Die casting Design Simulation

Transcription

1 Die casting Design Simulation

2 Introduction In the process of zinc alloy die casting, feeding the die is of crucial importance both for the quality, the strength and the finish of the product and for the life span of the die. The element that most influences the casting is the feeder duct that leads the alloy into the die. The simulation of filling and solidification phases may therefore predict possible defects caused by non-uniform liquid flow, high temperature gradient, local increase of velocity, and other issues. With this instrument it is possible to optimize the entire production with positive effect on the final product in terms of esthetics and efficiency and on the duration of die life. In this document we will analyze case studies in which simulation analysis has contributed to improve current production runs and avoid engineering mistakes that would have poorly impacted the finished product by generating waste of material, time and resources.

3 Magmasoft Magmasoft is the innovative software package that allows to simulate the die feed and the casting solidification, calculating the alloy flow, alloy temperature and the possible presence of gasses or porosity in the finished product. Thanks to the simulation it is possible to identify the best configuration for the feeders that convey the alloy into the die to achieve the best results in terms of precision and reliability, reduce risks to a minimum and achieve savings in time and resources. The software brings five distinct advantages that influence important decisions in the choice of technology for die casting components. 1. Esthetical quality 2. Scrap reduction 3. Mechanical characteristics 4. Maintenance and life cycle of the die 5. Optimization of set up parameters

4 CASE STUDIES Esthetical quality

5 1. Esthetical quality CASE STUDY AUTOMOTIVE Product: the thin plates in car vents that direct and control air flow in the car s passenger compartment (Audi) The vents for air flow control are usually plastic, but the object of our case-study is the central one, used by the driver to direct the other air ducts and hence made in alloy. This product poses challenges especially for the esthetical quality of the surface because of its exposed position in full view inside the passenger compartment compared to the other components, the high-end car on which the vent will be installed and possibly the criticalities of surface treatment after die casting. OBJECTIVE AND PHASES OF THE SIMULATION The simulation analysis will hence focus to engineer an optimal flow through the feeders to avoid defects such as flow marks and blisters in the product. In the first phase of the process we studied the effect of various runner configurations, one by one, to identify the runner with the best characteristics. Then we improved the shape of the runners, especially at the entry of the die. The objectives sought in the various phases of the simulations are filling consistency of the die according to a main flow direction, decrease of the probability of cavitation to increase the speed of flow, reducing the filling time and keeping the alloy at a high temperature during this phase and eventually the absence of air bubbles in the product.

6 RESULTS In the study of product surface quality, the main parameter to consider is temperature. The temperature must be maintained the highest and most consistent possible during and at the end of die filling, especially in the parts of the product that will be in full view. Top Bottom The images portray the analysis results at filling cycle end: the temperature is sufficiently high and homogeneous, with even better characteristics in the top surface which requires a higher level of quality being visible in the compartment of the car it will be installed in.

7 From the velocity field analysis it is possible to observe the feed direction and the speed modulus at the die entry point, a particularly critical point because often subject to cavitation phenomena, cause the die to wear. We notice filling consistency and that the last volumes to be filled are close to the overflows allowing the exit of the first alloy to arrive in the die, because the temperature will be lowest. The velocity field, depending other than on the runner geometry also on boundary conditions at die entry level, has been obtained by using process parameters common for die casting this kind of product. The speed pattern is sufficiently low even in the vicinity of the gates. This has a positive effect on manufacturing because it will allow to apply higher filling parameters without the risk of premature wear of the die.

8 The next field shows that the impact of the runners is well balanced: the two runners, identified by red and blue colorings, fill the die in a consistent manner. Proper functioning of the small runner on the left is confirmed: the presence of the orange color, as small as it is, is necessary to avoid flow marks. Reduced influence (orange liquid) but not zero: it s presence may be necessary in order to avoid flowmarks in the entry zone. And finally, the field that shows trapped air indicates a favorable situation: the air contained in the casting is not an issue because it distributed evenly across the volume and concentrated only at the sprues.

