Fachpublikationen

Multicycle Simulation: Closer to Reality

Once the entire mold can be modeled in the simulation, it no longer makes sense just to simulate one injection molding cycle. The potential of process-oriented simulation is rather to use the simulation of multiple sequential injection molding cycles to determine the thermal-rheological state of the mold during actual production. To achieve these production conditions, heating of the mold through multiple complete cycles (mold closing, injection, cooling, mold opening, demolding, mold closing …) is simulated, and the energy input and removal of the different components is simulated appropriately.

This makes it possible to determine, for example, how many starting cycles are used for the production start of a mold, or after a production interruption, in order to reach a steady state again – a simple approach for minimizing rejects. This method also illustrates the actual performance of the chosen cooling system, which can now be assessed in terms of cost efficiency, article quality and energy efficiency, as is shown by the following practical example.

Practical Design of the Cooling System

A typical housing application of PA66-GF50 (Fig.1, left) illustrates the potential of process-oriented injection molding simulation with the selection of a cooling concept. The result of the simulation of a standard cooling concept is shown by the cutaway view of the entire mold, and, in colors, by the local temperature distribution at the end of the filling phase in the 22nd production cycle. The heat exchange between the article, mold components and cooling system can be clearly seen, as can the temperature profile in the mold resulting from the transient heat transfer (Fig.1, right).

After approval of the part design, the development team was confronted with the question of whether a standard cooling system is sufficient for efficient cooling of the part, or whether conformal cooling improves the cycle time, part quality and energy efficiency. There are currently many conformal cooling systems on the market, and, without process-oriented injection molding simulation, it is difficult to quantify the advantages in advance.

In a first step, two alternative cooling concepts were therefore drafted and compared (Fig.2). Process simulation with Sigmasoft was carried out to analyze and evaluate the performance of the respective mold design, and provide facts to support the decision for the most suitable cooling system.

To assess the cooling systems, 15 successive injection molding cycles for the two models were simulated. A comparison of the temperatures at the stationary mold half shortly before the end of the filling phase makes it clear that the mandrel-shaped mold core in the foreground is only poorly cooled by the simple cooling concept (Fig.3, left). Here, the temperatures at the core surface are between 130 °C and about 142 °C, which prolongs the solidification time at these points, and therefore the cycle time as a whole. The solution with conformal cooling (Fig.3, right), on the other hand, shows a more homogeneous temperature distribution over the entire mold cavity with temperatures on the core surface of about 95 °C, which reduces the solidification time by up to 23 % (Fig.4).

The differences in the part solidification are illustrated by colored regions, which indicate hotspots in the part (Fig.5, top). At these points, the polymer stays fluid for longer than at the surface, and in particular longer than in adjacent part regions. In the comparison of the two results, the significantly more voluminous hotspots that occur in the mold with standard cooling are immediately conspicuous. With the simple cooling concept, liquid regions can actually still be found after 2s cooling time.

In this application, conformal cooling reduces the cycle time as well as the warpage (Fig.5, below). Although for a polymer reinforced with 50 % glass fibers, the fiber orientation clearly dominates the warpage, the differences in the mold temperatures still show an effect. More homogeneous cooling reduces internal tensions in the region of the core, and improves the warpage behavior at these points by up to 13 %.

Evaluation of the Energy Efficiency

Besides the cycle time and article quality, energy efficiency is also increasingly becoming a determining issue. If the heat balances of the two cooling concepts are compared, it can be read from the curves how much heat (in kW) per cycle is exchanged in the mold (Fig.6). The melt introduces energy into the mold; the mold platens lose energy to the surroundings, and the cooling systems are required to bring the entire energy balance into equilibrium by absorbing and emitting energy. Over time (multiple cycles), the absorbed energy becomes smaller. If this decrease stabilizes, the mold has reached the quasi-stationary thermal state, and production can begin.

The curve profile proves that the energy requirement for heating the mold with the simple cooling concept is greater than with conformal cooling. Conformal cooling achieves an asymptotic energy absorption at a comparatively early stage – after about eleven cycles – while simple cooling is still not stable even after 15 cycles. Conformal cooling thereby consumes less energy overall in this case, and production can start earlier.

Summary

Injection molding simulation is no longer only limited to the part development phase, but can also act as an effective aid in the subsequent development phases, and provide detailed information for supporting many decision-making processes during mold development, trial moldings, identification of process windows and troubleshooting during production. The application example clearly shows how conformal cooling has been quantitatively analyzed based on an integrated consideration of the results, and it can be assessed at an early stage whether the extra costs are worthwhile.