events. Typically, cooling time is the largest component in the overall cycle (Figure 1). A new operating paradigm, made possible with Vitrolite ®, significantly reduces cooling time, thus reducing overall cycle time (Figure 2).
Many injection molders have adopted a strategy of fast injection speeds because the process is stable and not subject to machine or polymer melt flow variations. Depending on the molder and application, the process is operated within a melt temperature range in which the viscosity of the polymer is low enough at the injection speed, to fill the part completely in the shortest time (Figure 3).
Addition of Vitrolite ® to plastics allows for a new operating paradigm – relatively low melt temperature and slow injection speed. A lower melt temperature is possible because the viscosity decrement from adding Vitrolite ® is greater at lower temperatures (Figure 4). The viscosity decrement due to Vitrolite ® increases with slower injection speed; however, a minimum injection speed is required to establish consistent part weight(Figure 4).
Temperature management includes more than obvious heat sources such as barrel, nozzle and hot runner heaters. In typical applications, a substantial amount of viscous heating is generated primarily during recovery. The degree of viscous heating during recovery is reduced by adjusting screw RPM and back pressure. Reduction in screw RPM is generally constrained by the requirement that recovery must be shorter than cooling time. However, Vitrolite ® is an excellent dispersant, so it is often possible to reduce or eliminate back pressure. This reduction in back pressure allows for the lowering of screw RPM, shortening of the recovery time and for more savings in the cooling time. Management of external heat and viscous heat optimizes cycle time and cooling time. This is possible due to the reduced plastic melt viscosity caused by the addition of Vitrolite ® (Figure 5).
Optimization for injection molding is an iterative process much like that shown in Figure 6. The principal difference between traditional approaches and the one used with Vitrolite ® is that individual process adjustments in the operation with Vitrolite ® may be counter-intuitive. For example, injection speed may be decreased with a consequent increase in fill time. However the slower injection speed may reduce melt viscosity, allowing a lower melt temperature and a shorter cooling cycle. The time saved in the cooling cycle will invariably be much larger than the very small increase in fill time, resulting in a shorter overall cycle time. The process is allowed to equilibrate between changes so that the effect of each change can be assessed. The process is adjusted so long as the increase in productivity is accompanied by acceptable part quality.
When a Vitrolite ® optimized machine is idled long enough to solidify the polymer, it may be necessary at re-start to heat to 20°F above the optimized temperature and then lower the temperature back to the optimized setting once production is resumed.
VitroCo has technical staff that will assist in establishing an optimal cycle and in training users to take advantage of the multiple benefits of Vitrolite ®. Adding Vitrolite ® reduces cycle time and improves productivity in injection molding.