9 CASE STUDIES Scrap reduction

10 2. Scrap reduction CASE STUDY ENERGY MANAGEMENT AND AUTOMATION Product: support for emergency switch (product is in view but high esthetical quality is not required) The die has been in use for several years and has always generated 5% scrap due to the formation of bubbles during the painting process. The origin of this phenomenon is the presence of air in the alloy which is forced out of the product by the high temperatures that are reached during the painting process and stays trapped between product and paint layer. After Powder Coating Defects observed near the corner OBJECTIVE AND PHASES OF THE SIMULATION In this case the simulation focuses on avoiding bubbles in the postproduction phase. The objective of the simulation is to reduce the quantity of scrapped parts caused by entrapped air. We examine the behaviors of the flow during the filling phase to ascertain the causes of this phenomenon. Subsequently, alternative way of feeding are studied.

11 RESULTS The analysis shows reflux and air encapsulation areas in proximity of the edges where bubbles are later identified. Numerical Simulation of the current die configuration turbulence and vorticity Turbulence is caused by the speed of the flow and by the geometrical characteristics of the part. The solution that is depicted in the image foresees in alternative feeding, with the feeder ducts rotated by 45 around the axis of the product. 45 Current Configuration 2 nd Configuration

12 The results of the modification are shows consecutively. The reflux area has been fully eliminated and the filling flow results more uniform. 2 nd Configuration After the simulation, the die has been modified with results confirming the validity of the study: the quantity of rejects cause by bubbles reduced by approximately 95%.

13 CASE STUDIES Mechanical characteristics

14 3. Mechanical characteristics CASE STUDY CONSTRUCTION/HARDWARE PRODUCT: Door hinge. Mechanical component subject to mediumlow stress intensity. Contrary to other products, esthetical characteristics are not relevant, mechanical characteristics and resistance to wear are fundamental. OBJECTIVE AND PHASES OF THE SIMULATION For the engineering of this die, previous experience with the dies for products of similar shape and utilization was applied. The objective is to minimize the defects that may influence the mechanical characteristics of the part: porosity and holes. The part needs to be free of internal cavities and bubbles in the holes caused by air entrapment, it must be filled correctly, there must be no missing details and low shrinking porosity both internally and in proximity of the feeders. Experience with production of similar pieces has shown that particularly porosity is an extremely critical aspect, with the presence of holes and cavities due to the shrinkage of the part in proximity of the feeder where the alloy solidifies later and contracts while cooling. Incorrect cooling of the parts causes hot spots on the cavity surface of the die. The simulation analysis focuses mainly on the solidifying phase, with the objective of finding a configuration that allows homogeneous cooling of the part avoiding the creation of hotter areas.

15 RESULTS The study of the filling phase is focused on the identification of the field of entrapped air. As you may observe, the quantity of air is minimal and evenly distributed in small amounts across the part.

16 The porosity is analyzed observing the behavior of the part during the solidifying phase. Contrary to the configuration used in similar parts that were previously made, where the feeder duct is perpendicular to the axis of the holes, in this simulation the duct is orientated parallel to them and its geometrical configuration is optimized to avoid the creation of hot spots on the part s surface. As can be observed in the image, in the new configuration the surface close to the feeders solidifies much more rapidly with the subsequently diminished risk of material shrinkage porosity by alloy flowing back into the feeder ducts or into the part itself. 50% Liquid 60% Liquid In comparison with mold 1672A: the Junction repositioning has reduced the shrinking porosity issue The junction area solidifies fast (colored in light blue ), together with the rest of the product surface, lowering the surface shrinking porosity risk. The most critical area is the one right below the junction (red circle) where the cooling is much slower because of the proximity to the feeder. That shouldn't be a problem as the next picture (50% liquid) shows the above mentioned area is solid while the core of the part is still liquid (colored in yellow) Mold 1672A simulation image: the part volume near the casting ingate leads to a really delayed solidification and to shrinking porosity as a direct consequence. The results of the simulation have been confirmed during the utilization of the die: the problem of shrinkage porosity, typical for this type of product, has been fully eliminated.

