Develop and produce a panel module for an air-conditioning system which meets the cost requirements and high quality specifications
Replace the conventional cold-runner manifold originally planned with a hot-runner system with small cold-runner sub-manifold.
Reduces the material consumption by 12.45g per shot; this is equivalent to a cost saving of around EUR 60,000 over a production time of six years.
In January 2007, a customer from the automotive industry contracted Schuster Kunststofftechnik GmbH to develop and produce a panel module for an air-conditioning system by IML. To allow it to meet the required price and high quality specifications, the process parameters, in particular the cycle time, gating system and the special requirements for processing décor films in the mold, must be specially matched to one another. For this, the IML specialist commissioned SimpaTec Simulation & Technology Consulting GmbH, a consulting engineer specializing in process and part optimization in the plastics industry, to model the entire injection molding process in a simulation.
Simulating the process from filling to warpage
From its own practical knowledge, Schuster realized that it would be necessary to precisely model the mechanical and thermal properties of the PC film in the simulation. Consequently, SimpaTec ran through the process using Moldex3D, the software from Taiwan for injection molding simulation, which has a successful track record over many years in German automotive industry. It is capable of simulating the entire process from filling to warpage.
Although Moldex3D also permits 2.5D simulation, SimpaTec practically only performs complete 3-D simulations in practice. That allows very reliable warpage analyses to be performed. The quality of the program is shown particularly in the high-quality mesh capabilities. Besides tetrahedrons, hexahedrons, prismatic and pyramidal elements can also be used. The mold, too, enters three-dimensionally into the simulation. This is particularly important for new developments in mold technology, such as conformal cooling.
The first task is to replace the conventional cold-runner manifold originally planned with a hot-runner system with small cold-runner sub-manifold. The simulation is solved by placing the hot-runner nozzles as close as possible to the cavity such that only a cold-runner such-manifold of 1.05g in total remains. Compared to the original cold-runner weight of 13.50g, this reduces the material consumption by 12.45g per shot; this is equivalent to a cost saving of around EUR 60,000 over a production time of six years. The precise gate geometry and flow aids are determined by the software. That allows the mold to be completely filled with a pressure reduced by 130 bar. That has a beneficial effect on the retaining of structure within the film at cores around which the melt flows. At the calculated ends of the flow path, particular attention is paid to venting.
What is the influence of warping?
A special challenge in the design of IML articles is the part warpage. The simulations for determining the resulting warpage show a surprising result: the warpage is largely independent of the gating system and process parameters. The geometry of the part and the asymmetric structure resulting from the film echnology generates significant warpage. The part becomes convex towards the visible side.
This behavior, in which the part folds slightly in the center, occurs because of the high volumetric shrinkage in the thick-walled regions of the domes, and because the stiffness of the base plate is reduced by cut-out sections. This phenomenon can only be compensated by making geometrical changes, not by relocating ht gating point.
In a detailed discussion among the project managers, the causes of warpage are analyzed and improvement suggestions worked out. All aspects of the process are completely simulated in Moldex3D. The 3-D simulation of the film thus makes the effects of isolation clearly visible. This property is indispensable for reliable simulation of the IML process. Ultimately, it is the temperature distribution in the boundary layer between the film and part that dominates the warpage. The thermal conditions in the mold can also be three-dimensionally reproduced by the software for efficiently tailoring the cooling system to the IML process. Based on these results, Schuster Kunststoffechnik implemented a more effective cooling circuit for the interior regions of the part-after mold production, such as change is extremely expensive, since it can usually only be realized by partial rebuilding of mold with relocation of the slides and ejectors.
Since the warpage depends to a large extend on the part geometry, the company informed the customer. The search for alternatives proved difficult, since the geometric conditions cannot be fundamentally changed because of the planned light reflection properties, and because of the design of the operating unit. The customer decided on a compromise solution: all the functional dimensions affected by warpage are investigated according to their tolerance with respect to adjacent parts. As a result of the analysis, openings and centerings could be redesigned before the start of mold manufacture.
After completion of the mold, the process parameters of the simulation are first imported. This step considerably reduces the sampling process. The simulated operating points are confirmed in practice to a good approximation, as is shown by the filling pressure comparison.
The simulated warpage agrees well, both qualitatively and quantitatively, with reality. The customer, who was initially surprised by the extent of the predicted warpage, is now relieved that this fact could be taken into account during mold design. This also includes the optimization of the snap connection to the housings, as a result of which warpage is now minimized during installation. In this manner, the project could be completed well within schedule.
The project flow confirms that 3-D simulation can identify the main problem areas in the manufacturing process. The realistic simulation of the films used in the IML process is new-it allows critical questions about process-oriented mold design to be answered. Without this aid, the warpage is particularly difficult for processors to predict. This allows the severe challenges of the automotive industry-quality and costs, freedom of design, attractive appearance and faithfulness to schedules. The applications demonstrated the versatility of complete 3-D injection molding simulation, and its potential for future part development, including for special injection molding processes.
* This article was appeared in Kunststoffe international 2007/11, Page 60-62