Stefan Magerl (left), Plat­form Devel­op­ment engi­neer at ebm-papst Land­shut and Remo Fedele (right), Advanced Devel­op­ment engi­neer at ebm-papst Mulfingen

In die-casting, the molten metal is pressed into a mold at high velocity and under great pres­sure. The velocity of the molten metal in the gate vg is usually between 30 and 60 m/s. Casting pres­sures attain values of between 400 and 800 bar. A high mold filling rate and high pres­sures make it possible to produce cast­ings with high dimen­sional accu­racy and extremely good surface prop­er­ties. The specific casting pres­sure occurs in the cavity of the perma­nent mold. The higher this is, the higher will be the gate velocity.

When calcu­lating the casting pres­sure it is however impor­tant to make allowance for the fact that flow resis­tances that have to be over­come occur at changes in cross-section and direc­tion, at corners, at edges and on account of wall rough­ness. These resis­tances are calcu­lated using a dimen­sion­less quan­tity, the resis­tance coef­fi­cient φ. According to the Bernoulli equa­tion p=( va²)/(2 φ</span²), a rela­tion­ship exists between the metal pres­sure and the gate velocity or flow rate. At low velocity there is a risk of solid­i­fi­ca­tion in the gate before the secondary compres­sion of the liquid metal takes effect in the mold cavity. It also results in irreg­u­lar­i­ties in the stream struc­ture, such as slag or oxide parti­cles, and in local clog­ging. Low flow rates do however cause less turbu­lence and are favor­able with respect to gas removal from the mold cavity.

Repre­sen­ta­tion of flow rate

The local wear and cavi­ta­tion asso­ci­ated with higher flow rates shorten the service life of the casting mold. These do however normally produce cast­ings with smooth surfaces and a fine-grained struc­ture.

Depending on the casting mate­rial and the casting concerned it is there­fore impor­tant to choose an appro­priate specific pres­sure to obtain a casting of optimum quality. Casting simu­la­tion can be helpful here. This numer­ical simu­la­tion is primarily a math­e­mat­ical descrip­tion of the Bernoulli equa­tion. It provides a graph­ical repre­sen­ta­tion of the rela­tion­ships between pres­sure and flow rate as a func­tion of the optimum casting temper­a­ture for the various alloys. With the results obtained, compo­nents and tools can be opti­mally designed.