How to prevent cavitation?

In the molding of articles by means of zamak die casting, considerable difficulties can arise in achieving the surface quality of the casting to meet the customer’s aesthetic requirements.
In order to achieve a good surface quality of the die casting, i.e., without marbling, it is necessary for the liquid metal to reach all areas of the impression as quickly as possible and without yielding temperature. Hence the need for high pressures and high flow velocities in the filling phase.
High pressures give rise to the phenomenon called “water hammer” at the end of the filling phase, while high velocities generate flow turbulence, both of which are crucial factors in the occurrence of cavitation.
We have found in our experience that using liquid zamak injection speeds below 60 m/s significantly reduces the occurrence of the phenomenon in the mold surface – an important fact to consider when designing the mold and the process.

The use of simulation programmes to fight cavitation

A very useful tool used daily in Bruschi’s engineering area is simulation software.
With these programmes, the engineering team can simulate the filling of the product in order to design casting and vent channels capable of achieving the quality required by our customers without, however, causing premature deterioration of the mold.
During the realisation of projects, from the design phase to the production phase, with the simulation software it is possible to prevent all possible defects on the mold. In this way, Bruschi can to save time by predicting possible problems, achieving customer goals in the best possible way.
These simulations are of such importance that they are also taken into great consideration during the bidding phase to the customer, as they prove essential in anticipating any eventuality.

Conclusion

The aim of engineering is to solve the cavitation problem by studying the behavior of the fluid inside the molds on a daily basis. These evaluations are carried out using simulation software, with which Bruschi can visualise parameters such as flow velocity, pressure inside the mould and filling temperature of customer products.
Considering that each product has a unique geometry, different issues and new daily challenges arise and are addressed to ensure quality products for the Bruschi team and its customers.

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On the subject of mechanical strength, and more in detail, breaking load, in this post, we will briefly define the function of rupture tests in the structural quality control of zinc die castings during production. This article will also describe the instrument used to perform these tests to ensure compliance with the specifications of the component and some case studies related to the main topic.

Ultimate tensile stress: breaking strength test of a part or assembly

As anticipated, the ultimate tensile stress is the force that must be applied to a component to cause yielding and/or rupture and is measured in mega-pascals, indicated by the symbol MPa, which is the primary unit of pressure measurement, that is, of the force on the unit of surface.

In order to proceed with this test, the force is applied at a point of the piece predetermined and agreed with the customer to observe the load necessary to cause its breakage or yielding. In this way, it is possible to test the amount of force needed for the test piece to lose the specific mechanical properties, that is, the ability of the component to withstand external loads and, consequently, be unusable.

The execution of this process requires a specific machine for laboratory tensile/compression tests.

Therefore, this machine is extremely crucial because breaking strength tests are among the key elements for the functional verification of a finished product. Often individual zinc die-cast components are mounted in an environment or in an assembly that undergoes various stresses from the outside, and these forces, if not adequately considered and analyzed, could compromise one or more elements of the system by jeopardizing the function of the product itself. An obvious example of this concept is undoubtedly the steering group of a car that undergoes different stresses due to different drivers. The individual elements and the system must necessarily be able to perform their function without breaking even in the event of excessive forces applied by drivers with extreme driving styles. Consequently, avoiding malfunctions and ensuring the correct mechanical strength of steering components is an essential safety factor for the driver.

The ultimate tensile stress: real tests and computer simulations (FEM)

This kind of test can also be simulated within CAD (FEM) applications, but as the die casting process is known, also using vacuum technology, it does not allow the molding of parts with theoretical material density because, as a result of the volumetric shrinkage during the cooling inside the zones of the die-cast with a higher mass, microcavities are created, typically called shrinkage porosity, which could compromise the performance of the product in terms of mechanical strength. These density variations are not completely intercepted by the simulation software systems, which can be very useful in the design phase but which, in terms of production, could call into question the achievement of the required specifications due to the inaccurate results detected by the simulation program. For this reason, it is recommended to use simulation programs to define an idea of reality, a production guideline, and then, directly in the foundry, instead proceed with sample checks during the molding and verify in detail every single aspect related to mechanical strength.

Having constant real-time control over the production, cyclically checking on a sample basis, is therefore essential to verify if the production complies with the customer’s specifications. Thus, the advantage of using these control instruments directly in production involves continuous control and the ability to act in real-time to avoid any line interruptions.

In conclusion

As we have briefly observed before, the tests in production of the ultimate tensile stress do not replace the simulations, but they place side by side with different objectives. These are critical, periodic tests to ensure a reliable and specific process during production.

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