17 CASE STUDIES Die maintenance

18 4. Die maintenance CASE STUDY an important AUTOMOTIVE Tire 1 for FCA PRODUCT: steering lock housing assembly (vehicles of Fiat Chrysler group) The part is the upper cover of the steering block assembly, containing an electronic card. Die wear is caused by the high speed of flow during the filling phase and by the specific geometry of the part. The consequences of poor die maintenance are rather severe and cause on the one hand the need for frequent maintenance work, on the other hand malfunctioning risk of the part caused by surface spurs as a result of die wear.

19 OBJECTIVE AND PHASES OF THE SIMULATION The objective of the simulation is to identify opportunities for slowing down or eliminating erosion on an existing die. Particularly, the analysis of alternative feeder solutions. Erosion is caused by the development and non-stationary implosion of alloy bubbles against the surface of the die. Due to high flow speeds through the narrow ducts, usually close to the feeder openings into the die, the pressure falls below the vapor pressure of the alloy, causing it to evaporate instantaneously. Subsequently, the alloy enters the die with a larger cross section than the feeders and the speed of flow reduces because of the resulting expansion. The pressure rises rapidly causing the implosion of vapor bubbles and freeing up of energy. This entire process is called cavitation and is the main cause of wear to the internal surface of the die.

20 RESULTS The filling velocity field represents the speed with which the flow initially enters inside the control volume. As cavitation is a non-stationary process which takes place in the first phase of filling, being the phase of highest instability, this field provides the highest opportunity for identifying the critical areas. As the images show, the new geometry positively influences the expansion of the flow, mitigating the velocity gradients along the green line. Internal part of the housing The area with largest cavitation is in the center of the field where the green line demarcates the separation between the two areas with different speeds. Exactly in this area, because the flow slows down and pressure increases, the alloy bubbles implode and damage the die. Old feeder New feeder With the new feeder duct the green line is moved generating a broadening of flow diffusion. This broadening should shift the cavitation area and reduce the risk of cavitation because of the lower speed gradient in the critical area of the die.

21 External part of the housing Vecchio Old feeder canale Nuovo canale New feeder On the fixed die as well the green line has moved, broadening the diffusion and diminishing the transversal speed gradient.

22 OBJECTIVES AND PHASES OF THE SIMULATION The objective of the simulation is to estimate the change in surface quality and cycle time at varying die temperatures. In reality, the die temperature depends on the geometry of the conditioning circuit (engineering phase) and on the temperature and nature of the coolant (production phase). In the simulation, a homogeneous die temperature was imposed in proximity of the cavity. Surface quality has been determined at the average value of the alloy temperature at the end of the filling phase whereas the production cycle has been extracted from the necessary time for the component to solidify and cool to a temperature of 340 C. The evolution of the two quantities has been calculated across a series of simulations in which the temperature was made variable.

23 RESULTS The following graph clearly indicates the results of the series of simulations. Strictly speaking this is not an optimization analysis because the two monotonously increasing curves do not allow to find an absolute minimum or maximum, it is a tradeoff analysis to define the optimal temperature range. The C temperature range is where the surface quality curve grows most significantly while the product cycle time stays fairly constant to then increase almost exponentially. For both these reasons we choose the range with bandwidth of about 10 C, placed at the extreme right of the interval (218 C < T < 228 C).

24 Conclusion Bruschi S.p.A. has more than 70 years of experience in die casting of zinc alloys. Over time, efficiency, precision, ability to listen to our customers and innovative drive have brought the organization to excel in both the engineering and technology aspects. We can assist you both in the design and engineering phase of your product as well as in the assembly and finishing of components to reduce scrap and production time to a minimum. For further information on the company, please visit website or write an to info@bruschispa.it

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